Appendix 2

A Short Course In Anatomy, Physiology, Mechanics And Biomechanics

It is not necessary to be an expert in anatomy, physiology, mechanics or biomechanics in order to be a weightlifting champion or a successful coach. If you have read the rest of this book, you have learned a great many scientific principles which apply to the sport of weightlifting without delving very far into their formal underpinnings in the more general sciences of anatomy, physiology, mechanics and biomechanics. I have known many successful athletes and coaches who had no formal or informal training in any of these general sciences, although all are quite familiar with many of the weightlifting-specific scientific principles that were presented earlier in this book and have at least an intuitive grasp of the more fundamental areas of sport science. No doubt you can be equally successful without further pursuit of subjects like physiology and biomechanics.

However, athletes and coaches who have achieved outstanding performance without such knowledge have been successful despite a lack of training in these areas, not because of it. It is difficult to fully understand some of the most basic principles of weightlifting training and technique without a grasp of the scientific bases of sports performance, at least those which pertain most directly to weightlifting. Therefore, in order for an athlete to live up to his or her potential, he or she should know some of the key concepts in these areas. Such knowledge is even more essential for the coach and/or the athlete who is self-coached. Space constraints, the essential purpose of this book and the availability of books which cover sports science thoroughly make it inappropriate to address more than the basics here. Those interested in further developing their understanding of sports science and their analytical skills with respect to weightlifting may wish to go well beyond what is presented in this section. The Bibliography of this book offers suggested readings for those who do wish to build further on the foundation that this section provides.

The Interrelationship Of Anatomy, Physiology And Biomechanics

Weightlifting is primarily concerned with imparting force to a projectile called a bar (although receiving force from and sustaining force against a bar are also of great importance). Those who perform and analyze weightlifting are of course interested in how the application of force can be accomplished most effectively. The effective application of force to a bar (or anything else) is influenced by a number of factors, and many of these factors fall within the realm of different branches of the sport sciences.

For instance, general principles relating to the motion of a projectile, the application of force and the use of energy fall within the purview of mechanics, a subdivision of the science of physics. Biomechanics applies the principles of mechanics to the specifics of human movement. Anatomy tells us about the structure of the body, including the nature of the tools (e.g., levers) that the human body gives us for producing and moderating the effects of forces. Muscle physiology tells us how our muscles function in order to move the levers of our body (the bones) and produce force. We will examine each of these subjects separately in some detail.

In some cases the discussions of different disciplines will overlap. For example, in examining the bases of muscular contraction (an area of physiology), it is necessary to understand the ultrastructure of muscle tissue (an area of anatomy). Therefore, an aspect of anatomy will be discussed in the physiology section.

This point should serve to remind the reader that all of aforementioned disciplines, as they pertain to sport, are highly interrelated. In fact, a branch of scientific study called kinesiology has evolved as a result of the need to integrate biomechanics, musculoskeletal anatomy and neuromuscular physiology in order to understand the movement of the body. Therefore, you should always be mindful that what you learn in one part of this section of the book needs to be integrated with what you learn in another. Perhaps more importantly, what you learn in this section of this book needs to be integrated with what you learn in the others if its value is to be maximized in terms of weightlifting performance.

Physiology

Human physiology is a vast subject. Today we understand so much about the way that our bodies function that a person who spent his or her entire life studying physiology could probably not learn it all, and yet there is still a great deal that we do not know. In the discussion that follows, the focus will be on muscle physiology, the aspect of physiology that impacts most strongly on weightlifting. It is assumed that the reader has some basic knowledge of other areas of human physiology. If you do not, the Bibliography lists some resources for further study in this area.

Muscle Action as the Basis for Human Movement

Virtually all internally generated human movement is caused by what are conventionally referred to as muscular contractions. A number of researchers have recently argued that the term “action” is preferable to “contraction” because while a contraction implies a shortening, the term contraction is used to describe muscles that are shortening (concentric contraction), lengthening (eccentric contraction) or remaining the same length (isometric contraction). Physical movements of the kind used in sport are caused by the contraction of a specific kind of muscle, called skeletal or striated muscle (as compared with the cardiac muscle which powers the heart and the “smooth” muscles that support certain internal organs and functions of the body).

The Structure and Action of Skeletal Muscle

What we commonly refer to as a “muscle” is really a complex unit of contractile tissue that is surrounded by a connective tissue called the epimysium or fascia. Within the fascia are bundles of muscle cells called fasciculi, each of which is covered by a connective tissue sheath called a perimysium. Within the fasciculi are typically 100 to 150 muscle cells or fibers. Within those muscle fibers are bundles of units called myofibrils, which run the length of the muscle fiber. Inside the myofibrils themselves, arranged in series (lengthwise, end to end), are entities called sacromeres. A sacromere is the smallest functional unit within a muscle (i.e., the smallest entity that can contract).Fig. 50 depicts the progressively smaller components of the muscle.

While the mechanism of contraction is not fully understood, a rather well developed explanation of how muscles contract has emerged. Called the “sliding filament theory,” it has undergone considerable amplification and clarification since it was first advanced by Huxley several decades ago.

Sliding filament theory can be explained as follows. Within a sacromere are “strings” of protein called filaments, which come in two varieties, thick and thin. The thick filaments are made primarily of a protein called myosin, and the thin filaments are made primarily of a protein called actin. These actin and myosin filaments run lengthwise in the sacromere, parallel to one another. Myosin filaments are typically two to three times the thickness of actin filaments, but the actin filaments outnumber the myosin filaments by a ratio of two to one. Fig. 51 depicts a cross section of the sacromere that highlights the arrangement of actin and myosin fibers.

 Hundreds of myosin molecules make up each myosin filament. Each myosin molecule is made up of two identical sub-units, each shaped somewhat like a golf club and arranged with the “shafts” of these golf club shaped units intertwined. Within a myosin filament, these molecules are arranged with the shaft portions parallel to one another but staggered lengthwise, so that the heads of different molecules are nearer or further from the center than the heads of other molecules.

The molecules are further arranged such that half face one direction and the other half face the opposite direction. The result is that the club-head-shaped portions of these molecules are at either end of the filament and the shafts shaped ends meet in the center of the filament. Therefore, the center portion of the myosin filament appears thinner and contains less protein than either end of it (see Fig. 52).

The club-head-shaped portions of the filament are referred to as its “cross-bridges.”  On each cross-bridge there is a site at which it can bind to a corresponding site on an actin molecule and another site called the myosin ATPase site. This latter site is capable of binding with the chemical ATP (which is the only direct source of energy for muscular contraction) and breaking it down into adenosine diphosphate (ADP) and inorganic phosphate (Pi), yielding the energy which is utilized by the myosin cross-bridges during the actions that they perform during contraction .

Actin molecules are essentially spherical, each with a site at which it can bind with a myosin molecule. (At rest these sites are prevented from coming in contact with the binding sites on the myosin cross-bridges by proteins called tropomyosin and troponin.) The actin molecules are arranged like two strings of “pearls” twisted together to form an actin myofilament (Figure 53). These myofilaments attach at either end of the sacromere to a structure called the “Z-line,” a connective tissue which separates myofibrils from one another at their ends (really a disc-like structure), so that the Z-lines connect (without direct contact) the ends of actin filaments of adjoining sacromeres and help to maintain their orderly arrangement.

When viewed from the side, sacromeres appear to organize themselves into several segments or “bands.” The schematic diagram in Figure 54 depicts in conceptual, if not visually accurate, terms the arrangement of these bands.

Within each sacromere in a relaxed state (schematic (a) of Figure 54), there is a significant gap between actin myofilaments (called the H-zone or H-band). The H-band contains the central, non-cross-bridge portion of the myosin filament and may contain some of the cross-bridges at either end of the myosin filament. The areas called the I-bands contain only actin filaments. The areas called A-bands contain the entire length of the myosin filament and the portion of the actin filament that overlaps either end of the myosin filament. In the center of the A-band is an area referred to as the M-line, which is believed to serve the function of holding the thick myosin filaments together in a vertical stack (much as the Z-lines help to maintain the order of sacromeres) ;like the Z-line, the M-line is really three dimensional in nature.

During contraction the binding sites on the cross-bridges of the myosin filaments make contact with the binding sites of actin filaments. This releases the energy stored during the breakdown of ATP, causing the cross-bridges to perform a swiveling or stroking action toward the center of the sacromere, pulling the actin filaments closer to the center of the sacromere (schematic (b) of Figure 59). At the same time ADP and Pi are rapidly released by the myosin filament. This frees the ATPase site so that it can attach to another ATP molecule (which occurs at the end of the stroking motion). The new ATP molecule is split by myosin ATPase, creating energy for the myosin cross-bridge to “stroke” once again. The attachment of ATP to the myosin site occurs before the cross-bridge link between myosin and actin is broken.

After each successive stroke (a process which results from “reloading” or “recharging” the cross-bridge with an ATP molecule), the cross-bridge returns to its original position, where it contacts another site closer to the end of the actin filament. In order for a complete shortening of the sacromere to take place, this process of attachment, stroking, detachment and repositioning of the cross-bridges must take place repeated times. In many cases, , relaxation (a chemical reaction which covers the actin binding sites with the two proteins that interfered with actin/myosin connecting before the contraction) occurs before maximal shortening is achieved.

The attachment and detachment process takes place in an asynchronous fashion (i.e., with different cross-bridges in the same filament attaching at different times) so that there is a continual shortening of the overall sacromere. (If all cross-bridges went through the same part of this cycle at the same time, the actin filaments would slide back to their original position during the period of no contact between the actin and myosin filaments.) It should be noted that the result of this asynchronous action within the sacromere is a smooth contraction of the whole sacromere.

As a result of the contraction or sliding filament process, the H-band becomes much narrower and may even disappear as the actin filaments are brought closer to one another. In addition, the width of the I-band at either end of the sacromere decreases. Neither the actin nor the myosin filaments themselves changes in length during contraction. Therefore, the width of the A-band (which is equal to the length of the myosin filament) remains the same throughout the contraction.

Contractile machinery comprises approximately 80% of muscle-fiber volume. The balance of the fiber volume is comprised of tissue that supplies energy to the muscle or is involved with the neural stimulation of the muscle.

The Neural Basis for Muscular Action

Contractions of skeletal muscles are caused by impulses or “action potentials” that are delivered to the junctions between specific kinds of nerves (called alpha motor neurons) and muscles. Because the process of muscle contraction involves the combination of neural and muscular activity, it is often described as neuromuscular in nature.

An alpha motor neuron consists of a cell body (or “soma”) with numerous short projections called dendrites. The dendrites carry impulses to the cell body. Extending from one side of the neuron is a long projection called an “axon”, which connects the alpha motor neuron to the muscle fibers that it innervates. The axon is almost fully covered with a white, fatty substance called myelin. Myelin insulates axons from other axons and dendrites in the same nerve. Small gaps in the myelin covering are called the “nodes of Ranvier.”  These breaks speed transmission of impulses along the axon as the impulses are actually able to “jump” from one node of Ranvier to another.

As the axon nears the muscle, it loses its myelin sheath and divides into many terminal branches. Each branch, or terminal, enlarges into a knoblike structure called a terminal button. That button fits into a small depression in the muscle fiber called the motor end plate), which delivers the contractile impulse to the muscle fiber itself. Figure 55 depicts an alpha motor neuron.

The speed with which the impulse travels along an axon is influenced by two primary factors: whether or not the axon is covered by myelin and the size (diameter) of the axon. Alpha motor neurons, which are large, myelinated (meduallated), neurons, can carry nerve impulses as fast as 120 meters per second. (Non-myelinated neurons that are very small carry nervous impulses at a speeds as slow—relatively speaking—as .5 meter per second.) There are no significant differences in speed related to gender, and these speeds remain relatively stable between the ages of twenty and forty, after which the speed begins to decline gradually.

One alpha motor neuron may control as few as several and as many as a thousand or more muscle fibers. However, each muscle fiber is innervated by no more than one neuron. The fibers controlled by a given motor neuron are distributed throughout the muscle.

All of the fibers stimulated by the same neuron tend to be similar in their physical, biochemical and ultrastructural characteristics, i.e., they are of the same muscle fiber “type.” (An explanation of muscle fiber types and their properties will be presented later in this section.) This suggests that innervation has an influence on the properties of muscles (indeed some experimental evidence exists to support this notion).

A neuron and the muscle fibers that it innervates are referred as motor unit, the smallest contractile unit that is under neural control. Motor units vary in the frequency with which they can generate impulses (i.e., in their “firing rates”). The motor units of small muscles reportedly fire at rates between nine and fifty pulses per second, while the motor units of larger muscles have a narrower range (from thirteen to thirty pulses per second).

An accepted principle of muscle action is that either all or no fibers within a motor unit contract. This is referred to as the “all or none” principle. While the all or none concept is generally true, some researchers have argued that under certain conditions an impulse delivered by given neuron may not activate all of the muscles it innervates. (A stimulus may not be of sufficient strength to activate some of the least irritable fibers in a motor unit because certain fibers may be compromised in their functioning by such factors as fatigue or limited circulation.)

Within a muscle, motor units vary widely in their maximum performance-potential and specific tension (the tension per unit of cross-sectional area that a muscle can develop).

When a muscle generates a sustained contraction of moderate or less than moderate effort, it avoids becoming fatigued by two forms of asynchronous recruitment of motor units (i.e., recruitment at different times). First, units that innervate the same types of muscle fibers will be recruited at different times, so that some are resting while others are working (this process is not possible at higher loads because more units are activated to contract simultaneously under the higher load). A second form of asynchronous recruitment of motor units occurs when fibers that are more fatigue-resistant are recruited before fibers that fatigue rapidly. It has been observed that during sustained voluntary contractions there is a dropout of active motor units and a recruitment of fresh fibers with similar but slightly greater thresholds of activation.

There is evidence that fatigue during fast and powerful activities may occur first at the neuromuscular junction. When neuromuscular fatigue occurs, the motor neurons cannot manufacture acetylcholine (a chemical which effectively transmits neural stimulation to the muscle) fast enough to maintain chemical transmission of action potential from the motor neurons to the muscles.

There is also a phenomenon referred to as central or psychological fatigue which occurs when the central nervous system (CNS) can no longer activate motor neurons. Fatigue, discomfort, boredom or lack of sleep may bring on CNS failure. The individual and combined influences of these mechanisms are not well understood.

The Chemical Basis for Muscle Action

While the preceding discussion of the neural basis for the stimulation of muscle contraction emphasized the “electrical” nature of that process (e.g., by discussing the effect of nerve impulses that are electrical in nature), chemical processes within nerves and muscles form the basis for the generation and transmission of electrical impulses. Therefore, it is now appropriate to discuss some key chemical processes that lead to muscle contraction. It should be noted that while the discussion focuses on activity within the muscle, similar processes occur within the nerve to maintain the neural activity there.

All living cells have the capacity to maintain the charged ions of different chemicals in a separate and electrically unbalanced state (ions are atoms that have greater or fewer electrons than electrically balanced atoms). In the case of muscle cells in their resting state, sodium ions (Na+) in the fluid outside the cell exist at a much higher level than on the inside (the opposite is true for potassium ions – K+). Overall, the outside of the cell is positively charged relative to the inside. This state of electrical imbalance is maintained in part by the permeability (or lack thereof) of the cell membrane (a covering that exists in some form around all types of cells) to Na+ and K+ ions and is referred to as a “membrane potential.”

As mentioned earlier, neurons and muscle cells meet at a point called the neuromuscular junction. There is a small space between the axon ending and the motor end plate at that junction, but it is too large for a direct transfer of the electrical impulse in the nerve to the end plate. Therefore, a chemical transmitter called acetylcholine (ACh) is required to accomplish transmission. When the terminal ending of the axon is simulated by a nerve impulse, it releases ACh into the gap between it and the motor end plate of the muscle, effecting transmission of the nerve impulse to the muscle. The mechanism of that transmission process is described below.

The release of ACh into what is referred to as the “synaptic space” between the motor neuron and special receptor sites on an area called the motor end plate of the muscle cell membrane triggers a change in the ion permeability of the muscle cell membrane. Sodium flows into the muscle cell and potassium flows out, the latter slightly later and more slowly (the amount and duration of this process, known as depolarization of the motor end plate, is determined by the amount of ACh released). As a result of this ion exchange, the inside of the cell becomes positively charged for a short time and the exterior becomes negatively charged, generating a small electrical current called an action potential. When depolarization occurs at the motor end plate, local current flow occurs between the depolarized end plate and the adjacent resting cell membrane in both directions. (The neuromuscular junction is generally located in the center of a muscle fiber, so impulses must travel outward from it in order to reach the entire fiber.) The action potential that is thereby transmitted throughout the fiber causes it to contract.

A contraction that is the result of a single nerve impulse is referred to as a “twitch.”  Single twitches of a muscle fiber are too short and weak to be of any practical use, and they do not normally occur. Functional tension is generated by multiple twitches in the same muscle fiber and contraction by multiple muscle fibers.

An action potential lasts only one to two msecs (msecs), and it is not until approximately three msecs later that muscle contraction commences. That time interval is known as the latent period (during this latency period, the muscle is unable to respond to another contraction stimuli). It takes approximately fifty msec (from the time that a muscle commences its contraction) for the muscle to reach its maximum tension (this contraction time varies considerably with muscle-fiber type). It takes a slightly longer period for the muscle to relax, so that the overall time of contraction and relaxation is approximately 100 msecs.

It should be noted that the chemical ACh plays another key role in muscle contraction. When ACh is released into the muscle, it causes a chemical reaction that alters the two proteins that, at rest, cover the binding sites on the actin filament (points at which the myosin cross-bridges can attach). The cross-bridges then make contact (there is some debate within the scientific community over whether this contact is direct or is mediated in some way).

The entire process that links excitation to contraction is referred to as the excitation-contraction coupling process.

Muscle Fiber Types

All muscle fibers are not the same. At least seven different types of muscle fibers have been identified, and some researchers believe there may be even more. In humans, only three muscle fiber types have been detected in significant quantities: Type I (also known as slow twitch, slow-oxidative or red fibers); Type II A (also known as fast twitch, fast oxidative or white fibers); and Type IIB (a second variety of white or fast twitch fiber often referred to as a fast-glycolytic fiber). The differences between Type IIB fibers and Type I fibers are more significant than those between Type II A and Type I fibers.

Fast twitch (FT) fibers are better suited for anaerobic activity than slow twitch (ST). Relative to ST fibers, FT have a higher myosin ATPase activity rate (which results in a faster rate of energy release in the muscle). FT fibers shorten and relax more quickly than ST fibers, which enables the former to deliver more power than ST fibers with the same cross-sectional area. FT fibers also have a lesser degree of capillarization than ST fibers (which causes their paler or “white” color relative to ST fibers). FT fibers are larger in diameter than ST fibers because of the greater presence of actin and myosin in the FT fibers. ST fibers have more intramuscular triglyceride stores (a source of energy);more myoglobin (a substance which facilitates the use of oxygen to create energy—subject discussed in some detail later in this appendix—and gives these muscle fibers their red color); more aerobic enzyme activity; greater capillary density; and greater mitochondrial density than FT fibers. (Mitochondria are responsible for manufacturing approximately 95% of the ATP that exists in muscle tissue.) The characteristics of FT fibers make them highly sensitive to fatigue, so they are best suited for generating a large force over a short duration. Type II A fibers have good aerobic and anaerobic qualities, but Type II B are good anerobically and poor aerobically.

Fast oxidative (Type II A) fibers have high ATPase activity like fast-glycolytic fibers, but a high oxidative capacity like slow oxidative fibers. They can maintain a contraction longer than fast glycolytic fibers and contract faster than slow oxidative fibers. ST fibers have capacity for long-term, low-intensity work ,so they are better suited for aerobic activities.

Motor units appear to have homogeneous fiber types, but muscle fibers from different motor units are mixed within muscles. FT fibers have significantly larger neurons than ST fibers, so they are activated with more difficulty and only after ST fibers have been activated, but the speed with which nerve impulses move down their motor neurons is greater. FT fibers are typically brought into play either by the effort to move a heavy load or the need to move an object faster than is possible through the use of ST fibers alone. There is a positive correlation between a muscle’s recruitment threshold (the point at which it is activated to contract) and its twitch tension (the degree of tension that it develops with a single twitch).

It should be noted that the speed with which muscle fibers contract varies. The average contraction time (time from the onset of a contraction to the point at which maximum tension in achieved) of a skeletal muscle is approximately fifty msecs, but there is more than a threefold difference between the contraction time of the fastest and slowest muscle fibers. Therefore, while an FT fiber may be activated later than a ST fiber, the former may complete its contraction at the same time as a slower fiber because of the differences in contraction speed.

Although the general order of muscle fiber recruitment is influenced by fiber type, the order of recruitment of particular fibers within fiber types is influenced by the nature of the motion that is undertaken. In fact, fiber-recruitment order appears to be fixed for a particular movement. For example, the recruitment order of particular muscle fibers in the hamstring muscles may be different for a leg curl than for a leg press, even if the tension developed in performing both exercises is identical.

Most muscles combine all three types of fibers, the percentage of each being correlated with the type of activity for which the muscle is specialized. For example, a higher proportion of slow oxidative fibers are found in the back and legs (which often contract at low intensity for long periods in resisting gravity). In contrast, fast glycolytic fibers dominate in the arm muscles. The distribution of FT and ST fibers varies among individuals as well as muscles, with most people having a fifty-fifty split. However, there can be very significant differences in the distribution of muscles types, presumably making some individuals better suited to certain activities.

Factors Influencing the Force Produced by Muscles

Despite the “all or none” principle of motor units, graduations in the force (i.e., tension) generated by motor units can be achieved by differences in the rate at which nerve impulses are generated by the neurons in those motor units (their “rate of firing”). Single twitches of muscle fibers occurring in rapid succession lead to stronger levels of contraction until fatigue begins to occur. This phenomenon of successive single twitches resulting in stronger contractions is referred to a “treppe.”

The frequency of firing can also affect the tension generated by a muscle because of differences in the speeds of neuronal firing and muscle contraction. Specifically, an action potential and the latency (rest) period that must occur before another action potential can be generated is far shorter than the period of muscle contraction and relaxation (a few msecs for the former and more than 100 msecs for the latter). Therefore, as soon as a few msecs after a muscle is stimulated by a neuron to contract (i.e., well before the point of full or peak contraction from that impulse has even been reached), a new impulse may stimulate the muscle to contract again. As long as the new impulse arrives soon enough, it will stimulate the muscle fiber to contract before it has fully lost the tension generated during its previous twitch. This causes the muscle to achieve a higher level of tension than it did as a result of the first twitch.

The increasing tension that results from the muscle’s receiving a stimulation to contract before relaxation has occurred is called summation. Summation continues as long as impulses of sufficient frequency are received by the fiber, until the fiber reaches its maximum level of tension, a state called tetanus (this normal state of maximum tension should be distinguished from the pathologic state of tetanic contraction that can result from a tetanus infection). It has been reported that the forces developed in muscle fibers as a result of summation are as much as four times greater than the forces that can be generated by single twitches. (The phenomenon of summation can occur in an entire muscles as well as in single fibers if stimulation is sufficient to make all muscle fibers achieve tetanus simultaneously.)

Within the ultrastructure of the muscle, tension increases when the thin actin filaments within the sacromere are brought closer together as a result of greater cross bridge cycling. With tetanus the maximum number of cross-bridge binding sites remain uncovered so cross-bridge attachment and, consequently, tension development, are at their peak.

The force developed isometrically by muscle fibers appears to be relatively independent of the fiber type but closely related to the cross-sectional area of the muscle fiber. However, since slower fibers tend to have smaller diameters, they tend to exert lower levels of force. Moreover, since ST fibers have slower speeds of contraction (ST fibers require 90 msecs to 140 msecs to contract, while FT require only 40 msecs to 90 msecs), their power output tends to be lower even if they can exert force equal to that of a given FT fiber.

The physiological cross-section of a muscle can be estimated by dividing the a muscle’s volume by its fiber length. Overall muscle size varies with the size of the individual muscle fibers within it and, to a lesser degree, by the genetically determined number of those fibers.

In addition to the differences in force that muscle fibers generate as consequence of their size and the rate at which neurons stimulate them, graduations in force are achieved by differences in the number of units that are stimulated to contract at any given time. The more units that are stimulated to contract, the greater is the force that is developed. Normally the impulses that stimulate different motor units are not simultaneous (asynchronous). When a rapid and maximum effort is required, the impulses that travel to many or even all motor units may occur simultaneously. Finally, additional muscles may be brought into play to supply extra force under extreme conditions.

The firing rate of each motor unit increases gradually as effort increases while the recruitment of more units represents a greater change. Consequently, the rate of firing is probably utilized for more precise changes in force and the number of units recruited is more suited to extreme changes in force.

Even a relaxed muscle has some tension, or tonus, which is due to a baseline level of neural activity. Muscles used more often tend to have more tonus. Muscles used while held in shortened position develop tonus in that position (as do the muscles that oppose the action of those muscles).

All Muscles Have an Optimal Length for Generating Force

Apart from neuronal influences, the force with which a muscle fiber can contract is influenced by the length of the muscle fiber at the onset of the contraction. Each muscle fiber has an optimal length at which maximum force can be achieved (though that force may not be fully reflected externally, due to mechanical constraints). This optimal length occurs at the point at which there is maximum cross-bridge formation. When the muscle length is greater, the actin filaments cannot make contact with as many sites as when the muscle is at its optimal length. (At 70% more than the optimal length—which can only be achieved in a laboratory—there is no overlap of the myosin filaments by the actin filaments.)

When the muscle is at a shorter than optimal length less tension can be developed for several reasons. First, the chemical processes taking place within the muscle are so altered that fewer actin sites are uncovered (the reasons are unknown). Second, thin filaments from opposite ends of the sacromere overlap one another, reducing the number of actin sites exposed to the cross-bridges. Third, the myosin filaments touch the Z-lines, impeding further shortening. The most extreme forms of these reactions to muscle shortening occur only in a laboratory setting.

Under normal circumstances the optimal length of a muscle is its relaxed length. Moreover, under normal conditions, a muscle cannot achieve more than a 30% shortening or lengthening beyond its normal length (and these are the outside limits). At such extreme points, the ability of the muscle to contract is lessened by about 50%.

Activity within the ultrastructure of muscle tissue appears to have an effect on the muscle’s resistance to injury as well as the work it can perform. It has been determined experimentally that the amount of energy that can be absorbed by a muscle before failure is greater when a muscle is active than when it is inactive.

It should be noted that in practice differences in the mechanical resistance offered due to the position and nature of the body levers involved (i.e., bones and soft tissues being moved by the muscle’s action) are more influential in determining the force generated externally (the “strength curve”) for that part of the joint motion than changes in the lengths of the muscles that are acting. The resistance offered agonistic muscles (muscles that cause a lever to move) by antagonistic muscles (muscles that can pull the lever in the opposite direction) can have an important influence on the practical expression of strength as well. Consequently, a decrease in the resistance encountered by the muscles, a change that occurs as the as a joint angle increases, generally outweighs the decrease in the ability of the muscle to generate tension, with the result that the ability of the muscles to express its force externally increases. An example would be in the extension of the arm. More weight can be lifted at the end of the extension than at the beginning, even though the triceps muscles that are responsible for the motion are at a relatively weak point when the arm is nearly extended.

The Force-Velocity Curve

The maximum velocity of a muscle’s contraction occurs when there is no resistance (i.e., when velocity is constrained essentially by the maximum rate at which cross-bridges can be formed and broken). The maximum speed of contraction appears to be unaffected by the number of cross-bridges that are interacting with actin filaments at any given point in time (because, unlike strength, speed of contraction is relatively stable over a wide range of muscle lengths). In contrast, maximum force develops at zero velocity (i.e., during an isometric contraction). It is presumed that the force-velocity curve is caused by a slower rate of cross bridge stroking under loaded conditions. This relatively smooth relationship between force and velocity pertains to entire muscles (see Fig. 56, in which the vertical axis represents contraction velocity and the horizontal the force of contraction).

For single muscle fibers, the force-velocity relationship is more complicated. Instead of one smooth curve or relationship between force and velocity, there are two distinct curves located on either side of a break-point at approximately 75% of the muscle’s maximum isometric force (Figure 57). A muscle fiber begins to lengthen when the force applied to it reaches its maximum isometric force (i.e., an eccentric contraction begins). Only when the force applied to a muscle reaches approximately 40% to 50% more than the muscle’s maximum isometric force does the speed of the eccentric lengthening of the muscle become great. At forces less than the maximum isometric force, the muscle shortens (the smaller the percentage the maximum isometric force the faster the shortening). At the point of maximum isometric force, the force and velocity relationship becomes nearly flat, and there is little change in velocity with a fairly wide range of force change. This zone of stability is what gives muscles the capability to be relatively stable when loads are high, such as when you walk down stairs.

Differentiation in the kinetic properties of muscles extends below the fiber level because the maximum speed of shortening (Vmax) and force-velocity relation vary from one part to another along the same fiber. (Variations in Vmax within a fiber may be as large as among different fibers in the same muscle, with Vmax generally falling near the distal ends of the muscles, essentially the points furthest away from the body’s center.) A muscle’s ability to produce force declines quickly as a muscle shortens, but this depressant effect on the strength of contraction falls over time, so that after about a second, a muscle that has shortened to a certain point is able to exert about the same force as a muscle that began contracting at that point.

A fall in a muscle’s ability to produce force as a result of fatigue is associated with a much smaller decline in the muscle’s stiffness, so it is thought that the decline in force is due only partially to fewer cross bridges attaching. The major factor is believed to be a reduced force-output of the individual bridge (there is also a reduced speed of recycling of bridges during fatigue). All three of these effects are probably due, at least partially, to the accumulation of the breakdown products of ATP with continued muscle action.

The Arrangement of Muscle Fibers Within a Muscle

Muscle fibers can be arranged in a number of different ways within a muscle. There are two main kinds of arrangements: fusiform (longitudinal) and penniform. Fusiform muscles have their fibers arranged essentially parallel to the surface of the muscle and to the pull of the muscle when it contracts. Penniform muscles are arranged diagonally with respect to the pull of the muscle when it is contracted (Figure 10).

Fusiform muscles tend to have smaller cross-sections than penniform muscles. As a consequence, fusiform muscles generally contract with less force than penniform muscles. However, the arrangement of muscle fibers along the direct line of pull in fusiform muscles enables the overall muscle to shorten more rapidly and to a greater extent (in relation to their length) than penniform muscles.

Penniform muscles tend to have larger cross-sections than fusiform muscles, but their diagonal arrangement within the muscle causes them to give up some mechanical efficiency in terms of generating force. But less efficiency in generating force is more than offset by the greater number of fibers that can be brought into play by such muscles.

The Nature and Function of Connective Tissue

As s noted above, muscle tissue is infiltrated and surrounded by connective tissue (e.g., the epimysium which covers an entire muscle). The thickness and strength of these connective tissues that surround muscles and their component parts vary significantly from muscle to muscle. Such variations probably explain, at least in part, the different findings with regard to the physical properties of muscle tissue.

Most skeletal muscles merge into a connective tissue (called a tendon) at either end. It is the tendon that connects the muscle to the bone (at a juncture called the aponeurosis). The load that a tendon can sustain is influenced by the size and shape of that tendon, the speed with which any loading is applied to the tendon and any training effect that activity has had on the tendon. In normal activity, tendons are stressed at only a fraction of their limits, but very rapid and unexpected stresses (such as a slip while bearing a load) can overstress a tendon, causing it to rupture. This is one reason why proper conditioning, technique and equipment are so necessary for safe athletic activity.

Elastic Components of Muscle and Their Relation to Function

Muscles appear to have at least three interdependent elements which contribute to force generation: a contractile element (CE), a series elastic component (SEC) and a parallel elastic component (PEC). The CE functions through the sliding filament theory that was described above.

The series elastic component (SEC) is connected in series with the contractile element of the muscle (i.e., along the length of the muscle). It acts like a spring within the muscle in that its function is related to tension development in the entire muscle-tendon unit, particularly when the muscle shortens from a previously stretched position. It is believed that both the tendinous tissues of muscle and the cross-bridges of the muscle fibers themselves contribute to the SEC. The SEC is capable of receiving the energy delivered by the contractile element and/or external forces and then returning that energy.

The PEC is comprised of connective tissue and is believed to be a major source of the opposing forces encountered when trying to elongate a passive muscle. It runs parallel to the CE and/or its line of pull. One of the functions of the PEC appears to be preventing a non-active CE from being damaged when external forces exert a sudden pull on it. It is believed to be activated when a muscle stretched. The PEC is also thought to contribute to the resting tension or tonus that is exhibited by muscles.

When a person is standing or moving slowly, the contractile element is a key factor in the movement. At higher speeds, contractions are generally immediately preceded by lengthening (negative) work. As a result, the muscles are stretched, and energy is stored in the elastic and viscoelastic components of the muscle (then released during an immediate shortening contraction). This entire cycle is known as the stretch-shortening cycle (SSC).

The extent to which elastic (not total) length-changes in a muscle-tendon unit are due to changes in tendinous tissue length can vary from less than half to as much as 90%. At least part of this difference can be attributed to significant differences between muscles in the share of the total length of the muscle-tendon unit made up of tendinous tissue.

The extent to which tendinous tissues change in length is rather limited. Researchers have estimated that increases in the length of tendinous tissues at low force levels range from 2% to 4%. When the forces on tendons are low, a small change in force can have a relatively great effect on tendon length. As force levels increase, the relative change in tendon length decreases significantly. However, regardless of the degree or change in tendon length, tendons can return a significant share of the energy that is applied to them if the rate of change in the force applied to the tendon is high enough. It is interesting to note that at long muscle lengths, the loss of power due to loss of cross-bridge overlap (and the concomitant decline in contractile capability) that results from the lengthening of the muscle appears to be more than offset by tension from stretching passive connective tissue.

Potential energy stored in elastic tissues that have been stretched is released when the force being applied to the elastic tissues is decreased; the faster the decrease, the faster the realization of the potential energy. Stored tendon energy can cause high velocities and power output without imposing those same velocities on muscle fibers. Therefore, as a consequence of the action of the elastic components of muscle, at a given velocity of shortening of the muscle-tendon complex, the muscle fibers can shorten more slowly than the overall complex. For example, during the end of the vertical jump push-off, more power is delivered by the tendon than by muscle fibers of the gastrocnemius and soleus (because a lowering of muscular force at this point permits the release of elastic energy).

When an active muscle-tendon unit is subjected to large changes in force, the unit acts eccentrically and a concentric contraction immediately follows; the degree of energy return from the unit muscle-tendon unit is relatively high. Such a sequence is often referred to as the stretch-shortening cycle (SSC). During the SSC, changes in muscle length are very small during the stretching phase, even though there is a very sharp increase in force. This suggests that the conditions presented by this kind of activity favor the use of short- range muscle stiffness to activate the SEC.

Different forms of muscle action have different mechanical efficiencies, and the velocities of stretching and shortening influence these efficiencies. The loading conditions during the start of an action from a static position are different from those during an SSC. This difference apparently leads to different mechanical efficiencies (ME) for the actions that result.

The ME of concentric exercise decreases with increasing shortening velocity. The mechanical efficiency of eccentric muscular contractions tends to be quite high and generally increases when mechanical work increases. The mechanical efficiency of eccentric contractions is improved with an increasing stretch velocity. In concentric contraction, electrical activity within the muscle, energy expenditure and mechanical work all change proportionally during slow muscle actions as effort increases. As velocity increases, the relationships between these activities change. The initial force peak becomes higher with fatigue, and the subsequent reduction of force becomes more pronounced. In addition, the contribution of reflexive action to sustaining repeated stretch loads is improved.

During an isometric contraction, the total length of the CE and SEC is constant, but the CE shortens and the SEC lengthens. The effect of the SEC is that the build up of force at the lever takes longer than it would be if only the CE existed. As force falls, the length of the SEC declines as well. In situations where the CE force falls rapidly, the contribution to the shortening velocity by the SEC is much greater than that of the CE.

It is a well-known physiological principle that stretching a muscle (up to a point) before it is contracted increases the force that the muscle can generate. Part of the advantage of pre-stretching can be understood from the perspective of the elastic components of muscle (e.g., in the enhanced recoil after a pre-stretch of the muscle). However, another reason for the increased force of contraction that follows a pre-stretch lies in the reflexive characteristics of the neuromuscular system, a subject that is addressed in the next section of this appendix.

Proprioceptive Receptors

The brain receives information from the muscles that enable it to control the actions of those muscles on a subconscious level. This information is provided by two special kinds of sensory organs, called proprioceptive muscle receptors. One type of muscle receptor is the golgi tendon organ. These organs are made up of afferent fibers entwined within the bundles of connective tissue that comprise the tendon. (Afferent neurons carry impulses from receptors in the body to the central nervous system, the system comprised of the brain and spinal chord.) These neurons respond to tension in the tendons that is created by the muscles to which they are attached. When the tendons are stretched, they cause the golgi tendon organs to be stretched as well, causing the afferent fibers to fire (the frequency of that firing being directly related to the tension that is developed). This information is sent to the brain, and at the same time, by means of interneurons (special neurons that connect afferent and efferent neurons), to the efferent alpha motor neurons of that muscle, thereby inhibiting its action. (Efferent neurons receive information from the central nervous system.) The sensory neuron of the golgi tendon organ also connects with the antagonists (the muscles that oppose, i.e., have a pull opposite to that of the muscle which in generating tension in the tendon the golgi tendon organ is monitoring—the agonist) and can cause the antagonists to contract, opposing the action of the agonists. This combination of effects on the acting muscle and its antagonists helps to protect the tendon of the acting muscle from overly high and potentially injurious tension.

The second kind of muscle receptor is known as a muscle spindle. A muscle spindle is a group of specialized muscle fibers known as intrafusal fibers. These fibers are contained within spindle-shaped connective tissue coverings that run parallel to normal (extrafusal) muscle fibers. The central or control area of the muscle spindle is non-contractile in nature, but it is sensitive to being stretched, while its ends are capable of contraction.

Each muscle spindle has its own afferent and efferent nerve supply. The afferent neuron has two types of sensory endings. Both of these sensory endings terminate on intrafusal fibers (i.e., muscle spindles). They serve as muscle-spindle receptors that are activated by a stretch. The primary endings are wrapped around the central portion of these intrafusal fibers. They detect both the change in length that takes place in the muscle and the rate at which that change occurs. The secondary endings are arranged at the end portions of many of the intrafusal fibers and are sensitive solely to length.

Muscle spindles have an action which is virtually the opposite of golgi tendon organs. When a muscle is stretched, the afferent neuron endings send an impulse to the spinal chord, synapsing directly on the motor neurons that reside in same muscle, causing contraction of the muscle and other muscles which cause the same movement. At the same time other neurons inhibit contraction of antagonist muscles. This overall reaction, known as a stretch reflex, resists any passive change in muscle length, so that the optimal resting length of the muscle is maintained. The best-known example of such a reflex is the “knee-jerk” reflex in the patellar tendon. The stretch can be and is used in athletics to maximize muscle power-outputs (as when the foot is planted in jumping events).

The efferent neuron of a muscle spindle is known as a gamma motor neuron. Gamma motor neurons initiate contraction of the muscular end regions of intrafusal fibers. Simultaneous activation, or “coactivation,” of the gamma motor neuron system together with the alpha motor neuron system occurs when reflex and voluntary contractions and remove the slack from the muscle spindle fibers as the entire muscle shortens, permitting these receptor structures to maintain their sensitivity to stretch over a full range of muscle lengths. Gamma stimulation triggers simultaneous contraction at both ends of the intrafusal fiber, removing any slack from the central region. The extent of the gamma activity in a muscle being voluntarily contracted depends on the anticipated distance of the shortening that occurs.

The Influence of Hormones on Muscle Function

There are a number of hormones which are considered anabolic (i.e., have a muscle-building effect). Among these are growth hormone, insulin and insulin growth factors, testosterone and thyroid hormones.

The level of anabolic hormones appears to be affected in a positive way by resistance training. While mere elevation of these hormones has no discernible effect on strength and functional muscle growth, their presence, in combination with the stimulation of training, apparently has growth- and strength-building effects. Testosterone and growth hormones (the latter through the mediators of the insulin growth factors) appear to enhance growth, while insulin itself appears to protecting against the breakdown of protein in the body rather than to enhance its production. The effects of testosterone may be as much due to neural factors and to the influence of testosterone on the transition of muscle fibers from slower to faster as to any anabolic effect. The effects of thyroid hormone are secondary in nature (stimulating anabolism indirectly).

Certain hormones, such as glucocorticoids (particularly cortisol), are believed to have anti-anabolic or catabolic (i.e., muscle-wasting) effects. Both anabolic and anti-catabolic factors contribute to overall growth.

In a completely different category are hormones that appear to have an acute effect on performance (i.e., have an immediate and positive effect on strength), such as those from the catecholomines category (e.g., noradrenaline and adrenaline). Their role on muscle growth is not well understood, but through their ability to increase the strength of contractions, they may increase the stimulation from training (although the release of such hormones at too high a level and too often may have a negative effect on growth by fostering the development of an overstress condition).

The balance of anabolic and catabolic factors in the body is subject to change. Factors such as training, rest, general health, nutritional factors, aging and stress all have an influence. This is one of the reasons why a healthy lifestyle is so critical for the athlete.

Energy for Muscular Contractions

There is only one fundamental source of energy for muscular contraction: ATP. As was discussed in the section on the bases for muscular contraction, when an ATP molecule binds to myosin at the ATPase binding site, it permits the detachment of the myosin cross-bridge from the actin myofilament at the completion of the power stroke and the resetting of the cross-bridge in its original position, where a new connection to the actin filament can occur. The ATP molecule is then split by myosin ATPase, releasing energy for the power stroke of the cross bridge. The energy for the chemical process which reset the neuron for another stimulation of the muscle fiber is derived from the breakdown of ATP.

A very small amount of ATP is stored in muscle tissue, perhaps enough for as much as one to a few seconds of maximum muscular effort. Therefore, ATP must be supplied to the muscles on a continuing basis if activity is to continue. There are three means of generating ATP: the breakdown of phosphocreatine (PC); glycolysis; and oxidative phosphorylation (the citric acid or “Kreb’s cycle” and electron transport). The latter process requires oxygen in order to occur and is referred to as aerobic, while the first two sources of ATP, which do not require oxygen to occur, so are called anaerobic.

PC, like ATP itself, is stored within the muscle (a rested muscle contains about five times as much PC as ATP). When an ATP molecule releases energy, it is broken down into adenosine diphosphate (ADP) and phosphate (Pi). When PC is broken down, the resulting energy is used to recombine ADP and Pi into ATP. ATP can be formed within a fraction of a second from the breakdown of CP. ATP stores remain fairly constant early in contraction, while creatine phosphate stores become depleted. ATP and the PC used to replenish it are exhausted in thirty seconds or less of all-out work. Although PC stores cannot provide energy for long-term work, they have the advantage of being immediately available and yielding a larger power capacity than other energy sources.

Also stored in the muscle is glycogen, a sugar molecule whose breakdown produces energy for the formation of ATP and a substance called pyruvic acid, which is ultimately converted by the body to lactic acid . This process is called anaerobic glycolysis.

An accumulation of lactic acid in the muscles causes pain in nerve endings during exercise. (Lactic acid accumulation was once thought to be the source of delayed onset muscle soreness, the soreness which people often experience twenty-four to forty-eight hours after a bout of exercise, particularly when they have not trained for some time, but this has not proven to be the case.) A lactic-acid buildup also makes the interior of muscle cells more acidic, which interferes with a number of chemical processes of the cell, including ATP formation and the chemical process that exposes actin filaments and permits cross-bridging. These combined effects, along with energy depletion, are believed to contribute to muscle fatigue. (The intensity of a given exercise as well as the type of muscle fiber influence the actual onset of fatigue.) Consequently, the energy provided by glycolysis has limitations. However, glycolysis can produce more energy than ATP-PC (though not as much per unit of time). It is a major source of energy in all-out exercise bouts lasting one to three minutes.

The production of ATP can occur aerobically through the process of oxidative phosphorylation mentioned above. Through oxidative phosphorylation the body metabolizes carbohydrates, fats and proteins to create energy (protein is normally used for energy only during starvation and long, intense, exercise bouts). The body’s use of carbohydrates and fats is in large part determined by the nature of the activity that is being undertaken. Carbohydrates are utilized more extensively when work is intense; utilization of carbohydrates is nearly 100% at maximum work levels, assuming carbohydrates are available. The body uses fats as its primary energy source during activities of low intensity and long duration.

When the body metabolizes the carbohydrate glycogen using oxygen, the pyruvate released does not form lactic acid but, rather, through an extended series of chemical reactions called the oxidative phosphorylation, ATP, carbon dioxide (CO2), which is expired via the lungs and water. In order to be metabolized, fats go through a process called beta oxidation and then through the oxidative phosphorylation process, yielding, as do carbohydrates, water, CO2 and ATP.

The amount of aerobic energy that the body can produce depends on the amount of oxygen it can obtain and utilize in a given unit of time (which is typically measured in terms of the liters of oxygen per minute that the body can process). During exercise the amount of oxygen that the body processes is increased through faster and deeper breathing, a faster heart rate, the diversion of blood to the exercising muscles and hemoglobin releasing more oxygen to those muscles. In addition, some muscles have a large supply of myoglobin, a substance similar to hemoglobin (a molecule that binds with and transports the majority of the oxygen in the blood). Myoglobin can store small amounts of oxygen, but its most important role is in increasing the rate of oxygen transfer from the blood into the muscle fibers.

The aerobic method of energy production cannot produce enough energy for maximum efforts, but it can supply a virtually unlimited amount of ATP over time and is a very efficient energy source. (The breakdown of one glucose molecule by this aerobic mechanism yields thirty-six molecules of ATP as compared with only two molecules of ATP when glycolysis is the mechanism for supplying energy.) However, it should be noted that aerobic energy-production is the indirect source of anaerobic energy-production as well. On an intuitive level, the athlete can appreciate this by the heavy breathing that takes place after a bout of intensive exercise. This breathing is used to replenish ATP, PC and glycogen stores. The restoration of ATP takes place over several minutes. This rate of restoration is often explained by the concept of a “half-life” of restoration. It is estimated that half of the ATP depleted by an all-out bout of exercise is replaced in approximately twenty seconds. Half of the remaining half is restored in another 20 seconds, so that 75% of the stores are restored at approximately forty seconds, and virtually full restoration occurs within several minutes. The rate of restoration is slowed if activity which depletes ATP is undertaken during the restoration period. Therefore, sufficient rest between heavy bouts (e.g. sets) of exercise is needed in order to maintain performance at a high level.

The glycogen energy source is replaced while breathing gradually returns to normal after heavy exercise. During this process lactic acid converts back into pyruvic acid, part of which is processed through oxidative phosphorylation to create ATP. The balance is converted back into glucose by the liver (most of that glucose is converted to glycogen, which is stored in the muscles and liver). The half-life of lactic acid restoration is approximately twenty-five minutes. Light activity helps to remove lactic acid accumulation faster than rest (part of lactic acid is aerobically metabolized to supply some of the needed ATP to perform the light activity). Therefore, light activity is good between bouts of intensive exercise sets if rest periods are at least several minutes. (This presupposes that the activity being undertaken is one of sufficient duration to require energy from lactic acid in the first place.) Full restoration of the energy derived from this source can take several hours. The restoration of the glycogen stores utilized during extremely long exercise bouts can take days.

Part of the increased oxygen uptake that continues after exercise is attributable to the general metabolic influence of exercise. For instance, the release of certain catecholomines during exercise increases oxygen consumption, as does the increased rate of chemical reactions that takes place in the muscle as a result of the local increase in muscle temperature.

It must be remembered that each of the body’s three energy systems is in use at all times, with one or another of those systems being the dominant source of energy at any given point in time, depending on the nature of the activity that is being performed and the availability of various energy stores in the body.

Anatomy

Human anatomy is an extremely complex subject. However, the gross anatomy of the structures that cause athletic movement (the muscles, connective tissue, bones and joints) is far simpler, though still complex. In this section we will examine the basics of human anatomy as they pertain to athletic movement.

The Development and Anatomy of Human Bones

At birth most human bones are fairly soft or cartilaginous in nature. As a child ages, the soft tissue is replaced by bone through a process called ossification. Ossification takes place in certain areas earlier than others. For example, the skull ossifies relatively early in life, but most long bones do not complete the ossification process until the late teens. Ossification also occurs in some bony protuberances, such as the tibial tuberosity (the small protrusion at the top of the shin).

There are approximately 200 bones in the human body that are involved in movement. These bones are generally divided into four categories: long, flat, short and irregular.

Long bones are the key components of human limbs and digits. Their primary purpose is to serve as levers. The largest portion of long bones is the dense shaft or diaphysis. At either end of the bone are areas called epiphyses, which have a larger diameter than the diaphysis. The epiphyses have a more porous or spongy inner area than the diaphysis and then a thin outer layer of denser bone. In the area at the ends of the bone where their bony surfaces come in contact with other bones to form a juncture or joint, there is a thin layer of articular cartilage. The cartilage here serves to cushion the shock when bones are pushed toward one another and also reduces the friction between the bones. Except where cartilage is present, a fibrous membrane called the periosteum covers the bone. It functions as a place for muscles and their tendons to attach to the bone. The ossification of long bones begins in the diaphysis and progresses to the epiphyses. Special areas called the epiphyseal plates lie between the epiphyses and diaphyses of immature bones. These areas gradually ossify with age. (As a child grows, new cartilage is created which is ultimately converted to bone through the ossification process.) This process of ossification takes place in other areas of the epiphyses as well. (It also takes place in the other types of bones, but the sites and patterns of ossification are somewhat different.) Growth in the bone’s width and length continues until the ossification process is complete.

The scapula, ilia and ribs are examples of flat bones. These bones protect the internal organs that they at least partially cover. They also provide a large area at which muscles can attach. Their composition tends to be like that of the epiphyses (e.g., spongy on the inside with a denser outside layer).

The carpal bones of the wrist and the tarsal bones of the ankle are examples of short bones. These bones are composed of spongy bones with a thin outer layer of more compact bone. They have a more blocky shape than long bones.

The irregular bones are not paired like the long, short or flat bones. Examples are the pubis and the vertebrae. These bones have an irregular shape and typically serve particular purposes, such as protecting a certain area of the body or supporting the body.

Bones have three essential components: minerals (nearly half of the bone’s total volume, primarily calcium compounds) and organic matter (nearly 40%, primarily collagen). The remainder of the bone’s volume consists of fluid-filled spaces of various shapes. The organic factors give bones their strength, and the mineral components supply rigidity.

Anatomy of the Joints

There are three basic kinds of joints: diarthrodial (synovial), amphiarthrodial (cartilaginous) and synarthrodial. Diarthrodial joints permit movement in a variety of ranges and directions. Examples are the knee and shoulder joints. During their movement, diarthrodial joints are stabilized by a combination of factors. Their shape has a strong effect, with one end of the bone designed to fit into or to move smoothly along another. The ligaments that surround the bones hold them together so that movement in the correct groove is facilitated. Finally, the tendons and/or muscles that are in contact with the joint guide the action of the joint, maintaining the joint’s integrity.

Amphiarthrodial joints permit a relatively small degree of movement. Typically a layer of fibrocartilage separates the bones. When movement occurs, it occurs primarily through the deformation of the cartilage. Ligaments connect these rather tightly to one another. The joints between the vertebrae and the sacroiliac joints are examples of this kind of joint.

Synarthrodial joints do not move; the bones merely merge together at these joints, joined by fibrous tissue that is essentially a continuation of the periosteum. Cranial sutures are examples of synarthrodial joints.

The ligamentous tissue that encloses the joint forms what is called an articular capsule. The thickness of that capsule varies within the same joint and among joints. A synovial membrane within the joint secretes a lubricating and shock-absorbing substance called synovial fluid into the articular capsule.

Tendons connect muscle to bone. They are often enclosed by cylindrical sheaths of connective tissue that are lined with a synovial membrane.

Two other kinds of tissues serve to absorb shock and facilitate movement at the joint. One is a fibrocartilage pad that rests between the bones. Examples are the discs between the vertebrae and the menisci of the knee. A second kind of shock absorber is the bursae sac. These soft, pad-like tissues are filled with synovial fluid. Both the bursa sacs and the articular capsule contain a relatively modest amount of synovial fluid under normal conditions. However, in the event of an injury to the joint, the amount of fluid can increase several fold or more. A common example is “water on the knee.”

Directions of Human Movement

Scientists have adopted specific terms to describe human movement. Many of these terms are relative in nature. For example, the term superior is used by anatomists to describe some part of the body that is higher than or above something else. To say that your head is superior to your feet is true when you are standing, but not when you are lying down. Therefore, in the study of anatomy, a reference position to which all other positions are related has been established. This position, called the anatomical position, is used as a reference for understanding the direction of human movement. The anatomical position is assumed when the body is standing erect, facing the observer with the arms at the sides, palms facing forward. It is in relation to this position that anatomical movements and locations are described.

For instance, since the term superior means higher or above another structure, in the context of the anatomical position, the portion of the upper arm that is nearest the shoulder is said to be superior to the portion of the upper arm that is nearest the elbow. Inferior has the opposite meaning (in this case, the portion of the arm nearest the elbow is inferior to the portion nearest the shoulder). Lateral means farther from the mid-line of the body, and medial means closer to it. When the arm is raised sideways, it is said to be moving laterally; when it is lowered it is moving medially. Proximal means nearer to and distal means further away. Anterior is nearer to or in front of the body, while posterior is nearer to or behind the body. Finally, superficial means nearer the surface, and deeper means further away from the surface of the body.

Anatomically, all motions of the joints are measured from stipulated zero degree starting positions. For example, when the arm is completely straight, the elbow is said to be at the zero starting position (i.e., 0⁰). At full flexion, the elbow is at an angle of approximately 150⁰. Movement from the zero starting position to the fully bent position is called flexion. Movement back to the fully extended position is called extension. When the arm continues its extension past the point where it is straight and moves until the arm goes past the zero position it is said to be hyperextending. (In many people, such movement is not possible without injury,  but in some people a hyperextension of as much as 20⁰ can be comfortably attained.

Movements are measured from Movements are also described in reference to three planes, imaginary flat surfaces, like thin boards, that pass through the body. The transverse plane is a horizontal plane passes through the body parallel to the ground (the mid-transverse plane divides the body into superior and inferior halves). The sagittal plane passes through the body from top to bottom, perpendicular to the ground, dividing it into right and left sides (the mid-sagittal plane divides the body into right and left halves, although right and left are not anatomical terms). The final plane, the frontal plane, is also vertical and perpendicular to the ground, but it divides the body into anterior and posterior sections (the mid-frontal plan divides the body into equal anterior and posterior halves).

Finally, movement can be described in terms of three axes that can be viewed as rods running through the center of the body. The transverse axis is like a rod running through the body from side to side. The anterior/posterior axis is like a rod running through the body from front to back. The longitudinal axis is like a rod running vertically through the body from top to bottom.

When movements are described, they often refer to the plane through which the body part is moving and the axis about which it is moving. Figure 58 depicts planes and axes.

General Kinds of Joint Actions

Joints can perform a number of actions. Many of these actions are quite universal in that they can be performed by a number of joints; others are functions limited to only one type of joint. The actions of joints and a definition of those actions are listed below.

Flexion: Flexion is a decrease in the angle between two bones or groups of bones. It occurs in the sagittal plane around the body’s transverse axis. Flexion occurs when the palm is raised toward the shoulder in the anatomical position. Here, when the lower arm is raised to a position parallel to the ground, the angle between the bones of the upper and lower arms has decreased from approximately 180^o to 90^o.

Extension: Extension is the opposite movement to flexion (e.g., the arm being straightened from a flexed position). Extension takes place in the same plane as flexion and around the same axis, but there is an increase in the angle between two bones when extension occurs.

Abduction: Abduction is moving away from the body’s (or body part’s) mid-line. Movement generally takes place in the frontal plane around the anterior/posterior axis. (Some examples of exceptions are the hands and feet, in which abduction takes place when the fingers and toes are spread apart.) Abduction occurs when the leg is raised sideways (laterally) from the anatomical position.

Adduction: Adduction is the opposite movement to abduction (e.g., the leg being lowered after having been raised sideways).

Elevation: Elevation occurs when body parts are moved to a superior position. Elevation occurs when the shoulder girdle is raised to a superior position.

Depression: Depression is the opposite movement to elevation.

Rotation: Rotation is a turning about the longitudinal axis. Rotation occurs when the head or trunk is turned from left to right or right to left. For the limbs, rotation is described as medial when the anterior surface of the limb is turned toward the mid-line of the body (inward or medially). Lateral rotation of the limbs is the opposite of medial rotation.

Supination: Supination is a special form of movement pertaining to the lower arm. In the anatomical position the arm is supinated (the palm is facing forward). The natural stance of humans is with their palms facing inward and to the rear. By rotating the forearm laterally (outward) from the natural position, the anatomical position is assumed.

Pronation: Pronation is the opposite of supination. Rotating the forearm medially (inward) from the anatomical position to a more natural position is an example of pronation.

Inversion: Inversion is lifting the (inner) medial border of the foot inward and upward.

Eversion: Eversion is the opposite of inversion. It consists of lifting the lateral (outside) border of the foot.

Dorsiflexion: Dorsiflexion is lifting the foot toward the shin (the starting or normal position of the foot is considered to be at an angle approximately 90^o to the shin).

Plantarflexion: Plantarflexion is pointing the foot downward or rising on the toes.

In addition to the movements described above, there are also combinations of movements, such as circumduction (moving the joint in a circular direction). However, these terms merely describe  some combination of simpler joint motions when they are executed in a specific sequence.

The Actions of Specific Major Joints

Joints vary considerably in their ability to move. Some joints are capable of only one basic kind of motion, while others can execute a variety of motions. The major joints and the movements of which they are capable are listed below. Types of motions are listed in the table that follows. The names of the motions have been abbreviated, but they appear from left to right in the order in which they were introduced in the preceding section.

Abbreviations of Headings: Flx = flexion, Ext – extension, Ab = abduction, Ad = adduction, Rotat = rotation, Elv = elevation, Dep = depression, Sup = supination, Prn = pronation, Inv = inversion, Ev = eversion, Dor = dorsiflexion, Pln = plantarflexion.

Abbreviations Within The Table: X indicates that both movements in the heading occur at that joint, H = hyperextension, Z = horizontal flexion, R = radial and ulnar flexion, and L = lateral flexion.

1. The scapula, or shoulder blade, can move if there is movement at the sternoclavicular articulation as well. The scapula can abduct, moving laterally, away from the spine, in conjunction with a movement known as a lateral tilt (adduction being the opposite movement). Elevation occurs when the shoulder blades are lifted (which occurs primarily in concert with the hunching forward of the shoulders). Upward rotation occurs when the arms are raised forward or to the sides and the scapulae are lifted, with the superior portion of the scapulae tilting inward and the inferior portion moving outward, creating an inward tilt of both scapulae from top to bottom. One final kind of motion, upward tilt, occurs when the shoulder joint is hyperextended.

2. In addition to flexion and extension (in which the arms are raised forward and upward about the transverse axis and down and back, respectively), the shoulder joint is capable of hyperextension (when the arm is moved rearward from the anatomical position) and horizontal flexion and extension. The latter two movements can be described in the context of the arm beginning at a point where it is held parallel to the ground and out to the side of the body and then moved medially, inward across the chest (flexion) and outward back to position at the side of the body (extension).

3. The shoulder can also rotate medially and laterally.

4. The wrist is capable of hyperextension (raising the posterior of the hand toward the posterior of the forearm in the anatomical position); radial and ulnar flexion (the outside—thumb side—portion of the hand raised to the outside of the forearm and the opposite side of the hand raised toward the inside of the forearm, respectively, in the anatomical position); as well as normal flexion and extension (raising the palm toward the anterior of the forearm and returning it to the anatomical position, respectively).

5. The vertebral joints are capable of lateral flexion to the left or right, as well as rotation to the left and right (when the head and/or shoulders turn to the right). Flexion is movement forward and down from the anatomical position, and extension is the return to that position.

6. The lumbarsacral joint permits pelvic tilt forward and backward.

7. The hip joint permits rotation medially and laterally.

Basic Muscular Anatomy

As was noted earlier in this section, there are approximately 600 muscles in the human body. However, there are far fewer muscles that have a significant effect on athletic movements in general and on weightlifting movements In particular. In this section we will limit our attention to depicting the shapes and locations of the major muscle groups that were identified above. Two illustrations of the muscles of the human body that most affect weightlifting performance appear in Figures 59 (a) and (b) Figure (a) shows the major muscles of the body as viewed from the posterior of the body. Fig. (b) shows a view from the front. By observing these muscles and relating them to their functions in joint action (see the discussion on pages 499-500), you should be able to gain a functional understanding of muscle anatomy.

Muscles Are Differentiated by Their Function in a Given Movement

A muscle that shortens, thereby causing a joint action is a mover or agonist for that movement. Some muscles are agonists for more than one action, and many have one or many actions on each of two or more joints that they happen to traverse. For example, the biceps brachii causes elbow flexion, radioulnar supination and several shoulder-joint actions. Muscles that are the most effective for a particular movement are often referred to as prime movers. Those which are less effective are termed assistant movers. Those muscles which assist the prime movers can vary according to the circumstances.

A category of muscles called emergency muscles are called into action only when exceptional force is needed. (Motor programs do not always activate all of muscles that can help to execute a particular movement; in fact, the opposite is the case, which is why motor skills must be developed.)

Antagonists have the opposite effect of the agonists of the same joint. (The hamstring muscles are antagonists to the leg extensors in the extension of the leg.)

Fixators or stabilizers support a bone, anchoring it in a given position. This provides a firm base from which a prime mover can exert its pull.

Synergists, or neutralizers, act to prevent or counteract actions of other active muscles that are unwanted. There are two categories of synergists: “helping” and “true” synergists. Helping synergists are two muscles that both cause a certain joint action. However, each has its own secondary action which is antagonistic to the other. An example of such muscles are the external obliques. These muscles act as spinal flexors while at the same time acting to cause flexion in their own direction. (Any actual flexion in either direction is counteracted by the actions of each oblique working against the other.)

True synergy only occurs in opposition to a muscle that acts across two or more joints. The true synergists act to preclude joint movement at one of the joints crossed by the multi-joint muscle by contracting statically. For example, when the fingers are closed to made a fist, a group of true synergists (the wrist extensors) act to prevent the muscles that flex the fingers from generating wrist flexion at the same time.

Skeletal muscles can also be categorized as spurt or shunt muscles. Spurt muscles impart the majority of their force across a bone instead of along it and thereby foster movement. Shunt muscles impart most of their force along the bones. This force has a tendency to stabilize the joints by pulling them toward one another. Many muscles act on more than one joint. In such a case the muscle is typically a spurt muscle relative to one joint while acting as a shunt to the other. A good example is the biceps brachii muscle. It serves as a spurt muscle at the elbow and a shunt muscle at the shoulder. There are even certain muscles that serve both functions at the same time (with certain fibers within the muscles acting as shunt muscles and other fibers acting as spurt muscles). In addition, the action of a particular muscle (or a portion thereof) as a shunt or spurt muscle is not fixed. The role of the muscle can change when the direction of movement changes.

Major Muscle Groups

Although the human body has approximately 600 skeletal muscles, there are only about seventy-five muscle pairs that are responsible for most skeletal movements and posture maintenance. The combination of muscles that causes a given movement at a particular joint is referred to as a muscle group. Such a group takes its name from the joint at which the movement takes place and the kind of movement it causes. For example, the muscles that are primarily responsible for flexion of the spinal column are referred to as the flexors. Those muscles are the external oblique, internal oblique and rectus abdominus. The following is an alphabetical list of the major joints of the body, along with the names of the major muscle groups which are responsible for the movements of those joints and the muscles that are considered to be the prime movers in those joints.

Ankle

Plantar flexors: gastrocnemius and soleus

Dorsiflexors: tibialis anterior, extensor digitorum longus and extensor peroneus tertius

Elbow

Flexors: brachialis, biceps brachii and brachioradialis

Extensors: triceps brachii

Hip

Flexors: illiopsoas, pectineus, retus femoris

Extensors and Hyperextensors: gluteus maximus, semitendinosus and semimembranosus

Abductors: gluteus medius

Adductors: adductor brevis, adductor longus, gracilis, pectineus

Lateral rotators: gluteus maximus, obturator externus and internus, gemellus superior and inferior, quadratus femoris and piriformis

Medial rotators: gluteus minimus and gluteus medius

Intertarsal

Inverters: tibialis anterior and tibialis posterior

Everters: extensor digitorum longus, peroneus brevis, perioneus longus and peroneus tertius

Knee

Flexors: biceps femoris, semimembranosus, semitendinosus

Extensors: rectus femoris, vastus medialis, vastus lateralis and vastus intermedius

Lumbosacral

Forward pelvic tilters: iliopsoas

Backward pelvic tilters: rectus abdominus and internal oblique

Radioulnar

Pronators: pronator quadratus, pronator teres and brachioradialis

Supinators: supinator, biceps brachii and brachioradialis

Shoulder Girdle

Adductors: serratus anterior and pectoralis minor

Adductors: mid trapezius and rhomboids

Upward rotators: lower and upper trapezius and serratus anterior

Downward rotators: pectoralis minor and rhomboids

Elevators: levator scapulae, rhomboids and upper trapezius

Depressors: lower trapezius and pectoralis minor

Shoulder Joint

Flexors: anterior deltoid and clavicular portion of pectoralis major

Extensors: sternal portion of pectoralis major, latissmus dorsi and teres major

Hyperextensors: latissmus dorsi and teres major

Abductors: middle deltoid and supraspinatus

Adductors: latissmus dorsi, teres major and sternal portion of pectoralis major

Lateral rotators: infraspinatus and teres major

Medial rotators: pectoralis major, subscapularis, latissmus dorsi and teres major

Horizontal flexors: pectoralis major and anterior deltoid

Horizontal extensors: infraspinatus, latissmus dorsi, teres major, teres minor and posterior deltoid

Spinal Column

Flexors: rectus abdonminus, external oblique and internal oblique

Extensors: erector spinae

Hyperextensors: erector spinae

Rotators: internal oblique, external oblique, erector spinae, rotators, multifidus

Lateral flexors: internal oblique, external oblique, quadratus lumborum, mutlifidus and rotators

Wrist

Flexors: flexor carpi radialis and flexor carpi radialis

Extensors: extensor carpi ulnaris, extensor carpi radialis longus and brevis

Abductors: flexor carpi radialis, extensor carpi radialis longus and brevis

Adductors: flexor carpi ulnaris and extensor carpi ulnaris

One-, Two- and Multi-Joint Muscles

Many muscles influence only one joint movement. That muscle contracts alone or in conjunction with other muscles, and there is a resulting movement or stabilization at a particular joint.

In other cases a muscle passes over two or more joints, acting on both whenever it contracts. Some multi-joint muscles, such as the group of muscles referred to as the hamstrings (the semitendinosus, semimenbranousus and biceps femoris), act to cause movement at the joint in opposite directions. (These muscles both flex the leg at the knee and extend the thigh at the hip.) Other multi-joint muscles cause flexion in the same direction at all of the joints that they cross (e.g., the flexors of the fingers).

None of the multi-joint muscles are capable of causing complete movement in both joints on which they act at the same time. One result of this limitation is that the contraction of one muscle group can cause another to contract (e.g., when the hamstrings act to extend the hip, they cause a contraction of the knee extensors).

The action of multi-joint muscles has been classified in two ways: concurrent and countercurrent. Simultaneous extension of the hip and knee joints is an example of concurrent movement. When this kind of motion occurs, the muscle groups lose tension at one end and gain it at another. (In hip and knee extension, the knee extensors lose tension in the knee area but gain tension in the hip area.)

In countercurrent movement, one of the multi-joint muscles shortens at both joints while its antagonist lengthens, gaining tension at both its ends. For example, if the knee is extended and the hip is flexed at the same time, a kick is executed.

The Influence of the Angles at Which Muscle Force Is Applied

The angle between a muscle and the bone to which it applies force has in important influence on the degree to which muscle contraction generates movement in the bone. The smaller the angle between the muscle’s line of pull and the bone to which force is being applied, the larger the movement in the bone. For instance, when the arm is fully straight, the angle between the elbow flexors and the radius and ulnar bones is small. At this point, even a small contraction of the elbow flexors generates a relatively large movement in the forearm. At the end of the curling motion, when the angle between these muscles is far greater, a given distance of muscle shortening results in a much smaller movement at the forearm.

From a mechanical standpoint, the most efficient angle for the application of muscle force to a bone is 90^o. This is true because in such a case the force of the muscle is being applied fully to rotating the lever about the joint. At angles larger than 90^o, at least some of the force applied by the muscle can pull the bone away from the joint, dissipating some of the efficiency of the force. At angles below 90 ^o, at least some of the force exerted by the muscle is used to pull the bone in towards the joint. This action of the muscle stabilizes the joint, but it increases the frictional force that is generated by the joint, lowering the efficiency of the muscle’s force.

Training Effects

Training has a profound influence on performance. Some portion of the training effect is attributable to learning. Another portion is due to changes that take place in composition of muscle tissue and its functional capabilities. But the effects of training are not limited learning to use muscles effectively or to changes in muscle tissue itself. Bone and connective tissue are capable of adapting to the imposed demands of training as well. While such changes generally do not have a significant direct effect on short-term performance, they do affect performance profoundly in the long term by influencing the ability to sustain stress without injury.

Training Effects on Muscle Tissue

Specific kinds of training can apparently cause a conversion among FT sub-types of fibers (e.g., Type IIB and Type IIA), but the available evidence does not suggest that Type I and II fibers are interconvertible. (Experimentally, the switching of motor neurons supplying fast and slow fibers has resulted in the gradual reversal of the speed with which the fibers contract.) It has been suggested that, on a practical level, the transformation from one muscle fiber type to another is impeded by a number of natural conditions. In the case of the transformation of slow twitch into fast twitch fibers, any transformation stimulated by training may be countered by the use of the trained muscles for postural reasons. (The low intensity and long- term kind of muscle action that is needed to maintain posture stimulates the slow-twitch qualities of the muscle, perhaps offsetting any stimulation for those muscles to transform into FT fibers.) The transformation of FT fiber types to ST is probably impeded by the fact that considerable effort is required in order to reach the threshold necessary to activate the FT muscles often enough to transfer them to a slower type.

Training appears to selectively hypertrophy muscle fiber types. Most of hypertrophy is due to increases in the diameter of fast glycolytic fibers. For example, one study of bodybuilders found that their Type II fibers were 58% larger than normal while their Type I fibers were only 38% larger. In weightlifters and powerlifters, hypertrophy is probably even more selective in favor of Type II hypertrophy (because of the emphasis of these athletes on low-repetition training). It should be noted that an increase in the diameter of the muscles fiber is caused primarily by increased synthesis of actin and myosin filaments, which leads to a greater opportunity for cross-bridge interaction and hence an increase in contractile strength.

Training with maximal and near-maximal weights may lead to the recruitment of the high-threshold neurons that are not normally within the realm of voluntary control (heavy training may also cause an increase motor unit firing rates). FT fibers (particularly type II B) are rarely recruited, but when they are, hypertrophy of them is relatively rapid. ST fibers appear to grow less easily. Partial splitting (hyperplasia) of muscle fibers (not myofibrils) is observed in surgically overloaded muscle but little evidence of such an occurrence in live human muscle exists. Nevertheless, hyperplasia through a lengthwise split of an enlarged muscle fiber may occur to some small extent with unusual levels of training stress.

Endurance training can increase the amount and size of mitochondria, the muscle’s ATP-synthesizing capacity, as well as capillarization within muscle tissue. Resistance training generally has no effect on the ratio of capillaries to muscle fibers, but capillary density within a muscle falls as hypertrophying muscle fibers comprise a greater share of total muscle. Bodybuilders who employ high reps in their training may be an exception to this rule (for these athletes the capillary density may not change).

It takes time for the influences of training to cause a change in the composition of muscle fibers. The half life of contractile proteins has been estimated to be seven to fifteen days. This is the time it takes for half of the contractile proteins to be synthesized (with synthesis taking place in FT fibers faster than it does in ST fibers). Therefore, the training effects on muscle tissue cannot begin to take widespread effect for several weeks.

Muscle fibers can adapt to stresses placed on them by increasing in length as well a girth (the former by adding sacromeres in series to the same muscle fiber). For example, the immobilization of muscles in a shortened position will result in a decrease in the number of sacromeres along the fiber in series (immobilization in the lengthened position has the opposite effect). It appears that length has a greater effect than tension on the number of sacromeres.

The Response of Muscle Tissue to Immobilization

It is well known that the immobilization of a muscle leads to atrophy (a decrease in the size) of that muscle. That atrophy is due in part to a decrease in the diameter of muscle fibers. However, a number of less obvious changes in muscle tissue typically take place during immobilization as well. For instance, when a muscle is immobilized in a shortened position, there is an increase in the ratio of collagen to muscle fiber; both decrease, but muscle tissue decreases faster. There is also a loss in the number of sacromeres in series within the muscle.

Electrical stimulation of an immobilized muscle can reduce the loss of serial sacromeres and minimize or eliminate the change in the ratio of muscle to collagen. Immobilization in a lengthened position also helps to reduce the loss of sacromeres and any change in the ratio of muscle to collagen. Atrophy is also mitigated by stretching (which can increase protein synthesis as well as the number of sacromeres is series). One study showed that stretching a muscle for fifteen minutes every forty-eight hours was enough to sustain the ratio of muscle to connective tissue at the same level. Stretching or stimulation alone showed signs of activation in slow fibers and the suppression of activity in the genes of fast fiber types. (Naturally, any of these activities might be dangerous to the tissues that were intended to be protected during immobilization so they should not be attempted without the permission and supervision of the physician who ordered the immobilization.) Once immobilization has been terminated, it takes several weeks for sacromere numbers to return to normal.

It should be noted that similar effects occur when a muscle is simply not used, , though they far less severe. Size decreases are partly due to reduction in the actin and myosin content of muscles that are used less often or less intensely.

After damage to a muscle, myoblasts (a small population of undifferentiated cells that reside close to a muscle’s surface) can fuse to form a large multinucleated cell that then assembles the structure of a muscle. When an injury is extensive, this process cannot replace all of the lost fibers, and the remaining fibers may hypertrophy in order to compensate for the net loss of muscle tissue.

Training Effects on Bones and Connective Tissue

The bones and connective tissues of the body respond to stresses from the loads that are placed on them. The quality and quantity of the loads determine the body’s response (e.g., an extreme stress can lead to an immediate fracture of otherwise healthy bone tissues, while a repetitive loading somewhat below that level can lead to stress fractures and stress at a still lower level can induce positive changes in bone tissue).

Electrical effects are probably responsible for the link between the mechanical deformation of bone tissue and that tissue’s cellular adaptation response to that deformation. Dynamic strain appears to have a greater effect than static on adaptation. The thickness of bones is affected significantly and positively by training in general, and by resistance training in particular. The bone density (mineralization) of weightlifters is the greatest of all athletes. However, particularly strenuous training by an athlete with an immature skeleton may delay collagen maturation in connective tissues, slow the rate of long bone growth and/or negatively affect bone mechanical characteristics. Training which begins prior to middle age and continues into an advanced age appears to affect positively bone mass and mineralization. The effects of training commenced at middle age or later are not fully known at this time but the prospects of a positive effect appear to be good.

Exercise increases the maximum load which the tendons and ligaments can withstand before they separate from bone tissue. Exercise also increases collagen synthesis, but this synthesis is matched by degradation as a result of the stress of the exercise. However, overload leads to an increase in the number of fibroblasts in the tendon (fibroblasts are cells that aid in the formation of connective tissue). Overall, there is evidence that this process of synthesis and destruction leads to the development of stronger connective tissue. Training also appears to maintain tendon strength and integrity with aging. Despite the fact that circulation within collagenous fibers is limited, cyclic compression of these fibers apparently enhances the synthesis of collagen and perhaps meniscal fibrocartilage as well.

Activation of the SSC seems to provide neural training and metabolic stimuli to muscular tissue, specifically the loading components related to stiffness regulation, especially in explosive type force production. For example, after plyometric training, subjects preactivated their leg extensors earlier, before the impact of the landing, adding to the possibility of increased power during the breaking phase.

Finally, different forms of exercise produce different patterns of neuronal discharge to the muscle fibers. As was noted above, neural stimulation has an influence on muscle structure.

Physics And Mechanics

Mechanics is the branch of physics that deals with forces and their effects on objects. Kinematics is the subdivision of mechanics that studies the nature of motion. Kinetics studies the causes of motion. Biomechanics is the subdivision of the science of mechanics that deals with the application of the laws of mechanics to living organisms. Knowing certain principles of mechanics can help us to understand a number of principles of optimal technique, such as how best to impart force to the movement of projectiles like bars.

On one level the principles of mechanics can be used to analyze a number of the most fundamental aspects of force delivery. For example, you can say that in order to lift a bar of a specific weight to a certain height, you must impart a force of x over a period y in direction z. At a more fundamental level you can say that certain levers (e.g., the bones of the legs and spine) must pass through the certain angles with a given angular velocity in order to impart the force necessary to lift a bar (assuming there is solid contact between the levers and the bar. At a still more fundamental level, mechanics can be used to determine the amount of force with which the muscles of the legs and back must contract, and for how long a period, in order for the levers comprised by the bones and joints of the legs and back to be moved at the velocity required to lift the bar in the appropriate way.

A discussion of some of the basic principles of mechanics follows. That discussion will avoid the mathematical aspects of mechanics, but will attempt to cover relevant concepts in sufficient detail so that the reader will be able to understand both the concepts and how to apply them to the sport of weightlifting.

Some Basic Definitions Used in the Science of Mechanics

We will begin our discussion of the laws of mechanics with some key definitions. These definitions are important because the science of physics uses certain familiar words in a special ways as well as some unfamiliar words.

Kinds of Motion

There are four basic kinds of motion: linear, angular, curvilinear and general. In linear motion (also referred to as translation), all parts of a body move the same distance in the same direction and at the same time. Examples would be a box being pushed along the floor or a bar being pulled from the floor.

In angular motion a body moves along a circular path around a central line that is perpendicular to the plane of the motion. An example would be when plates of a bar are being spun around the bar. Angular speed is measured in terms of the angle traversed by an object in a given time interval. Because of this means of measure, all points on the rotating object are moving with the same angular speed, although the points on the object that are further away from its center are moving through a greater linear distance and at a faster linear rate than points that are closer to the object’s center.

In curvilinear motion an object moves in a curved path but does not necessarily rotate as it does so. An example would be the movement of a ball swung on a string.

General motion is a combination of two or more motions. The combination might be motions of the same type (e.g., two angular motions) or of different types (e.g., an angular motion combined with a linear motion). General motions are the most common motions that we encounter. An example would be a weightlifter descending under a bar in the jerk. Roughly speaking, the lifter’s center of gravity and overall body are moving in the same direction, at the same speed and at the same time. The lifter’s legs are rotating about the hip joint as one is moved forward and the other rearward to assume a split position. On the leg that is being moved forward, the lifter’s foot is undergoing angular motion about his or her knee joint as the front leg is being bent in stepping forward and at the same time undergoing linear motion from the position at which it began (under the lifter’s torso) to a position well ahead of the lifter’s torso.

Scalars and Vectors

In the language of mechanics, motion can be described in two basic ways. One way is to consider only the magnitude of a motion and not its direction. An example of such a means of description is the term distance. Linear distance describes how far an object has traveled in a straight line, but not its direction. (An automobile that has gone one way between two cities that are 100 miles apart has traveled the same distance as an auto that has made a round trip between two cities that are fifty miles apart.)

Similarly, angular distances are measured by the number of degrees of angle through which an entity has passed. (For example, a pendulum that has swung forward 45^o from its starting point, returned to its starting position and stopped, has traveled an angular distance 90^o, as has a pendulum that has swung forward 90^o and stopped at that position.) Any description of motion which is comprised only of a one- dimensional measure is termed a scalar quantity.

Another means of describing motion is to consider magnitude and direction. Linear displacement is a measure of motion that consists of a straight line between the beginning point of the object’s travel and its end point and an indication of the direction of that motion (e.g., 10 miles, north). Continuing the example of the automobile presented earlier, the auto that travels 100 miles north of its starting point and stops has undergone a displacement of 100 miles. In contrast, an auto that has traveled 50 miles north and then 50 miles south along the same line has undergone a displacement of 0 miles.

Angular displacement is the angle between an object’s initial and final positions. Continuing the earlier example of the pendulum, the pendulum that has traveled 45^o forward and then the same number of degrees back has undergone a displacement of 0 ^o while a pendulum that has swung forward 90^o and stopped has undergone a displacement of 90^o. Descriptions of motion that include magnitude and direction are referred to as vector quantities.

Speed and distance are scalar quantities, while velocity, acceleration and displacement are vector quantities. Vector quantities have a distinct advantage over scalar quantities, in that the information they contain permits predictive calculations.

For instance, two vector quantities, such as the velocities of two objects before they collide, can be added to determine their combined effect, or the resultant vector. Consider a situation in which a bar is traveling at a speed of 2 meters per second vertically when it collides with the thighs of a lifter (which impart a forward horizontal velocity to the bar of 2 meters per second). The bar will move forward away from the lifter, at a 45^o angle from the vertical at a speed of 2.828 meters per second. In a similar fashion, the horizontal and vertical components of a given vector can be calculated.

Speed. Speed in physics is defined as the rate at which a distance is covered. For example, linear motion (motion in a straight line) might be expressed as 50 mph or 30 meters per second. Speed is called a scalar quantity because it expresses a quantity in only one way. Speed measures only a rate of motion; no particular direction is implied. Since speed is by nature an average quantity (the distance that is covered by an object over a given interval), it technically has little to say about how fast an object is moving at any given point during the interval being considered. When scientists wish to address the speed of an object at a specific point in time and motion, they refer to its instantaneous speed (a term which is much closer to what most people mean when they talk about speed in conventional terms).

Velocity. Lay persons often use the terms speed and velocity interchangeably, but to physicists, velocity and speed have very different meanings. Speed is purely a rate of motion. Velocity is speed in a specific direction (e.g., 50 mph, north). Velocity is called a vector, or directed quantity, because it is described by its direction as well as the rate of motion. Since velocity is measured in the two dimensions of speed and direction, it will change if either the speed or direction of an object changes (an object traveling at the same speed that changes direction has changed its velocity). Both speed and velocity are used to describe linear motion, motion in a continuous direction.

Angular Speed and Velocity. When experts in mechanics speak about the rate of angular or rotational motion (movement in a circular path around a central line) they measure such motion in terms of the angular distance or displacement. For example, the speed of a rotating wheel might be described as 10^o per second. While all points on such a wheel are moving through the same number of degrees of angular motion in the same interval, different points on the wheel are traversing different linear distances during the same period. Specifically, the further a point is from the center of the wheel, the greater its linear speed (the more distance it is covering in the same period).

Newton’s Three Fundamental Laws of Motion

During the 1600s one of the greatest geniuses in the history of science identified many of the most important laws of physics. His name was Isaac Newton. Newton single-handedly developed an astonishing number of physical laws that continue to serve as the basis of the science of physics to this day. Among his many discoveries and insights, perhaps the most influential laws that Newton conceived were his three laws of motion: the laws of inertia, acceleration and reaction.

Newton’s First Law of Motion: The Law of Inertia

The law of inertia (also known an the law of conservation of motion) states that a body at rest, or a body moving with a constant velocity in a straight line, will remain in that state until it is compelled to change its state by an external force acting on it. In short, all objects that are in motion have a tendency to remain in that same motion and all objects that are at rest tend to stay at rest. This tendency or property of objects is referred to as inertia.

The concept of inertia seems counter to our everyday experience, because we constantly encounter two forces that counteract the inertia of moving objects: friction and gravity. Friction acts to reduce the rate of motion of objects, and gravity acts to increase their rate of motion in the direction of the gravitational pull. However, in the absence of those two forces, objects once set in motion would remain in motion at the same speed and in the same direction (without the force of gravity, objects at rest, whether in space or on a solid object, would tend to remain in place).

Mass. The inertial property of an object is influenced by the number and type of atoms that an object contains. We measure an object’s inertia with a concept known as mass. A kilogram is a measure of mass. Mass is a constant property. Regardless of whether an object is located on earth, on the moon or in space, its mass, its tendency to resist a change in motion remains the same.

Weight. In contrast to mass, weight is a measure of the gravitational force that the earth (or another celestial body) exerts on an object. The force of gravity does vary with the body exerting it (e.g., the moon versus the earth) and the distance of the object on which the force is being exerted from the surface of the object which is exerting the gravitational pull. Therefore, objects may weigh six times more on the earth than the moon (as a result of the earth’s greater gravitational force relative to the moon), but the tendency of an object to remain in constant velocity motion (its inertia) is the same on the moon as on the earth (essentially because the makeup of the object’s atoms does not alter when its location changes). Moreover, even objects in space that are far enough away from the earth to be considered weightless still have the same tendency to remain at constant velocity as they did on the earth, the same inertia (they simply are not affected by gravity and inertia). Weight is a product of mass and gravitational acceleration (g) and is measured in pounds or newtons (which weigh slightly less than a quarter of a pound).

Since the gravitational force that the earth exerts on an object at any point on the earth’s surface is proportional to the object’s mass, there is a fixed relationship between the mass of an object and its weight at that point on the earth’s surface (although the relationship actually changes slightly over the surface of the earth, because not all points on the earth’s surface are an equal distance from its center). It is because of this relationship that we speak of converting pounds (a measure of weight) to kilograms (a measure of mass or an object’s inertia) and use kilograms as a proxy for weight, even though, at least to a physicist, a kilogram is a measure of mass. However, as noted above, the weight of an object will be one-sixth as much on the moon as on earth, while the mass of the object will remain the same. Therefore, while an object with a mass of 1 kg. weighs a little more than 2 lb. on the earth’s surface, it will weigh about a third of a pound on the moon’s surface, because of the moon’s weaker gravitational pull on an object with the same mass.

It should be remembered that although mass and weight have a fixed relationship to one another at any given point on the earth’s surface, they are really separate properties. That is, when lifting an object (e.g., a bar) from the earth’s surface, the force exerted on that object must be great enough to overcome both the weight of the object and its inertial tendency to remain at rest. (If the force of the upward lift and the weight of the object are equal, the object will not move; it will merely achieve a virtually weightless state in which motion up, down or sideways takes an equal amount of force, the force required to overcome the object’s inertia.) Ignoring any effects of friction, the act of moving a bar in a purely horizontal direction involves overcoming only the inertia of the bar and not its weight.

Perhaps we can come closest to directly experiencing the inertial property of an object on earth when we begin to push an object suspended from a string or slide an object on a nearly frictionless surface. For example, imagine the effort that it would take to push an object with a smooth surface across a surface covered with the slickest ice that you have ever experienced versus the effort that would be required to lift that object. The former gives a sense of an object’s inertia (although only an approximation, since there is still some friction even on the slickest of ice).

Newton’s Second Law of Motion: The Law of Acceleration

The law of acceleration states that, for bodies of constant mass, acceleration is proportional to the force that causes it and takes place in the direction that the force acts. What is acceleration and what is force?

Acceleration. Acceleration is the rate of change in velocity. It is normally expressed in terms of an amount of change in velocity that occurs in a given interval. For example, an object that falls toward the earth in a vacuum moves toward the earth 32 feet per second faster with each passing second (i.e., it is accelerating at a rate of 32 feet per second, per second, which is also expressed as 32 ft/sec2). Therefore, if the object falls from a resting position, it will have reached a downward velocity of 32 feet per second by the end of one second and 64 feet per second at the end of two seconds. Objects that are stationary and those that are moving at a constant speed, are both undergoing zero acceleration, because their rate of motion is not changing over time. Acceleration can be positive or negative (negative acceleration is often referred to as deceleration). Acceleration only occurs when a force acts on an object (in the example of acceleration in an object falling to earth, gravitational pull is the force that causes acceleration).

Any object that is moving in a curved direction is constantly accelerating, because it is always changing its velocity. (Velocity is a function or speed and direction, and from the perspective of a straight line, the direction of an object moving in a curve is continually changing direction .) Therefore, an object that is moving in a curved direction must have a force acting on it. In contrast, linear motion at a constant velocity can occur in a frictionless and gravityless world (such an object would not be accelerating because it would be moving in the same direction and at the same speed).

Force. Force is a quantity that has a tendency to change the motion (i.e., the speed or direction) of an object. Force will always cause a change in velocity (acceleration), unless there is an equal or greater force that resists the acceleration. For example, if an athlete tries to lift an object that weighs 200 kg. by applying a force equal to 100 kg., the object will not move because it will exert an equal and opposite force of 100 kg.. It is only when the upward force applied to the object exceeds 200 kg., plus the inertia of that object, that it will accelerate (i.e., move upward) from its resting position.

Without a force of some kind acting on a object, it can experience no acceleration. Force is a function of mass and acceleration (mass x acceleration = force). Therefore, if two objects of unequal mass are to be accelerated at the same rate, more force will be required to accelerate the object with the greater mass. If two objects with the same mass are to be accelerated at different rates, the one that is to have a greater rate of acceleration will require more force. Similarly, if the force applied to an object is increased, so must its rate of acceleration, and if an equal force is applied to two objects, the object with the greater mass will be accelerated less.

The actual reaction that a force creates results from the magnitude of the force and the direction in which the force is applied. A force that is directed through the center of a body (centric force) causes that body to translate (i.e., to move in a linear fashion). An example would be an upward force applied to the exact center of a boulder that was larger than the forces of inertia and gravity combined (such an upward force would cause vertical translation of the boulder).

A force that does not act through the center of an object is known as an eccentric force. Such a force causes translation and rotation at the same time. An example would be a person applying upward force to one side of a boulder. If that force is of sufficient magnitude, the boulder will rise and rotate at the same time.

A combination of two forces acting in opposite directions is known as a couple. An example of a couple would be a person pushing down on one side of a boulder while pulling up on the other side. In such a case the upward and downward translatory effects of the forces applied by the person to the opposite sides of the boulder would be canceled. However, the rotational effects of the forces applied would combine to yield a purely rotational force and, consequently, a purely rotational movement.

Forces can also be categorized by another measure of effectiveness. Static force acts on an object but does not produce motion because of the counterbalancing force that it encounters. Dynamic force is one that causes acceleration because it is not completely counterbalanced.

Newton’s Third Law of Motion: The Law of Reaction

The law of reaction states that for every force exerted by one body on another, there is an equal and opposite force exerted by the second on the first. In essence, there is always a pairing of forces, with those forces really being interactions between two objects that occur simultaneously or not at all. A force never acts in isolation.

The mass of each of two objects upon which equal forces are acting determines how much acceleration each experiences as a result of the contact with the other force. For example, if a baseball collides with the earth, the baseball and the earth exert an equal and opposite force on one another, which tends to accelerate the other in the opposite direction. However, the force received by the earth is insufficient to overcome its inertia, so that the earth’s position is unaffected (it undergoes no acceleration, no change in motion, as a result of the contact. In contrast, the baseball is accelerated significantly away from the earth’s surface because the force it encounters easily overcomes its inertia.

Newton’s Three Laws of Motion Have Implications for Angular Motion

Newton’s laws of motion, as described above, apply to linear motion. However, these laws all have counterparts that apply to angular motion. For example, Newton’s first law of angular motion says that an object in angular motion will tend to remain in motion as long as it is not acted on by some force. This is because rotating bodies have their own version of inertia which is referred to as their rotational inertia or moment of inertia.

An object’s moment of inertia is a function of its mass and the distribution of the mass around the axis of rotation. The greater the mass of the object and the greater its distance from the axis of rotation, the greater is the object’s moment of inertia. For example, the arm has a greater moment of inertia when it is extended than when it is bent (because in the extended position the mass of the arm is distributed further from the shoulder joint). Similarly, the extended leg has a greater moment of inertia than the extended arm because the mass of the leg is greater and the leg is longer (so that mass is distributed further from its axis of rotation- -the hip joint—than the arm is from its axis of rotation—the shoulder). An arm with weights held in the hand has a greater moment of inertia than an empty hand. Such an arm requires more force to move and more force to stop because of its greater moment of inertia.

The quantity of motion experienced by an object in angular motion is referred to as its angular momentum. Angular momentum is the product of the object’s rotational inertia and its angular velocity.

Torque is a measure of the eccentric force that causes (a torque produces angular acceleration). Torque is a function of the force applied to an object that is being rotated and the perpendicular distance from the point at which force is applied (the axis of rotation). Shorten the resistance arm and you reduce the torque.

Newton’s second law of angular motion states that the angular acceleration experienced by a body is proportional to the torque causing it and takes place in the direction in which the torque acts. Finally, Newton’s third law of angular motion states that for every torque exerted by one body on another, there is an equal and opposite torque exerted by the second body (if both bodies have the same axis). Examples of the latter are the dancer who moves the feet counterclockwise and the hands in a clockwise direction when jumping. This is because when a body is in the air, if the angular momentum of any part of the body is changed, the angular momentum of another part must also change so that the total remains the same.

Other Concepts of Motion

Momentum

While the force required to move an object at rest (ignoring the influence of gravity) is only that needed to overcome its inertia, the force needed to change the motion of a moving object is different. In order to stop the motion of a moving object, one must overcome the quantity of the object’s motion. That quantity is referred to as the object’s momentum. Momentum is a product of the object’s mass and its velocity. The faster an object is moving or the greater its mass, the greater its momentum. Therefore, if two objects are moving at the same velocity, the object with the greater mass has greater momentum. Similarly, if two objects have the same mass, the object moving with greater velocity has greater momentum.

The principle of momentum explains why when a larger automobile collides with a smaller automobile that is moving in the opposite direction, the occupants of the smaller vehicle tend to be more severely injured. Both vehicles are brought to a stop, but the larger vehicle is brought to a stop over a distance (which affords the occupants some opportunity to decelerate over time), while the smaller vehicle suffers a reversal of its direction (a much more rapid deceleration).

The law of conservation of linear momentum says that in any collision or interaction between two objects, the objects exert an equal and opposite force on each other for the same period and have equal and opposite changes in momentum. This means that if one object’s momentum increases by a given amount, the other object loses an equal amount. The combined momentum has not changed; it has only been transferred. (If we know how much one object’s momentum has changed, we know how much the other’s has changed as well.) However, in the case of the automobiles given above, the heavier auto had more momentum to begin with, so even though its loss of momentum was equal to that of the lighter vehicle, its change in motion was smaller.

Impulse

An impulse is the average force exerted by a body in a given direction. The impulse is a function of the net force and the time over which a force is applied. The following equation expresses the impulse- momentum relationship: Momentum = Mass x Velocity.

The left side of the equation is the impulse of a force (a force that acts for a finite period versus those which act continuously, such as gravity). The impulse momentum principle says the impulse force is equal to the change in momentum it produces. The pressure is the average load supported per unit of area. (Very large loads or forces can be harmless, depending in the area over which it is distributed.) That is why objects are decelerated over distances and why protective equipment distributes its force over a wider area.

Impact

Impact occurs when two bodies collide. Whenever an impact occurs, the bodies which come in contact either remain in contact or separate. The velocity at which any separation occurs depends on the velocity of the impact and the elasticity of the objects (elasticity is the property that causes a body to return to its original shape).

The elasticity of a given entity can be described by its coefficient of restitution (COR), a constant that expresses the relationship between the velocity of impact of that object and its velocity of separation after impact occurs. It is derived by dividing the speed of an object after a collision with its speed before the collision. An object with a high coefficient of restitution returns a large share of the velocity it had prior to impact. An object with a low coefficient of restitution returns only a small share of its velocity prior to impact. If an object moving in a purely downward direction were to impact with the hard surface of an immovable object and separate with the same speed at which it had been moving immediately prior to impact, it would be said to have a coefficient of restitution of 1 (the maximum, which has never been observed experimentally). Rubber balls, such as basketballs, have a fairly high COR (NBA basketballs have CORs in the .76 to .80 range), while a ball made of cast iron (such the shot used in shot putting) has a relatively low COR.

The velocity with which a basketball bounces when it impacts with an inelastic surface is influenced by the ball’s vertical force, any horizontal force component that the ball has when it impacts with the floor (including spin) and the friction of the ball against the floor.

When two free bodies collide (e.g., two basketball players jumping for a ball), the greater the velocity of player A prior to impact, the greater the velocity of player B after hitting player A. The greater the mass of player B, the less that player’s velocity will change as a result of impact. Finally, the greater the coefficient of restitution of player B, the greater will be that player’s force of separation upon contact with player A.

Work . Work is a function of the force applied to a body and the distance through which that body moves in the direction in which the force is applied. In terms of the science of mechanics, an athlete holding a weight overhead is performing no work, even though he or she is exerting considerable effort. When the object against which force is applied is raised, positive work is being performed. When such an object is being lowered, negative work is being performed.

Power. Power is the rate at which work is performed. It is determined by dividing the total work accomplished by the time it took to perform that work.

Energy. Energy is the capacity to do work. Energy can take many forms (e.g., mechanical, electrical and heat). Moreover, different kinds of energy can be transferred to one another. Mechanical energy can take several forms. It is the energy a body possesses as the result of its motion, of being pushed or pulled out of its normal shape or its position relative to the earth’s surface.

Kinetic Energy. Kinetic energy is the energy an object possesses because of its motion, which can be the energy of translation or rotation. The kinetic energy of an object is determined by the object’s mass and speed. Strain energy is the work capacity that results from that entity’s being out of its normal shape. (The timing of movements associated with the development of strain energy and its release have an important relationship to the technique of many sports, including weightlifting.) Potential energy is determined by an object’s position in relation to the surface of the earth. All things being equal, the farther an object is from the surface of the earth, the greater is its potential energy.

Virtually all work consists of transforming one form of energy into another. Lifting an object gives it potential energy (stored energy that can be later released). In an elastic collision, energy is transferred from kinetic to elastic and then back to kinetic. It is rare for kinetic energy that is turned in to elastic energy and then back to kinetic energy not to lose some of that energy in the process (energy that is “lost” has really been transferred to friction or heat). The coefficient of restitution is a measure of a object’s ability to return the kinetic energy it receives.

When an upward force is imparted to an external body (e.g., when a projectile is launched), the more the reaction force against the ground exceeds the weight of the object, the greater the upward acceleration will be. The longer the period and distance of acceleration, the greater the height that body will reach.

In jumping, an upswing of the arms just as the legs leave the ground transmits force from the shoulder muscles that are lifting the arms to the ground. This increases the launching force. Therefore, a faster upward thrust of the arms as a lifter splits in the jerk will cause the body to descend under the bar more quickly, and a fast and forceful rearrangement of the feet in the split-position stop will impart upward force to the bar (assuming that the arms and torso are in a position to transmit that force). This faster and more forceful rearrangement of the feet will place more stress on the body than a slower and less forceful movement, but it will also result in the athlete being able to fix the bar with the body in a higher position, which places less stress on the body, one of the many technique trade-offs in weightlifting).

Some Basic Principles of Levers

Torque is the principle behind a lever. A lever consists of a fulcrum (a pivot point) and a rigid object called a lever arm (the moment arm). The force that is applied to the lever arm of a lever is typically referred to as F and the force arm is referred to as FA. Gravity or some other force that opposes the action of the lever is referred to as R, and the resistance arm is referred to as RA. All things being equal, the longer the lever, the greater the torque it can exert.

There are three types of levers. A type I lever is one in which the force is applied on one side of the fulcrum and the resistance arm is on the other. The classic example is the seesaw. In the seesaw, the fulcrum is the point at which the seesaw pivots (the crossbar at the seesaw’s center). A child who sits on one side of the seesaw represents the resistance (R) and the length of the seesaw between the second child and the fulcrum represents the resistance arm (RA). An adult on the opposite side of the seesaw who wanted to raise that child up from the ground would represent the force (F) being applied to the lever and the portion of the seesaw between where the adult applies the force and the fulcrum at the center of the seesaw represents the force arm (FA).

If the child were to merely sit with his or her feet on the board the force required to lift the child would be equal to that child’s weight and inertia (seesaw riders normally sit with their feet on the ground rather than the board so that they can push against the ground to assist the person on the other side who is doing the lifting). If the child were to move closer to the center of board, he or she would shorten the resistance arm. This would reduce the force required to accomplish lifting the child. If the adult applied force to the lever at a point that was further from the fulcrum than the child on the opposite side, less force would be required to lift the child than if the distance of the adult and child from the fulcrum were the same.

When the force arm of a lever is longer than the resistance arm, it is said to create a mechanical advantage relative to the resistance arm. The greater the length differential of the force arm over the resistance arm, the greater the mechanical advantage of the force arm and the smaller the force needed to overcome the resistance. But a mechanical advantage comes at a price. A greater resistance can be overcome, but the distance moved at the shortened resistance arm is smaller than the distance moved by the lever arm. Pliers and crowbars are common examples of tools which assist workers by creating a mechanical advantage.

In a type II lever, the force arm and the resistance arm are on the same side of the fulcrum but the force is applied further away from the fulcrum than the resistance. Using our seesaw example, if the adult moved to the same side of the seesaw as the child but was positioned behind the child, he or she would be using a type II lever to lift the child. A wheelbarrow is a common example of a type II lever. In this case, the axle of the wheel represents the fulcrum, the load in the wheelbarrow represents the resistance and the force is applied to the handles of the wheelbarrow, which are placed behind the load.

In the type III lever, the force and resistance are also on the same side of the fulcrum. The difference between type II and III levers is that in the latter the force is applied closer to the fulcrum than the resistance. In the case of the seesaw, the adult would be positioned on the same side of the seesaw as the child but would be closer to the fulcrum than the child (i.e., between the child and the fulcrum). In such a case, the adult would be at a mechanical disadvantage in that he or she would have to apply a force greater than weight and inertia of the child in order to lift the child. The closer the adult was to the fulcrum, and the further child was away, the more difficult it would be to lift the child. However, for any distance the force arm was moved by the adult, the resistance arm would move a greater distance. Therefore, while a type III lever reduces the effectiveness of a force applied to it, it amplifies the distance over which force is applied.

This principle is of great importance in human movement. It enables muscles which are capable of exerting force over relatively short distances to cause skeletal movements many times larger (although rather large forces developed in the muscles generate much smaller forces at the ends of the bones upon which they act). Most of the muscles that are responsible for major body movements are in the type III category. One example is the biceps brachii, one of the muscles responsible for elbow flexion. In the case of arm flexion (such as when performing the curl), the elbow joint serves as the fulcrum, the radius and ulna form the bulk of the resistance arm, with the bar comprising the resistance. The biceps brachii is one of the four muscles responsible for elbow flexion. It attaches to the radius at a point near the elbow joint and is one source of the forces that are responsible for elbow flexion. The distance from the elbow joint to the point at which the biceps brachii attaches to the radius represents the force arm. This attachment point is far closer to the fulcrum than the resistance (as is the case for all four of the elbow flexors). The result is that a relatively large force generated by the elbow flexors is translated into a much smaller force at the end of the resistance arm, but a relatively short distance of contraction by the elbow flexors generates a large range of movement at the end of the resistance arm.

It is interesting to note that, technically, when the bar is being lowered in the curl exercise, the elbow flexors act to resist the force supplied by the bar. Therefore, the distance from the point at which the elbow flexors attach to the elbow joint is the resistance arm, and the distance from the elbow joint to the bar represents the force arm and the bar comprises the force. This is true for all eccentric actions.

The discussion of levers thus far has implicitly assumed that the force and resistance applied to a lever occur at an angle of 90 ^o to that lever. In such cases the lengths of the force and lever arms are simply the distance from the point at which force or resistance is applied to the fulcrum.

The muscles in the human body are not typically acting in a direction that is perpendicular to the lever arm. This does not change any of the principles of levers, but it does change the way in which the length of the true lever- and resistance-arms are calculated. For instance, the true force arm of a human lever is the length of a line perpendicular to the line of pull of the muscle that intersects with the fulcrum (represented by the joint around which motion is occurring), which is generally far shorter than would intuitively be assumed.

Given the nature of levers, it should not be surprising that under certain circumstances, one of the objectives of sports technique is to shorten the lever arm as much as possible so that the muscle tension required to perform a given movement is minimized. In other cases the objective is to maximize the lever arm so that any distance of muscular contraction is multiplied as much as possible. The result is that the movement in the lever arm will be great and the acceleration created by that lever arm will occur over a long distance. In fact, many sports rely on this principle of amplification of distance to enable the participants to launch projectiles great distances. The golf club, for example, effectively lengthens the arms of the player, thereby converting a relatively small distance of contraction of the muscles that move the club to a much greater movement of the club head, which causes the golf ball to travel a great distance.

The Concept of the Center of Gravity

The concept of the center of gravity bears some discussion here because it is not typically explained well in the literature of weightlifting (if it is explained at all). In such literature an object’s line of gravity is often mistakenly referred to as its center of gravity. In certain contexts these concepts are quite similar, but they are not the same. A line of gravity is essentially one dimensional, whereas the center of gravity is three dimensional. Lines of gravity are the focus of most weightlifting analysis (at least partly because they are much easier to determine). We will explain both concepts, beginning with the concept of the line of gravity.

As was noted in the discussion of anatomy, the body has three principle planes: 1) the mid-frontal plane, which divides the body into its anterior and posterior sections; 2) the mid-transverse or horizontal plane, which divides the body into superior (upper) and inferior (lower) halves; and 3) the mid-saggital plane (which runs the length of a body from top to bottom, dividing its right and left sides). These planes also represent the lines of gravity for the object in each of these dimensions, the line of gravity being the point at which the object would be balanced if it were supported there. In objects that are symmetrical in every plane and homogeneous in composition, such as perfect solid spheres or cubes, the location of these three planes can be easily calculated from the external dimensions of the object. For objects that have more irregular shapes (such as the human body) the determination of the object’s mid planes is normally done experimentally by finding where it balances objects in each of three directions.

For every position in which an object is placed, a line can be drawn through the object to a point where it would be balanced if it were supported at that point. This is referred to as the line of gravity for that position of the object. For example, consider a statue of a human. We could determine the mid-frontal plane of the statue by placing it on a sharp edge running side to side and determining the point at which the statue was balanced (i.e., had no tendency to fall forward or backward). Similarly, we could identify the statue’s mid-saggital plane by balancing it on a sharp edge that ran from the front to back of the statue perpendicular to its sides. If the statue were placed in a prone or supine position on the sharp edge, its mid-transverse plane could be found by determining its line of gravity in that position. The point at which all three of these planes intersect is the center of gravity of the statue.

The center of gravity of a bar is relatively easy to locate because of the essentially spherical and symmetrical nature of the bar. Since the plates and bar are round and evenly balanced, one line of gravity runs through the exact center of the bar (viewed from either side), dividing the front and rear portions of the bar. Another line of gravity runs vertically through the center of the bar when it is viewed from the front or back, dividing it into right and left sides. A third line of gravity runs horizontally, parallel to the ground, and through the center of the bar, dividing it into upper and lower halves.

The center of gravity (COG) of the lifter is far harder to determine than the COG of the bar, because the lifter’s COG is influenced by the position of the lifter’s body and the anatomical proportions of the lifter. For example, an average male standing in an erect (e.g., anatomical) position has a center of gravity at a point that is approximately 55% of his height, and an average female has a her COG at about 54% of her height, approximately in the middle of the body when viewed from the front of back and from either side. If a person raises his or her arms above the head, the center of gravity of that person will rise. If a person moves only his or her right arm laterally, that person’s center of gravity shifts to the right slightly. If a weightlifter strengthens his or her legs regularly, the mass of the athlete’s legs will tend to increase in relation to the lifter’s overall weight. In such a case the center of gravity of that lifter will be lower than it was at the outset of the lifter’s career.

Much analysis of weightlifting technique involves the vertical lines of gravity run through the lifter or the bar when both are viewed from the side. These lines represent the mid-frontal planes of the bar and the lifter (the lines that divide the front from the back, the anterior and posterior aspects, respectively). This emphasis is appropriate because the horizontal level of the line of gravity (the mid-transverse plane) is not as crucial as the vertical one in the performance of the lifts (although the horizontal line of gravity is of some significance in weightlifting and is crucial in other sports). Similarly, the vertical line of gravity of the bar and athlete from side to side (the mid-sagittal plane) is assumed to remain the same during the performance of a lift (although this is not always a valid assumption). Hence, there has been little, if any, analysis of the actions of either of the latter two gravity lines.

In addition to analysis of the movement of the mid-frontal planes of the bar and the athlete, the analysis of weightlifting technique concerns itself with the single line of gravity that runs through the lifter and the bar combined, that is, the point at which, if the lifter and bar were viewed as one entity, where the lifter and the bar would be in balance along the mid-frontal line of gravity. Many Eastern European weightlifting analysts believe that the lifter and bar should be viewed as one system, and they do so in much of their analysis.

From an analytical standpoint, the center of gravity is important because if the action of a force passes through the COG, the force produces a linear motion in that object. If the force is off center (i.e., is an eccentric force), it produces angular acceleration (change in motion) as well as linear motion. It is also important because the center of gravity has an important influence on the stability an athlete and the athlete-bar system.

To remain balanced, an object’s line of gravity must remain within the area of its base of support. An object’s base of support is generally the perimeter of the portions of an object that are in contact with the earth. For instance, to determine the base of support of a human standing upright, we could draw a line around the outside of the person’s feet and connect the distance between the person’s feet with straight lines (i.e., trace a straight line from the rearmost point on each heel and from large toe to large toe). The area within those lines would be the person’s base of support. (In reality, the base of support is slightly smaller than this, because the majority of a person’s support is produced by the area between the ball of the foot and the middle the heel ; the toes provide only limited support, as does the back of the heel.) Using this method, we can see that the wider the feet, the larger the person’s base of support would be laterally (i.e., from side to side). In addition, a person with longer feet would have a larger base of support from front to back.

The lower an object’s center of gravity in relation to that object’s base of support, the greater the force that is needed to upset the object. A simple example of this characteristic is the difference in the force it takes to upset the balance of a stick four feet long and one inch square when that stick is standing on end rather than lying on its side. If the stick is standing on end, it is extremely unstable, because the center of the stick (.5” from its surface) must remain over a narrow (1” square) base of support. Any movement in excess of .5” forward, back or to either side, brings the stick’s center of gravity outside that base. When the stick is on its side, its base of support is quite large, with the center of gravity being a full 2’ from either edge of the stick. Only a very large movement in the stick with respect to its length could cause it to become unstable.

Similarly, when a lifter is in a deep squat position, he or she has greater stability than when standing (even though the lifter may find it easier to sustain balance in a standing position because he or she is able to move his or her base of support more easily than when in a squat position).

Understanding the concept of the center of gravity will help the coach to identify the causes of an athlete’s or a barbell’s movement (e.g., the athlete is jumping forward) and how any inappropriate movements can be corrected.

The Motion of Projectiles

A projectile is a body that has been launched into the air. Once a projectile has been launched, it is acted on by air resistance and gravity (the former a negligible factor in weightlifting and the latter a critical factor). Whenever a projectile is in flight, its horizontal velocity (ignoring any effect of the friction contributed by air) is a constant throughout its flight. The horizontal distance the projectile travels is determined solely by the time it remains in flight before the pull of gravity returns it to the earth. Gravity exerts a constant downward acceleration on any projectile of 9.81 meters per second each second. A projectile that is rising rapidly loses speed at the rate of 32 feet per second.

The Action of Friction

Friction is a force which acts tangential to the points of contact that are made between two bodies opposing their motion. It occurs whenever one body moves, or tends to move, over the surface of another. The extent of friction experienced between two objects is influenced by the nature of the surfaces that are in contact (this is expressed as the coefficient of friction of that kind of surface) and the force that is holding the two surfaces together. The greater the coefficient of friction and the force holding the two objects together, the greater will be the resulting friction.

Athletes take various measures to increase and decrease friction, as appropriate for their events (or at least as they believe is appropriate). For example, magnesium carbonate placed on the hands increases the friction of the hands against the bar, thereby making the grip more secure for the weightlifter and gymnast. In contrast, lubricants applied to the thighs of weightlifters (though currently illegal) are used to reduce the friction between the bar and the athlete’s thighs during the pull. This not only enhances the force that an athlete can transmit to the bar during the pull; it also reduces the possibility of contact between the bar and the lifter causing an abrasion of the lifter’s skin . (On the down side, such lubricants can make the bar and/or platform slippery, one of the reasons the use of lubricants is prohibited.)

Summary

This appendix has provided more information regarding physiology, anatomy and mechanics than most coaches possess. Many coaches have been quite effective with far less knowledge of these branches of science. However, if you carefully connect what you have learned in this section with the practical information that has preceded it, you cannot help but improve your ability as a coach. Just as the actual practice of weightlifting will provide a coach with insights about the sport that could never be gained by merely reading about it or coaching it, knowing the science that underlies the principles of weightlifting technique and training can give a coach insights that mere coaching can never provide. Therefore, it is well worth the effort to digest this information.