Chapter 6. The Biomechanics of Human Skeletal Muscle

After completing this chapter, you will be able to:

• Identify the basic behavioral properties of the musculotendinous unit.
• Explain the relationships of fiber types and fiber architecture to muscle function.
• Explain how skeletal muscles function to produce coordinated movement of the human body.
• Discuss the effects of the force–velocity and length–tension relationships and electromechanical delay on muscle function.
• Discuss the concepts of strength, power, and endurance from a biomechanical perspective.

What enables some athletes to excel at endurance events such as the marathon and others to dominate in power events such as the shot put or sprinting? What characteristics of the neuromuscular system contribute to quickness of movement? What exercises tend to cause muscular soreness? From a biomechanical perspective, what is muscular strength?

Muscle is the only tissue capable of actively developing tension. This characteristic enables skeletal, or striated, muscle to perform the important functions of maintaining upright body posture, moving the body limbs, and absorbing shock. Because muscle can only perform these functions when appropriately stimulated, the human nervous system and the muscular system are often referred to collectively as the neuromuscular system. This chapter discusses the behavioral properties of muscle tissue, the functional organization of skeletal muscle, and the biomechanical aspects of muscle function.

The four behavioral properties of muscle tissue are extensibility, elasticity, irritability, and the ability to develop tension. These properties are common to all muscle, including the cardiac, smooth, and skeletal muscle of human beings, as well as the muscles of other mammals, reptiles, amphibians, birds, and insects.

Extensibility and Elasticity

The properties of extensibility and elasticity are common to many biological tissues. As shown in Figure 6-1, extensibility is the ability to be stretched or to increase in length, and elasticity is the ability to return to normal length after a stretch. Muscle’s elasticity returns it to normal resting length following a stretch and provides for the smooth transmission of tension from muscle to bone.

The elastic behavior of muscle has been described as consisting of two major components (41, 65). The parallel elastic component (PEC), provided by the muscle membranes, supplies resistance when a muscle is passively stretched. The series elastic component (SEC), residing in the tendons, acts as a spring to store elastic energy when a tensed muscle is stretched. These components of muscle elasticity are so named because the membranes and tendons are respectively parallel to and in series (or in line) with the muscle fibers, which provide the contractile component (Figure 6-2). The elasticity of human skeletal muscle is believed to be due primarily to the SEC. Modeling studies show that the height of a jump increases when a countermovement (knee flexion) immediately precedes it due to increased elasticity of the SEC in the lower-extremity muscles (72). Other research supporting the increase in muscle force following stretch has shown that part of the force enhancement also comes from the PEC (81).

Both the SEC and the PEC have a viscous property that enables muscle to stretch and recoil in a time-dependent fashion. When a static stretch of a muscle group such as the hamstrings is maintained over time, the muscle progressively lengthens, increasing joint range of motion. Likewise, after a muscle group has been stretched, it does not recoil to resting length immediately, but shortens gradually over time. This viscoelastic response of muscle is independent of gender (70).

Irritability and the Ability to Develop Tension

Another of muscle’s characteristic properties, irritability, is the ability to respond to a stimulus. Stimuli affecting muscles are either electrochemical, such as an action potential from the attaching nerve, or mechanical, such as an external blow to a portion of a muscle. When activated by a stimulus, muscle responds by developing tension.

The ability to develop tension is the one behavioral characteristic unique to muscle tissue. Historically, the development of tension by muscle has been referred to as contraction, or the contractile component of muscle function. Contractility is the ability to shorten in length. However, as discussed in a later section, tension in a muscle may not result in the muscle’s shortening.

There are approximately 434 muscles in the human body, making up 40–45% of the body weight of most adults. Muscles are distributed in pairs on the right and left sides of the body. About 75 muscle pairs are responsible for body movements and posture, with the remainder involved in activities such as eye control and swallowing. When tension is developed in a muscle, biomechanical considerations such as the magnitude of the force generated, the speed with which the force is developed, and the length of time that the force may be maintained are affected by the particular anatomical and physiological characteristics of the muscle.

Muscle Fibers

A single muscle cell is termed a muscle fiber because of its threadlike shape. The membrane surrounding the muscle fiber is sometimes called the sarcolemma, and the specialized cytoplasm is termed sarcoplasm. The sarcoplasm of each fiber contains a number of nuclei and mitochondria, as well as numerous threadlike myofibrils that are aligned parallel to one another. The myofibrils contain two types of protein filaments whose arrangement produces the characteristic striated pattern after which skeletal, or striated, muscle is named.

Observations through the microscope of the changes in the visible bands and lines in skeletal muscle during muscle contraction have prompted the naming of these structures for purposes of reference (Figure 6-3). The sarcomere, compartmentalized between two Z lines, is the basic structural unit of the muscle fiber (Figure 6-4). Each sarcomere is bisected by an M line. The A bands contain thick, rough myosin filaments, each of which is surrounded by six thin, smooth actin filaments. The I bands contain only thin actin filaments. In both bands, the protein filaments are held in place by attachment to Z lines, which adhere to the sarcolemma. In the center of the A bands are the H zones, which contain only the thick myosin filaments. (See Table 6-1 for the origins of the names of these bands.)

During muscle contraction, the thin actin filaments from either end of the sarcomere slide toward each other. As viewed through a microscope, the Z lines move toward the A bands, which maintain their original size, while the I bands narrow and the H zone disappears. Projections from the myosin filaments called cross-bridges form physical linkages with the actin filaments during muscle contraction, with the number of linkages proportional to both force production and energy expenditure.

A network of membranous channels known as the sarcoplasmic reticulum is associated with each fiber externally (Figure 6-5). Internally, the fibers are transected by tiny tunnels called transverse tubules that pass completely through the fiber and open only externally. The sarcoplasmic reticulum and transverse tubules provide the channels for transport of the electrochemical mediators of muscle activation.

Several layers of connective tissue provide the superstructure for muscle fiber organization (Figure 6-6). Each fiber membrane, or sarcolemma, is surrounded by a thin connective tissue called the endomysium. Fibers are bundled into fascicles by connective tissue sheaths referred to as the perimysium. Groups of fascicles forming the whole muscles are then surrounded by the epimysium, which is continuous with the muscle tendons.

Considerable variation in the length and diameter of muscle fibers within muscles is seen in adults. Some fibers may run the entire length of a muscle, whereas others are much shorter. Fibers over 30 cm in length have been identified in the sartorius (6). Skeletal muscle fibers grow in length and diameter from birth to adulthood, with a fivefold increase in fiber diameter during this period (67). Fiber diameter can also be increased by resistance training with few repetitions of large loads in adults of all ages (24, 92).

In animals such as amphibians, the number of muscle fibers present also increases with the age and size of the organism. However, this does not appear to occur in human beings. The number of muscle fibers present in humans is genetically determined and varies from person to person. The same number of fibers present at birth is apparently maintained throughout life, except for the occasional loss from injury (26). The increase in muscle size after resistance training is generally believed to represent an increase in fiber diameters rather than in the number of fibers (58). There is, however, a possibility that hyperplasia, or increase in fiber number, may occur among some individuals in response to training (69).

Motor Units

Muscle fibers are organized into functional groups of different sizes. Composed of a single motor neuron and all fibers innervated by it, these groups are known as motor units (Figure 6-7). The axon of each motor neuron subdivides many times so that each individual fiber is supplied with a motor end plate (Figure 6-8). Typically, there is only one end plate per fiber, although multiple innervation of fibers has been reported in vertebrates other than humans (37). The fibers of a motor unit may be spread over a several-centimeter area and be interspersed with the fibers of other motor units. With rare exceptions (22), motor units are confined to a single muscle and are localized within that muscle (14). A single mammalian motor unit may contain from less than 100 to nearly 2000 fibers, depending on the type of movements the muscle executes (12). Movements that are precisely controlled, such as those of the eyes or fingers, are produced by motor units with small numbers of fibers. Gross, forceful movements, such as those produced by the gastrocnemius, are usually the result of the activity of large motor units.

Most skeletal motor units in mammals are composed of twitch-type cells that respond to a single stimulus by developing tension in a twitchlike fashion. The tension in a twitch fiber following the stimulus of a single nerve impulse rises to a peak value in less than 100 msec and then immediately declines.

In the human body, however, motor units are generally activated by a volley of nerve impulses. When rapid, successive impulses activate a fiber already in tension, summation occurs and tension is progressively elevated until a maximum value for that fiber is reached (Figure 6-9). A fiber repetitively activated so that its maximum tension level is maintained for a time is in tetanus. The tension present during tetanus may be as much as four times peak tension during a single twitch (88). As tetanus is prolonged, fatigue causes a gradual decline in the level of tension produced.

Not all human skeletal motor units are of the twitch type. Motor units of the tonic type are found in the oculomotor apparatus. These motor units require more than a single stimulus before the initial development of tension.

Fiber Types

Skeletal muscle fibers exhibit many different structural, histochemical, and behavioral characteristics. Because these differences have direct implications for muscle function, they are of particular interest to many scientists. The fibers of some motor units contract to reach maximum tension more quickly than do others after being stimulated. Based on this distinguishing characteristic, fibers may be divided into the umbrella categories of fast twitch (FT) and slow twitch (ST). It takes FT fibers only about one-seventh the time required by ST fibers to reach peak tension (Figure 6-10) (25). However, a wide range of twitch times to achieve maximum tension exists within both categories (13). This difference in time to peak tension is attributed to higher concentrations of myosin-ATPase in FT fibers. The FT fibers are also larger in diameter than ST fibers. Because of these and other differences, FT fibers usually fatigue more quickly than do ST fibers. Although intact FT and ST muscles generate approximately the same amount of peak isometricforce per cross-sectional area of muscle, individuals with a high percentage of FT fibers are able to generate higher magnitudes of torque and power during movement than are those with more ST fibers (25).

FT fibers are divided into two categories based on histochemical properties. The first type of FT fiber shares the resistance to fatigue that characterizes ST fibers. The second type of FT fiber is larger in diameter, contains fewer mitochondria, and fatigues more rapidly than the first type (20).

Researchers have proposed several categorization schemes based on metabolic and contractile properties of the three different general types of fiber (Table 6-2). In one scheme, ST fibers are referred to as Type I, and the FT fibers as Type IIa and Type IIb (11). Another system terms the ST fibers as slow-twitch oxidative (SO), with FT fibers divided into fast-twitch oxidative glycolytic (FOG) and fast-twitch glycolytic (FG) fibers (76). An additional categorization includes ST fibers, and fast-twitch fatigue-resistant (FFR) and fast-twitch fast-fatigue (FF) fibers (14). These systems of classification are based on different fiber properties, however, and are not interchangeable (31). While three categories of muscle fiber are useful for describing gross functional differences, today scientists recognize that it is more accurate to think in terms of a continuum of fiber characteristics based on multiple combinations of thin and thick filaments, genetic expression of protein isoforms, metabolic potential, and calcium processing (48).

Although all fibers in a motor unit are the same type, most skeletal muscles contain both FT and ST fibers, with the relative amounts varying from muscle to muscle and from individual to individual. For example, the soleus, which is generally used only for postural adjustments, contains primarily ST fibers. In contrast, the overlying gastrocnemius may contain more FT than ST fibers. The vastus lateralis, which is the largest of the quadriceps muscles, typically has about 55% ST fibers, 30% FTa fibers, and 15% FTb fibers (86). Muscle fiber composition is the same across genders in the normal population, although men tend to have larger fibers than do women (95).

FT fibers are important contributors to a performer’s success in events requiring fast, powerful muscular contraction, such as sprinting and jumping. Endurance events such as distance running, cycling, and swimming require effective functioning of the more fatigue-resistant ST fibers. Using muscle biopsies, researchers have shown that highly successful athletes in events requiring strength and power tend to have unusually high proportions of FT fibers (8), and that elite endurance athletes usually have abnormally high proportions of ST fibers (59). As might be expected, in an event such as cycling, the most energetically optimal cycling cadence has been shown to be related to lower-extremity fiber type composition, with a faster pedaling frequency being better for athletes with a higher percentage of FT fibers (98).

As these findings suggest, exercise training over time can result in changes in fiber types within an individual (48). Today it is accepted that FT fibers can be converted to ST fibers with endurance training and that within the FT fibers conversions from Type IIb to Type IIa fibers can occur with heavy resistance (strength) training, endurance training, and concentric and eccentric isokinetic training (3, 17, 45).

Individuals genetically endowed with a high percentage of FT fibers may gravitate to sports requiring strength, and those with a high percentage of ST fibers may choose endurance sports. However, the fiber type distributions of both elite strength-trained and elite endurance-trained athletes fall within the range of fiber type compositions found in untrained individuals (30). Within the general population, a bell-shaped distribution of FT versus ST muscle composition exists, with most people having an approximate balance of FT and ST fibers, and relatively small percentages having a much greater number of FT or ST fibers.

Two factors known to affect muscle fiber type composition are age and obesity. There is a progressive, age-related reduction in the number of motor units and muscle fibers and in the size of Type II fibers that is not related to gender or to training (66, 68, 78, 79). A longitudinal study of 28 male distance runners showed a significantly increased proportion of Type I fibers over a 20-year period, presumably due to selective loss of Type II fibers (96). These age-related changes may vary with the muscle, however, as the numbers of Type I and Type II fibers have been found not to change with age in the biceps brachii (55). Infants and young children, on the other hand, also have significantly smaller proportions of Type IIb fibers than do adults, and significantly lower proportions of Type IIb fibers are found in obese than in nonobese adults (62).

Exciting new evidence underscores the role of genetic expression on fiber type and suggests that skeletal muscle adapts to altered functional demands with changes in the genetic phenotype of individual fibers (105). Myogenic stem cells called satellite cells are normally inactive, but can be stimulated by a change in habitual muscle activity to proliferate and form new muscle fibers (9). It has been hypothesized that muscle regeneration following exercise may provide a stimulus for satellite cell involvement in remodeling muscle by altering genetic expression in terms of muscle fiber appearance and function within the muscle (105).

Fiber Architecture

Another variable influencing muscle function is the arrangement of fibers within a muscle. The orientations of fibers within a muscle and the arrangements by which fibers attach to muscle tendons vary considerably among the muscles of the human body. These structural considerations affect the strength of muscular contraction and the range of motion through which a muscle group can move a body segment.

The two umbrella categories of muscle fiber arrangement are termed parallel and pennate. Although numerous subcategories of parallel and pennate fiber arrangements have been proposed (29), the distinction between these two broad categories is sufficient for discussing biomechanical features.

In a parallel fiber arrangement, the fibers are oriented largely in parallel with the longitudinal axis of the muscle (Figure 6-11). The sartorius, rectus abdominis, and biceps brachii have parallel fiber orientations. In most parallel-fibered muscles, there are fibers that do not extend the entire length of the muscle, but terminate somewhere in the muscle belly. Such fibers have structural specializations that provide interconnections with neighboring fibers at many points along the fiber’s surface to enable delivery of tension when the fiber is stimulated (90).

A pennate fiber arrangement is one in which the fibers lie at an angle to the muscle’s longitudinal axis. Each fiber in a pennate muscle attaches to one or more tendons, some of which extend the entire length of the muscle. The fibers of a muscle may exhibit more than one angle of pennation (angle of attachment) to a tendon. The tibialis posterior, rectus femoris, and deltoid muscles have pennate fiber arrangements.

When tension is developed in a parallel-fibered muscle, any shortening of the muscle is primarily the result of the shortening of its fibers. When the fibers of a pennate muscle shorten, they rotate about their tendon attachment or attachments, progressively increasing the angle of pennation (89) (Figure 6-12). As demonstrated in Sample Problem 6.1, the greater the angle of pennation, the smaller the amount of effective force actually transmitted to the tendon or tendons to move the attached bones. Once the angle of pennation exceeds 60°, the amount of effective force transferred to the tendon is less than one-half of the force actually produced by the muscle fibers. Sprinters have been found to have leg muscle pennation angles less than those in distance runners, with the smaller pennation angle favoring greater shortening velocity for faster running speeds (93).

Although pennation reduces the effective force generated at a given level of fiber tension, this arrangement allows the packing of more fibers than can be packed into a longitudinal muscle occupying equal space. Because pennate muscles contain more fibers per unit of muscle volume, they can generate more force than parallel-fibered muscles of the same size. Interestingly, when muscle hypertrophies, there is a concomitant increase in the angulation of the constituent fibers, and even in the absence of hypertrophy, thicker muscles have larger pennation angles (53).

The parallel fiber arrangement, on the other hand, enables greater shortening of the entire muscle than is possible with a pennate arrangement. Parallel-fibered muscles can move body segments through larger ranges of motion than can comparably sized pennate-fibered muscles. Increasing research findings point to differences in regional structural organization and regional functional differences within a given muscle (33).

Sample Problem 6.1

How much force is exerted by the tendon of a pennate muscle when the tension in the fibers is 100 N, given the following angles of pennation?

1. 1. 40°

2. 2. 60°

3. 3. 80°

Known

angle of pennation = 40°, 60°, 80°

Solution

The relationship between the tension in the fibers and the tension in the tendon is

1.

2.

3.

When an activated muscle develops tension, the amount of tension present is constant throughout the length of the muscle, as well as in the tendons, and at the sites of the musculotendinous attachments to bone. The tensile force developed by the muscle pulls on the attached bones and creates torque at the joints crossed by the muscle. As discussed in Chapter 3, the magnitude of the torque generated is the product of the muscle force and the force’s moment arm (Figure 6-13). In keeping with the laws of vector addition, the net torque present at a joint determines the direction of any resulting movement. The weight of the attached body segment, external forces acting on the body, and tension in any muscle crossing a joint can all generate torques at that joint (Figure 6-14).

Recruitment of Motor Units

The central nervous system exerts an elaborate system of control that enables matching of the speed and magnitude of muscle contraction to the requirements of the movement so that smooth, delicate, and precise movements can be executed. The neurons that innervate ST motor units generally have low thresholds and are relatively easy to activate, whereas FT motor units are supplied by nerves more difficult to activate. Consequently, the ST fibers are the first to be activated, even when the resulting limb movement is rapid (18).

As the force requirement, speed requirement, and/or duration of the activity increase, motor units with higher thresholds are progressively activated, with Type IIa, or FOG, fibers added before the Type IIb, or FG, fibers (29). Within each fiber type, a continuum of ease of activation exists, and the central nervous system may selectively activate more or fewer motor units.

During low-intensity exercise, the central nervous system may recruit ST fibers almost exclusively. As activity continues and fatigue sets in, Type IIa and then Type IIb motor units are activated until all motor units are involved (32).

Change in Muscle Length with Tension Development

When muscular tension produces a torque larger than the resistive torque at a joint, the muscle shortens, causing a change in the angle at the joint. When a muscle shortens, the contraction is concentric, and the resulting joint movement is in the same direction as the net torque generated by the muscles. A single muscle fiber is capable of shortening to approximately one-half of its normal resting length.

Muscles can also develop tension without shortening. If the opposing torque at the joint crossed by the muscle is equal to the torque produced by the muscle (with zero net torque present), muscle length remains unchanged, and no movement occurs at the joint. When muscular tension develops but no change in muscle length occurs, the contraction is isometric. Because the development of tension increases the diameter of the muscle, body builders develop isometric tension to display their muscles when competing. Developing isometric tension simultaneously in muscles on opposite sides of a limb, such as in the triceps brachii and the biceps brachii, enlarges the cross-sectional area of the tensed muscles, although no movement occurs at either the shoulder or the elbow joints.

When opposing joint torque exceeds that produced by tension in a muscle, the muscle lengthens. When a muscle lengthens as it is being stimulated to develop tension, the contraction is eccentric, and the direction of joint motion is opposite that of the net muscle torque. Eccentric tension occurs in the elbow flexors during the elbow extension or weight-lowering phase of a curl exercise. The eccentric tension acts as a braking mechanism to control movement speed. Without the presence of eccentric tension in the muscles, the forearm, hand, and weight would drop in an uncontrolled way because of the force of gravity. Research indicates that enhanced ability to develop tension under concentric, isometric, and eccentric conditions is best achieved by training in the same respective exercise mode (104).

Roles Assumed by Muscles

An activated muscle can do only one thing: Develop tension. Because one muscle rarely acts in isolation, however, we sometimes speak in terms of the function or role that a given muscle is carrying out when it acts in concert with other muscles crossing the same joint.

When a muscle contracts and causes movement of a body segment at a joint, it is acting as an agonist, or mover. Because several different muscles often contribute to a movement, the distinction between primary and assistant agonists is sometimes also made. For example, during the elbow flexion phase of a forearm curl, the brachialis and the biceps brachii act as the primary agonists, with the brachioradialis, extensor carpi radialis longus, and pronator teres serving as assistant agonists. All one-joint muscles functioning as agonists either develop tension simultaneously or are quiescent (2).

Muscles with actions opposite those of the agonists can act as antagonists, or opposers, by developing eccentrictension at the same time that the agonists are causing movement. Agonists and antagonists are typically positioned on opposite sides of a joint. During elbow flexion, when the brachialis and the biceps brachii are primary agonists, the triceps could act as antagonists by developing resistive tension. Conversely, during elbow extension, when the triceps are the agonists, the brachialis and biceps brachii could perform as antagonists. Although skillful movement is not characterized by continuous tension in antagonist muscles, antagonists often provide controlling or braking actions, particularly at the end of fast, forceful movements. Whereas agonists are particularly active during acceleration of a body segment, antagonists are primarily active during deceleration, or negative acceleration (49). When a person runs down a hill, for example, the quadriceps function eccentrically as antagonists to control the amount of knee flexion occurring. Co-contraction of agonist and antagonist muscles also enhances stability at the joint the muscles cross (27). Simultaneous tension development in the quadriceps and hamstrings helps stabilize the knee against potentially injurious rotational forces.

Another role assumed by muscles involves stabilizing a portion of the body against a particular force. The force may be internal, from tension in other muscles, or external, as provided by the weight of an object being lifted. The rhomboids act as stabilizers by developing tension to stabilize the scapulae against the pull of the tow rope during waterskiing.

A fourth role assumed by muscles is that of neutralizer. Neutralizers prevent unwanted accessory actions that normally occur when agonists develop concentrictension. For example, if a muscle causes both flexion and abduction at a joint but only flexion is desired, the action of a neutralizer causing adduction can eliminate the unwanted abduction. When the biceps brachii develops concentric tension, it produces both flexion at the elbow and supination of the forearm. If only elbow flexion is desired, the pronator teres act as a neutralizer to counteract the supination of the forearm.

Performance of human movements typically involves the cooperative actions of many muscle groups acting sequentially and in concert. For example, even the simple task of lifting a glass of water from a table requires several different muscle groups to function in different ways. Stabilizing roles are performed by the scapular muscles and both flexor and extensor muscles of the wrist. The agonist function is performed by the flexor muscles of the fingers, elbow, and shoulder. Because the major shoulder flexors, the anterior deltoid and pectoralis major, also produce horizontal adduction, horizontal abductors such as the middle deltoid and supraspinatus act as neutralizers. Movement speed during the motion may also be partially controlled by antagonist activity in the elbow extensors. When the glass of water is returned to the table, gravity serves as the prime mover, with antagonist activity in the elbow and shoulder flexors controlling movement speed.

Two-Joint and Multijoint Muscles

Many muscles in the human body cross two or more joints. Examples are the biceps brachii, the long head of the triceps brachii, the hamstrings, the rectus femoris, and a number of muscles crossing the wrist and all finger joints. Since the amount of tension present in any muscle is essentially constant throughout its length, as well as at the sites of its tendinous attachments to bone, these muscles affect motion at both or all of the joints crossed simultaneously. The effectiveness of a two-joint or multijoint muscle in causing movement at any joint crossed depends on the location and orientation of the muscle’s attachment relative to the joint, the tightness or laxity present in the musculotendinous unit, and the actions of other muscles that cross the joint. Whereas one-joint muscles produce force directed primarily in line with a body segment, two-joint muscles can produce force with a significant transverse component (43). During power-based activities such as jumping and sprinting, the biarticular muscles crossing the hip and knee have been shown to be particularly effective in converting body segment rotations into the desired translational motion of the total-body center of gravity (51).

There are also, however, two disadvantages associated with the function of two-joint and multijoint muscles. They are incapable of shortening to the extent required to produce a full range of motion at all joints crossed simultaneously, a limitation that is termed active insufficiency. For example, the finger flexors cannot produce as tight a fist when the wrist is in flexion as when it is in a neutral position (Figure 6-15). Some two-joint muscles are not able to produce force at all when the positions of both joints crossed place the muscles in a severely slackened state (40). A second problem is that for most people, two-joint and multijoint muscles cannot stretch to the extent required for full range of motion in the opposite direction at all joints crossed. This problem is referred to as passive insufficiency (67). For example, a larger range of hyperextension is possible at the wrist when the fingers are not fully extended (Figure 6-16). Likewise, a larger range of ankle dorsiflexion can be accomplished when the knee is in flexion due to the change in the tightness of the gastrocnemius.

The magnitude of the force generated by muscle is also related to the velocity of muscle shortening, the length of the muscle when it is stimulated, and the period of time since the muscle received a stimulus. Because these factors are significant determiners of muscle force, they have been extensively studied.

Force–Velocity Relationship

The maximal force that a muscle can develop is governed by the velocity of the muscle’s shortening or lengthening, with the relationship respectively shown in the concentric and eccentric zones of the graph in Figure 6-17. This force–velocity relationship was first described for concentric tension development in muscle by Hill in 1938 (41). Because the relationship holds true only for maximally activated muscle, it does not apply to muscle actions during most daily activities.

Accordingly, the force–velocity relationship does not imply that it is impossible to move a heavy resistance at a fast speed. The stronger the muscle is, the greater the magnitude of maximum isometrictension (shown in the center of Figure 6-17). This is the maximum amount of force that a muscle can generate before actually lengthening as the resistance is increased. However, the general shape of the force–velocity curve remains the same, regardless of the magnitude of maximum isometric tension.

The force–velocity relationship also does not imply that it is impossible to move a light load at a slow speed. Most activities of daily living require slow, controlled movements of submaximal loads. With submaximal loads, the velocity of muscle shortening is subject to volitional control. Only the number of motor units required are activated. For example, a pencil can be picked up from a desktop quickly or slowly, depending on the controlled pattern of motor unit recruitment in the muscle groups involved.

The force–velocity relationship has been tested for skeletal, smooth, and cardiac muscle in humans, as well as for muscle tissues from other species (42). The general pattern holds true for all types of muscle, even the tiny muscles responsible for the rapid fluttering of insect wings. Maximum values of force at zero velocity and maximum values of velocity at a minimal load vary with the size and type of muscle. Although the physiological basis for the force–velocity relationship is not completely understood, the shape of the concentric portion of the curve corresponds to the rate of energy production in a muscle.

The force–velocity relationship for muscle loaded beyond the isometric maximum is shown in the top half of Figure 6-17 (52). Under eccentric conditions, the maximal force a muscle can produce exceeds the isometric maximum by a factor of 1.5–2.0 (39). Achievement of such a high force level, however, appears to require electrical stimulation of the motor neuron (102). Maximal eccentric forces produced volitionally are similar to the isometric maximum (102). It is likely that this is true because the nervous system provides inhibition through reflex pathways to protect against injury to muscles and tendons (102). Elevated force production under eccentric conditions with volitional muscle activation is not a function of greater neural activation of the muscle, but appears to represent the contribution of the elastic components of muscle (54, 57).

Eccentric strength training involves the use of resistances that are greater than the athlete’s maximum isometricforce generation capability. As soon as the load is assumed, the muscle begins to lengthen. Research shows this type of training to be more effective than concentric training in increasing muscle size and strength (45). As compared with concentric and isometric training, however, eccentric training is also associated with increased muscle soreness and structural damage (16).

Length–Tension Relationship

The amount of maximum isometrictension a muscle is capable of producing is partly dependent on the muscle’s length. In single muscle fibers, isolated muscle preparations, and in vivo human muscles, force generation is at its peak when the muscle is slightly stretched (81). Conversely, muscle tension development capability is less following muscle shortening (85). Both the duration of muscle stretch or shortening and the time since stretch or shortening affect force generation capability (38, 82).

Within the human body, force generation capability increases when the muscle is slightly stretched. Parallel-fibered muscles produce maximum tensions at just over resting length, and pennate-fibered muscles generate maximum tensions at between 120% and 130% of resting length (35). This phenomenon is due to the contribution of the elastic components of muscle (primarily the SEC), which add to the tension present in the muscle when the muscle is stretched. Figure 6-18 shows the pattern of maximum tension development as a function of muscle length, with the active contribution of the contractile component and the passive contribution of the SEC and PEC indicated. Research indicates that following eccentric exercise there may be a slight, transient increase in muscle length that impairs force development when joint angle does not place the muscle in sufficient stretch (88).

Stretch-Shortening Cycle

When an actively tensed muscle is stretched just prior to contraction, the resulting contraction is more forceful than in the absence of the pre-stretch. This pattern of eccentric contraction followed immediately by concentric contraction is known as the stretch-shortening cycle (SSC). A muscle can perform substantially more work when it is actively stretched prior to shortening than when it simply contracts. In an experiment involving forceful dorsiflexion followed by plantar flexion at slow and fast frequencies, the SSC contributed an estimated 20.2% and 42.5%, respectively, to the positive work done (64). The metabolic cost of performing a given amount of mechanical work is also less when the SSC is invoked than the cost without it.

The mechanisms responsible for the SSC are not fully understood. However, one contributor in at least some cases is likely to be the SEC, with the elastic recoil effect of the actively stretched muscle enhancing force production. It has been estimated that during running at a slow speed, the triceps surae complex stores 45 J of elastic energy in the first half of stance, with 60 J produced during the second half (44). Eccentric training enhances the ability of the musculotendinous unit to store and return more elastic energy (83). Another potential contributor to the SSC is activation of the stretch reflex provoked by the forced lengthening of the muscle. Muscle spindle activity has been shown to provide a brief but substantial facilitation of neural drive during volitional contraction following prestretch (97). Force potentiation has also been shown to be significantly diminished following fatiguing exercise involving the SSC and following cooling-induced suppression of muscle spindle activity (56, 75).

Regardless of its cause, the SSC contributes to effective development of concentric muscular force in many sport activities. Quarterbacks and pitchers typically initiate a forceful stretch of the shoulder flexors and horizontal adductors immediately before throwing the ball. The same action occurs in muscle groups of the trunk and shoulders at the peak of the backswing of a golf club and a baseball bat. Competitive weight lifters use quick knee flexion during the transition phase of the snatch to invoke the SSC and enhance performance (34). The SSC also promotes storage and use of elastic energy during running, particularly with the alternating eccentric and concentric tension present in the gastrocnemius (50). Researchers have found that the muscles, tendons, and ligaments in the lower extremity behave very much like a spring during running, with higher stride frequencies associated with increased spring stiffness (23). Research also shows that positioning the starting blocks at a 30° or 50° angle as compared to a 70° angle to the track also increases the velocity of the start during sprinting, because of the added stretch placed on the Achilles tendon (36).

Electromechanical Delay

When a muscle is stimulated, a brief period elapses before the muscle begins to develop tension (Figure 6-19). Referred to as electromechanical delay (EMD), this time is believed to be needed for the contractile component of the muscle to stretch the SEC. During this time, muscle laxity is eliminated. Once the SEC is sufficiently stretched, tension development proceeds.

The length of EMD varies considerably among human muscles, with values of 20–100 msec reported (58). Researchers have found shorter EMDs produced by muscles with high percentages of FT fibers as compared to muscles with high percentages of ST fibers (73). Development of higher contraction forces is also associated with shorter EMDs (101). Factors such as muscle length, contraction type, contraction velocity, and fatigue, however, do not appear to affect EMD (101). EMD is longer when contraction is initiated from a resting state as compared to an activated state (100). EMD in children is also significantly longer than that in adults (5).

The time required for a muscle to develop maximum isometrictension may be a full second following EMD (45). Shorter maximum force development times are associated with a high percentage of FT fibers in the muscle and with a trained state (99).

In practical evaluations of muscular function, the force-generating characteristics of muscle are discussed within the concepts of muscular strength, power, and endurance. These characteristics of muscle function have significant implications for success in different forms of strenuous physical activity, such as splitting wood, throwing a javelin, or hiking up a mountain trail. Among senior citizens and individuals with neuromuscular disorders or injuries, maintaining adequate muscular strength and endurance is essential for carrying out daily activities and avoiding injury.

Muscular Strength

When scientists excise a muscle from an experimental animal and electrically stimulate it in a laboratory, they can directly measure the force generated by the muscle. It is largely from controlled experimental work of this kind that our understandings of the force–velocity and length-tension relationships for muscle tissue are derived.

In the human body, however, it is not convenient to directly assess the force produced by a given muscle. The most direct assessment of “muscular strength” commonly practiced is a measurement of the maximum torque generated by an entire muscle group at a joint. Muscular strength, then, is measured as a function of the collective force-generating capability of a given functional muscle group. More specifically, muscular strength is the ability of a given muscle group to generate torque at a particular joint.

As discussed in Chapter 3, torque is the product of force and the force’s moment arm, or the perpendicular distance at which the force acts from an axis of rotation. Resolving a muscle force into two orthogonal components, perpendicular and parallel to the attached bone, provides a clear picture of the muscle’s torque-producing effect (Figure 6-20). Because the component of muscle force directed perpendicular to the attached bone produces torque, or a rotary effect, this component is termed the rotary component of muscle force. The size of the rotary component is maximum when the muscle is oriented at 90° to the bone, with change in angle of orientation in either direction progressively diminishing it. Isokinetic resistance machines are designed to match the size of the rotary component of muscle force throughout the joint range of motion. Sample Problem 6.2 demonstrates how the torque generated by a given muscle force changes as the angle of the muscle’s attachment to the bone changes.

The component of muscle force acting parallel to the attached bone does not produce torque, since it is directed through the joint center and therefore has a moment arm of zero (Figure 6-21). This component, however, can provide either a stabilizing influence or a dislocating influence, depending on whether it is directed toward or away from the joint center. Actual dislocation of a joint rarely occurs from the tension developed by a muscle, but if a dislocating component of muscle force is present, a tendency for dislocation occurs. If the elbow is at an acute angle in greater than 90° of flexion, for example, tension produced by the biceps tends to pull the radius away from its articulation with the humerus, thereby lessening the stability of the elbow in that particular position.

Therefore, muscular strength is derived both from the amount of tension the muscles can generate and from the moment arms of the contributing muscles with respect to the joint center. Both sources are affected by several factors.

The tension-generating capability of a muscle is related to its cross-sectional area and its training state. The force generation capability per cross-sectional area of muscle is approximately 90 N/cm2 (74), as illustrated in Sample Problem 6.3. With both concentric and eccentric strength training, gains in strength over approximately the first 12 weeks appear to be related more to improved innervation of the trained muscle than to the increase in its cross-sectional area (61). This notion is further strengthened by the finding that unilateral strength training also produces strength gains in the untrained contralateral limb (46). The neural adaptations that occur with resistance training may include increased neuronal firing rates, increased motoneuron excitability and decreased presynaptic inhibition, lessening of inhibitory neural pathways, and increased levels of motor output from the central nervous system (1). Recent research findings suggest that muscle hypertrophy in response to resistance exercise is at least partially regulated by each individual’s genetic composition (10).

A muscle’s moment arm is affected by two equally important factors: (a) the distance between the muscle’s anatomical attachment to bone and the axis of rotation at the joint center, and (b) the angle of the muscle’s attachment to bone, which is typically a function of relative joint angle. The greatest amount of torque is produced by maximum tension in a muscle that is oriented at a 90° angle to the bone, and anatomically attached as far from the joint center as possible.

Sample Problem 6.2

How much torque is produced at the elbow by the biceps brachii inserting at an angle of 60° on the radius when the tension in the muscle is 400 N? (Assume that the muscle attachment to the radius is 3 cm from the center of rotation at the elbow joint.)

Known

Solution

Wanted: Tm

Only the component of muscle force perpendicular to the bone generates torque at the joint. From the diagram, the perpendicular component of muscle force is

Sample Problem 6.3

How much tension may be developed in muscles with the following cross-sectional areas?

1. 4 cm2

2. 10 cm2

3. 12 cm2

Known

muscle cross-sectional areas = 4 cm2, 10 cm2, and 12 cm2

Solution

Wanted: tension development capability

The tension-generating capability of muscle tissue is 90 N/cm2. The force produced by a muscle is the product of 90 N/cm2 and the muscle’s cross-sectional area. So,

1.

2.

3.

Muscular Power

Mechanical power (discussed in Chapter 12) is the product of force and velocity. Muscular power is therefore the product of muscular force and the velocity of muscle shortening. Maximum power occurs at approximately one-third of maximum velocity (41) and at approximately one-third of maximum concentric force (58) (Figure 6-22). Research indicates that training designed to increase muscular power over a range of resistance occurs most effectively with loads of one-third of one maximum repetition (71).

Because neither muscular force nor the speed of muscle shortening can be directly measured in an intact human being, muscular power is more generally defined as the rate of torque production at a joint, or the product of the net torque and the angular velocity at the joint. Accordingly, muscular power is affected by both muscular strength and movement speed.

Muscular power is an important contributor to activities requiring both strength and speed. The strongest shot-putter on a team is not necessarily the best shot-putter, because the ability to accelerate the shot is a critical component of success in the event. Athletic endeavors that require explosive movements, such as Olympic weight lifting, throwing, jumping, and sprinting, are based on the ability to generate muscular power.

Since FT fibers develop tension more rapidly than do ST fibers, a large percentage of FT fibers in a muscle is an asset for an individual training for a muscular power–based event. Individuals with a predominance of FT fibers generate more power at a given load than do individuals with a high percentage of ST compositions. Those with primarily FT compositions also develop their maximum power at faster velocities of muscle shortening (94). The ratio for mean peak power production by Type IIb, Type IIa, and Type I fibers in human skeletal muscle is 10:5:1 (25).

Muscular Endurance

Muscular endurance is the ability of the muscle to exert tension over time. The tension may be constant, as when a gymnast performs an iron cross, or may vary cyclically, as during rowing, running, and cycling. The longer the time tension is exerted, the greater the endurance. Although maximum muscular strength and maximum muscular power are relatively specific concepts, muscular endurance is less well understood because the force and speed requirements of the activity dramatically affect the length of time it can be maintained.

Training for muscular endurance typically involves large numbers of repetitions against relatively light resistance. This type of training does not increase muscle fiber diameter.

Muscle Fatigue

Muscle fatigue has been defined as an exercise-induced reduction in the maximal force capacity of muscle (28). Fatigability is also the opposite of endurance. The more rapidly a muscle fatigues, the less endurance it has. A complex array of factors affects the rate at which a muscle fatigues, including the type and intensity of exercise, the specific muscle groups involved, and the physical environment in which the activity occurs (47). Moreover, within a given muscle, fiber type composition and the pattern of motor unit activation play a role in determining the rate at which a muscle fatigues. However, this is an evolving area of understanding, with a considerable amount of related research in progress (15).

Characteristics of muscle fatigue include reduction in muscle force production capability and shortening velocity, as well as prolonged relaxation of motor units between recruitment (4). High-intensity muscle activity over time also results in prolonged twitch duration and a prolonged sarcolemma action potential of reduced amplitude (26).

A muscle fiber reaches absolute fatigue once it is unable to develop tension when stimulated by its motor axon. Fatigue may also occur in the motor neuron itself, rendering it unable to generate an action potential. FG fibers fatigue more rapidly than FOG fibers, and SO fibers are the most resistant to fatigue. Research has shown that the proportion of ST fibers in the vastus lateralis is directly related to the length of time that a level of 50% of maximum isometric tension can be maintained (60).

The specific causes of muscle fatigue are not well understood. However, a growing body of evidence indicates that reduction in the rate of intracellular calcium release and uptake by the sarcoplasmic reticulum is involved (103). As many as three different mechanisms of reduced calcium release have been identified, but are incompletely understood (4). Some experimental evidence suggests that sliding of the actin and myosin filaments during repeated muscle contraction reduces the affinity for calcium at uptake sites on the thin actin filaments (21). A variety of other factors have also been implicated in the development of fatigue, including increases in muscle acidity and intracellular potassium (7) and decreases in muscle energy supplies and intracellular oxygen (77).

Effect of Muscle Temperature

As body temperature elevates, the speeds of nerve and muscle functions increase. This causes a shift in the force–velocity curve, with a higher value of maximum isometrictension and a higher maximum velocity of shortening possible at any given load (Figure 6-23). At an elevated temperature, the activation of fewer motor units is needed to sustain a given load (84). The metabolic processes supplying oxygen and removing waste products for the working muscle also quicken with higher body temperatures. These benefits result in increased muscular strength, power, and endurance, and provide the rationale for warming up before an athletic endeavor. Notably, these benefits are independent of any change in the elasticity of the musculotendinous units, as research has demonstrated that the mechanical properties of muscle and tendon are not altered with either heating or cooling over the physiological range (63).

Muscle function is most efficient at 38.5°C (101°F) (6). Elevation of body temperature beyond this point may occur during strenuous exercise under conditions of high ambient temperature and/or humidity and can be extremely dangerous, possibly resulting in heat exhaustion or heat stroke. Organizers of long-distance events involving running or cycling should be particularly cognizant of the potential hazards associated with competition in such environments.

Muscle injuries are common, with most being relatively minor. Fortunately, healthy skeletal muscle has considerable ability to self-repair.

Strains

Muscular strains result from overstretching of muscle tissue. Most typically, an active muscle is overloaded, with the magnitude of the injury related to the size of the overload and the rate of overloading. Strains can be mild, moderate, or severe. Mild strains involve minimal structural damage and are characterized by a feeling of tightness or tension in the muscle. Second-degree strains involve a partial tear in the muscle tissue, with symptoms of pain, weakness, and some loss of function. With third-degree sprains, there is severe tearing of the muscle, functional loss, and accompanying hemorrhage and swelling. The hamstrings are the most frequently strained muscles in the human body.

Contusions

Contusions, or muscle bruises, are caused by compressive forces sustained during impacts. They consist of hematomas within the muscle tissue. A serious muscle contusion, or a contusion that is repeatedly impacted, can lead to the development of a much more serious condition known as myositis ossificans. Myositis ossificans consists of the presence of a calcified mass within the muscle. Apparently, the fibroblasts recruited during the healing process begin to differentiate into osteoblasts, with calcification becoming visible on a radiograph after three or four weeks (87). After six or seven weeks, resorption of the calcified mass usually begins, although sometimes a bony lesion in the muscle remains present.

Cramps

The etiology of muscle cramps is not well understood, with possible causative factors including electrolyte imbalances, deficiencies in calcium and magnesium, and dehydration. Cramps can also occur secondary to direct impacts. Cramps may involve moderate to severe muscle spasms, with proportional levels of accompanying pain.

Delayed-Onset Muscle Soreness

Muscle soreness often occurs after some period of time following unaccustomed exercise. Delayed-onset muscle soreness (DOMS) arises 24–72 hours after participation in a long or strenuous bout of exercise and is characterized by pain, swelling, and the same kinds of histological changes that accompany acute inflammation (91). Microtearing of the muscle tissue is involved, with symptoms of pain, stiffness, and restricted range of motion. Researchers have hypothesized that the increase in joint stiffness may serve as a protective mechanism that helps prevent added damage and pain (19).

Compartment Syndrome

Hemorrhage or edema within a muscle compartment can result from injury or excessive muscular exertion. Pressure increases within the compartment, and severe damage to the neural and vascular structures within the compartment follows in the absence of pressure release. Swelling, discoloration, diminished distal pulse, loss of sensation, and loss of motor function are all progressively apparent symptoms.

Muscle is elastic and extensible, and responds to stimulation. Most importantly, however, it is the only biological tissue capable of developing tension.

The functional unit of the neuromuscular system is the motor unit, consisting of a single motor neuron and all the fibers it innervates. The fibers of a given motor unit are either slow twitch, fast-twitch fatigue-resistant, or fast-twitch fast-fatigue. Both ST and FT fibers are typically found in all human muscles, although the proportional fiber composition varies. The number and distribution of fibers within muscles appear to be genetically determined and related to age. Within human skeletal muscles, fiber arrangements are parallel or pennate. Pennate fiber arrangements promote force production, whereas parallel fiber arrangement enables greater shortening of the muscle.

Muscle responds to stimulation by developing tension. Depending on what other forces act, however, the resulting action can be concentric, eccentric, or isometric, for muscle shortening, lengthening, or remaining unchanged in length. The central nervous system directs the recruitment of motor units such that the speed and magnitude of muscle tension development are well matched to the requirements of the activity.

There are well-defined relationships between muscle force output and the velocity of muscle shortening, the length of the muscle at the time of stimulation, and the time since the onset of the stimulus. Because of the added contribution of the elastic components of muscle and neural facilitation, force production is enhanced when a muscle is actively prestretched.

Muscle performance is typically described in terms of muscular strength, power, and endurance. From a biomechanical perspective, strength is the ability of a muscle group to generate torque at a joint, power is the rate of torque production at a joint, and endurance is resistance to fatigue.

1. 1. List three examples of activities requiring concentric muscle action and three examples of activities requiring eccentric muscle action, and identify the specific muscles or muscle groups involved.

2. 2. List five movement skills for which a high percentage of fast-twitch muscle fibers is an asset and five movement skills for which a high percentage of slow-twitch fibers is an asset. Provide brief statements of rationale for each of your lists.

3. 3. Hypothesize about the pattern of recruitment of motor units in the major muscle group or groups involved during each of the following activities:

   • a. Walking up a flight of stairs

   • b. Sprinting up a flight of stairs

   • c. Throwing a ball

   • d. Cycling in a 100 km race

   • e. Threading a needle

4. 4. Identify three muscles that have parallel fiber arrangements, and explain the ways in which the muscles’ functions are enhanced by this arrangement.

5. 5. Answer Problem 4 for pennate fiber arrangement.

6. 6. How is the force–velocity curve affected by muscular strength training?

7. 7. Write a paragraph describing the biomechanical factors determining muscular strength.

8. 8. List five activities in which the production of muscular force is enhanced by the series elastic component and the stretch reflex.

9. 9. Muscle can generate approximately 90 N of force per square centimeter of cross-sectional area. If a biceps brachii has a cross-sectional area of 10 cm2, how much force can it exert? (Answer: 900 N)

10. 10. Using the same force/cross-sectional area estimate as in Problem 9, and estimating the cross-sectional area of your own biceps brachii, how much force should the muscle be able to produce?

1. 1. Identify the direction of motion (flexion, extension, etc.) at the hip, knee, and ankle, and the source of the force(s) causing motion at each joint for each of the following activities:

   • a. Sitting down in a chair

   • b. Taking a step up on a flight of stairs

   • c. Kicking a ball

2. 2. Considering both the force–length relationship and the rotary component of muscle force, sketch what you would hypothesize to be the shape of a force versus joint angle curve for the elbow flexors. Write a brief rationale in support of the shape of your graph.

3. 3. Certain animals, such as kangaroos and cats, are well known for their jumping abilities. What would you hypothesize about the biomechanical characteristics of their muscles?

4. 4. Identify the functional roles played by the muscle groups that contribute to each of the following activities:

   • a. Carrying a suitcase

   • b. Throwing a ball

   • c. Rising from a seated position

5. 5. If the fibers of a pennate muscle are oriented at a 45° angle to a central tendon, how much tension is produced in the tendon when the muscle fibers contract with 150 N of force? (Answer: 106 N)

6. 6. How much force must be produced by the fibers of a pennate muscle aligned at a 60° angle to a central tendon to create a tensile force of 200 N in the tendon? (Answer: 400 N)

7. 7. What must be the effective minimal cross-sectional areas of the muscles in Problems 5 and 6, given an estimated 90 N of force-producing capacity per square centimeter of muscle cross-sectional area? (Answer: 1.2 cm2; 4.4 cm2)

8. 8. If the biceps brachii, attaching to the radius 2.5 cm from the elbow joint, produces 250 N of tension perpendicular to the bone, and the triceps brachii, attaching 3 cm away from the elbow joint, exerts 200 N of tension perpendicular to the bone, how much net torque is present at the joint? Will there be flexion, extension, or no movement at the joint? (Answer: 0.25 N-m; flexion)

9. 9. Calculate the amount of torque generated at a joint when a muscle attaching to a bone 3 cm from the joint center exerts 100 N of tension at the following angles of attachment:

   • a. 30°

   • b. 60°

   • c. 90°

   • d. 120°

   • e. 150°

   • (Answers: a. 1.5 N-m; b. 2.6 N-m; c. 3 N-m; d. 2.6 N-m; e. 1.5 N-m)

10. 10. Write a quantitative problem of your own involving the following variables: muscle tension, angle of muscle attachment to bone, distance of the attachment from the joint center, and torque at the joint. Provide a solution for your problem.

• The characteristic behavioral properties of muscle are extensibility, elasticity, irritability, and the ability to develop tension.

• Muscle’s viscoelastic property enables it to progressively lengthen over time when stretched.

• A high percentage of FT fibers is advantageous for generating fast movements, and a high percentage of ST fibers is beneficial for activities requiring endurance.

• Pennate fiber arrangement promotes muscle force production, and parallel fiber arrangement facilitates muscle shortening.

• The net torque at a joint is the vector sum of the muscle torque and the resistive torque.

• Slow-twitch motor units always produce tension first, whether the resulting movement is slow or fast.

• Two-joint muscles can fail to produce force when slack (active insufficiency) and can restrict range of motion when fully stretched (passive insufficiency).

• The stronger a muscle, the greater the magnitude of its isometric maximum on the force–velocity curve.

• Muscular strength is most commonly measured as the amount of torque a muscle group can generate at a joint.

• Explosive movements require muscular power.

Fitts RH and Widrick JJ: Muscle mechanics; adaptations with exercise-training, Exerc Sport Sci Rev 24:427, 1996.

Provides detailed review of the literature related to adaptations in muscle resulting from exercise training.

Kraemer WJ, Fleck SJ, and Evans WJ: Strength and power training: physiological mechanisms of adaptation, Exerc Sport Sci Rev 24:363, 1996.

Comprehensively reviews current knowledge about adaptations occurring in the neuromuscular, cardiovascular, and endocrine systems with resistance training.

Reich TE, Lindstedt SL, LaStayo PC, and Pierotti DJ: Is the spring quality of muscle plastic? Am J Physiol 278:R1661, 2000.

Discusses the springlike properties of muscle and the contribution of elastic energy storage and recovery in the stretch-shortening cycle.

Rogers MA and Evans WJ: Changes in skeletal muscle with aging: effects of exercise training, Exerc Sport Sci Rev, 21:65, 1993.

Discusses changes in body composition, muscle function, muscle morphology, and metabolic capacity with aging, as well as responses to exercise training in the older population.

Duke University Presents Wheeless’ Textbook of Orthopaedics

• www.wheelessonline.com/
• A comprehensive review of orthopedics; includes a search engine to locate topics related to fractures, joints, muscles, nerves, and so on.

E-Tech: An Orthopaedics and Biomechanics Resource

• http://dspace.dial.pipex.com/town/square/fk14/index.htm
• Includes links to additional pages on a number of orthopedic and biomechanics topics, including linear viscoelasticity.

Guided Tour of the Visible Human

• www.madsci.org/~lynn/VH/tour.html
• Introduces key concepts in human anatomy with images and animations from the Visible Human Project data set, which contains over 18,000 digitized sections of the human body.

Martindale’s “Virtual” Medical Center: Muscles

• www.martindalecenter.com/Medical1_1_MhU.html#Mus
• Contains numerous images, movies, and course links for human anatomy.

Myology Section from Gray’s Anatomy

• www.bartleby.com/107/102.html
• A description of muscle mechanics from this classic anatomy text.

Nicholas Institute of Sports Medicine and Athletic Trauma

• www.nismat.org/
• Provides links to pages on skeletal muscle anatomy and physiology.