Chapter 3. The Musculoskeletal System: The Musculature

At the conclusion of this chapter, the student should be able to:

• 1. Describe the structure and properties of the whole muscle, fast- and slow-twitch muscle fiber, and the myofibril.
• 2. Explain how the relationship of the muscle’s line of pull to the joint axis affects the movement produced by the muscle.
• 3. Describe the relationship between the skeletal muscle’s fiber arrangement and its function.
• 4. Define the roles a muscle may play (agonist, antagonist, and synergist), and explain the cooperative action of muscles in controllingjoint actions by naming and ex-plainingthemuscle roles in a specified movement.
• 5. Define the types of muscular contraction (concentric, eccentric, and static), and name and demonstrate each type of action.
• 6. Demonstrate an understanding of the influence of gravity and other external forces on muscular action by correctly analyzing several movement patterns in which these forces influence the muscular action.
• 7. Describe the various methods of studying muscle action, citing the advantages and disadvantages of each method.
• 8. State the force–velocity and length–tension relationships of muscular contraction, and explain the significance of these relationships in static and dynamic movements.
• 9. Identify the muscle groups active in a variety of motor skills.

Body parts are moved by external or internal forces. The internal force responsible for the movement and positioning of the bony segments of the body is the action of skeletal muscles. These muscles are able to serve this function because they can contract, they are attached to the bones, and they cross a joint. In addition, they are constructed of bundles of striated muscle fibers, which differ in both structure and function from the highly specialized cardiac muscle and from the smooth muscle of blood vessels, digestive organs, and urogenital organs.

Properties of Muscular Tissue

The properties of striated muscle tissue are extensibility, elasticity, and contractility. The first two enable a muscle to be stretched like an elastic band and, when the stretching force is discontinued, to return again to its normal resting length. Tendons, which are simply continuations of the muscle’s connective tissue, also possess these properties. Contractility, the ability to shorten and produce tension at its ends, is a unique property possessed by muscle tissue only. The average muscle fiber can shorten to approximately one-half its resting length. It can also be stretched until it is approximately one-half again as long as its resting length. The range between the maximal and minimal lengths of a muscle fiber is known as the amplitude of its action. The elongation varies proportionately with the length of the fiber and inversely with its cross section.

The Muscle Fiber

A single muscle cell is a threadlike fiber about 1 to 20 inches in length and containing the cell nucleus, mitochondria (important in cell metabolism), myoglobin (similar to hemoglobin), and glycogen (form of sugar). In addition, microscopic examination has also revealed that the fiber consists of many hundreds of myofibrils embedded in sarcoplasm, held together by a delicate membrane known as sarcolemma, which has the ability to propagate nerve impulses (Figure 3.1). Each fiber is enclosed within a thin connective tissue sheath called endomysium. The microscopic myofibrils, which are the contractile elements, are arranged in parallel formation within the fiber and are made of alternating dark and light bands that give the muscle fibers their striated appearance.

The electron microscope has revealed the striations to be a repeating pattern of bands and lines caused by an interdigitating arrangement of two sets of filaments. These myofilaments of contractile proteins, mainly actin (thin ones) and myosin (thick ones), when stimulated, slide past each other. This is due to the coupling and uncoupling of the cross-bridges, which appear as projections (heads) of the myosin filament attaching to the actin filaments. The myofibril is divided into a series of several sarcomeres, with each sarcomere consisting of the portion of the myofibril between two Z lines (Figure 3.2). The sarcomere is considered the functional contractile unit of skeletal muscle. A concentric contraction (shortening) occurs when the actin filament slides over the myosin filament, pulling the Z lines toward one another; an eccentric contraction is the reverse, with the actin filament sliding outward, resulting in the return of the sarcomere to its original length or longer. An isometric contraction involving no length change results from the continual breaking and rebuilding of the cross-bridges. This is a condensed and highly simplified explanation of contraction, a function that is the unique property of muscle tissue.

The muscle fibers are bound into bundles within bundles (see Figure 3.1). Each individual bundle of muscle fibers, called a fasciculus, is enclosed in a fibrous tissue sheath called perimysium; the group of bundles that constitutes a complete muscle is in turn encased within a tougher connective tissue sheath called epimysium. In long muscles whose fibers run parallel to the long axis of the muscle, the bundles form “chains,” which function as though the individual fibers ran the entire length of the muscle. Each enclosing sheath (the endomysium, perimysium, and epimysium) is made of connective tissue, which contains elastic fibers arranged both in series (end to end) and parallel (in the same direction) to the contractile elements. The elements that are in series are those which are important in controlling how the tension is applied to the bony levers in moving a load. Those in parallel remain slack in contraction and have a negligible effect during stretching of the muscle fiber.

Muscles are attached to bone by means of their connective tissue, which continues beyond the muscle belly in the form of a tendon or an aponeurosis (a fibrous sheet). The muscle–tendon complex formed by the two tissue types provides the primary series elastic component of muscle. The stiffness of the tendon is largely responsible for the transmission of force from the contracting muscle to the affected bone.

Slow- and Fast-Twitch Fibers

Although four different fiber types (one slow- and three fast-twitch fibers) have been identified in human skeletal muscle, only the two major categories of slow- and fast-twitch fibers are pertinent for the study of kinesiology. In humans, most limb muscles contain a relatively equal distribution of each muscle fiber type, whereas the muscles of the back (i.e., posture muscles) contain more slow-twitch fibers. Where differences exist, the usual variation may be of the order of 2 percent to 10 percent. However, rare instances of significant variation between fiber types may be found among muscles and individuals, although not between genders.

The two primary types of fast-twitch muscle fiber are types IIa (fast oxidative glycolytic) and IIb (fast glycolytic). Fast-twitch muscle fibers are large and pale and have a less elaborate blood supply than slow-twitch muscle fibers. Once stimulated, fast-twitch fibers respond rapidly but fatigue easily. This characteristic makes them suitable for activities requiring intense responses over a short period of time, such as those responses found in sprints or weight-training exercises.

Slow-twitch fibers are small and red and have a rich blood supply. They also contain more myoglobin than fast-twitch fibers. Slow-twitch fibers take relatively longer to reach peak isometric tension (80–100 milliseconds [ms] vs. approximately 40–60 ms for fast-twitch fibers), are highly efficient, and do not fatigue easily. Slow-twitch fibers are suitable for activities of long duration, including posture and endurance events.

Research has shown that training does not produce an increase in the number of muscle fibers or major shifts in muscle fiber types. There is evidence that some shift from fast-twitch to slow-twitch fibers, or the reverse, can occur with training (MacIntosh et al. 2006). Training does result in muscle fiber hypertrophy and may change some fast-twitch fiber types, but these changes, such as increased endurance and a greater number of capillaries, are reversible when use is discontinued. This is one reason that activity that places demands on the muscle fibers should be done on a lifelong basis.

Muscular Attachment

Historically, anatomy texts designated attachments of the two ends of a muscle as “origin” and “insertion.” The origin is usually characterized by stability and closeness of the muscle fibers to the bone. It is usually the more proximal of the two attachments. The insertion, on the other hand, is usually the distal attachment; it frequently involves a relatively long tendon, and the bone into which the muscle’s tendon inserts is ordinarily the one that moves. It should be understood, however, that the muscle does not pull in one direction or the other. When it contracts, it exerts equal force on the two attachments and attempts to pull them toward each other. Which bone is to remain stationary and which one is to move depends on the purpose of the movement. A muscle spanning the inside of a hinge joint, for instance, tends to draw the two bones toward one another. However, most precision movements require that the proximal bone be stabilized, whereas the distal bone performs the movement. The stabilization of the proximal bone is achieved by the action of other muscles. Sometimes the greater weight or the more limited mobility of the proximal structure is sufficient to stabilize it against the pull of the contracting muscles.

Reverse Muscle Action

Students often receive the impression that there is some physiological reason for a muscle to pull in a single direction. They fail to grasp the concept of a muscle merely contracting, not realizing that it cannot pull in a predetermined direction. This misconception makes it difficult for students to understand seeming exceptions. Actually, in many movements the insertion or distal attachment of the muscle is stationary and the origin or proximal attachment is the one that moves. Such is the case in the familiar act of performing pull-ups. The movement of the elbow joint is flexion, but it is the upper arm that moves toward the forearm, just the reverse of what happens when one lifts a book from the table. The grasp of the hands on the bar serves to immobilize the forearm, and thus it provides a stable base for the contracting muscles. To avoid the erroneous idea of a muscle always pulling from its insertion toward its origin, the following terminology for muscular attachments is used in this text.

Terminology for Muscular Attachments

Structural Classification of Muscles on the Basis of Fiber Arrangement

The arrangement of the fibers and the method of attachment vary considerably among different muscles. These structural variations form the basis for a classification of the skeletal muscles.

Longitudinal

This is a long, straplike muscle whose fibers lie parallel to its long axis. Two examples are the sartorius, which slants across the front of the thigh, and the rectus abdominis on the front of the abdomen (see Figures 7.15 and 9.17).

Quadrate or Quadrilateral (Figures 3.3e and f)

Muscles of this type are four sided and usually flat. They consist of parallel fibers. Examples include the pronator quadratus on the front of the wrist and the rhomboid muscle between the spine and the scapula.

Triangular or Fan Shaped (Figure 3.3d)

This is a relatively flat type of muscle whose fibers radiate from a narrow attachment at one end to a broad attachment at the other. The pectoralis major on the front of the chest is an excellent example.

Fusiform or Spindle Shaped (Figure 3.3a)

This is usually a rounded muscle that tapers at either end. It may be long or short, large or small. Good examples are the brachialis and the brachioradialis muscles of the upper extremity.

Pennate (Figure 3.3b)

In this type of muscle, a series of short, parallel, featherlike fibers extends diagonally from the side of a long tendon, giving the muscle as a whole the appearance of a wing feather. Examples include the extensor digitorum longus and tibialis posterior muscles of the leg.

Bipennate (Figure 3.3c)

This double pennate muscle is characterized by a long central tendon with the fibers extending diagonally in pairs from either side of the tendon. It resembles a symmetrical tail feather. Examples include the flexor hallucis longus and rectus femoris of the leg and thigh, respectively.

Multipennate

In this type of muscle several tendons are present, with the muscle fibers running diagonally between them. The middle portion of the deltoid muscle of the shoulder and upper arm is a prime example of a multipennate muscle (see Figures 5.11 and 5.14).

Effect of Muscle Structure on Force and Range of Motion

The force a muscle can exert is proportional to its physiological cross section, a measure that accounts for the diameter of every fiber and whose size depends on the number and thickness of the fibers. A broad, thick, longitudinal muscle exerts more force than a thin one, but a pennate muscle of the same thickness as a longitudinal muscle can exert greater force. The oblique arrangement of the fibers in the various classifications of pennate muscle allows for a larger number of fibers than in comparable sizes of the other classifications. Pennate muscles are the most common type of skeletal muscle and predominate when forceful movements are needed.

The range through which a muscle shortens depends on the length of its fibers, with the average muscle fiber capable of shortening to half its resting length. Those long muscles with fibers longitudinally arranged along the long axis of the muscle, such as the sartorius, can exert force over a longer distance than muscles with shorter fibers. Muscles of the pennate type, with their oblique fiber arrangement and short fiber length, can exert their superior force through only a short range.

The basis for all muscle function is the ability of muscular tissue to contract. This should be kept in mind as various aspects of muscular function are considered.

Line of Pull

The movement that the contracting muscle produces—flexion, extension, abduction, adduction, or rotation—is determined by two factors: the type of joint that it spans and the relation of the muscle’s line of pull to the joint. For instance, the contraction of a muscle whose line of pull is directly anterior to the knee joint may cause the joint to extend, whereas a muscle whose line of pull is anterior to the elbow joint may cause this joint to flex. The possible axes of motion are, of course, determined by the structure of the joint itself. It will be recalled that, from the anatomical standing position, hinge joints have only a bilateral axis, and condyloid (ovoid) joints have both a bilateral and a anteroposterior axis. Ball-and-socket joints have three axes, bilateral and anteroposterior and vertical (longitudinal), whereas pivot joints have a vertical axis only.

A muscle whose line of pull is lateral to the hip joint is a potential abductor of the thigh, but muscles whose lines of pull are lateral to the elbow joint cannot cause abduction of the forearm because the construction of the elbow joint is such that no provision is made for abduction or adduction. Because it is a hinge joint, its only axis of motion is a bilateral one, and the only movements possible are flexion and extension.

The importance of the relation of a muscle’s action line to the joint’s axis of motion is especially seen in some of the muscles that act on triaxial joints. Occasionally it happens that a muscle’s line of pull for one of its secondary movements shifts from one side of the joint’s center of motion to the other during the course of the movement. For instance, the clavicular portion of the pectoralis major is primarily a flexor, but it also adducts the humerus. When the arm is elevated sideward (abducted) to a position slightly above shoulder level, however, the line of pull of some of the fibers of the clavicular portion shifts from below to above the anteroposterior axis of the shoulder joint (Figure 3.4). Contraction of these fibers in this position contributes to abduction of the humerus, rather than to adduction. Similarly, several muscles or parts of muscles of the hip joint appear to reverse their customary function. Steindler (1970) pointed out that as the adductor longus adducts the hip joint, it also flexes it until the flexion exceeds 70 degrees. Beyond this point the adductor longus helps extend the hip.

It is not uncommon to classify and name muscles according to the relation of their line of pull to the joint structure. In other words, muscles whose line of pull is such that they produce flexion are called flexors, and those in a position to cause extension are extensors. Similarly there would also be abductors, adductors, and rotators. The difficulty with this type of classification is that it may mislead the student into believing that the muscle is responsible for the related joint action under all circumstances, when in reality this is not the case. The biceps brachii muscle, for instance, is usually listed as a flexor and supinator of the forearm, when in fact the results of electromyographic studies have established that the biceps plays little if any part in flexion of the prone forearm or supination of the extended forearm unless the movements are resisted (Basmajian and DeLuca 1985). This example is not unusual. Many situations occur in which the presence or absence of resistance governs a muscle’s participation in a joint action. Other factors, such as the starting position for the joint action, the direction of the movement, and the speed of the movement may also alter a muscle’s involvement in the joint action.

Knowing the general location of a muscle with respect to the joint axis—anterior, superior, lateral, or medial—and knowing the line of pull of the muscle is important information for deducing possible muscle participation during a body movement, but confirmation of those muscle actions must rely on the evidence of electromyography.

Angle of Attachment

The efficiency of a muscle in producing movement at a joint is also affected by the angle of attachment of a muscle to the stationary bone. If the angle of attachment is very shallow (the muscle lies along the line of the bone), most of the tension developed in the muscle will produce a force pulling along the bone. In other words, this muscle will have a very large stabilizing component. Force produced by this muscle will tend to stabilize the joint rather than produce motion. Conversely, a muscle whose angle of attachment is fairly large will have a much larger rotary component of force. The development of tension in this muscle will produce a force that will cause action at the joint. In many muscles the angle between muscle and bone changes as movement occurs. A good example of this is the biceps, whose tendon can be easily palpated where it attaches to the radius. Starting from the neutral position, one can feel that the biceps tendon lies almost parallel to the humerus. As the forearm is slowly moved through elbow flexion, it is possible to feel the change in the angle between the biceps tendon and the radius. When the elbow is at a 90-degree angle, notice that the biceps tendon forms approximately a 90-degree angle with the radius. In this position, all of the force generated by the biceps can act to produce joint motion. As elbow flexion continues beyond 90 degrees, notice that the angle between the biceps tendon and the radius again begins to decrease as the line of pull shifts closer to the bone. As this change occurs, the biceps is becoming less efficient at producing joint motion.

Types of Contraction

Because the word contract literally means to “draw together” or to shorten, the nature of muscular contraction may cause some initial confusion. A muscle contraction occurs whenever the muscle fibers generate tension in themselves, a situation that may exist when the muscle is actually shortening, remaining the same length, or lengthening.

Concentric or Shortening Contraction

Concentric (toward the middle) contraction occurs when the tension generated by the muscle is sufficient to overcome a resistance and to move the body segment of one attachment toward the segment of its other attachment. As the arm is raised sideward, the abductor shoulder muscles shorten to overcome the resistance of the arm. The muscle actually shortens and, when one end is stabilized, the other pulls the bone to which it is attached and turns it about the joint axis (Figure 3.5c).

Eccentric or Lengthening Contraction

When a muscle slowly lengthens as it gives in to an external force (such as gravity) that is greater than the contractile force it is exerting, it is in eccentric (away from the middle) contraction. The term lengthening is misleading because in most instances the muscle does not actually lengthen. It merely returns from its shortened condition to its normal resting length (Figure 3.5a). The abductor shoulder muscles are in eccentric contraction when the arm is slowly lowered from an abducted position. In most instances in which muscles contract eccentrically, the muscles are acting as a “brake,” or resistive force, against the moving force of gravity or other external forces. When the muscles contract in this manner, they are said to perform negative work.

Isometric or Static Contraction

Isometric means “equal length.” Tension of the muscle in partial or complete contraction without any appreciable change in length is isometric contraction. Isometric contraction is likely to occur under two different conditions.

• 1. Muscles that are antagonistic to each other contract with equal strength, thus balancing or counteracting each other. The part affected is held tensely in place without moving. Tensing the biceps to show off its bulge is an example of this. The contraction of the triceps prevents the elbow from further flexing.
• 2. A muscle is held in either partial or maximal contraction against another force, such as the pull of gravity or an external mechanical or muscular force. Examples of this are holding a book with outstretched arm, a tug of war between two equally matched opponents, and attempting to move an object that is too heavy to move.

Isotonic Contraction

Isotonic means “equal tension.” Isotonic contraction is a contraction in which the tension remains constant as the muscle shortens or lengthens. It is commonly, although erroneously, used as a synonym for either eccentric or concentric contraction. The latter terms, however, do not indicate the degree of tension; they merely indicate an increase or decrease in length.

Isokinetic Contraction

Literally, isokinetic means “equal or same motion” (iso + kinetic). Through the use of special equipment, it is possible to have maximum muscle effort at the same speed throughout the entire range of motion of the related lever. The response of the muscles in maximum contraction to the “accommodating resistance” of the machinery is called isokinetic contraction.

Influence of Gravity

Movements of the body or its segments may be in the direction of gravitational forces (downward), opposing gravity (upward), or perpendicular to gravity (horizontal). It is essential to consider the direction and speed of the movement when identifying the nature of the muscular involvement of any movement. The muscles may be contracting, either to provide the force for a movement or the force to resist and control the movement, or they may be completely relaxed (Figure 3.5). It may surprise the student to learn that the muscles used when placing a book on a low table or a suitcase on the floor are the same as those used for lifting it. They are used in a different way, however. When the book or suitcase is lifted, the muscles provide the force, and the weight of the object (gravity) is the resistance. In this case the muscles shorten in concentric contraction. When the object is slowly lowered, however, the muscles lengthen in eccentric contraction as they resist, but gradually give in to, the force of gravity. Without the muscular resistance, the force of gravity would lower the object at a far more rapid rate! Another example of the influence of gravity on muscular action is the action of the lower extremity muscles when one lowers the body weight by bending the knees to assume a squat or semisquat position and then returns to the erect position. As the body is lowered, the extensor muscles of the hips and knees are undergoing eccentric contraction. They are indeed lengthening in this instance, yet their tension is increasing as they assume the burden of the body weight and gradually allow it to be lower in a controlled manner. When the joint action is reversed and the body weight is lifted, the extensor muscles are contracting concentrically until the hips and knees are straight and the body is erect.

As has been demonstrated, any slow, controlled movement in the downward direction of gravity’s force uses the same muscles in eccentric contraction that would be used in concentric contraction to perform the opposite upward movement (against gravity). Slow lowering of the forearm from a flexed position to one of extension is controlled by eccentric contraction of the elbow flexors, the same muscles that, by contracting concentrically, cause flexion. A forceful movement downward does not follow this pattern, however. When done forcefully, as in the downswing of a golf swing, the same movement of elbow extension uses the elbow extensors in concentric contraction, because gravitational pull is now being exceeded (Figure 3.6).

Muscular involvement in movements performed horizontally is not affected by gravity in the manner just described. Regardless of the force or speed of the movement, the muscle force for extension of the forearm at the elbow in the transverse plane is provided by the elbow extensors. The only exception to this is seen when one is opposing an external force other than gravity, and it proves to be too strong. In a tug of war, for instance, the weaker opponents will find their elbows being pulled out straight in spite of themselves. The elbow flexor muscles are in eccentric contraction as they attempt to resist the force causing the extension at the elbow.

Finally, at times the movement of the body or its segments occurs without muscular action. In these instances the external force causing the movement is not resisted by eccentric muscle action. Examples are allowing the force of gravity to cause the arm to drop to the side from a flexed or abducted position (Figure 3.5b) and the passive moving of a body segment by another person.

Length–Tension Relationship

There is an optimum length at which a muscle, when stimulated, can exert maximum tension. This length varies somewhat according to both the muscle’s structure and its function but, as a general rule, it is slightly greater than the resting length of the muscle. Lengths that are either greater or less produce less tension. This relationship applies for all three types of contraction: isometric, concentric, or eccentric. A typical length–tension curve is depicted in Figure 3.7. The maximum resting length occurs at 100 percent. Curve 1 represents the tension generated when the muscle is passively stretched, and curve 2 represents the total tension generated in the muscle. The active tension of the muscle would thus be the difference between the passive and the total tension, curve 2 minus curve 1. This relationship suggests that when maximum force is desired, the muscle should be longer than the resting length. It should also be noted that a longer tendon can generate a higher level of stored elasticity than a shorter tendon. The Achilles tendon, for instance, is able to generate greater velocity than the shorter quadriceps tendon.

Other factors such as the muscle’s angle of pull also must be considered. It is rare for the angle of pull for optimum force application to occur when the muscle is in the resting position.

Force–Velocity Relationship

As the speed of a muscular contraction increases, the force it is able to exert decreases. The velocity of contraction is maximal when the load is zero, and the load is maximal when the velocity is zero (Figure 3.8). For any given load there is an optimum velocity that is somewhere between the slowest and fastest rates. As the load increases, the optimum rate decreases. So does the need for more cross-bridges. Because it takes time for the cross-bridge sites to establish or break down, the inverse relationship between load and rate is readily understood. Thus, if an activity requires the development of large forces, only a small amount of muscle shortening should be expected. If, on the other hand, high limb or implement velocities are needed, then little force should be expected from the contracting muscles.

Stretch–Shortening Cycle

The synergistic response of a muscle and associated tendinous structures gives rise to the phenomenon known as the stretch–shortening cycle. Both muscle and tendon possess elastic properties. That is, when they are stretched, they store energy and will release this energy when they return to their original length. When a concentric muscle contraction is preceded by an active stretch, the elastic energy stored in the stretch phase is available for use in the contractile phase. The work done by the concentric contraction of muscle immediately following a prestretch is greater than that done by muscles contracting from their resting length. In addition, the work done increases with the speed of the stretch.

In the muscle–tendon complex, the tendons are most responsive to speed of stretch and provide the largest percentage of elastic energy release. Tendons, therefore, are the prime providers of the series elastic component. A parallel elastic component of muscle is also active when a prestretch is applied. This parallel component is considered to be the result of cross-bridge and fascicle elasticity (the contractile elements) and possibly the stretch reflex (see Chapter 4).

Many common movement patterns make use of the stretch–shortening cycle to enhance the effects of muscle action. The countermovement action common in the preparatory phase of many sport motions is an example of this. A fast squat before a jump, a rapid backswing before a tennis serve, or a kick are all examples of the utilization of the stretch–shortening cycle and the series elastic component of the muscle–tendon complex.

An effective, purposeful movement of the body or any of its parts involves considerable muscular activity in addition to that of the muscles that are directly responsible for the movement itself. To begin with, the muscles causing the movement must have a stable base. This means that the bone (or bones) not engaged in the movement but providing attachment for one end of such muscles must be stabilized by other muscles. In some movements, such as those in which the hands are used at a high level, the upper arms may need to be maintained in an elevated position. This necessitates contraction of the shoulder muscles to support the weight of the arms. Many muscles, especially those of biaxial and triaxial joints, can cause movements involving more than one axis, yet it may be that only one of their actions is needed for the movement in question. A similar situation exists in regard to the muscles of the scapula. A muscle cannot voluntarily choose to effect one of its movements and not another; it must depend on other muscles to contract and prevent the unwanted movement. Even a simple movement such as threading a needle or hammering a nail may require the cooperative action of a relatively large number of muscles, each performing its own particular task in producing a single, well-coordinated movement.

Roles of Muscles

Muscles have various roles. Their particular roles in a given movement depend on the requirements of that movement. These roles are designated prime movers (or agonists), antagonists, and synergists. Furthermore, if one concedes that the negative function of remaining relaxed can be viewed as a role, then the muscles that are antagonistic to the movers may also be included as participants in the total cooperative effort. The definitions of these roles are as follows.

Movers, or Agonists

A mover is a muscle that is directly responsible for producing a movement. In the majority of movements there are several movers, some of them of greater importance than others. These are the principal, or prime, movers. The muscles that help perform the movement but seem to be of less importance, or contract only under certain circumstances, are the assistant movers. Muscles that help only when an extra amount of force is needed, as when a movement is performed against resistance, are sometimes called emergency muscles. This distinction between the various muscles that contribute to a movement is an arbitrary one. There may well be some difference of opinion as to whether a muscle is a prime or an assistant mover in a given movement.

Synergists

The term synergy is often used to describe muscles in various ways, from mutual neutralizer to stabilizer. In this text, synergistic muscle action will be used to indicate cooperative muscle functioning in various roles. Muscles acting as synergists may fill one of several roles. The way in which a muscle aids in the production of the desired motion determines its synergistic role.

Stabilizing, Fixator, and Supporting Muscles

This group includes the muscles that contract statically to steady or support some part of the body against the pull of the contracting muscles, against the pull of gravity, or against the effect of momentum and recoil in certain vigorous movements. One of the most common functions of stabilizers is steadying or fixating the bone to which a contracting muscle is attached. It is only by the stabilizing of one of its attachments that the muscle is able to cause an effective movement of the bone at which it has its other attachment (Figure 3.9). The term supporting may be used when a limb or the trunk must be supported against the pull of gravity while a distal segment such as the hand, foot, or head is engaging in the essential movement.

Neutralizers

A neutralizer is a muscle that acts to prevent an undesired action of one of the movers. Thus, if a muscle both flexes and abducts, but only flexion is desired in the movement, an adductor contracts to prevent the abduction action of the mover.

Occasionally two of the movers have one action in common but can also perform second actions that are antagonistic to each other. For instance, one muscle may upward rotate and adduct while the other may downward rotate and adduct. When they contract together to cause adduction, their rotary functions counteract each other (Figure 3.10). Muscles that behave this way in a movement are mutual synergists as well as movers.

Antagonists

Muscles that have an effect opposite to that of movers, or agonists, are labeled antagonists. Because they are located on the opposite side of the joint from the movers, they are also called contralateral muscles. The elbow flexors, located on the anterior arm, are antagonistic to the elbow extensors located on the posterior arm. When the forearm is extended at the elbow, as in doing a push-up, the extensors are the movers and contract concentrically to provide the force for the movement. The flexors are the antagonists and are relaxed. In the return movement, the letdown, the joint action at the elbow is flexion but the flexors do not contract—they remain relaxed. The contracting muscles, once again, are the extensors, but now they are contracting eccentrically to resist gravity and control the speed of the elbow flexion. When a body segment is moved by muscular effort, the contracting muscles are the movers in concentric contraction. When movement of a segment is effected by the force of gravity and resisted by muscle force, the contracting muscles are the antagonists in eccentric contraction.

Now that the general rule has been stated, it is necessary to describe what may at first appear to be a contradiction. If a movement performed with great force and rapidity is not checked, it will subject the ligamentous reinforcements of the joint to sudden strain. The tissues would probably be severely damaged. This is particularly true of quick movements of the arm or leg because of the tremendous momentum that can be developed in a long lever. This is often referred to as a ballistic movement, which by definition is a movement carried on by its own momentum. To prevent injury from the momentum of the limbs, the muscles that are antagonistic to the movers contract momentarily to check the movement. As they contract, the movers relax, if indeed they have not already relaxed, allowing momentum to complete the movement. The situation is a little like taking the foot off the accelerator to put it on the brake. At the moment when the movement is being checked, the so-called antagonistic muscles are not truly antagonistic. In a vigorous movement, the antagonistic muscles may be said to perform two functions. Their first function is to relax in order to permit the movement to be made without hindrance; their second function is to act as a brake at the completion of the movement and, by doing so, to protect the joint.

Summary of Muscle Classification Based on Role in Total Movement

• Mover or agonist: A muscle that is directly responsible for effecting a movement.
• Synergist:
  • Fixator, stabilizer, supporting muscle: Muscles that contract statically to steady or to support some part of the body against the pull of contracting muscles, the pull of gravity, or any other force that interferes with the desired movement.
  • Neutralizer: A muscle that acts to prevent an undesired action of one of the movers.
• Antagonist: A muscle that causes the opposite movement from that of the movers.

Example of the Different Roles of Muscles in a Total Movement

Let us consider the movement of the right upper arm when playing shuffleboard (pushing a disk with a cue). The movement of the humerus at the shoulder joint is flexion, and this is accompanied by slight upward rotation and abduction of the scapula. The movers of the humerus are the anterior deltoid, the clavicular portion of the pectoralis major, and the coracobrachialis. (Look up these terms if you are not yet familiar with them.) The two former muscles are also inward rotators of the humerus, but because this movement is not desired, it must be prevented. The infraspinatus and the teres minor take care of this. Therefore, they are serving as neutralizers in this pushing action. Meanwhile, the scapula is rotating upward through the action of the serratus anterior and trapezium II and IV, which are serving as shoulder girdle movers. Trapezius II, which is an elevator as well as an upward rotator of the scapula, and trapezium IV, which is a depressor as well as an upward rotator, mutually neutralize each other with respect to elevation and depression while they are cooperating in rotating the scapula upward. Hence they are both movers and mutual neutralizers.

Consider the movers of the humerus once again, keeping in mind that muscles tend to pull both their distal and proximal ends toward each other. Note that the proximal attachments of both the anterior deltoid and the pectoralis major are side by side on the anterior border of the clavicle. As they contract, they not only raise the humerus forward but also tend to pull the clavicle laterally, a movement that would put a strain on the sternoclavicular joint. They are prevented from doing this by the action of the subclavius muscle, which pulls the clavicle medially and thus serves as a stabilizer. So in this simple pushing movement we see that we have muscles acting as movers, neutralizers, and stabilizers, which, by their cooperative action, ensure an efficient movement.

Cocontraction

The patterns of muscle activation that can occur in the production of human movement are vast in number. Recent research in muscle activation has shown that the relationships between muscle actions is, by nature, task dependent. That is, the relationships will depend on the nature of the motion to be produced. Simultaneous cocontraction or coactivation of agonist and antagonist muscles across one or more joints does occur. Cocontraction is most often associated with stabilization, either during postural loading or in dynamic, unstable situations. The cocontraction of the abdominal muscles, for instance, helps stabilize the trunk during lifting tasks. In a similar fashion, cocontraction of the muscles of the arm stabilizes the arm when learning an accuracy task (Granata and Marras 2000; Gribble et al. 2003).

Cocontraction of biarticular muscles is common. The muscles that act as an agonist–antagonist pair for the motion of one joint often reverse roles for the second joint. In this situation, both muscles are active, but for different reasons. A more detailed description of the actions of biarticular muscles follows.

Action of Biarticular Muscles

Another type of coordination of the muscular system may be seen in the action of the biarticular muscles—that is, the muscles that pass over and act on two joints. Examples of these are the hamstrings (the semitendinosus, the semimembranosus, and the biceps femoris), which flex the leg at the knee and extend the thigh at the hip; the rectus femoris, which flexes the thigh and extends the leg; the sartorius, which flexes both the thigh and the leg; the gastrocnemius, which helps flex the leg in addition to its primary function of extending the foot; and the long flexors and extensors of the fingers. The latter are actually multijoint muscles because they cross the wrist and at least two of the joints of the fingers. A characteristic of all these muscles, whether they act on joints that flex in the same direction, as in the case of the wrist and fingers, or in the opposite direction, as in the case of the knee and hip, is that they are not long enough to permit complete movement in both joints at the same time. This results in the tension of one muscle being transmitted to the other, in much the same manner that a downward pull on a rope that passes through an overhead pulley is transmitted in the form of a pull in the reverse direction to the rope on the other side of the pulley. Thus, if the hamstrings contract to help extend the hip, tension in the form of a stretch is placed on the rectus femoris, causing it to extend the knee. Or if the rectus femoris contracts to help flex the hip, tension in the form of a stretch is placed on the hamstrings, causing them to flex the knee. This is a simplified version of what in actuality is a rather complex coordination.

When monoarticular (one-joint) muscles contract, their shortening is accompanied by a corresponding loss of tension. In quick movements of the limbs, monoarticular muscles rapidly lose their tension. The advantage of biarticular muscles is that they can continue to exert tension without shortening. The biarticular muscles have two different patterns of action, described as concurrent and countercurrent movements.

Concurrent Movements

An example of concurrent movement is seen in the simultaneous extension of the hip and knee and also in the simultaneous flexion of these joints. As the muscles contract, they act on each other in such a way that they do not lose length; thus their tension is retained. It is as though the pull traveled up one muscle and down the other in a continuous circuit. In simultaneous extension of the hip and knee, for instance, the rectus femoris’s loss of tension at the distal or knee end is balanced by a gain in tension at the proximal or hip end. Similarly, the hamstrings, which are losing tension at their proximal end, are gaining it at their distal end.

Countercurrent Movements

The countercurrent pattern presents a different picture. In this type of movement, while one of the biarticular muscles shortens rapidly at both joints, its antagonist lengthens correspondingly and thereby gains tension at both ends. An example of this action is seen in the rapid loss of tension in the rectus femoris and corresponding gain of tension in the hamstrings when the hip is flexed and the knee is extended simultaneously. A vigorous kick is a dramatic illustration of this kind of muscle action. The backward swing of the lower extremity preparatory to kicking is a less spectacular but equally valid example. The forward swing in walking is another. In both patterns of movement the monoarticular and biarticular muscles appear to supplement each other and thus, by their cooperative action, produce smooth, coordinated, efficient movements (Figure 3.11).

When a biarticular muscle contracts, it acts on both joints it crosses. If action is desired in only one joint, the other joint must be stabilized by another muscle or by some external force. In such circumstances, Basmajian (Basmajian and DeLuca 1985) found that the rectus femoris shows maximum activity in either pure hip flexion or pure knee extension, and the medial hamstrings, similarly, are most active in either hip extension or knee flexion. In the biarticular countercurrent movements of hip flexion and knee extension, the rectus femoris exhibited strong activity. The medial hamstrings exhibited similar strong activity during hip extension and knee flexion. When the movement was concurrent hip and knee flexion, however, the rectus femoris showed no activity, but the hamstring did. Similarly, during knee and hip extension, the hamstrings were inhibited and the rectus femoris was active. The activity of the rectus femoris and hamstrings was thus inhibited when they were antagonists.

Types of Bodily Movements

Movements may be passive or active, and if active, they may be slow or rapid. They may involve the constant application of force, or after the initial impetus has been given, they may continue without further muscular effort.

A passive movement requires no effort on the part of the person involved. It is performed by another person such as a therapist giving a treatment or an instructor or partner stretching tight ligaments, fascia, and muscles in an attempt to increase the range of motion of a particular joint. In some cases, it is a movement that has been started by the subject’s own effort but is continued by momentum. It might also be caused by the force of gravity if the subject remained relaxed and used no muscular effort to aid, restrain, or guide the moving part. To avoid injury, care must be taken that the subject does not resist the movement and that the person applying the force to cause the movement is not too aggressive.

An active movement is produced by the subject’s own muscular activity. It is usually performed volitionally, but it may be a reflex reaction to an external or internal stimulus. It may be rapid or slow. In slow movements muscular tension is maintained throughout the range of motion. Pushing a heavy piece of furniture across the room is an example of this type of movement. In rapid movements, tension could also be maintained throughout the range of motion, but this would be an inefficient way of performing. For efficiency, rapid movements should be performed ballistically. The concept of ballistic movement was introduced a number of years ago by experimental psychologists. They used the term for movements that were initiated by vigorous muscular contraction and completed by momentum. This type of movement is characteristic of throwing, striking, and kicking. It is also seen in the finger movements used for typing and piano playing. When such movements are performed nonballistically—that is, with constant muscular contraction—they are uneconomical and hence not skillful. In fact, they are characteristic of the way in which beginners tend to attempt new coordinations, especially if they are concentrating on accuracy of aim rather than on a ballistic type of motion. They need to be encouraged in the early stages of learning a skill to concentrate on form rather than accuracy if they are to master the skill of moving ballistically.

Ballistic movements may be terminated by one of three methods: (1) by contracting antagonistic muscles, as in the forehand drive in tennis; (2) by allowing the moving part to reach the limit of motion, in which case it will be stopped by the passive resistance of ligaments, other tissues, or the braking action of antagonistic muscles, as in the case of the forceful overarm throw; or (3) by the interference of an obstacle, as when chopping wood.

In many movements found in both sports and activities of living, three types of muscular action cooperate to produce a single act. This kind of cooperation is seen especially in striking activities that require the use of an implement, such as a tennis racket, golf club, or ax. Movements such as these involve (1) fixation to support the moving part and to maintain the necessary position, (2) ballistic movement of the active limb, and (3) fixation in the fingers as they grasp the implement.

In addition to the obvious method of studying the actions of muscles in a textbook, a number of procedures may be more meaningful to the student.

Conjecture and Reasoning

Using knowledge of the location and attachments of a muscle and the nature of the joint or joints it spans, one can deduce a great deal about a muscle’s actions. Conjecture and reasoning is a valuable method to use in conjunction with other methods. Although the use of this method does not identify which muscles are actually contracting in a given action, a careful study of the muscles’ attachments and line of pull (Figure 3.12) will enable one to see what movements a muscle is capable of causing.

Dissection

An excellent way of studying the location and attachments of a muscle and its relation to the joint it spans is dissection. This method provides a more meaningful basis for visualizing the muscle’s potential movements, but it sometimes leads to misinterpretations and erroneous conclusions.

Inspection and Palpation

Even though its use is limited to superficial muscles, inspection and palpation of normal living subjects is a valuable method, as far as it goes. Much of the information in early anatomy textbooks was based on the combination of dissection of the cadaver and inspection and palpation of living subjects. Before the days of electromyography, these were the chief methods of determining the actions of the muscles. There is a possibility of misinterpretation, however, against which students should be warned. When the force of a muscle contraction is weak, it is sometimes difficult to feel its contraction. If the subject repeats the action against greater resistance, the contraction may be stronger and more easily felt. The conclusion may be drawn erroneously that the muscle is a major mover for the action involved, when in fact it comes into action only as an assistant under conditions of heavy resistance.

Models

Numerous devices, both commercial and homemade, can be used for demonstrating and studying the actions of muscles. Probably the most commonly used device is the simplest of all—a long rubber band (or chain of short ones) held against the bones of a skeleton in such a way as to represent a single muscle. The movement is demonstrated by holding the elastic on a stretch with one end representing the proximal attachment and the other the distal attachment. The tendency of both bones to move toward each other can easily be demonstrated, as well as the necessity for stabilizing one bone so that the muscle can be effective in moving the other bone.

Muscle Stimulation

To most kinesiologists, the term muscle stimulation means G. B. Duchenne, the pioneer in the use of electrical stimulation as a means of studying the actions of the muscles and the author of Physiologie des Mouvements. This classic work was translated into English and published in 1949. It made a tremendous contribution to the science of kinesiology, yet its limitations must be recognized. It demonstrates the contraction of individual muscles when they are stimulated electrically. Unfortunately, it cannot analyze the sequence of muscular actions that occur in an everyday act such as walking, lifting a package, or working with a common tool. Neither can it reveal the complex combinations of muscular actions in ordinary sport techniques.

In spite of its limitations, muscle stimulation is being used in many modern kinesiology laboratories as a device for studying the responses of individual muscles to electrical stimulation and to rehabilitate muscles. One of the more promising areas of research in the rehabilitation field has been that of functional electrical stimulation (FES). FES combines electrical stimulation with computer control systems to offer some movement in muscles that have no functional nerves. The development of FES technology is aimed at providing mobility to disabled individuals. At this writing, much work remains to be done. FES systems have been used with some success in a number of settings, but the complexity of human movement, even the simple act of walking, has made the task of programming computers to stimulate the appropriate muscles in the appropriate sequence a daunting one. It is possible, however, that in this century, disabled individuals may be able to walk with the aid of electrical stimulation and computer control.

Electromyography (EMG)

Although few undergraduate students may have the opportunity of personal experience with electromyography, all may benefit by reading the reports of EMG investigations. A wealth of information concerning muscular action is available in such reports. Electromyography is based on the fact that contracting muscles generate electrical impulses. It is a technique of recording such impulses or action potentials, as they are also called (Figure 3.13). It provides specific information about muscular actions that we have only been able to guess at in the past, information that has proved much of our guessing to be inaccurate. The unique advantages of EMG are that it reveals both the intensity and duration of a muscle’s action and, in fact, discloses the precise time sequences of muscular activity in a movement. Furthermore, it reveals the actions not only of the agonists and antagonists but also of the muscles serving as stabilizers and neutralizers. Perhaps its greatest contribution to our knowledge of muscular action is its ability to record the impulses of deep as well as superficial muscles. Basmajian, the apostle of electromyography, says that it surpasses all the older methods of studying muscular action in that it reveals what the individual muscles are actually doing, not just what they can do or probably do (Basmajian and DeLuca 1985).

Imaging

The introduction of magnetic resonance imaging (MRI) has provided a method for actually “seeing” a muscle or muscle group. MRI images are based on the water and compounds that make up tissue. An MRI image clearly shows the structure of muscle and can be used to determine such factors as cross-sectional area, fiber orientation, fiber length, and tendon structure. From the changes that occur with muscle activation, it is possible to determine which muscles are active during movement. This is done in several ways. One might examine the change in muscle fiber length on the image, or the relative change in the pennations of a muscle. MRI findings have been found to agree well with EMG and are much more accurate that palpation (Yuen and Orendurff 2006). Currently, MRI imaging is expensive and available primarily to hospitals and large research institutions.

The approach to the analysis of the muscle participation in a given motion is exactly the same as that used for joint action. Using the same anatomical analysis model (Table 1.1), the analyst adds a description of muscular involvement to the previously completed analysis of joint and segment involvement. The muscular action is identified for each joint movement and recorded next to the joint actions on the chart (see Table 1.2). This implies identifying not only the muscle groups (flexors, extensors, abductors, adductors, etc.) that are contracting but also their function in the movement and the kind of contraction they are undergoing (concentric, eccentric, or static). Identification of the force causing the motion facilitates the subsequent identification of the muscle and type of contraction involved. In those instances where the force causing the motion is gravity, the muscle contraction type is most likely to be eccentric. If muscle is the force causing the motion, the contraction is concentric. It is critical to remember the influence of gravity during this portion of the analysis. When gravity is the force producing motion, the muscle group that is active will be that which is antagonistic to the observed motion. In other words, extension produced by the pull of gravity is controlled by eccentric contraction of the flexor muscles.

Any of the methods described for studying muscle actions may be used to determine which muscle groups are active during a performance. Once the general muscle groups have been identified, the analyst may wish to identify specific prime movers (agonists) and synergists. Chapters 5, 6, 7, 8, and 9 provide the knowledge required concerning individual muscles and their actions.

A condition of efficient movement is to use only those muscles that are appropriate to the motion. Tension in muscles that are not active in the skill produces fatigue and often a performance that is both inefficient and ineffective. Failure to use all requisite muscle groups can also seriously impair movement. A common fault in unskilled jumpers, for example, is tension in the anterior muscles of the lower leg (the ankle dorsiflexors). This tension can restrict the forceful plantar–flexion muscle action and related ankle motion necessary for an efficient and effective jump.

• 1. Take two sticks that are joined at one end by a hinge. Attach a single piece of elastic or a long rubber band to the opposite ends of the two sticks.
  • a. Separate the ends of the sticks as far as the elastic will permit and then demonstrate the way the elastic will pull both sticks together.
  • b. Demonstrate the way in which the elastic will move only one of the sticks if the other one is stabilized.
  • c. Repeat both a and b, using the arm of the skeleton instead of the sticks.
• 2. Get a subject to hold a heavy dumbbell in the right hand and slowly raise the arm sideward/upward without bending the elbow. Keep your fingers on the clavicular portion of the pectoralis major. Does it contract? If so, at what position of the arm does it begin?
• 3. Flex the fingers hard. Keep them flexed and flex the hand at the wrist as far as possible. What happens to the fingers? Explain.
• 4. Extend the fingers, and then hyperextend the hand at the wrist as far as possible. What happens to the fingers? Explain.
• 5. Get a subject to lie on the left side with the hip and knee in a partly flexed position and the right leg fully extended. The right thigh should now be flexed passively by an operator. The subject should attempt to keep the knee straight but not to the point of interfering with the hip flexion. What happens? Where does the subject feel discomfort? Explain.
• 6. Have the subject, in the same starting position as in Experience 5, flex both the right thigh and right leg completely. The right thigh should now be passively extended by an operator, with the subject attempting to keep the leg flexed at the knee. As the thigh becomes fully extended, what happens to the knee? Where does the subject feel discomfort? Explain.

(Caution: Do not use an acrobat or acrobatic dancer as a subject for Experience 5 or 6, or the experiments may not work. Why?)

• 7. Have a subject lie supine on a table with the knees at the edge and the lower legs hanging down. Ask the subject to extend one leg at the knee while you attempt to stop the motion by applying strong resistance at the front of the ankle. Note the amount of resistance you have to apply. Now ask the subject to flex the leg at the knee while you attempt to stop the movement by resisting at the rear of the ankle. Which action requires more resistance on your part?
  • Now ask the subject to sit up and flex the trunk well forward from the hips. Repeat the same two actions of knee flexion and extension against resistance. Which action requires more effort on your part this time? How does this compare with the first result? Explain.
  • It may help you to know that the rectus femoris crosses the front of the hip joint and the front of the knee joint. Hence, it is a flexor of the hip and an extensor of the knee. The biceps femoris is located on the back of the thigh and therefore is an extensor of the hip and flexor of the knee.
• 8. Following the analysis model presented in Table 1.2, do an analysis of the muscle groups (i.e., flexors, extensors, etc.) active in the performance of a motor skill that involves a number of joints.
  • Note: Laboratory exercises on the action of muscles as movers, stabilizers, and neutralizers are not included here because, in order to do them, it is necessary to know the individual muscles. They will be found in the laboratory sections of Chapters 5, 6, 7, 8, and 9.