After reading this chapter, the student will be able to:
Muscles exert forces and thus are the major contributor to human movement. Muscles are used to hold a position, to raise or lower a body part, to slow down a fast-moving segment, and to generate great speed in the body or in an object that is propelled into the air. A muscle only has the ability to pull and creates a motion because it crosses a joint. The tension developed by muscles applies compression to the joints, enhancing their stability. In some joint positions, however, the tension generated by the muscles can act to pull the segments apart and create instability.
Exercise programming for a young, healthy population incorporates exercises that push the muscular system to high levels of performance. Muscles can exert force and develop power to produce the desired movement outcomes. The same exercise principles used with young, active individuals can be scaled down for use by persons of limited ability. Using the elderly as an example, it is apparent that strength decrement is one of the major factors influencing efficiency in daily living activities. The loss of strength in the muscular system can create a variety of problems, ranging from inability to reach overhead or open a jar lid to difficulty using stairs and getting up out of a chair. Another example is an overweight individual who has difficulty walking any distance because the muscular system cannot generate sufficient power and the person fatigues easily. These two examples are really no different from the power lifter trying to perform a maximum lift in the squat. In all three cases, the muscular system is overloaded, with only the magnitude of the load and output varying.
Muscle tissue is an excitable tissue and is either striated or smooth. Striated muscles include skeletal and cardiac muscle. Both cardiac and smooth muscles are under the control of the autonomic nervous system. That is, they are not under voluntary control. Skeletal muscle, on the other hand, is under direct voluntary control. Of primary interest in this chapter is skeletal muscle. All aspects of muscle structure and function related to human movement and efficiency of muscular contribution are explored in this chapter. Because muscles are responsible for locomotion, limb movements, and posture and joint stability, a good understanding of the features and limitations of muscle action is necessary. Although it is not the function of this chapter to describe all of the muscles and their actions, it is necessary for the reader to have a good understanding of the location and action of the primary skeletal muscles. Figure 3-1 illustrates the surface skeletal muscles of the human body.
Muscle Tissue Properties
Muscle tissue is very resilient and can be stretched or shortened at fairly high speeds without major damage to the tissue. The performance of muscle tissue under varying loads and velocities is determined by the four properties of the muscle tissue: irritability, contractility, extensibility, and elasticity. A closer examination of these properties as they relate specifically to skeletal muscle tissue will enhance understanding of skeletal muscle actions described later in the chapter.
Irritability, or excitability, is the ability to respond to stimulation. In a muscle, the stimulation is provided by a motor neuron releasing a chemical neurotransmitter. Skeletal muscle tissue is one of the most sensitive and responsive tissues in the body. Only nerve tissue is more sensitive than skeletal muscle. As an excitable tissue, skeletal muscle can be recruited quickly, with significant control over how many muscle fibers and which ones will be stimulated for a movement.
Contractility is the ability of a muscle to generate tension and shorten when it receives sufficient stimulation. Some skeletal muscles can shorten as much as 50% to 70% of their resting length. The average range is about 57% of resting length for all skeletal muscles. The distance through which a muscle shortens is usually limited by the physical confinement of the body. For example, the sartorius muscle can shorten more than half of its length if it is removed and stimulated in a laboratory, but in the body,
the shortening distance is restrained by the hip joint and positioning of the trunk and thigh.
FIGURE 3-1 Skeletal muscles of the human body: anterior and posterior views. (Reprinted with permission from
Willis, M. C. . Medical Terminology: The Language of Health Care. Baltimore: Williams & Wilkins.
Extensibility is the muscle's ability to lengthen, or stretch beyond the resting length. The skeletal muscle itself cannot produce the elongation; another muscle or an external force is required. Taking a joint through a passive range of motion, that is, pushing another's limb past its resting length, is a good example of elongation in muscle tissue. The amount of extensibility in the muscle is determined by the connective tissue surrounding and within the muscle.
Elasticity is the ability of muscle fiber to return to its resting length after the stretch is removed. Elasticity in the
muscle is determined by the connective tissue in the muscle rather than the fibrils themselves. The properties of elasticity and extensibility are protective mechanisms that maintain the integrity and basic length of the muscle. Elasticity is also a critical component in facilitating output in a shortening muscle action that is preceded by a stretch.
Using a ligament as a comparison makes it easy to see how elasticity benefits muscle tissue. Ligaments, which are largely collagenous, have little elasticity, and if they are stretched beyond their resting length, they will not return to the original length but rather will remain extended. This can create laxity around the joint when the ligament is too long to exert much control over the joint motion. On the other hand, muscle tissue always returns to its original length. If the muscle is stretched too far, it eventually tears.
Functions of Muscle
Skeletal muscle performs a variety of different functions, all of which are important to efficient performance of the human body. The three functions relating specifically to human movement are contributing to the production of skeletal movement, assisting in joint stability, and maintaining posture and body positioning.
Skeletal movement is created as muscle actions generate tensions that are transferred to the bone. The resulting movements are necessary for locomotion and other segmental manipulations.
Maintain Postures and Positions
Muscle actions of a lesser magnitude are used to maintain postures. This muscle activity is continuous and results in small adjustments as the head is maintained in position and the body weight is balanced over the feet.
Muscle actions also contribute significantly to stability of the joints. Muscle tensions are generated and applied across the joints via the tendons, providing stability where they cross the joint. In most joints, especially the shoulder and the knee, the muscles spanning the joint via the tendons are among the primary stabilizers.
The skeletal muscles also provide four other functions that are not directly related to human movement. First, muscles support and protect the visceral organs and protect the internal tissues from injury. Second, tension in the muscle tissue can alter and control pressures within the cavities. Third, skeletal muscle contributes to the maintenance of body temperature by producing heat. Fourth, the muscles control the entrances and exits to the body through voluntary control over swallowing, defecation, and urination.
Skeletal Muscle Structure
Physical Organization of Muscle
Muscles and muscle groups are arranged so that they may contribute individually or collectively to produce a very small, fine movement or a very large, powerful movement. Muscles rarely act individually but rather interact with other muscles in a multitude of roles. To understand muscle function, the structural organization of muscle from the macroscopic external anatomy all the way down to the microscopic level of muscular action must be examined. A good starting point is the gross anatomy and external arrangement of muscles and the microscopic view of the muscle fiber.
Groups of Muscles
Groups of muscles are contained within compartments that are defined by fascia, a sheet of fibrous tissue. The compartments divide the muscles into functional groups, and it is common for muscles in a compartment to be innervated by the same nerve. The thigh has three compartments: the anterior compartment, containing the quadriceps femoris; the posterior compartment, containing the hamstrings; and the medial compartment, containing the adductors. Compartments for the thigh and the leg are illustrated in Figure 3-2.
The compartments keep the muscles organized and contained in one region, but sometimes the compartment is not large enough to accommodate the muscle or muscle groups. In the anterior tibial region, the compartment is small, and problems arise if the muscles are overdeveloped for the amount of space defined by the compartment. This is known as anterior compartment syndrome, and it can be serious if the cramped compartment impinges on nerves or blood supply to the leg and foot.
Two major fiber arrangements are found in the muscle: parallel and pennate.
Parallel Fiber Arrangements In the parallel fiber arrangement, the fascicles are parallel to the long axis of the muscle. The five different shapes of parallel fiber arrangements are flat,
fusiform, strap, radiate or convergent, and circular (Fig. 3-3). The flat, parallel fiber arrangement is usually thin and broad and originates from sheet-like aponeuroses. Forces generated in the flat-shaped muscle can be spread over a larger area. Examples of flat muscles are the rectus abdominus and the external oblique. The fusiform muscle is spindle shaped with a central belly that tapers to tendons on each ends. This muscle shape allows force transmission to small bony sites. Examples of fusiform muscles are the brachialis, biceps brachii, and brachioradialis. Strap muscles do not have muscle belly regions with a uniform diameter along the length of the muscle. This muscle shape allows for force transmission to targeted sites. The sartorius is an example of a strap-shaped muscle. The radiate or convergent muscle shape has a combined arrangement of flat and fusiform fiber shapes that originate on a broad aponeuroses and converge onto a tendon. The pectoralis major and the trapezius muscles are examples of convergent muscle shapes. Circular muscles are concentric arrangements of strap muscles, and this muscle surrounds openings to close the openings upon contraction. The orbicularis oris surrounding the mouth is an example of a circular muscle.
Many of the more than 600 muscles in the body are organized in right and left pairs. About 70 to 80 pairs of muscles are responsible for the majority of movements.
FIGURE 3-2 Muscles are grouped into compartments in each segment. Each compartment is maintained by fascial sheaths. The muscles in each compartment are functionally similar and define groups of muscles that are classified according to function, such as extensors and flexors.
FIGURE 3-3 A. Parallel muscles have fibers running in the same direction as the whole muscle. B. Penniform muscles have fibers that run diagonally to a central tendon through the muscle. The muscle fibers of a penniform muscle do not pull in the same direction as the whole muscle.
The fiber force in a parallel muscle fiber arrangement is in the same direction as the musculature (23). This results in a greater range of shortening and yields greater movement velocity. This is basically because the parallel muscles are often longer than other types of muscles and the muscle fiber is longer than the tendon. The fiber length of the biceps brachii muscle (fusiform) is shown in Figure 3-4 and can be equal to the muscle length.
Penniform Fiber Arrangements
In the second type of fiber arrangement, penniform, the fibers run diagonally with respect to a central tendon running the length of the muscle. The general shape of the penniform muscle is featherlike because the fascicles are short and run at an angle to the length of the muscle. Because the fibers of the penniform
muscle run at an angle relative to the line of pull of the muscle, the force generated by each fiber is in a different direction than the muscle force (23). The fibers are shorter than the muscle, and the change in the individual fiber length is not equal to the change in the muscle length (23). The fibers can run diagonally off one side of the tendon, termed unipennate (e.g., biceps femoris, extensor digitorum longus, flexor pollicis longus, semimembranosus, tibialis posterior), off both sides of the tendon, termed bipennate (e.g., rectus femoris, flexor hallucis longus, gastrocnemius, vastus medialis, vastus lateralis, infraspinatus) or both, termed multipennate (e.g., deltoid, gluteus maximus).
FIGURE 3-4 A. The muscle length (ML) is equal to the fiber length (FL) in the biceps brachii, and it has a small physiological cross-section (PCSA), making it more suitable for a larger range of motion. B. The vastus lateralis is capable of greater force production because it has a larger physiological cross-section. Additionally, the fiber length is shorter than the muscle length, making it less suitable for moving through a large distance. C. The largest physiological cross-section is seen in the gluteus medius.
Because the muscle fibers are shorter and run diagonally into the tendon, the penniform fibers create slower movements through a smaller range of motion than a fusiform muscle. The tradeoff is that a penniform muscle has a much greater physiological cross-section that can generally produce more strength than can a fusiform muscle.
The pennation angle is the angle made by the fascicles and the line of action (pull) of the muscle (Fig. 3-5). The greater the angle of pennation, the smaller the amount of force transmitted to the tendon, and because the pennation angle increases with contraction, the force-producing capabilities will reduce. For example, the medial gastrocnemius working at the ankle joint is at a disadvantageous position when the knee is positioned at 90 degrees because of pennation angles of approximately 60 degrees, allowing only half of the force to be applied to the tendon (29). When the pennation angle is low, as with the quadriceps muscles, the pennation angle is not a significant factor.
Muscle Volume and Cross-Section
A number of parameters can be calculated to describe the muscle potential in relationship to muscle architecture. Muscle mass, muscle length, and surface pennation angle can be measured directly after dissection of whole-muscle cadaver models. Ultrasonography and magnetic resonance imaging can also be to collect some of these parameters. Muscle volume (cm3) can be calculated after these initial factors are known using the following equation:
MV = m/ρ
where m is the mass of the muscle (g) and p (g/cm3) is the density of the muscle (1.056 g/cm2). Cross-section area (cm2) can be calculated with the following equation:
CSA = MV/L
where MV is the muscle volume (cm3) and L is fiber length (cm). This is an estimate for the whole muscle. In one study (58), the largest muscle volume recorded in the thigh and the lower leg was in the vastus lateralis (1505 cm3) and the soleus (552 cm3). Large cross-sectional areas were recorded in the gluteus maximus (145.7 cm2) and vastus medialis (63 cm2). A measurement of the cross-sectional area perpendicular to the longitudinal axis of the muscle is called the anatomical cross-section and is only relevant to the site where the slice is taken.
The physiological cross-section is the sum total of all of the cross-sections of fibers in the muscle in the plane perpendicular to the direction of the fibers. The formula for physiologic cross-section area (PCSA) is:
PCSA = m cos θ/ρL
where m is the mass of the muscle, p is the density of the muscle (1.056 g/cm2), L is the muscle length, and θ is the surface pennation angle. The soleus muscle has a PCSA of 230 cm2, which is three to eight times larger than those of the medial gastrocnemius (68 cm2) and the lateral gastrocnemius (28 cm2), making its potential for force production greater (14). The PCSA is directly proportional to the amount of force generated by a muscle. Muscles such as the quadriceps femoris that have a large PCSA and short fibers (low fiber length/muscle length) can generate large forces. Conversely, muscles such as the hamstrings that have smaller PCSA and long fibers (high fiber length/muscle length) are more suited to developing high velocities. Figure 3-4 illustrates the difference between fiber length, muscle length, and physiological cross-section in fusiform (biceps brachii) and pennate muscles (vastus lateralis and gluteus medius).
Each muscle contains a combination of fiber types that are categorized as slow-twitch fibers (type I) or fast-twitch fibers (type II). Fast-twitch fibers are further broken down into types IIa and IIb. Fiber type is an important consideration in muscle metabolism and energy consumption, and muscle fiber type is thoroughly studied in exercise physiology. Mechanical differences in the response of slow- and fast-twitch muscle fibers warrant an examination of fiber type.
FIGURE 3-5 The pennation angle is the angle made between the fibers and the line of action of pull of the muscle.
Slow-Twitch Fiber Types
Slow-twitch, or type I, fibers are oxidative. The fibers are red because of the high content of myoglobin in the muscle. These fibers have slow contraction times and are well suited for prolonged, low-intensity work. Endurance athletes usually have a high quantity of slow-twitch fibers.
Intermediate- and Fast-Twitch Fiber Types
Fast-twitch, or type II, fibers are further broken down into type IIa, oxidative-glycolytic, and type IIb, glycolytic. The type IIa fiber is a red muscle fiber known as the intermediate fast-twitch fiber because it can sustain activity for long periods or contract with a burst of force and then fatigue. The white type IIb fiber provides us with rapid force production and then fatigues quickly.
Most, if not all, muscles contain both fiber types. An example is the vastus lateralis, which is typically half fast-twitch and half slow-twitch fibers (31). The fiber type influences how the muscle is trained and developed and what techniques will best suit individuals with specific fiber types. For example, sprinters and jumpers usually have great concentrations of fast-twitch fibers. These fiber types are also found in high concentrations in muscles on which these athletes rely, such as the gastrocnemius. On the other hand, distance runners usually have greater concentrations of slow-twitch fibers.
Individual Muscle Structure
The anatomy of a skeletal muscle is presented in Figure 3-6. Each individual muscle usually has a thick central portion, the belly of the muscle. Some muscles, such as the biceps brachii, have very pronounced bellies, but other muscles, such as the wrist flexors and extensors, have bellies that are not as apparent.
Covering the outside of the muscle is another fibrous tissue, the epimysium. This structure plays a vital role in the transfer of muscular tension to the bone. Tension in the muscle is generated at various sites, and the epimysium transfers the various tensions to the tendon, providing a smooth application of the muscular force to the bone.
Each muscle contains hundreds to tens of thousands of muscle fibers, which are carefully organized into compartments within the muscle itself. Bundles of muscle fibers are called fascicles. Each fascicle may contain as many as 200 muscle fibers. A fascicle is covered with a dense connective sheath called the perimysium that protects the muscle fibers and provides pathways for the nerves and blood vessels. The connective tissue in the perimysium and the epimysium gives muscle much of its ability to stretch and return to a normal resting length. The perimysium is also the focus of flexibility training because the connective tissue in the muscle can be stretched, allowing the muscle to elongate.
The fascicles run parallel to each other. Each fascicle contains the long, cylindrical, threadlike muscle fibers, the cells of skeletal muscles, where the force is generated. Muscle fibers are 10 to 100 (m in width and 15 to 30 cm in length. Fibers also run parallel to each other and are covered with a membrane, the endomysium. The endomysium is a very fine sheath carrying the capillaries and nerves that nourish and innervate each muscle fiber. The vessels and the nerves usually enter in the middle of the muscle and are distributed throughout the muscle by a path through the endomysium. The endomysium also serves as an insulator for the neurological activity within the muscle.
Directly underneath the endomysium is the sarcolemma. This is a thin plasma membrane surface that branches into the muscle. The neurological innervation of the muscle travels through the sarcolemma and eventually reaches each individual contractile unit by means of a chemical neurotransmission.
At the microscopic level, a fiber can be further broken down into numerous myofibrils. These delicate rodlike strands run the total length of the muscle and contain the contractile proteins of the muscle. Hundreds or even thousands of myofibrils are in each muscle fiber, and each fiber is filled with 80% myofibrils (5). The remainder of the fiber consists of the usual organelles, such as the mitochondria, the sarcoplasm, sarcoplasmic reticulum, and the t-tubules (or transverse tubules). Myofibrils are 1 to 2 µm in diameter (about a 4 millionth of an inch wide) and run the length of the muscle fiber (5). Figure 3-7 illustrates muscle myofibrils and some of these organelles.
The myofibrils are cross-striated by light and dark filaments placed in an order that forms repeating patterns of bands. The dark banding is the thick protein myosin, and the light band is a thin polypeptide, actin. One unit of these bands is called a sarcomere. This structure is the actual contractile unit of the muscle that develops tension. Sarcomeres are in series along a myofibril. That is, sarcomeres form units along the length of the myofibril much like the links in a chain.
FIGURE 3-6 A. Each muscle connects to the bone via a tendon or aponeurosis. B. Within the muscle, the fibers are bundled into fascicles. C. Each fiber contains myofibril strands that run the length of the fiber. D. The actual contractile unit is the sarcomere. Many sarcomeres are connected in series down the length of each myofibril. Muscle shortening occurs in the sarcomere as the myofilaments in the sarcomere, actin, and myosin slide toward each other.
FIGURE 3-7 A portion of a skeletal muscle fiber illustrating the sarcoplasmic reticulum that surrounds the myofibril. (Adapted with permission from
Pittman, M. I., Peterson, L. . Biomechanics of skeletal muscle. In M. Nordin, V. H. Frankel [Eds.]. Basic Biomechanics of the Musculoskeletal System (2nd ed.). Philadelphia: Lea & Febiger, 89-111.
Force Generation in the Muscle
Skeletal muscle is organized into functional groups called motor units. A motor unit consists of a group of muscle fibers that are innervated by the same motor neuron. Motor units are discussed in more detail in Chapter 4, but it is important to discuss some aspects in this chapter. Motor units can consist of only a few muscle fibers (e.g., the optic muscles) or may have up to 2000 muscle fibers (e.g., the gastrocnemius). The signal to contract that is transmitted from the motor neuron to the muscle is called an action potential. When a motor neuron is stimulated enough to cause a contraction, all muscle fibers innervated by that motor neuron contract. The size of the action potential and resulting muscle action are proportional to the number of fibers in the motor unit. An increase in output of force from the muscle requires an increase in the number of motor units activated.
The action potential from a motor neuron reaches a muscle fiber at a neuromuscular junction or motor end plate that lies near the center of the fiber. At this point, a
synapse, or space, exists between the motor neuron and the fiber membrane. When the action potential reaches the synapse, a series of chemical reactions take place, and acetylcholine (ACH) is released. ACH diffuses across the synapse and causes an increase in permeability of the membrane of the fiber. The ACH rapidly breaks down to prevent continuous stimulation of the muscle fiber. The velocity at which the action potential is propagated along the membrane is the conduction velocity.
The muscle resting membrane potential inside is – 70 mV to – 95mV with respect to the outside. At the threshold level of the membrane potential (approximately – 50mV), a change in potential of the fiber membrane or sarcolemma occurs. The action potential is characterized by a depolarization from the resting potential of the membrane so that the potential becomes positive (approximately +40mV) and is said to overshoot. There is a hyperpolarized state (hyperpolarization) before returning to the resting potential. This is followed by a repolarization, or a return to the polarized state.
FIGURE 3-8 Excitation-contraction coupling occurs when the action potential traveling down the motor neuron reaches the muscle fiber where acetylcholine (Ach) is released. This causes depolarization and the release of Ca2+ ions that promote cross-bridge formation between actin and myosin, resulting in shortening of the sarcomere. AchE, acetylcholinesterase; ADP, adenosine diphosphate.
The wave of depolarization of the action potential moves along the nerve until it reaches the muscle fibers, where it spreads to the muscle membrane as calcium ions (Ca2+) are released into the area surrounding the myofibrils. These Ca2+ ions promote cross-bridge formation, which results in an interaction between the actin and myosin filaments (see the discussion of sliding filament theory in the next section). When the stimulation stops, ions are actively removed from the area surrounding the myofibrils, releasing the cross-bridges. This process is excitation-contraction coupling (Fig. 3-8). The calcium ions link action potentials in a muscle fiber to contraction by binding to the filaments
and turning on the interaction of the actin and myosin to start contraction of the sarcomere.
Muscle force production is achieved in two ways. First, muscle force can be increased by recruiting increasingly larger motor units. Initially, during a muscle contraction, smaller motor units are activated. As muscle force increases, more and larger motor units are engaged. This is the size principle (20). Second, a motor unit may be activated at any of several frequencies. A single action potential that activates a fiber will cause the force to increase and decrease. This is referred to as a twitch. If a second stimulus occurs before the initial twitch has subsided, another twitch builds upon the first. With subsequent high frequency of stimulations, the force continues to build and forms a state called unfused tetanus. Finally, the force builds to a level in which there is no increase in muscle force. At this point, the force level has reached tetanus. This scenario is illustrated in Figure 3-9. In a muscle contraction, both size recruitment and frequency of stimulation are simultaneously used to increase muscle force.
Sliding Filament Theory
How a muscle generates tension has been an area of much research. An explanation of the shortening of the sarcomere has been presented via the sliding filament theory presented by Huxley (26). This theory is the most widely accepted explanation of muscular contraction but certainly is not the only one. In the past, muscle contraction was thought, for example, to be similar to the principle of blood clotting, the behavior of India rubber, a chain of circular elastic rings, and a sliding movement caused by opposite electric charges in the different filaments (42).
In Huxley's sliding filament theory, when calcium is released into the muscle through neurochemical stimulation, the contracting process begins. The sarcomere contracts as the myosin filament walks along the actin filament, forming cross-bridges between the head of the myosin and a prepared site on the actin filament. In the contracted state, the actin and myosin filaments overlap along most of their lengths (Fig. 3-10).
FIGURE 3-9 When a single stimulus is given, a twitch occurs. When a series of stimuli is given, muscle force increases to an uneven plateau or unfused tetanus. As the frequency of stimuli increases, the muscle force ultimately reaches a limit, or tetanus. (Adapted from
McMahon, T. A. . Muscles, Reflexes, and Locomotion. Princeton, NJ: Princeton University Press.
The simultaneous sliding of many thousands of sarcomeres in series changes the length and force of the muscle (5). The amount of force that can be developed in the muscle is proportional to the number of cross-bridges formed. The shortening of many sarcomeres, myofibrils, and fibers develops tension running through the muscle and to the bone at both ends to create a movement.
Transmission of Muscle Force to Bone
Tendon versus Aponeurosis
A muscle attaches to bone in one of three ways: directly into the bone, via a tendon, or via an aponeurosis, a flat tendon.
These three types of attachments are presented in Figure 3-11. Muscle can attach directly to the periosteum of the bone through fusion between the epimysium and the surface of the bone, such as the attachment of the trapezius (56). Muscle can attach via a tendon that is fused with the muscle fascia, such as in the hamstrings, biceps brachii, and flexor carpi radialis. Last, muscle can attach to a bone via a sheath of fibrous tissue known as an aponeurosis seen in the abdominals and the trunk attachment of the latissimus dorsi.
FIGURE 3-10 The sliding filament theory. Shortening of the muscle has been explained by this theory. Shortening takes place in the sarcomere as the myosin heads bind to sites on the actin filament to form a cross-bridge. The myosin head attaches and turns, moving the actin filament toward the center. It then detaches and moves on to the next actin site.
Characteristics of the Tendon
The most common form of attachment, the tendon, transmits the force of the associated muscle to bone. The tendon connects to the muscle at the myotendinous junction, where the muscle fibers are woven in with the collagen fibers of the tendon. Tendons are powerful and carry large loads via connections where fibers perforate the surfaces of bones. Tendons can resist stretch, are flexible, and can turn corners running over cartilage, sesamoid bones, or bursae. Tendons can be arranged in a cord or in strips and can be circular, oval, or flat. Tendons consist of an inelastic bundle of collagen fibers arranged parallel to the direction of the force application of the muscle. Even though the fibers are inelastic, tendons can respond in an elastic fashion through recoiling and the elasticity of connective tissue. Tendons can withstand high tensile forces produced by the muscles, and they exhibit viscoelastic behavior in response to loading. The Achilles tendon has been reported to resist tensile loads to a degree equal to or greater than that of steel of similar dimensions.
The stress-strain response of a tendon is viscoelastic. That is, tendons show a nonlinear response and exhibit hysteresis. Tendons are relatively stiff and much stronger than other structures. Tendons respond very stiffly when exposed to a high rate of loading. This stiff behavior of tendons is thought to be related to their relatively high collagen content. Tendons are also very resilient and show relatively little hysteresis or energy loss. These characteristics are necessary to the function of tendons.
FIGURE 3-11 A muscle can attach directly into the bone (A) or indirectly via a tendon (B) or aponeurosis (C).
Tendons must be stiff and strong enough to transmit force to bone without deforming much. Also, because of the low hysteresis of tendons, they are capable of storing and releasing elastic strain energy. The differences in the strength and performance characteristics of tendons versus muscles and bones is presented in Figure 3-12.
Tendons and muscles join at myotendinous junctions, where the actual myofibrils of the muscle fiber join the collagen fibers of the tendon to produce a multilayered interface (62). The tendon connection to the bone consists of fibrocartilage that joins to mineralized fibrocartilage and
then to the lamellar bone. This interface blends with the periosteum and the subchondral bone.
FIGURE 3-12 The stress-strain curves for muscle, tendon, and bone tissue. Top. Muscle is viscoelastic and thus deforms under low load and then responds stiffly. Middle. Tendon is capable of handling high loads. The end of the elastic limits of the tendon is also the ultimate strength level (no plastic phase). Bottom. Bone is a brittle material that responds stiffly and then undergoes minimal deformation before failure.
Tendons and muscles work together to absorb or generate tension in the system. Tendons are arranged in series, or in line with the muscles. Consequently, tendons bear the same tension as muscles (46). The mechanical interaction between muscles and tendons depends on the amount of force that is being applied or generated, the speed of the muscle action, and the slack in the tendon.
Tendons are composed of parallel fibers that are not perfectly aligned, forming a wavy, crimped appearance. If tension is generated in the muscle fibers while the tendon is slack, there is initial compliance in the tendon as it straightens out. It will begin to recoil or spring back to its initial length (Fig. 3-13). As the slack in the tendon is taken up by the recoiling action, the time taken to stretch the tendon causes a delay in the achievement of the required level of tension in the muscle fibers (46).
Recoiling of the tendon also reduces the speed at which a muscle may shorten, which in turn increases the load a muscle can support (46). If the tendon is stiff and has no recoil, the tension will be transmitted directly to the muscle fibers, creating higher velocities and decreasing the load the muscle can support. The stiff response in a tendon allows for the development of rapid tensions in the muscle and results in brisk, accurate movements.
The tendon and the muscle are very susceptible to injury if the muscle is contracting as it is being stretched. An example is the follow-through phase of throwing. Here, the posterior rotator cuff stretches as it contracts to slow the movement. Another example is the lengthening and contraction of the quadriceps femoris muscle group during the support phase of running as the center of mass is lowered via knee flexion. The tendon picks up the initial stretch of the relaxed muscle, and if the muscle contracts as it is stretched, the tension increases steeply in both the muscle and tendon (46).
When tension is generated in a tendon at a slow rate, injury is more likely to occur at the tendon-bone junction than other regions. At a faster rate of tension development, the actual tendon is the more common site of failure (54). For the total muscle-tendon unit, the likely site of injury is the belly of the muscle or the myotendinous junction.
Many tendons travel over bony protuberances that reduce some of the tension on the tendon by changing the angle of pull of the muscle and reducing the tension generated in the muscle. Examples of this can be found with the quadriceps femoris muscles and the patella and with the tendons of the hamstrings and the gastrocnemius as they travel over condyles on the femur. Some tendons are covered with synovial sheaths to keep the tendon in place and protect the tendon.
The tension in the tendons also produces the actual ridges and protuberances on bone. The apophyses found on a bone are developed by tension forces applied to the bone through the tendon (see Chapter 2). This is of interest to physical anthropologists because they can study skeletal remains and make sound predictions about lifestyle and occupations of a civilization by evaluating prominent ridges, size of the trochanters and tuberosities, and basic size of the specimen.
Tendon Influences on Force Development (Force-Time Characteristics)
When a muscle begins to develop tension through the contractile component of the muscle, the force increases nonlinearly over time because the passive elastic components in the tendon and the connective tissue stretch and absorb some of the force. After the elastic components are stretched, the tension that the muscle exerts on the bone increases linearly over time until maximum force is achieved.
The time to achieve maximum force and the magnitude of the force vary with a change in joint position. In one joint position, maximum force may be produced very quickly, but in other joint positions, it may occur later in the contraction. This reflects the changes in tendon laxity, not changes in the tension-generating capabilities of the
contractile components. If the tendon is slack, the maximum force occurs later and vice versa.
FIGURE 3-13 A. In a relaxed state, the fibers in many tendons are slack and wavy. B. When tension is applied, the tendon springs back to its initial length, causing a delay in the achievement of the required muscle tension.
Mechanical Model of Muscle: The Musculotendinous Unit
A series of experiments by A. V. Hill gave rise to a behavioral model that predicted the mechanical nature of muscle. The Hill model has three components that act together in a manner that describes the behavior of a whole muscle (21,22). A schematic of configurations of the Hill model is presented in Figure 3-14. Hill used the techniques of a systems engineer to perform experiments that helped him identify key phenomena of muscle function. The model contained components referred to as the contractile component (CC), parallel elastic component (PEC), and series elastic component (SEC). Because this is a behavioral model, it is inappropriate to ascribe these mechanical components to specific structures in the muscle itself. However, the model has given great insight into how muscle functions to develop tension and is often used as a basis for many computer models of muscle.
The contractile component is the element of the muscle model that converts the stimulation of the nervous system into a force and reflects the shortening of the muscle through the actin and myosin structures. The contractile component has mechanical characteristics that determine the efficiency of a contraction, that is, how well the signal from the nervous system translates into a force. We have already discussed the first of these mechanical characteristics, the relationship between stimulation and activation. Two others, the force-velocity and force-length relationships, are discussed later in this chapter.
FIGURE 3-14 A. The most common form of the Hill muscle model. B. An alternative form. Because series elastic component (SEC) is usually stiffer than parallel elastic component (PEC) for most muscles, it generally does not matter which form of the model is used. (Adapted with permission from
Winters, J. M. . Terminology and Foundations of Movement Science. In J. M. Winters, P. E. Crago [Eds.]. Biomechanics and Neural Control of Posture and Movement. New York: Springer-Verlag, 3-35.
The elasticity inherent in muscle is represented by the series elastic and the parallel elastic components. Because the SEC is in series with the CC, any force produced by the CC is also applied to the SEC. It first appears that the SEC is the tendon of the muscle, but the SEC represents the elasticity of all elastic elements in series with the force-generating structures of the muscle. The SEC is a highly nonlinearly elastic structure.
Muscle displays elastic behavior even when the CC is not producing force. An external force applied to a muscle causes the muscle to resist, but the muscle also stretches. This inactive elastic response is produced by structures that must be in parallel to the CC rather than in series to the CC. Thus, we have the PEC. The PEC is often associated with the connective tissue that surrounds the muscle and its compartments, but again, this is a behavioral model rather than a structural model, so this association cannot be made. The PEC, similar to the SEC, is highly nonlinear, and increases in stiffness as the muscle lengthens. Both the SEC and the PEC also behave like springs when acting quickly.
Role of Muscle
In the performance of a motor skill, only a small portion of the potential movement capability of the musculoskeletal system is used. Twenty to 30 degrees of freedom may be available to raise your arm above your head and comb your hair (35). Many of the available movements, however, may be inefficient in terms of the desired movement (e.g., combing the hair). To eliminate the undesirable movements and create the skill or desired movement, muscles or groups of muscles play a variety of roles. To perform a motor skill at a given time, only a small percentage of the potential movement capability of the motor system is used.
Origin Versus Insertion
A muscle typically attaches to a bone at both ends. The attachment closest to the middle of the body, or more proximal, is termed the origin, and this attachment is usually broader. The attachment farther from the midline, or more distal, is called the insertion; this attachment usually converges to a tendon. There can be more than one attachment site at both ends of the muscle. Traditional anatomy classes usually incorporate a study of the origins and insertions of the muscles. It is a common mistake to view the origin as the bony attachment that does not move when the muscle contracts. Muscle force is generated and applied to both skeletal connections, resulting in movement of one bone or both. The reason that both bones do not move when a muscle contracts is the stabilizing force of adjacent muscles or the difference in the mass of the two segments or bones to which the muscle is attached. Additionally, many muscles cross more than one
joint and have the potential to generate multiple movements on more than one segment.
Numerous examples are available of a muscle shifting between moving one end of its attachment and the other, depending on the activity. One example is the psoas muscle, which crosses the hip joint. This muscle flexes the thigh, as in leg raises, or raises the trunk, as in a curl-up or sit-up (Fig. 3-15). Another example is the gluteus medius, which moves the pelvis when the foot is on the ground and the leg when the foot is off the ground. The effect of tension in a muscle should be evaluated at all attachment sites even if no movement is resulting from the force. Evaluating all attachment sites allows assessment of the magnitude of the required stabilizing forces and the actual forces applied at the bony insertion.
A muscle controls or creates a movement through the development of torque. Torque is defined as the tendency of a force to produce rotation about a specific axis. In the case of a muscle, a force is generated in the muscle along the line of action of the force and applied to a bone, which causes a rotation about the joint (axis). The muscle's line of action or line of pull is the direction of the resultant muscle force running between the attachment sites on both ends of the muscle. The two components of torque are the magnitude of the force and the shortest or perpendicular distance from the pivot point to the line of action of the force, often termed the moment arm. Mathematically, torque is:
FIGURE 3-15 The origin of the psoas muscle is on the bodies of the last thoracic and all of the lumbar vertebrae, and the insertion is on the lesser trochanter of the femur. It is incorrect to assume that the origin remains stable in a movement. Here the psoas pulls on both the vertebrae and the femur. With the trunk stabilized, the femur moves (leg raise), and with the legs stabilized, the trunk moves (sit-up).
where T is torque, F is the applied force in newtons, and r is the perpendicular distance in meters from the line of action of the force to the pivot point (moment arm). The amount of torque generated by the muscle is influenced by the capacity to generate force in the muscle itself and the muscle's moment arm. During any movement, both of these factors are changing. In particular, the moment arm increases or decreases depending on the line of pull of the muscle relative to the joint. If the muscle's moment arm increases anywhere in the movement, the muscle can produce less force and still produce the same torque around the joint. Conversely, if the moment arm decreases, more muscle force is required to produce the same torque around the joint (Fig. 3-16). A more thorough discussion of torque is presented in a later chapter.
Muscle Role Versus Angle of Attachment
The muscle supplies a certain amount of tension that is transferred via the tendon or aponeurosis to the bone. Not all of the tension or force produced by the muscle is put to use in generating rotation of the segment. Depending on the angle of insertion of the muscle, some force is directed to stabilizing or destabilizing the segment by pulling the bone into or away from the joint.
Muscular force is primarily directed along the length of the bone and into the joint when the tendon angle is acute or lying flat on the bone. When the forearm is extended, the tendon of the biceps brachii inserts into the radius at a low angle. Initiating an arm curl from this position requires greater muscle force than from other positions because most of the force generated by the biceps brachii is directed into the elbow rather than into moving the segments around the joint. Fortunately, the resistance offered by the forearm weight is at a minimum in the extended position. Thus, the small muscular force available to move the segment is usually sufficient. Both the force directed along the length of the bone and that which is applied perpendicular to the bone to create joint movement can be determined by resolving the angle of the muscular force application into its respective parallel and rotary components. Figure 3-17 shows the parallel and rotatory components of the biceps brachii force for various attachment angles.
Even though muscular tension may be maintained during a joint movement, the rotary component and the torque varies with the angle of insertion. Many neutral starting positions are weak because most of the muscular force is directed along the length of the bone. As segments move through the midrange of the joint motion, the angle of insertion usually increases and directs more of the muscular force into moving the segment. Consequently, when starting a weight-lifting movement from the fully extended position, less weight can be lifted than if the person started the lift with some flexion in the joint. Figure 3-18 shows the isometric force output of the shoulder flexors and extensors for a range of joint positions.
FIGURE 3-16 A muscle with a small moment arm (A) needs to produce more force to generate the same torque as a muscle with a larger moment arm (B).
In addition, at the end of some joint movements, the angle of insertion may move past 90°, the point at which the moving force again begins to decrease and the force along the length of the bone acts to pull the bone away from the joint. This dislocating force is present in the elbow and shoulder joints when a high degree of flexion is present in the joints.
The mechanical actions of broad muscles that have fibers attaching directly into bone over a large attachment site, such as the pectoralis major and trapezius, are difficult to describe using one movement for the whole muscle (56). For example, the lower trapezius attaches to the scapula at an angle opposite that of the upper trapezius; thus, these sections of the same muscle are functionally independent. When the shoulder girdle is elevated and abducted as the arm is moved up in front of the body, the lower portion of the trapezius may be inactive. This presents a complicated problem when studying the function of the muscle as a whole and requires multiple lines of action and effect (56).
FIGURE 3-17 When muscle attachment angles are acute, the parallel component of the force (P) is highest and is stabilizing the joint. The rotatory component (R) is low (A). As the angle increases, the rotatory component also increases (B). The rotatory component increases to its maximum level at a 90° angle of attachment (C). Beyond a 90° angle of attachment, the rotatory component diminishes, and the parallel component increases to produce a dislocating force (D and E).
FIGURE 3-18 The isometric force output varies with the joint angle. As the shoulder angle increases, the shoulder extension force increases. The reverse happens with shoulder flexion force values, which decrease with an increase of the shoulder angle. (Adapted with permission from
Kulig, K., et al. . Human strength curves. In R. L. Terjund [Ed.]. Exercise and Sport Sciences Reviews, 12:417-466.
Muscle Actions Creating, Opposing, and Stabilizing Movements
Agonists and Antagonists
The various roles of selected muscles in a simple arm abduction exercise are presented in Figure 3-19. Muscles creating the same joint movement are termed agonists. Conversely, muscles opposing or producing the opposite joint movement are called antagonists. The antagonists must relax to allow a movement to occur or contract concurrently with the agonists to control or slow a joint movement. Because of this, the most sizable changes in relative position of muscles occur in the antagonists (25). Thus, when the thigh swings forward and upward, the agonists producing the movement are the hip flexors, that is, the iliopsoas, rectus femoris, pectineus, sartorius, and gracilis muscles. The antagonists, or the muscles opposing the motion of hip flexion, are the hip extensors, hamstrings, and gluteus maximus. The antagonists combined with the effect of gravity slow down the movement of hip flexion and terminate the joint action. Both the agonists and antagonists are jointly involved in controlling or moderating movement.
FIGURE 3-19 Muscles perform a variety of roles in movement. In arm abduction, the deltoid is the agonist because it is responsible for the abduction movement. The latissimus dorsi is the antagonistic muscle because it resists abduction. There are also muscles stabilizing in the region so the movement can occur. Here, the trapezius is shown stabilizing and holding the scapula in place. Last, there may be some neutralizing action: The teres minor may neutralize via external rotation any internal rotation produced by the latissimus dorsi.
When a muscle is playing the role of an antagonist, it is more susceptible to injury at the site of muscle attachment or in the muscle fiber itself. This is because the muscle is contracting to slow the limb while being stretched.
Stabilizers and Neutralizers
Muscles are also used as stabilizers, acting in one segment so that a specific movement in an adjacent joint can occur. Stabilization is important, for example, in the shoulder girdle, which must be supported so that arm movements can occur smoothly and efficiently. It is also important in the pelvic girdle and hip region during gait. When one foot is on the ground in walking or running, the gluteus medius contracts to maintain the stability of the pelvis so it does not drop to one side.
The last role muscles are required to play is that of synergist, or neutralizer, in which a muscle contracts to eliminate an undesired joint action of another muscle. Forces can be transferred between two adjacent muscles and supplement the force in the target muscle (25). For example, the gluteus maximus is contracted at the hip joint to produce thigh extension, but the gluteus maximus also attempts to rotate the thigh externally. If external rotation is an undesired action, the gluteus minimus and the tensor fascia latae contract to produce a neutralizing internal rotation action that cancels out the external rotation action of the gluteus maximus, leaving the desired extension movement.
Net Muscle Actions
Isometric Muscle Action
Muscle tension is generated against resistance to maintain position, raise a segment or object, or lower or control a segment. If the muscle is active and develops tension with no visible or external change in joint position, the muscle action is termed isometric (31). Examples of isometric muscle actions are illustrated in Figure 3-20. To bend over into 30° of trunk flexion and hold that position, the muscle action used to hold the position is termed isometric because no movement is taking place. The muscles contracting isometrically to hold the trunk in a position of flexion are the back muscles because they are resisting the force of gravity that tends to farther flex the trunk.
To take the opposite perspective, consider the movement in which the trunk is curled up to 30° and that position is held. To hold this position of trunk flexion, an isometric muscle action using the trunk flexors is produced. This muscle action resists the action of gravity that is forcing the trunk to extend.
FIGURE 3-20 A muscle action is isometric when the tension creates no change in joint position. A concentric muscle action occurs when the tension shortens the muscle. An eccentric muscle action is generated by an external force when the muscle lengthens.
Concentric Muscle Action
If a muscle visibly shortens while generating tension actively, the muscle action is termed concentric (31). In concentric joint action, the net muscle forces producing movement are in the same direction as the change in joint angle, meaning that the agonists are the controlling muscles (Fig. 3-20). Also, the limb movement produced in a concentric muscle action is termed positive because the joint actions are usually against gravity or are the initiating source of movement of a mass.
Many joint movements are created by a concentric muscle action. For example, flexion of the arm or forearm from the standing position is produced by a concentric muscle action from the respective agonists or flexor muscles. Additionally, to initiate a movement of the arm across the body in a horizontal adduction movement, the horizontal adductors initiate the movement via a concentric muscle action. Concentric muscle actions are used to generate forces against external resistances, such as raising a weight, pushing off the ground, or throwing an implement.
Eccentric Muscle Action
When a muscle is subjected to an external torque that is greater than the torque generated by the muscle, the muscle lengthens, and the action is known as eccentric (31). The source of the external force developing the external torque that produces an eccentric muscle action is usually gravity or the muscle action of an antagonistic muscle group (5).
In eccentric joint action, the net muscular forces producing the rotation are in the opposite direction of the change in joint angle, meaning that the antagonists are the controlling muscles (Fig. 3-20). Also, the limb movement produced in eccentric muscle action is termed negative because the joint actions are usually moving down with gravity or are controlling rather than initiating the movement of a mass. In an activity such as walking downhill, the muscles act as shock absorbers as they resist the downward movement while lengthening.
Most movements downward, unless they are very fast, are controlled by an eccentric action of the antagonistic muscle groups. To reverse the example shown in Figure 3-20, during adduction of the arm from the abducted position, the muscle action is eccentrically produced by the abductors or antagonistic muscle group. Likewise, lowering into a squat position, which involves hip and knee flexion, requires an eccentric movement controlled by the hip and knee extensors. Conversely, the reverse thigh and shank extension movements up against gravity are produced concentrically by the extensors.
From these examples, the potential sites of muscular imbalances in the body can be identified because the extensors in the trunk and the lower extremity are used to both lower and raise the segments. In the upper extremity, the flexors both raise the segments concentrically and lower the segments eccentrically, thereby obtaining more use.
Eccentric actions are also used to slow a movement. When the thigh flexes rapidly, as in a kicking action, the antagonists (extensors) eccentrically control and slow the joint action near the end of the range of motion. Injury can be a risk in a movement requiring rapid deceleration for athletes with impaired eccentric strength.
Eccentric muscle actions preceding concentric muscle actions increase the force output because of the contribution elastic strain energy in the muscle. For example, in throwing, the trunk, lower extremity, and shoulder internal rotation are active eccentrically in the windup, cocking, and late cocking phases. Elastic strain energy is stored in these muscles, which enhances the concentric phase of the throwing motion (39).
Examples of muscles and actions
What is the muscle action of the quadriceps femoris in the lowering action of a squat?
What is the muscle action of the posterior deltoid in the follow-through phase of a throw?
Comparison of Isometric, Concentric, and Eccentric Muscle Actions
Isometric, concentric, and eccentric muscle actions are not used in isolation but rather in combination. Typically, isometric actions are used to stabilize a body part, and eccentric and concentric muscle actions are used sequentially to maximize energy storage and muscle performance. This natural sequence of muscle function, during which an eccentric action precedes a concentric action, is known as the stretch-shortening cycle, which is described later in this chapter.
These three muscle actions are very different in terms of their energy cost and force output. The eccentric muscle action can develop the same force output as the other two types of muscle actions with fewer muscle fibers activated. Consequently, eccentric action is more efficient and can produce the same force output with less oxygen consumption than the others (3) (Fig. 3-21).
FIGURE 3-21 It has been illustrated that eccentric muscle action can produce high workloads at lower oxygen uptake levels than the same loads produced with concentric muscle action. (Adapted with permission from
Asmussen, E. . Positive and negative muscular work. Acta Physiologica Scandinavica, 28:364-382.
In addition, the eccentric muscle action is capable of greater force output using fewer motor units than isometric or concentric actions (Fig. 3-22). This occurs at the level of the sarcomere, where the force increases beyond the maximum isometric force if the myofibril is stretched and stimulated (10,13).
Concentric muscle actions generate the lowest force output of the three types. Force is related to the number of cross-bridges formed in the myofibril. In isometric muscle action, the number of bridges attached remains constant. As the muscle shortens, the number of attached bridges is reduced with increased velocity (13). This reduces the level of force output generated by tension in the muscle fibers. A hypothetical torque output curve for the three muscle actions is presented in Figure 3-23.
An additional factor contributing to noticeable force output differences between eccentric and concentric
muscle actions is present when the actions are producing vertical movements. In this case, the force output in both concentric and eccentric actions is influenced by torques created by gravity. The gravitational force creates torque that contributes to the force output in an eccentric action as the muscles generate torque that controls the lowering of the limb or body. The total force output in a lowering action is the result of both muscular torques and gravitational torques.
FIGURE 3-22 The integrated EMG activity (IEMG) in the biceps brachii muscle is higher as the same forces are generated using concentric muscle action compared with eccentric muscle action. (Adapted with permission from
Komi, P. V. . The stretch-shortening cycle and human power output. In N. L. Jones, et al. [Eds.]. Human Muscle Power.Champaign, IL: Human Kinetics, 27-40.
FIGURE 3-23 Eccentric muscle action can generate the greatest amount of torque through a given range of motion. Isometric muscle action can generate the next highest level of torque, and concentric muscle action generates the least torque (Adapted with permission from
Enoka, R. M. . Neuromuscular Basis of Kinesiology. Champaign, IL: Human Kinetics.
The force of gravity inhibits the movement of a limb upward, and before any movement can occur, the concentric muscle action must develop a force output that is greater than the force of gravity acting on the limb or body (weight). The total force output in a raising action is predominantly muscle force. This is another reason concentric muscle action is more demanding than the eccentric or isometric action.
This information is useful when considering exercise programs for unconditioned individuals or rehabilitation programs. Even the individual with the least amount of strength may be able to perform a controlled lowering of a body part or a small weight but may not be able to hold or raise the weight. A program that starts with eccentric exercises and then leads into isometric followed by concentric exercises may prove to beneficial in the progression of strength or in rehabilitation of a body part. Thus, a person unable to do a push-up should start at the extended position and lower into the push-up, then receiving assistance on the up phase until enough strength is developed for the concentric portion of the skill. Factors to consider in the use of eccentric exercises are the control of the speed at which the limb or weight is lowered and control over the magnitude of the load imposed eccentrically because muscle injury and soreness can occur more readily with eccentric muscle action in high-load and high-speed conditions.
One- and Two-Joint Muscles
As stated earlier, one cannot determine the function or contribution of a muscle to a joint movement by simply locating the attachment sites. A muscle action can move a segment at one end of its attachment or two segments at both ends of its attachment. In fact, a muscle can accelerate and create movement at all joints, whether the muscle spans the joint or not. For example, the soleus is a plantar flexor of the ankle, but it can also force the knee into extension even though it does not cross the knee joint (61). This can occur in the standing posture. The soleus contracts and creates plantarflexion at the ankle. Because the foot is on the ground, the plantarflexion movement necessitates extension of the knee joint. In this manner, the soleus accelerates the knee joint twice as much as it accelerates the ankle, even though the soleus does not even span the knee.
Most muscles cross only one joint, so the dominating action of the one-joint muscle is at the joint it crosses. The two-joint muscle is a special case in which the muscle crosses two joints, creating a multitude of movements that often occur in opposite sequences to each other. For example, the rectus femoris is a two-joint muscle that creates both hip flexion and knee extension. Take the example of jumping. Hip extension and knee extension propel the body upward. Does the rectus femoris, a hip flexor and knee extensor, contribute to the extension of the knee, does it resist the movement of hip extension, or does it do both?
The action of a two-joint, or biarticulate, muscle depends on the position of the body and the muscle's interaction with external objects such as the ground (61). In the case of the rectus femoris, the muscle contributes primarily to the extension of the knee because of the hip joint position. This position results in the force of the rectus femoris acting close to the hip, thereby limiting the action of the muscle and its effectiveness in producing hip flexion (Fig. 3-24).
The perpendicular distance from the action line of the force of the muscle over to the hip joint is the moment arm, and the product of the force and the moment arm is the muscle torque. If the moment arm increases, torque at the joint increases, even if the applied muscle force is the same. Thus, in the case of a two-joint muscle, the muscle primarily acts on the joint where it has the largest moment arm or where it is farther from the joint. The hamstring group primarily creates hip extension rather than knee flexion because of the greater moment arm at the hip (Fig. 3-24). The gastrocnemius produces plantarflexion at the ankle rather than flexion at the knee joint because the moment arm is greater at the ankle.
For example, in vertical jumping, maximum height is achieved by extending the proximal joints first and then moving distally to where extension (plantarflexion) occurs in the ankle joint. By the time the ankle joint is involved in the sequence, very high joint moments and
extension velocities are required (57). The role of the two-joint muscle becomes very important. The biarticular gastrocnemius muscle crosses both the knee and ankle joints. Its contribution to jumping is influenced by the knee joint. In jumping, the knee joint extends and effectively optimizes the length of the gastrocnemius (6). This keeps the contraction velocity in the gastrocnemius muscle low even when the ankle is plantarflexing very quickly. With the velocity lowered, the gastrocnemius is able to produce greater force in the jumping action.
FIGURE 3-24 The rectus femoris moment arms at the hip and knee while standing (A) and in a squat (B) demonstrate why this muscle is more effective as an extender of the knee than as a flexor at the hip. Likewise, the hamstring moment arm while standing (C) and in a squat (D) demonstrates why the hamstrings are more effective as hip extensors than as knee flexors.
The most important contribution of the two-joint muscle in the lower extremity is the reduction of the work required from the single-joint muscles. Two joint muscles initiate a mechanical coupling of the joints that allows for a rapid release of stored elastic energy in the system (61). Two-joint muscles save energy by allowing positive work at one joint and negative work at the adjacent joint. Thus, while the muscles acting at the ankle are producing a concentric action and positive work, the knee muscles can be eccentrically storing elastic energy through negative work (61).
FIGURE 3-25 Two-joint muscles work synergistically to optimize performance, shown here for walking.
The two-joint muscle actions for walking are presented in Figure 3-25. Two-joint muscles that work together in walking are the sartorius and rectus femoris at heel strike; the hamstrings and gastrocnemius at midsupport; the gastrocnemius and rectus femoris at toe-off; the rectus femoris, sartorius, and hamstrings at forward swing; and the hamstrings and gastrocnemius at foot descent (60). At heel strike, the sartorius, a hip flexor and a knee flexor, works with the rectus femoris, a hip flexor and knee extensor. As the heel strikes the surface, the rectus femoris performs negative work, absorbing energy at the knee as it moves into flexion. The sartorius, on the other hand, performs positive work as the knee and the hip both flex with gravity (60).
The two-joint muscle is limited in function at specific joint positions. When the two-joint muscle is constrained in elongation, it is termed passive insufficiency. This occurs when the antagonistic muscle cannot be elongated
any farther and the full range of motion cannot be achieved. An example of passive insufficiency is the prevention of the full range of motion in knee extension by a tight hamstring. A two-joint muscle can also be restrained in contraction through active insufficiency where the muscle is slackened to the point where it has lost its ability to generate maximum tension. An example of active insufficiency is seen at the wrist where the finger flexors cannot generate maximum force in a grip when they are shortened by an accompanying wrist flexion movement.
Force-Velocity Relationships in Skeletal Muscle
Muscle fibers will shorten at a specific speed or velocity while concurrently developing a force used to move a segment or external load. Muscles create an active force to match the load in shortening, and the active force continuously adjusts to the speed at which the contractile system moves (10). When load is low, the active force is adjusted by increasing the speed of contraction. With greater loads, the muscle adjusts the active force by reducing the speed of shortening.
Force-Velocity and Muscle Action or Load
Force-Velocity Relationship in Concentric Muscle Actions
In concentric muscle action, velocity increases at the expense of a decrease in force and vice versa. The maximum force can be generated at zero velocity, and the maximum velocity can be achieved with the lightest load. An optimal force can be created at zero velocity because a large number of cross-bridges are formed. As the velocity of the muscle shortening increases, the cycling rate of the cross-bridges increases, leaving fewer cross-bridges attached at one time (24). This equates to less force, and at high velocities, when all of the cross-bridges are cycling, the force production is negligible (Fig. 3-26). This is opposite to what happens in a stretch in which an increase in the velocity of deformation of the passive components of the muscle results in higher force values. Maximum velocity in a concentric muscle action is determined by the cross-bridge cycling rates and the whole muscle fiber length over which the shortening can occur.
Force-Velocity in the Muscle Fiber versus External Load
The force-velocity relationship relates to the behavior of muscle fiber, and it is sometimes confusing to relate this concept to an activity such as weight lifting. As an athlete increases the load in a lift, the speed of movement is likely to decrease. Although the force-velocity relationship is still present in the muscle fiber itself, the total system is responding to the increase in the external load or weight.
FIGURE 3-26 The force-velocity relationship in a concentric muscle action is inverse. The amount of tension or force-developing capability in the muscle decreases with an increase in velocity because fewer cross-bridges can be maintained. Maximum tension can be generated in the isometric or zero velocity condition, in which many cross-bridges can be formed. Maximum power can be generated in concentric muscle action with the velocity and force levels at 30% of maximum.
The muscle may be generating the same amount of force in the fiber, but the addition of the weight slows the movement of the total system. In this case, the action velocity of the muscle is high, but the movement velocity of the high load is low (48). Muscles generate forces greater than the weight of the load in the early stages on the activity to move the weight and at the later stages of the lift, less muscle force may be required after the weight is moving.
The product of force and velocity, power, is one of the major distinguishing features between successful and average athletes. Many sports require large power outputs, with the athlete expected to move his or her body or some external object very quickly. Because velocity diminishes with the increase of load, the most power can be achieved if the athlete produces one third of maximum force at one third of maximum velocity (43,44). In this way, the power output is maximized even though the velocities or the forces may not be at their maximum level.
To train athletes for power, coaches must schedule high-velocity activities at 30% of maximum force (43). The development of power is also enhanced by fast-twitch muscle fibers, which are capable of generating four times more peak power than slow-twitch fibers.
Force-Velocity Relationships in Eccentric Muscle Actions
The force-velocity relationship in an eccentric muscle action is opposite to that in the shortening or concentric action. An eccentric muscle action is generated by antagonistic muscles, gravity, or some other external force. When a load greater than the maximum isometric strength value is applied to a muscle fiber, the fiber begins to lengthen eccentrically. At the initial stages of lengthening, when the load is slightly greater than the isometric maximum, the
speed of lengthening and the length changes in the sarcomeres are small (10).
FIGURE 3-27 The relationship between force and velocity in eccentric muscle action is opposite to that of concentric muscle action. In eccentric muscle action, the force increases as the velocity of the lengthening increases. The force continues to increase until the eccentric action can no longer control lengthening of the muscle.
If a load is as high as 50% greater than the isometric maximum, the muscle elongates at a high velocity. In eccentric muscle action, the tension increases with the speed of lengthening because the muscle is stretching as it contracts (Fig. 3-27). The eccentric force-velocity curve ends abruptly at some lengthening velocity when the muscle can no longer control the movement of the load.
Factors Influencing Force and Velocity Generated by Skeletal Muscle
Many factors influence how much force, or how fast of a contraction, a muscle can produce. Muscle contraction force is opposed by many factors, including the passive internal resistance of the muscle and tissue, the opposing muscles and soft tissue, and gravity or the effect of the load being moved or controlled. Let's examine some of the major factors that influence force and velocity development. These include muscle cross-section, muscle length, muscle fiber length, preloading of the muscle before contraction, neural activation of the muscle, fiber type, and the age of the muscle.
What are some of the factors that determine force production in the muscle?
What are some of the factors that determine velocity production in the muscle?
Muscle Cross-Section and Whole Muscle Length
Muscle architecture determines whether the muscle can generate large amounts of force and whether it can change its length significantly to develop higher velocity of movement. In the case of the latter, the shortening ability of a muscle is reflected by changes in both length and speed, depending on the situation.
Generally speaking, the strength of a muscle and the potential for force development are determined mainly by its size. Muscle can produce a maximum contractile force between 2.5 and 3.5 kg per muscle cross-section, so a bigger muscle produces more force.
In the penniform muscle, the fibers are typically shorter and not aligned with the line of pull. An increased number of sarcomeres are aligned in parallel, which enhances the force-producing capacity. With an increased diameter and cross-section, the penniform muscle is able to exert more force than similar-sized parallel fibers.
Parallel fibers with longer fiber lengths typically have a longer working range, producing a larger range of motion and a higher contraction velocity. With the fibers aligned parallel to the line of pull, an increased number of sarcomeres are attached end to end in series. This results in the increased fiber lengths and the capacity to generate greater shortening velocity.
A muscle with a greater ratio of muscle length to tendon length has the potential to shorten over a greater distance. Consequently, muscles attaching to the bone with a short tendon (e.g., the rectus abdominus) can move through a greater shortening distance than muscles with longer tendons (e.g., the gastrocnemius) (19). Great amounts of shortening also occur because skeletal muscle can shorten up to approximately 30% to 50% of its resting length. Similarly, a muscle having a greater ratio of fiber length to muscle length can also shorten over a longer distance and generate higher velocities (i.e., hamstrings, dorsiflexor). In contrast, a muscle having a shorter fiber length compared with muscle length (e.g., the gastrocnemius) can generate larger forces.
Muscle Fiber Length
The magnitude of force produced by a muscle during a contraction is also related to the length at which the muscle is held (10). Muscle length may increase, decrease, or remain constant during a contraction depending on the external opposing forces. Muscle length is restricted by the anatomy of the region and the attachment to the bone. The maximum tension that can be generated in the
muscle fiber occurs when a muscle is activated at a length slightly greater than resting length, somewhere between 80% and 120% of the resting length. Fortunately, the length of most muscles in the body is within this maximum force production range. Figure 3-28 shows the length-tension relationship and demonstrates the contribution of active and passive components in the muscle during an isometric contraction.
Tension at Shortened Lengths
The tension-developing capacity drops off when the muscle is activated at both short and elongated lengths. The optimal length at the sarcomere level is when there is maximum overlap of myofilaments, allowing for the maximal number of cross-bridges. When a muscle has shortened to half its length, it is not capable of generating much more contractile tension. At short lengths, less tension is present because the filaments have exceeded their overlapping capability, creating an incomplete activation of the cross-bridges because fewer of these can be formed (10) (Fig. 3-28). Thus, at the end of a joint movement or range of motion of a segment, the muscle is weak and incapable of generating large amounts of force.
Tension at Elongated Lengths
When a muscle is lengthened and then activated, muscle fiber tension is initially greater because the cross-bridges are pulled apart after initially joining (49). This continues until the muscle length is increased slightly past the resting length. When the muscle is further lengthened and contracted, the tension generated in the muscle drop off because of slippage of the cross-bridges, resulting in fewer cross-bridges being formed (Fig. 3-28).
Contribution of the Elastic Components
The contractile component is not the only contributor to tension at different muscle lengths. The tension generated in a shortened muscle is shared by the series elastic component, that is, most tension develops in the tendon. The tension in the muscle is equal to the tension in the series elastic component when the muscle contracts in a shortened length.
FIGURE 3-28 Muscle fibers cannot generate high tensions in the shortened state (A) because the actin and myosin filaments are maximally overlapped. The greatest tension in the muscle fiber can be generated at a length slightly greater than resting length (B). In the elongated muscle (C), the fibers are incapable of generating tension because the cross-bridges are pulled apart. The total muscle tension increases, however, because the elastic components increase their tension development. PEC, parallel elastic component; SEC, series elastic component.
As the tension-developing characteristics of the active components of the muscle fibers diminish with elongation, tension in the total muscle increases because of the contribution of the passive elements in the muscle. The series elastic component is stretched, and tension is developed in the tendon and the cross-bridges as they are rotated back (24). Significant tension is also developed in the parallel elastic component as the connective tissue in the muscle offers resistance to the stretch. As the muscle is lengthened, passive tension is generated in these structures, so that the total tension is a combination of contractile and passive components (Fig. 3-28). At extreme muscle lengths, the tension in the muscle is almost exclusively elastic, or passive, tension.
Optimal Length for Tension
The optimal muscle length for generating muscle tension is slightly greater than the resting length because the contractile components are optimally producing tension and the passive components are storing elastic energy and adding to the total tension in the unit (18). This relationship lends support for placing the muscle on a stretch before using the muscle for a joint action. One of the major purposes of a windup or preparatory phase is to put the muscle on stretch to facilitate output from the muscle in the movement.
How does the tension in the active and passive components contribute to force generation in the muscle?
Neural Activation of the Muscle
The amount of force generated in the muscle is determined by the number of cross-bridges formed at the sarcomere level. The nature of stimulation of the motor units and the types of motor units recruited both affect force output. Force output increases from light to higher levels as motor unit recruitment expands from type I slow-twitch to type IIa and then type IIb fast-twitch fibers. Recruitment of additional motor units or recruitment of fast-twitch fibers increases force output.
At any given velocity of movement, the force generated by the muscle depends on the fiber type. A fast-twitch fiber generates more force than a slow-twitch fiber when the muscle is lengthening or shortening. Type IIb fast-twitch fibers produce the highest maximal force of all the fiber types. At any given absolute force level, the velocity is also greater in muscles with a greater percentage of fast-twitch fibers. Fast-twitch fibers generate faster velocities because of a quicker release of Ca++ and higher ATPase activity. Slow-twitch fibers, which are recruited first, are the predominately active fiber type in low-load situations, and their maximal shortening velocity is slower than that of fast-twitch fibers.
Preloading of the Muscle Before Contraction
If concentric, or shortening, muscle action is preceded by a prestretch through eccentric muscle action, the resulting concentric action is capable of generating greater force. Termed stretch-contract or stretch-shortening cycle, the stretch on the muscle increases its tension through storage of potential elastic energy in the series elastic component of the muscle (32) (Fig. 3-29). When a muscle is stretched, there is a small change in the muscle and tendon length (33) and maximum accumulation of stored energy. Thus, when a concentric muscle action follows, an enhanced recoil effect adds to the force output through the muscle-tendon complex (24).
A concentric muscle action beginning at the end of a prestretch is also enhanced by the stored elastic energy in the connective tissue around the muscle fibers. This contributes to a high-force output at the initial portion of the concentric muscle action as these tissues return to their normal length.
FIGURE 3-29 If a stretch of the muscle precedes a concentric muscle action, the resulting force output is greater. The increased force output is attributable to contributions from stored elastic energy in the muscle, tendon, and connective tissue and through some neural facilitation.
If the shortening contraction of the muscle occurs within a reasonable time after the stretch (up to 0.9 second), the stored energy is recovered and used. If the stretch is held too long before the shortening occurs, the stored elastic energy is lost through conversion to heat (31).
The stretch preceding the concentric muscle action also initiates a stimulation of the muscle group through reflex potentiation. This activation accounts for only approximately 30% of the increase in the concentric muscle action (31). The remaining increase is attributed to stored energy. The actual process of proprioceptive activation through the reflex loop is presented in the next chapter.
Use of the Prestretch
A short-range or low-amplitude prestretch occurring over a short time is the best technique to significantly improve the output of concentric muscle action through return of elastic energy and increased activation of the muscle (4,31). To get the greatest return of energy absorbed in the negative or eccentric action, the athlete should go into the stretch quickly but not too far. Also, the athlete should not pause at the end of the stretch but move immediately into the concentric muscle action. In jumping, for example, a quick counterjump from the anatomical position, featuring a drop-stop-pop action, lowering only through 8 to 12 inches, is much more effective than a jump from a squat position or a jump from a height that forces the limbs into more flexion (4). The influence of this type of jumping technique on the gastrocnemius muscle is presented in Figure 3-30.
Slow- and fast-twitch fibers handle a prestretch differently. Muscles with predominantly fast-twitch fibers benefit from a very high-velocity prestretch over a small distance because they can store more elastic energy (31). The fast-twitch fibers can handle a fast stretch because myosin cross-bridging occurs quickly. In slow-twitch fibers, the cross-bridging is slower (17).
In a slow-twitch fiber, the small-amplitude prestretch is not advantageous because the energy cannot be stored fast enough and the cross-bridging is slower (17,31). Therefore, slow-twitch fibers benefit from a prestretch that is slower and advances through a greater range of motion. Some athletes with predominantly slow-twitch fibers should be encouraged to use longer prestretches of the muscle to gain the benefits of the stretch. For most athletes, however, the quick prestretch through a small range of motion is the preferred method.
The use of a quick prestretch is part of a conditioning protocol known as plyometrics. In this protocol, the muscle is put on a rapid stretch, and a concentric muscle action is initiated at the end of the stretch. Single-leg
bounding, depth jumps, and stair hopping are all plyo-metric activities for the lower extremity. Surgical tubing or elastic bands are also used to produce a rapid stretch on muscles in the upper extremity. Plyometrics is covered in greater detail in Chapter 4.
FIGURE 3-30 Neural facilitation in the gastrocnemius. In trained jumpers, the prestretch is used to facilitate the neural activity of the lower extremity muscles. Neural facilitation coupled with the recoil effect of the elastic components adds to the jump if it is performed with the correct timing and amplitude. (Adapted with permission from
Sale, D. G. . Neural adaptation in strength and power training. In N. L. Jones, et al. [Eds.]. Human Muscle Power. Champaign, IL: Human Kinetics, 289-308.
Age of Muscle
Sarcopenia is the term for loss of muscle mass and decline in muscle quality seen in aging. Sarcopenia results in a loss of muscle force that impacts bone density, function, glucose intolerance, and a number of other factors leading to
disability in the elderly. Both anatomic and biochemical changes occur in the aging muscle to lead to sarcopenia. Anatomically, a number of changes take place in the aging muscle, including decreased muscle mass and cross-section, more fat and connective tissue, decreases in type II fiber size, decrease in the number of both type I and II fibers, changes at the sarcomere level, and a decreased number of motor units (28). Biochemically, a reduction in protein synthesis, some impact on enzyme activity, and changes in muscle protein expression take place.
Muscle force decreases with aging at the rate of about 12% to 15% per decade after age 50 years (28). The rate of strength loss increases with age and is related to many factors, some of which are anatomical, biochemical, nutritional, and environmental. Progressive resistance training is the best intervention to slow or reverse the effects of aging on the muscle.
Other Factors Influencing Force and Velocity Development
A number of other factors can influence the development of force and velocity in the skeletal muscle. Muscle fatigue can influence force development as the muscle becomes progressively weaker, the shortening velocity is reduced, and the rate of relaxation slows. Gender differences and psychological factors can also influence force and velocity development.
Strength is defined as the maximum amount of force produced by a muscle or muscle group at a site of attachment on the skeleton (38). Mechanically, strength is equal to maximum isometric torque at a specific angle. Strength, however, is usually measured by moving the heaviest possible external load through one repetition of a specific range of motion. The movement of the load is not performed at a constant speed because joint movements are usually done at speeds that vary considerably through the range of motion. Many variables influence strength measurement. Some of these include the muscle action (eccentric, concentric, isometric) and the speed of the limb movement (30). Also, length-tension, force-angle, and force-time characteristics influence strength measurements as strength varies throughout the range of motion. Strength measurements are limited by the weakest joint position.
Training of the muscle for strength focuses on developing a greater cross-sectional area in the muscle and on developing more tension per unit of cross-section area (59). This holds true for all people, both young and old. Greater cross-section, or hypertrophy, associated with weight training is caused by an increase in the size of the actual muscle fibers and more capillaries to the muscle, which creates greater mean fiber area in the muscle (32,40). The size increase is attributed to increase in size of the actual myofibrils or separation of the myofibrils, as shown in Figure 3-31. Some researchers speculate that the actual muscle fibers may split (Fig. 3-31), but this has not been experimentally substantiated in humans (40). The increase in tension per unit of cross-section reflects the neural influence on the development of strength (47). In the early stages of strength development, the nervous system adaptation accounts for a significant portion of the strength gains through improvement in motor unit recruitment, firing rates, and synchronization (37). Hypertrophy follows as the quality of the fibers improves. Figure 3-32 illustrates the strength progression.
FIGURE 3-31 A. During strength training, the muscle fibers increase in cross-section as the myofibrils become larger and separate. B. It has been hypothesized that the fibers may also actually split, but this has yet to be demonstrated in humans. (Adapted with permission from
MacDougall, J. D. . Hypertrophy or hyperplasia. In P. Komi [Ed.].Strength and Power in Sport. Boston: Blackwell Scientific, 230-238.
What are the components of a resistance training program?
Source: Kraemer and Rotamess, 2004 (37)
FIGURE 3-32 In the initial stages of a strength training program, the majority of the strength gain is because of neural adaptation, which is followed by hypertrophy of the muscle fibers. Both of these changes contribute to the overall increase in strength.
Principles of Resistance Training
Training specificity, relating to the specific muscles, is important in strength training. Only the muscles used in a specific movement pattern gain strength. This principle, specific adaptation to imposed demands, should direct the choice of lifts toward movement patterns related to the sport or activity in which the pattern might be used (59). This training specificity has a neurological basis, somewhat like learning a new motor skill–one is usually clumsy until the neurological patterning is established. Figure 3-33 shows two sport skills, football lineman drives and basketball rebounding, along with lifts specific to the movement. Decisions concerning muscle actions, speed of movement, range of motion, muscle groups, and intensity and volume are all important in terms of training specificity (Table 3-1) (37).
A learning process takes place in the early stages of strength training. This process continues into the later stages of training, but it has its greatest influence at the beginning of the program. In the beginning stages of a program, the novice lifter demonstrates strength gains as a consequence of learning the lift rather than any noticeable increase in the physical determinants of strength, such as increase in fiber size (15,59). This is the basis for using submaximal resistance and high-repetition lifting at the beginning of a strength-training program, so that the lift can first be learned safely.
In addition to the specificity of the pattern of joint movement, specificity of training of the muscle also relates to the speed of training. If a muscle is trained at slow speeds, it will improve strength at slow speeds but may not be strengthened at higher speeds, although training at a faster speed of lifting can promote greater strength gains (53). It is important that if power is the ultimate goal for an athlete, the strength-training routine should contain movements focusing on force and velocity components to maximize and emulate power. After a strength base is established, power is obtained with high-intensity loads and a low number of repetitions (48).
The intensity of the training routine is another important factor to monitor in the development of strength. Strength gains are directly related to the tension produced in the muscle. A muscle must be overloaded to a particular threshold before it will respond and adapt to the training (60). The amount of tension in the muscle rather than the number of
repetitions is the stimulus for strength. The amount of overload is usually determined as a percentage of the maximum amount of tension a muscle or muscle group can develop.
FIGURE 3-33 Weight-lifting exercises should be selected so that they reproduce some of the movements used in the sport. For football lineman (A-C), the dead lift and the power clean include similar joint actions. Likewise, for basketball players who use a jumping action, the squat and heel-raising exercises are helpful (D-F).
Athletes attempt to work at the highest percentage of their maximal lifting capability to increase the magnitude of their strength gains. If the athlete trains regularly using a high number of repetitions with low amounts of tension per repetition, the strength gains will be minimal because the muscle has not been overloaded beyond its threshold. The greatest strength gains are achieved when the muscle is worked near its maximum tension before it reaches a fatigue state (two to six repetitions).
The muscle adapts to increased demands placed on it, and a systematic increase through progressive overload can lead to positive improvements in strength, power, and local muscular endurance (36). Overload of the muscle can be accomplished by increasing the load, increasing the repetitions, altering the repetition speed, reducing the rest period between exercises, and increasing the volume (37).
The quality and success of a strength-development routine are also directly related to the rest provided to the muscles between sets, between days of training, and before
competition. Rest of skeletal muscle that has been stressed through resistive training is important for the recovery and rebuilding of the muscle fiber. As the skeletal muscle fatigues, the tension-development capability deteriorates, and the muscle is not operating at optimal overload.
TABLE 3-1 Sample Weight-Training Cycle
The volume of work that a muscle performs may be the important factor in terms of rest of the muscle. Volume of work on a muscle is the sum of the number of repetitions multiplied by the load or weight lifted (59). Volume can be computed per week, month, or year and should include all of the major lifts and the number of lifts. In a week, the volume of lifting for two lifters may be the same even though their regimens are not the same. For example, one lifter lifts three sets of 10 repetitions at 100 lb for a volume of 3000 lb, and another lifts three sets of two repetitions at 500 lb, also for a volume of 3000 lb.
Considerable discussion has focused on the number of sets that are optimal for strength development. Some evidence suggests that similar strength gains can be obtained with single rather than multiple sets (7). On the other side, an increasing amount of evidence supports considerably higher strength gains with three sets as compared to single sets (53).
At the beginning of a weight-training program, the volume is usually high, with more sessions per week, more lifts per session, more sets per exercise, and more repetitions per set taking place than later in the program (15). As one progresses through the training program, the volume decreases. This is done by lifting fewer times per week, performing fewer sets per exercise, increasing the intensity of the lifts, and performing fewer repetitions.
The yearly repetition recommendation is 20,000 lifts, which can be divided into monthly and weekly volumes as the weights are increased or decreased (15). In a month or in a week, the volume of lifting varies to offer higher and lower volume days and weeks.
A lifter performing heavy-resistance exercises with a low number of repetitions must allow 5 to 10 minutes between sets for the energy systems to be replenished (59). If the rest is less than 3 minutes, a different energy system is used, resulting in lactic acid accumulation in the muscle.
Bodybuilders use the short-rest and high-intensity training to build up the size of the muscle at the expense of losing some strength gains achieved with a longer rest period. If a longer rest period is not possible, it is believed that a high-repetition, low-resistance form of circuit training between the high-resistance lifts may reduce the buildup of lactic acid in the muscle. Bodybuilders also exercise at loads less than those of power lifters and weight lifters (6 to 12 RM). This is the major reason for the strength differences between the weight lifter (greater strength) and the bodybuilder (less strength).
The development of strength for performance enhancement usually follows a detailed plan that has been outlined in the literature for numerous sports and activities. The long-term picture usually involves some form of peri-odization during which the loads are increased and the volume of lifting is decreased over a period of months. Variation through periodization is important for long-term progression to overcome plateaus or strength decrements caused by slowed physical adaptations to the loads. As the athlete heads into a performance season, the lifting volume may be reduced by as much as 60%, which will actually increase the strength of the muscles. If an athlete stops lifting in preparation for a performance, strength can be maintained for at least 5 days and may be even higher after a few days of rest (59).
Strength Training for the Nonathlete
The principles of strength or resistance training have been discussed using the athlete as an example. It is important to recognize that these principles are applicable to rehabilitation situations, the elderly, children, and unconditioned individuals. Strength training is now recommended as part of one's total fitness development. The American College of Sports Medicine recommends at least one set of resistance training 2 days a week and including eight to
12 exercises for adults (1). The untrained responds favorably to most protocols demonstrating high rates of improvement compared with the trained (37).
Strength training is recognized as an effective form of exercise for elderly individuals. A marked strength decrement occurs with aging and is believed to be related to reduced activity levels (27). Strength training that is maintained into the later years may counteract atrophy of bone tissue and moderate the progression of degenerative joint alterations. Eccentric training has also been shown to be effective in developing strength in the elderly (39). The muscle groups identified for special attention in a weight-training program for the elderly include the neck flexors, shoulder girdle muscles, abdominals, gluteals, and knee extensors.
Only the magnitude of the resistance should vary in weight training for athletes, elderly individuals, young individuals, and others. Whereas a conditioned athlete may perform a dumbbell lateral raise with a 50-lb weight in the hand, an elderly person may simply raise the arm to the side using the arm weight as the resistance. High-resistance weight lifting must be implemented with caution, especially with young and elderly individuals. Excessive loading of the skeletal system through high-intensity lifting can fracture bone in elderly individuals, especially in the individual with osteoporosis.
The epiphyseal plates in young people are also susceptible to injury under high loads or improper lifting technique; thus, high-intensity programs for children are not recommended. If safety is observed, however, children and adolescents can achieve training-induced strength gains (11). Regular participation in a progressive resistance training program by children and adolescents has many potential benefits, including increased bone strength, weight control, injury reduction, sports performance enhancement, and increased muscle endurance (11).
There are various ways of loading the muscle, all of which have advantages and disadvantages in terms of strength development. Isometric training loads the muscle in one joint position so the muscle torque equals the resistance torque and no movement results (2). Individuals have demonstrated moderate strength gains using isometric exercises, and power lifters may use heavy-resistance isometric training to enhance muscle size.
Isometric exercise is also used in rehabilitation and with unconditioned individuals because it is easier to perform than concentric exercise. The major problem associated with isometric exercise is that there is minimal transfer to the real world because most real-world activities involve eccentric and concentric muscle actions. Furthermore, isometric exercise only enhances the strength of the muscle group at the joint angle in which the muscle is stressed, which limits development of strength throughout the range of motion.
The most popular strength-training modality is isotonic exercise. An exercise is considered isotonic when the segment moves a specified weight through a range of motion. Although the weight of the barbell or body segment is constant, the actual load imposed on the muscle varies throughout the range of motion. In an isotonic lift, the initial load or resistance is overcome and then moved through the motion (2). The resistance cannot be heavier than the amount of muscle torque developed by the weakest joint position because the maximum load lifted is only as great as this position. Examples of isotonic modalities are the use of free weights and multijoint machines, such as universal gyms, in which the external resistance can be adjusted (Fig. 3-34).
FIGURE 3-34 Two forms of isotonic exercises for the upper extremity. A. The use of free weights (bench press). B. The use of a machine.
The use of free weights versus machines has generated considerable discussion. Free weights include dumbbells, barbells, weighted vests, medicine balls, and other added loads that allow the lifter to generate normal movements with the added weight. The advocates for free weights promote stabilization and control as major benefits to the use of free weights. Machines apply a resistance in a guided or restricted manner and are seen as requiring less overall control. Both training techniques can generate strength, power, hypertrophy, or endurance, so the choice should be left to the individual. Free weights may be preferable to enhance specificity of training, but correct technique is mandatory.
An isotonic movement can be produced with an eccentric or concentric muscle action. For example, the squat exercise involves eccentrically lowering a weight and concentrically raising the same weight. Even though the weight in an isotonic lift is constant, the torque developed by the muscle is not. This is because of the changes in length-tension or force-angle or to the speed of the lift. To initiate flexion of the elbow while holding a 2.5-kg weight, a person generates maximum tension in the flexors at the beginning of the lift to get the weight moving. Remember that this is also one of the weakest joint positions because of the angle of attachment of the muscle. Moving through the midrange of the motion requires reduced muscular tension because the weight is moving and the musculoskeletal lever is more efficient. The resistive torque also peaks in this stage of the movement.
The isotonic lift may not adequately overload the muscle in the midrange, where it is typically the strongest. This is especially magnified if the lift is performed very quickly. If the person performs isotonic lifts with a constant speed (no acceleration) so that the midrange is exercised, the motive torque created by the muscle will match the load offered by the resistance. Strength assessment using isotonic lifting is sometimes difficult because specific joint actions are hard to isolate. Most isotonic exercises involve action or stabilization of adjacent segments.
A third training modality is the isokinetic exercise, an exercise performed at a controlled velocity with varying resistance. This exercise must be performed on an isokinetic dynamometer, allowing for isolation of a limb; stabilization of adjacent segments; and adjustment of the speed of movement, which typically ranges from 0° to 600°/sec (Fig. 3-35).
When an individual applies a muscular force against the speed-controlled bar of the isokinetic device, an attempt is made to push the bar at the predetermined speed. As the individual attempts to generate maximum tension at the specific speed of contraction, the tension varies because of changes in leverage and muscular attachment throughout the range of motion. Isokinetic testing has been used for quantifying strength in the laboratory and in the rehabilitation setting. An extensive body of literature presents a wide array of norms for isokinetic testing of different joints, joint positions, speeds, and populations.
FIGURE 3-35 An isokinetic exercise for knee extension. The machine is the Biodex isokinetic dynamometer.
The velocity of the devices significantly influences the results. Therefore, testing must be conducted at a variety of speeds or at a speed close to that which will be used in activity. This is often the major limitation of isokinetic dynamometers. For example, the isokinetic strength of the shoulder internal rotators of a baseball pitcher may be assessed at 300°/sec on the isokinetic dynamometer, but the actual speed of the movement in the pitch has been shown to average 6000°/sec (8). Isokinetic testing allows for a quantitative measurement of power that has previously been difficult to measure in the field.
Using isokinetic testing and training has some drawbacks. The movement at a constant velocity is not the type of movement typically found in the activities of daily living or in sport, and the cost of most isokinetic systems and lack of mass usage make isokinetic training or testing prohibitive for many.
Closed and Open Kinetic Chain Exercise
Although most therapists still use isokinetic testing for assessment, many have discontinued its use for training and have gone to closed-chain training, in which individuals use body weight and eccentric and concentric muscle actions. A closed-chain exercise is an isotonic exercise in which the end of the chain is fixed, as in the case of a foot or hand on the floor. An example of a closed-chain exercise for the quadriceps is a simple squat movement with the feet on the floor (Fig. 3-36). It is believed that this form of exercise is more effective than an open-chain exercise, such as a knee extension on the isokinetic dynamometer or
knee extension machine, because it uses body weight, maintains muscle relationships, and is more transferable to normal human function. The use of closed-chain kinetic exercise for the knee joint has been shown to promote a more balanced quadriceps activation than open-chain exercise (51). With the promotion of accelerated rehabilitation after anterior cruciate ligament surgery, the trend in physical therapy is toward the use of closed-chain kinetic exercises. Research has shown no difference in the strains produced at the anterior cruciate ligament in open- versus closed-chain exercise, however, even with more anterior tibial translation seen in the open-chain exercise (12).
FIGURE 3-36 A. An open-chain exercise for the same muscles (leg extension). B. A closed-chain exercise for the quadriceps femoris muscles (squat).
The final training modality presented is functional training, a specialized training protocol for specific purposes. With the goal of enhancement of specificity of training, functional training uses different equipment to individualize training for each functional purpose. This training typically incorporates balance and coordination into each exercise so that stability is inherent in the movement. The use of medicine balls, stability balls, Bosu®, rubber tubing, and pulley systems are examples of various tools used in functional training. Examples of functional training exercises include throwing a medicine ball, performing an overhead press while sitting on a stability ball, applying variable resistance to an exercise by using rubberized tubing, standing on a balance board or Bosu® while performing an exercise, and applying resistance via a cable system during a diagonal movement pattern. A specific type of functional training, multivector strength training, is resistance training in which the individual must coordinate muscle action occurring in three directions or planes of motion at the same time.
Whatever form of exercise selected, the training should simulate the contraction characteristics of the activity. Improved strength alone does not necessarily transfer to better functional performance (55). After improvement in strength, muscle stiffness and physical changes take place as well as changes in neural input, which require more coordination. Thus, improvement in function does not always occur because of strength improvements.
Injury to Skeletal Muscle
Cause and Site of Muscle Injury
Injury to the skeletal muscle can occur through a bout of intense exercise, exercising a muscle over a long duration, or in eccentric exercise. The actual injury is usually a microinjury with small lesions in the muscle fiber. The result of a muscle strain or microtear in the muscle is manifested by pain or muscle soreness, swelling, possible anatomical deformity, and athletic dysfunction.
Muscles at greatest risk of strain are two-joint muscles, muscles limiting the range of motion, and muscles used eccentrically (16). The two-joint muscles are at risk because they can be put on stretch at two joints (Fig. 3-37). Extension at the hip joint with flexion at the knee joint puts the rectus femoris on extreme stretch and renders it very vulnerable to injury.
Eccentric exercise has been identified as a primary contributor to muscle strain (50). After a prolonged concentric or isometric exercise session, the muscles are fatigued, but it is usually a temporary state. After an
unaccustomed session of eccentric exercise, the muscles remain weak longer and are also stiff and sore (45). The muscle damage process brought on by eccentric exercise starts with initial damage at the sarcomere level followed by a secondary adaptation to protect the muscle from further damage (45). It has also been documented that rest may play a big role in determining the force decrement after eccentric muscle actions. Shorter work-rest cycles (10 sec vs. 5 min) have been shown to result in more force decrement 2 days after exercise (9). Although it is known that high forces in muscles working eccentrically can cause tissue damage, some believe that this type of eccentric contraction may actually promote positive adaptations in the muscle and tendon, resulting in increased size and strength (39).
FIGURE 3-37 Muscles undergoing an eccentric muscle action are at increased risk of injury. A. The quadriceps femoris performing an eccentric muscle action in lowering as they control the knee flexion on the way down. B. The quadriceps and gastrocnemius eccentrically acting during the support phase of running. Two-joint muscles are also placed in injury-prone positions, making them more susceptible to strain. C. The hamstrings on extreme stretch when the hip is flexed and the knee is extended during hurdling.
How is muscle postulated to be damaged in eccentric exercise?
Source: Proske, U., Allen, T. J. (2005). Damage to skeletal muscle from eccentric exercise. Exercise and Sport Sciences Reviews, 33:98-104.
Muscles used to terminate a range of motion are at risk because they are used to eccentrically slow a limb moving very quickly. Common sites where muscles are strained as they slow a movement are the hamstrings as they slow hip flexion and the posterior rotator cuff muscles as they slow the arm in the follow-through phase of throwing (16).
Although the muscle fiber itself may be the site of damage, it is believed that the source of muscle soreness immediately after exercise and strain to the system is the connective tissue. This can be in the muscle sheaths, epimysium, perimysium, or endomysium, or it can be injury to the tendon or ligament (49). In fact, a common site of muscle strain is at the muscle-tendon junction because of the high tensions transmitted through this region. Injuries at this site are common in the gastrocnemius, pectoralis major, rectus femoris, adductor longus, triceps brachii, semimembranosus, semitendinosus, and biceps femoris muscles (16).
It is important to identify those who are at risk for muscle strain. First, the chance of injury increases with muscular fatigue as the neuromuscular system loses its ability to control the forces imposed on the system. This commonly results in an alteration in the mechanics of movement and a shifting of shock-absorbing load responsibilities. Repetitive muscle strain can occur after the threshold of mechanical activity has been exceeded. Practice times should be controlled, and events late in the practice should not emphasize maximum load or stress conditions.
Second, an individual can incur a muscle strain at the onset of practice if it begins with muscles that are weak from recent usage (49). Muscles should be given ample time to recover from heavy usage. After extreme bouts of exercise, rest periods may have to be 1 week or more, but normally, a muscle can recover from moderate usage within 1or 2 days.
Third, if trained or untrained individuals perform a unique task for the first time, they will probably have pain, swelling, and loss of range of motion after performing the exercise. This swelling and injury are most likely to occur in the passive elements of the muscle and generally lessen or be reduced as the number of practices increase (49).
Last, an individual with an injury is susceptible to a recurrence of the injury or development of an injury elsewhere in the system resulting from compensatory actions. For example, if the gastrocnemius is sore from a minor muscle strain, an individual may eccentrically load the lower extremity with a weak and inflexible gastrocnemius. This forces the person to pronate more during the support phase and run more on the balls of the feet, indirectly producing knee injuries or metatarsal fractures. With every injury, a functional substitution happens elsewhere in the system; this is where the new injury will occur.
Preventing Muscle Injury
Conditioning of the connective tissue in the muscle can greatly reduce the incidence of injury. Connective tissue responds to loading by becoming stronger, although the rate of strengthening of connective tissue lags behind the rate of strengthening of the muscle. Therefore, base work with low loads and high repetitions should be instituted for 3 to 4 weeks at the beginning of a strength and conditioning program to begin the strengthening process of the connective tissue before muscle strength is increased (53).
Different types of training influence the connective tissue in different ways. Endurance training has been shown to increase the size and tensile strength of both ligaments and tendons. Sprint training improves ligament weight and thickness, and heavy loading strengthens the muscle sheaths by stimulating the production of more collagen. When a muscle produces a maximum voluntary contraction, only 30% of the maximum tensile strength of the tendon is used (53). The remaining tensile strength serves as an excess to be used for very high dynamic loading. If this margin is exceeded, muscle injury occurs.
Other important considerations in preventing muscle injury are a warm-up before beginning exercise routines, a progressive strength program, and attention to strength and flexibility balance in the musculoskeletal system. Finally, early recognition of signs of fatigue also helps prevent injury if corrective actions are taken.
Inactivity, Injury, and Immobilization Effects On Muscle
Changes in the muscle with disuse or immobilization can be dramatic. Atrophy is one of the first signs of immobilization of a limb, showing as much as a 20% to 30% decrease in cross-sectional area after 8 weeks of cast immobilization (52). Disuse or inactivity leads to atrophy because of muscle remodeling, resulting in loss of proteins and changes in the muscle metabolism. The level of atrophy appears to be muscle specific where lower extremity muscles lose more cross-section than back or upper extremity muscles (52). The greatest change occurs in the initial weeks of disuse, and this should be a focus of attention in rehabilitation and exercise.
Muscle regrowth after inactivity or immobilization varies between young, adult, and elderly individuals (41). Regrowth in young muscle is more successful than in the aging muscle, and the regrowth process varies between fast and slow muscles. Also, when successfully rebuilding cross-section of the atrophied muscle, the force output of the muscle lags behind (52).
When a muscle is injured, the force-producing capabilities usually decrease. Compensation occurs where other muscles change in function to make up for the injured muscle or the motion can be changed to minimize the use of the injured muscle (34). For example, injury to a hip flexor can cause a large reduction of force in the soleus, an ankle muscle, because of its role in propelling the trunk forward via pushoff in plantarflexion. Injury to the gluteus maximus (hip extensor) can shift duties of hip extension over to the gluteus medius and hamstrings. Loss of function in one muscle can impact all of the joints in the linked segments such as the lower extremity, so the whole musculoskeletal system should be the focus of retraining efforts.
Skeletal muscle has four properties: irritability, contractility, extensibility, and elasticity. These properties allow muscle to respond to stimulation, shorten, lengthen beyond resting length, and return to resting length after a stretch, respectively.
Muscles can perform a variety of functions, including producing movement, maintaining postures and positions, stabilizing joints, supporting internal organs, controlling pressures in the cavities, maintaining body temperature, and controlling entrances and exits to the body.
Groups of muscles are contained in compartments that can be categorized by common function. The individual muscles in the group are covered by an epimysium and usually have a central portion called the belly. The muscle can be further divided internally into fascicles covered by the perimysium; the fascicles contain the actual muscle fibers covered by the endomysium. Muscle fibers can be organized in a parallel arrangement, in which the fibers run parallel and connect to a tendon at both ends, or in a penniform arrangement, in which the fibers run diagonally to a tendon running through the muscle. In penniform muscle, the anatomical cross-section, situated at right angles to the direction of the fibers, is less than the physiological cross-section, the sum of all of the cross-sections in the fiber. In parallel muscle, the anatomical and physiological cross-sections are equal. Muscle volume and physiological cross-section are larger in the penniform muscle. The force applied in the penniform muscle is influenced by the pennation angle, where a smaller force is applied to the tendon at greater pennation angles.
Each muscle contains different fiber types that influence the muscle's ability to produce tension. Slow-twitch fiber types have slow contraction times and are well suited for prolonged, low-intensity workouts. Intermediate- and fast-twitch fiber types are better suited for higher force outputs over shorter periods.
A motor unit is a group of muscle fibers innervated by a single motor neuron. Muscle contraction occurs as the action potential traveling along the axon reaches the muscle fiber and stimulates a chemical transmission across the synapse. Once at the muscle, excitation-contraction coupling occurs as the release of Ca+ ions promotes cross-bridge formation.
Each muscle fiber contains myofibrils that house the contractile unit of the muscle fiber, the sarcomere. It is at the sarcomere level that cross-bridging occurs between the actin and myosin filaments, resulting in shortening or lengthening of the muscle fiber.
A muscle attaches to bone via an aponeurosis, or tendon. Tendons can withstand high tensile forces and respond stiffly to high rates of loading and less stiffly at lower loading rates. Tendons recoil during muscle contraction and delay the development of tension in the muscle. This recoiling action increases the load that a muscle can support. Tendons and muscles are more prone to injury during eccentric muscle actions.
A mechanical model of muscular contraction breaks the muscle down into active and passive components. The active component includes the contractile components found in the myofibrils and cross-bridging of the actin and myosin filaments. The passive or elastic components are in the tendon and the cross-bridges and in the sarcolemma and the connective tissue.
Muscles perform various roles, such as agonist or antagonist and stabilizer or neutralizer. Torque is generated in a muscle, developing tension at both ends of the muscle. The amount of tension is influenced by the angle of attachment of the muscle.
Muscle tension is generated to produce three types of muscle actions: isometric, concentric, and eccentric. The isometric muscle action is used to stabilize a segment, the concentric action creates a movement, and the eccentric muscle action controls a movement. The concentric muscle actions generate the lowest force output of the three, and the eccentric muscle action generates the highest.
Two-joint muscles are unique in that they act at two adjacent joints. Their effectiveness at one joint depends on the positioning of the other joint, the moment arms at each joint, and the muscle synergies in the movement.
Numerous factors influence the amount of force that can be generated by a muscle, including the angle of attachment of the tendon, muscle cross-section, laxity or stiffness in the tendon that influences the force-time relationship, fiber type, neural activation, length of the muscle, contributions of the elastic component, age of the muscle, and velocity of the muscle action.
Greater force can be developed in a concentric muscle action if it is preceded by an eccentric muscle action, or prestretch (stretch-shortening cycle). The muscle force is increased by facilitation via stored elastic energy and neurological facilitation. A quick, short-range prestretch is optimal for developing maximum tension in fast-twitch fibers, and a slow, larger-range prestretch is beneficial for tension development in slow-twitch fibers.
The development of strength in a muscle is influenced by genetic predisposition, training specificity, training intensity, muscle rest during training, and total training volume. Training principles apply to all groups, including conditioned and unconditioned individuals, and only the magnitude of the resistance needs to be altered. Muscles can be exercised isometrically, isotonically, isokinetically, or through specific functional training. Another important exercise consideration should be the decision on the use of open- or closed-chain exercises.
Muscle injury is common and occurs most frequently in two-joint muscles and during eccentric muscle action. To prevent muscle injury, proper training and conditioning principles should be followed.
True or False
A protein of the myofibril, noticeable by its light banding. Along with myosin, it is responsible for the contraction and relaxation of muscle.
The signal propagated of a neuron and muscle fibers.
The inability of a two-joint muscle to produce force when joint position shortens the muscle to the point where it cannot contract.
A muscle responsible for producing a specific movement through concentric muscle action.
The cross-section at a right angle to the direction of the fibers.
A muscle responsible for opposing the concentric muscle action of the agonist.
A flattened or ribbonlike tendinous expansion from the muscle that connects into the bone.
A decrease in muscle mass from the original mass.
The fleshy central portion of a muscle.
A feather-shaped fiber arrangement, in which the fibers run off of both sides of a tendon running through the muscle.
Concentrically arranged muscle around an opening or recess.
Exercises using eccentric and concentric muscle actions with the feet fixed on the floor. Movements begin with segments distal to the feet (trunk and thigh) and move toward the feet, as in the squat.
A condition in which the circulation and function of the tissues within a muscle compartment are impaired by an increase in pressure within the compartment.
Muscle action in which tension causes visible shortening in the length of the muscle; positive work is performed.
The speed at which an action potential is propagated.
The active component in a muscle where behavioral shortening takes place.
The ability of muscle tissue to shorten when the muscle tissue receives sufficient stimulation.
The state of muscle when tension is generated across a number of actin and myosin filaments.
Fan-shaped muscle with broad fibers that converge to a common insertion site.
The connection and intertwining of the actin and myosin filaments of the myofibrils.
A reduction in the potential of a membrane.
Muscle action in which tension is developed in the muscle and the muscle lengthens; negative work is performed.
Electrochemical stimulation of the muscle fiber that initiates the release of calcium and the subsequent cross-bridging between actin and myosin filaments, which leads to contraction.
Capable of being stretched, compressed, or distorted and then returning to the original shape.
The ability of muscle tissue to return to its resting length after a stretch is removed.
The sheath surrounding each muscle fiber.
A dense, fibrous sheath covering an entire muscle.
The ability of muscle tissue to lengthen beyond resting length.
Sheet or band of fibrous tissue.
A bundle or cluster of muscle fibers.
Large skeletal muscle fiber innervated by the alpha-I motor neuron; has fast contraction times. The two subtypes of fast-twitch fibers are the low oxidative and high glycolytic (type IIb) and the medium oxidative and high glycolytic (type IIa).
Elongated cylindrical structures containing cells that constitute the contractile elements of muscle tissue.
Thin and broad-shaped muscle.
The relationship between the tension development in the muscle and velocity of shortening or lengthening.
Spindle-shaped fiber arrangement in a muscle.
An increase in the potential of a membrane.
An enlargement or growth of tissue caused by an increase in the size of cells.
Lacking the ability to withstand compression, stretch, or distortion and return to the original shape or length.
The more distal attachment site of the muscle.
In weight training, the load or percentage of maximum lifting capacity lifted with each repetition.
The capacity of muscle tissue to respond to a stimulus.
An exercise in which concentric muscle action is generated to move a limb against a device that is speed controlled. Individuals attempt to develop maximum tension through the full range of motion at the specified speed of movement.
Muscle action in which tension develops but no visible or external change is seen in joint position; no external work is produced.
An exercise that loads the muscle in one joint position.
An exercise in which an eccentric or concentric muscle action (or both) is generated to move a specified weight through a range of motion.
The relationship between the length of the muscle and the tension produced by the muscle; highest tensions are developed slightly past resting length.
The process whereby a rectified electromyography signal has most of the high-frequency components removed via a low-pass filter.
Line of Action or Pull
An infinite line extending along the direction of the force from the point where the force acts.
The perpendicular distance from the line of action of the force to the pivot point.
The nerve and all of the muscle fibers that it innervates.
A feather-shaped fiber arrangement in which the muscle fibers run diagonally off one or both sides of a tendon running through the muscle.
Exercise-induced reduction in the maximal force capacity of the muscle.
The amount of muscle space determined by the ratio of muscle mass divided by density.
Rodlike strand contained within and running the length of the muscle fibers; contains the contractile elements of the muscle.
A thick protein of the myofibril, noticeable by its dark banding. Aong with actin, it is responsible for contraction and relaxation of the muscle.
The site where the muscle and tendon join, consisting of a layered interface as the myofibrils and the collagen fibers of the tendon meet.
The chemical synapse between the motor neuron and the muscle fiber.
A muscle responsible for eliminating or canceling out an undesired movement.
The more proximal attachment site of a muscle.
Parallel Elastic Component
The passive component in a muscle model that behaviorally develops tension with elongation.
The inability of a two-joint muscle to be stretched sufficiently to allow a complete range of motion at all the joints it crosses because the antagonists cannot be further elongated.
The angle made by the fascicles and the line of action (pull) of the muscle.
A feather-shaped fiber arrangement in a muscle in which the fibers run diagonally to a tendon running through the muscle.
A dense connective tissue sheath covering the fascicles.
An area that is the sum total of all of the cross-sections of fibers in the muscle; the area perpendicular to the direction of the fibers.
A training technique that uses the stretch-shortening cycle to increase athletic power.
The product of force and velocity.
A gradual increase in the stress placed on the body during exercise by varying factors such as load, repetitions, speed, rest, and volume.
Progressive Resistance Training
The continued improvement in a desired variable such as resistance and number of sets and repetitions until a target has been reached.
To spring back to the original position, as seen in the elastic components in the muscle.
The process whereby the negative portion of a raw electromyography signal is made positive so that the complete signal is positive.
A return to the resting potential of a membrane.
The voltage across the membrane at steady-state conditions.
A thin plasma membrane covering the muscle that branches into the muscle, carrying nerve impulses.
One contractile unit of banding on the myofibril, running Z-band to Z-band.
Loss of muscle mass and decline in muscle quality with increased aging.
The fluid enclosed within a muscle fiber by the sarcolemma.
A membranous system within a muscle fiber that forms lateral sacs near the t-tubules.
Series Elastic Component
The passive component in a muscle model that behaviorally develops tension in contraction and during elongation.
Sliding Filament Theory
A theory describing muscle contraction whereby tension is developed in the myofibrils as the head of the myosin filament attaches to a site on the actin filament.
Small skeletal muscle fiber innervated by the alpha-2 motor neuron, having a slow contraction time. This fiber is highly oxidative and poorly glycolytic.
Training principle suggesting that specific training movements should be done in the same manner and position in which the movements are performed in the sport or activity.
A muscle responsible for stabilizing an adjacent segment.
A muscle shape lacking a central belly.
The maximum amount of force produced by a muscle or muscle group at a site of attachment on the skeleton; one maximal effort.
A common sequence of joint actions in which an eccentric muscle action, or prestretch, precedes a concentric muscle action.
A fibrous cord, consisting primarily of collagen, by which muscles attach to bone.
The force response of muscle to a series of excitatory inputs, resulting in a summation of twitch responses.
A parameter that is presented as a function of time.
The product of the magnitude of a force and the perpendicular distance from the line of action of the force to the axis of rotation.
T-tubule (Transverse Tubule)
Structure in the sarcolemma that facilitates rapid communication between action potentials and myofilaments in the interior of the muscle.
The force response of a muscle to a single stimulation.
A feather-shaped fiber arrangement in which the muscle fibers run diagonally off one side of the tendon.
Variable Resistive Exercise
Exercise performed on a machine that alters the amount of resistance through the range of motion.
In weight training, the sum of the number of repetitions multiplied by the load or weight lifted. Usually calculated over a week, month, or year.