Structure and Function of Skeletal Muscle

Nearly all physical rehabilitation programs involve stretching, strengthening, or retraining of muscles. As the sole producer of active force in the body, muscle is ultimately responsible for all active motions and therefore plays a fundamental role in kinesiology. Muscles also control and stabilize our posture by their actions at joints. Clinicians therefore often advocate strengthening muscles to stabilize the underlying joints, especially when structures such as ligaments have been weakened by disease or trauma. This chapter provides a basic overview of the structure and function of skeletal muscle and reviews the important features of muscle as it relates to our study of kinesiology.

Fundamental Nature of Muscle

Muscles develop active force after receiving input from the nervous system. Once stimulated, a muscle produces a contractile, or pulling, force. By pulling on bones, muscles create movement. Although not always obvious, it is important to understand that muscles act by pulling, not pushing, regardless of whether the muscle is shortening, lengthening, or remaining a constant length.

A fundamental principle of kinesiology states that when a muscle contracts, the freest (or least constrained) segment moves. This principle applies whether a muscle is pulling its distal attachment toward its proximal attachment or vice versa (Figure 3-1).

Figure 3-1 When a muscle contracts, the freest kinematic segment moves. This figure illustrates the knee extensor muscles contracting in an open and closed chain. A, The tibia (distal segment) is most free to move. B, The femur (proximal segment) is most free to move.

Consider this…

Eccentric Activation: The Lowering Force of Muscle

Eccentric activation occurs when a muscle is active but lengthening. Almost invariably, eccentric activation of a muscle is used to control the rate of descent, effectively lowering or decelerating the body or body segment in the direction of gravity. Lowering one’s self from standing to sitting, lowering an arm to one’s side, and lowering a weight to one’s chest, as during the lowering phase of a bench press, all require eccentric muscular activation.

If an action is described as “lowering,” it is almost 100% certain that the muscles controlling the action are eccentrically activated. During an eccentric activation, gravity usually powers the movement; the eccentric activation of muscle is used to decelerate the rate of descent of the body.

Isometric

Isometric activation occurs when a muscle generates an active force while remaining at a constant length (Figure 3-2, C). This occurs when the muscle generates an internal torque equal to the external torque; as a consequence, there is no motion and no change in joint angle.

Muscle Terminology

Specific terminology is commonly used when describing muscles or the actions of muscles. The following paragraphs outline some of these terms and their definitions.

The terms proximal attachment and distal attachment are used throughout this text to describe the relative points of attachment of muscle to bone. The proximal attachment, or origin, of a muscle refers to the point of attachment that is closest to the midline, or core, of the body when in the anatomic position. The distal attachment, or insertion, refers to the muscle’s point of attachment that is farthest from the midline, or core, of the body.

An agonist is a muscle or muscle group that is most directly related to performing a specific movement. For example, the quadriceps (knee extensors) are the agonists for knee extension. An antagonist, on the contrary, is the muscle or muscle group that can oppose the action or actions of the agonist. Usually, the antagonist muscle passively elongates as the agonist actively contracts. For example, when elbow flexion is performed, the biceps are considered the agonists as they perform elbow flexion. The triceps (elbow extensors), which are the antagonists of this action, passively elongate as the elbow is flexed. Therefore, an overly stiff antagonist muscle that fails to elongate can significantly limit the action of an agonist muscle.

A co-contraction occurs when agonist and antagonist muscles are simultaneously activated in a pure or near-isometric fashion. Co-contractions of muscle often stabilize and therefore protect a joint. Similarly, a muscle that fixes or holds a body segment relatively stationary so that another muscle can more effectively perform an action is referred to as a stabilizer.

Muscles that work together to perform a particular action are known as synergists; furthermore, most meaningful movements of the body involve the synergistic action of muscles. A force-couple is a type of synergistic action that occurs when two or more muscles produce force in different linear directions but produce torque in the same rotary direction. Figure 3-3 illustrates the force-couple generated by three different shoulder muscles to upwardly rotate the scapula.

Figure 3-3 A muscular force-couple producing upward rotation of the scapula. All three muscles have different lines of pull, but all assist in rotating the scapula in the same direction.

Muscles are elastic in nature and therefore are constantly being lengthened or shortened. This change in the length of a muscle is known as its excursion. Typically, a muscle can shorten or elongate only about half of its resting length. For example, a muscle that is 8 inches long at its resting length could contract to roughly 4 inches or could elongate to about 12 inches in length.

Figure 3-4 illustrates the primary functional components that constitute skeletal muscle, whereas Box 3-1 describes each of these components. A whole muscle consists of three main components, each surrounded by a particular type of connective tissue that supports its function.

Box 3-1 Functional Components of Skeletal Muscle

• Muscle belly: The muscle belly is the bulk, or body, of the muscle and is composed of numerous fasciculi.

• Surrounding connective tissue: The epimysium surrounds the outer layer, or belly, of the muscle and helps to hold the shape of a muscle.

• Fasciculus: Each fasciculus consists of a bundle of muscle fibers.

• Surrounding connective tissue: The perimysium surrounds individual fasciculi. It functions to support the fasciculi and serves as a vehicle to support the nerves and blood vessels.

• Muscle fiber: A muscle fiber is actually an individual cell with multiple nuclei. The fiber contains all contractile elements within muscle.

• Surrounding connective tissue: The endomysium surrounds each muscle fiber. It is composed of a relatively dense meshwork of collagen fibrils that help to transfer contractile force to the tendon.

• Myofibril: Each muscle fiber is composed of several myofibrils. Myofibrils contain contractile proteins, packaged within each sarcomere.

The Sarcomere: The Basic Contractile Unit of Muscle

A sarcomere is the basic contractile unit of muscle fiber. Each sarcomere is composed of two main protein filaments—actin and myosin—which are the active structures responsible for muscular contraction. The most popular model that describes muscular contraction is called the sliding filament theory. In this theory, active force is generated as actin filaments slide past the myosin filaments, resulting in contraction of an individual sarcomere.

Figure 3-5 illustrates a sarcomere and emphasizes the physical orientation of the actin and myosin filaments. The thick myosin filament contains numerous heads, which when attached to the thinner actin filaments create actin-myosin cross bridges. In essence, a myosin head is similar to a cocked spring, which on binding with an actin filament flexes and produces a power stroke. The power stroke slides the actin filament past the myosin, resulting in force generation and shortening of an individual sarcomere (Figure 3-6). Because sarcomeres are joined end to end throughout an entire muscle fiber, their simultaneous contraction shortens the entire muscle.