The muscle consists of muscle fibers and connective tissue (Fig. 11-1). There are three layers of connective tissue: the epimysium, the perimysium, and the endomysium. The epimysium surrounds the muscle and (along with the perimysium and the endomysium) is continuous with the muscle’s tendons. Within the muscle, groups of muscle fibers (known as a fasciculus) are surrounded by the perimysium. Within each fasciculus, each individual muscle fiber is surrounded by the endomysium.

Fig. 11-1 Macro- and microstructure of human skeletal muscle. A, Skeletal muscle organ, composed of bundles of contractile muscle fibers held together by connective tissue. B, Greater magnification of single fiber showing small fibers, myofibrils, in the sarcoplasm. C, Myofibril magnified further to show sarcomere between successive Z lines. Cross striae are visible. D, Molecular structure of myofibril showing thick myofilaments and thin myofilaments. (From Cameron MH, Monroe LG: *Physical rehabilitation: evidence-based examination, evaluation, and intervention*, St Louis, 2006, Saunders.)

A muscle fiber (muscle cell) is further divided into myofibrils and myofilaments (see Fig. 11-1). Each muscle fiber is surrounded by a sarcolemma, the muscle fiber’s cellular membrane. Within the muscle fiber, the myofibrils are contained within the sarcoplasm. The myofibrils are bundles of the contractile proteins myosin and actin grouped together as a sarcomere (the basic functional unit of the myofibril). The interaction between the myosin and actin during the muscle’s contraction contributes to functional movement. The structural relationship between the contractile and noncontractile components allows skeletal muscles to perform both concentric (shortening) and eccentric (lengthening) muscle contractions as well as the ability to return to a resting position.¹⁸

Muscle Fiber Types

Several types of muscle fibers have been identified in skeletal muscle. The differences between muscle fibers are based on the shape of the fiber and its function. There are two general categories of muscle fiber types: fast twitch (FT) and slow twitch (ST). The “twitch” term relates to the time it takes muscle fibers (and its associated nerve) to contract and relax.

ST muscle fibers are also known as type I muscle fibers. The type I fibers appear red because of the presence of myoglobin, an oxygen-binding protein. This is an important feature of the type I muscle fiber because it is specifically adapted for continuous aerobic activity; therefore the type I muscle fibers are generally fatigue resistant. Examples of muscle groups consisting of a high number of type I fibers are the soleus and the erector spinae, postural muscles that must contract either repetitively or require being held for long durations.

Whereas the type I (ST) muscle fibers possess a high endurance capacity, the type II (FT) muscle fibers differ in that they are specialized for more anaerobic activities. There are several types of FT fibers that are differentiated by their resistance to fatigue, their functional use, and the type of myosin heavy chain (MHC) protein complex present (Table 11-1).²⁷

Table 11-1

Summary of Physical and Physiologic Differences Among the Three Most Common Skeletal Muscle Fiber Types

Parameter	Type I	Type IIA	Type IIB
Cell body size	Small	Medium	Large
Conduction velocity	Slow	Fast	Fast
Number of muscle fibers	Few	More	Most
Rate of force development	Slow	Moderate	Fast
Absolute force generation	Low	Moderate	High
Resistance to fatigue	High	Moderate	Low
Mitochondrial density	High	High	Low
Aerobic capacity	High	Moderate	Low
Myoglobin content	High	Low	Low

Within the literature, there are at least eight types of named muscle fibers: I, IC, IIA, IIB, IIC, IID, IIX (IIB), and IIAX.7,9,12,15,27,30,31,35 However, this does not mean that there are eight distinct fiber types. Rather, there appears to be an inconsistency in the literature as to how the fibers are named.^{7,9,12,15,27,30,31,35} While the PTA can appreciate that there is variability among sources about the actual number of different fiber types, it is more important to appreciate the two broad categories of muscle fibers: type I and type II. Type I are the slow, oxidative fibers and type II are the fast, glycolytic fibers. There are significant differences between the two main types that have functional implications. The exercise prescription strategy used during different phases of the healing process should take into account the primary fiber composition of the injured skeletal muscle. The characteristics of the three most common fiber types in human skeletal muscle characteristics—types I, IIA, and IIB—will be compared in this chapter (see Table 11-1).^7,31,35

Microstructure of Tendon

Tendons serve two functions: to facilitate movement or joint stability by transmitting forces between muscle and bone, and to store energy for later movement. An example of this is the energy created during plyometric exercises. The series elastic component stores elastic energy created during plyometric exercises in response to an eccentric stretch of the muscle. Tendons have the ability to withstand significant physiologic loads; however, they may be at risk of injury by either trauma, degeneration, or overuse. A tendon’s structure is divided into three locations: the muscle–tendon junction, the midsubstance, and the bone–tendon junction.

Tendons are dense connective tissue consisting of fibroblasts (cells) and an extracellular matrix composed of collagen fibers, elastin, and ground substance (Fig. 11-2). The fibroblasts are responsible for synthesizing components of the extracellular matrix.²⁸ The ground substance provides support to collagen fibers.²⁸ The collagen fibers, which are oriented in a parallel arrangement, provide strength to resist tensile forces.

Fig. 11-2 Macro- and microstructure of tendon. (From Brinker MR, Miller DS, eds: *Fundamentals of orthopaedics*, Philadelphia, 1999, Saunders.)

The structural hierarchy of tendons is similar to skeletal muscle. Collagen fibrils are grouped together into the tendon’s basic functional unit, the collagen fiber. Groups of collagen fibers (primary fiber bundle) form a subfascicle.¹⁶ Several subfascicles grouped together form a fascicle (secondary fiber bundle).¹⁶ Several fascicles grouped together form a tertiary bundle.¹⁶ These three groups (bundles) are surrounded by a connective tissue called the endotenon.¹⁶ The epitenon surrounds the entire tendon as a fine, loose connective tissue sheath.^16,28 Blood vessels and nerves are also contained within the epitenon.²⁸ The paratenon is a loose areolar connective tissue attaching to the outer portion of the epitenon.^16,28 The function of the paratenon is to allow gliding over adjacent structures.¹⁶

The transition from tendon into its osseous attachment site includes several connective tissue changes.¹⁶ The first change is from the primary tendon structure into fibrocartilage. Next, the fibrocartilage converts into a mineralized fibrocartilage. The mineralized fibrocartilage secures the attachment of the tendon to the bone. In addition to this arrangement, there are collagen fibers from the tendon that pass through these transition zones and attach to the bone. These fibers are known as Sharpey’s fibers.

MUSCLE AND TENDON INJURIES

The human body experiences a range of physical loads and stresses during activities, work, and sports. These forces on the body, whether they are a onetime supraphysiologic load that is greater than a tissue’s tolerance or a subfailure load that is experienced for a prolonged period of time, may contribute to a musculoskeletal injury.

Examples from the world of sports will help illustrate the significant loads and stresses that may be experienced by some athletes. During the deceleration phase of pitching a baseball, the pitcher’s shoulder rotates up to 7000° per second with 400 newtons (N) of posterior shear forces, 300 N of inferior shear forces, and 1000 N of compressive forces experienced at the glenohumeral joint (Fig. 11-3, A).²⁰ Over time, these shear and compressive forces may increase the baseball pitcher’s risk of sustaining a shoulder injury.^20,21,34 During the golf swing, a golfer may experience compressive loads in the lumbar spine that are eight times his or her body weight (Fig. 11-3, B).¹¹ To provide some perspective, a distance runner will experience compressive loads that are equal to three times his or her body weight during the run. To reduce the risk of injury, muscles and tendons must possess the ability to stretch or deform in response to the applied load. When an applied load supersedes the muscle or tendon’s ability to resist tensile forces, the muscle or tendon is at a higher risk of injury.