Supination NeutralPronation

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Figure 3.5. Frontal plane view of rear-foot motion in the first half of the stance phase of running. The foot lands in a supinated position. The motion of the foot and ankle to accommodate to the surface and absorb shock is called pronation.

journal space but places a burden on the kinesiology professional to be knowledgeable about variations in descriptive terminology.

Review of Muscle Structure

The anatomical structure and microstructure of skeletal muscle has considerable functional importance. We will see later that the function of the complex structures of skeletal muscle can be easily modeled as coming from active and passive sources. This section will review a few of the structural components of skeletal muscle that are believed to be important in these active and passive tissue properties.

Careful dissection of skeletal muscle shows that muscles are composed of many distinct bundles of muscle fibers called fascicles. In cutting across a piece of beef or chicken you may have noticed the tissue is in small bundles. The connective tissue sheath that surrounds the whole muscle, bundling the fascicles together, is called epimysium (meaning over/above the muscle). Each fascicle is covered by connective tissue called perimysium, meaning "around the muscle." There are hundreds of muscle fibers within a fascicle, and an individual fiber is essentially a muscle cell. Muscle fibers are also covered with connective tissue called endomysium (within the muscle). The gradual blending of these connective tissue components of muscle forms a distinct tendon or fuses with the calcified connective tissue, the periosteum of bones. A schematic of the macrostructure of skeletal muscle is shown in Figure 3.6.

The specific arrangement of fascicles has a dramatic effect on the force and range-of-motion capability of the muscle

Figure 3.6. The macroscopic structure of muscle includes several layers of connective tissue and bundles of muscle fibers called fascicles. Muscle fibers (cells) are multinucleated and composed of many myofibrils.
Pinnate Muscle

Pennate

Unipennate

Bipennate

Figure 3.7. (a) Parallel arrangement of muscle fibers with the tendon favors range of motion over force. (b) Pennate arrangement of fibers are angled into the tendon and create greater force but less range of motion.

(Lieber & Friden, 2000). Anatomically, this fiber arrangement has been classified as either parallel or pennate. A parallel arrangement means that the muscle fascicles are aligned parallel to the long axis or line of pull of the muscle. Muscles like the rectus abdominis, sartorius, and biceps brachii have predominantly a parallel architecture (Figure 3.7a). Pennate muscles have fibers aligned at a small angle (usually less than 15°) to a tendon or aponeurosis running along the long axis of the muscle. An aponeurosis is a distinct connective tissue band within a muscle. This arrangement is called pennate because of the feathered appearance. The tibialis posterior and semimembranosus are primarily unipennate, while rectus femoris and gastrocnemius are bipennate (Figure 3.7b). An example of a multipennate muscle is the deltoid.

Muscles with parallel architecture favor range of motion over force development. The greater muscle excursion and velocity of parallel muscles comes from the greater number of sarcomeres aligned in se ries. The rectus abdominis can shorten from 1/3 to 1/2 of its length because of the parallel arrangement of fibers and fascicles. Small muscles may have a simple parallel design with fibers that run the length of the muscle, while larger parallel muscles have fibers aligned in series or end to end. These end-to-end connections and transverse connections within muscles make force transmission in muscle quite complex (Patel & Lieber, 1997; Sheard, 2000). Fiber architecture also interacts with the connective tissue within muscle to affect force or fiber shortening. The fibers in the center of the biceps do not shorten uniformly due to differences in the distal and proximal aponeurosis (Pappas, Asakawa, Delp, Zajac, & Draceet, 2002). The amount of tendon a muscle has and the ratio of tendon to fibers also affects the force and range-of-motion potential of a muscle.

In essence, pennate muscles can create a greater tension because of a greater physiological cross-sectional area per anatomical cross-sectional area, but have less range of shortening than a muscle with a parallel architecture. Physiological cross-sectional area is the total area of the muscle at right angles to the muscle fibers.

Muscle fibers are some of the largest cells in the body and are long cylindrical structures with multiple nuclei. A typical muscle cell is between 10 and 100 pm in diameter. The lengths of muscle fibers varies widely from a few centimeters to 30 cm long. Besides many nuclei there are hundreds to thousands of smaller protein filaments called myofibrils in every muscle fiber. If a muscle cell were to be imagined as a cylindrical straw dispenser, the myofibrils would be like the straws packed in this dispenser. Figure 3.8 illustrates the microstructure of a muscle fiber.

The microstructure of a muscle becomes even more fascinating and complex as you pull out a straw (myofibril), only to notice that there are even smaller threads or cylindrical structures within a myofibril. These many smaller fibers within each my-ofibril are all well organized and aligned with other adjacent myofibrils in a fiber. This is why looking at skeletal muscle under a light microscope gives the appearance of a consistent pattern of dark and light bands. This is how skeletal muscle came to be called striated muscle (Figure 3.8). These small sections of a myofibril between two Z lines (thin dark band) are called sarcomeres. Sarcomeres are the basic contractile structures of muscle.

Biomechanists model the active tension of whole muscles based on the behavior of the interaction of two contractile proteins in sarcomeres: actin and myosin. Actin is the thin protein filaments within the sarcomeres of a myofibril, and myosin the thicker protein filaments. Cross-bridges between myosin and actin are attached and detached with the chemical energy stored in adenosine triphosphate (ATP). You may be familiar with the names of the various zones (Z line, A band, and I band) and other substructures of a sarcomere.

While most biomechanists use simple models of the active tension of whole muscles, some biomechanists are interested in researching the mechanical behavior of the microstructures of myofibrils to increase our understanding of where active and passive forces originate. Considerable research is being done to understand muscle actions

Figure 3.8. The microscopic structure of myofibril components of muscle fibers. Schematics of the sarcomere, as well as of the actin and myosin filaments are illustrated.

at this microscopic level from variations in myosin isoforms (Lutz & Lieber, 1999) to force transmission throughout the muscle fiber and muscle (Patel & Lieber, 1997; Sheard, 2000). Some muscle injuries could be due to this complex force production behavior and to nonuniform stresses in the sarcomeres of fibers (Morgan, Whitehead, Wise, Gregory, & Proske, 2000; Talbot & Morgan, 1996).

Many kinesiology students are familiar with muscular hypertrophy (increased muscle fiber diameter as a result of training), but they are unaware that chronic elongation of muscles (like in stretching) increases the number of sarcomeres in series within muscle fibers to increase their functional range of motion (Cox et al., 2000; Williams & Goldspink, 1978). The number of sarcomeres and muscle fiber length are adaptable and strongly related to muscle performance (Burkholder, Fingado, Baron, & Lieber, 1994).

It is clear that biomechanics plays a role in understanding the functional significance of the gross and microstructural factors of the muscletendon unit. Most general concepts related to human movement, like muscular strength or range of motion, have many biomechanical factors and levels of structure that interact to determine how the concept actually affects movement. This is our first example of the paradox of learning: the more you know, the more you know what you don't know. Now that we have reviewed some of the major structural factors that affect muscle force and range of motion, let's define the kinds of actions muscles have.

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