The basic function of muscle is to generate force. Secondarily, muscles can provide some shape and form to the organism. Anatomically and functionally, muscle can be divided into two types, smooth and striated. Striated or striped muscle can be further divided into skeletal muscle and cardiac (heart) muscle. Regardless of the type, all muscles share the following basic properties (Gowitzke and Milner 1988):
• Conductivity: A muscle has the ability to conduct an action potential.
• Irritability: When stimulated, the muscle will react.
• Contractility: A muscle can shorten or produce tension between its ends.
• Relaxation: A muscle can return to resting properties after contraction.
• Distensibility: A muscle can be stretched by a force outside of the muscle itself. The muscle is not injured as long as it is not stretched past its physiological limits.
• Elasticity: The muscle will resist elongation and will return to its original position after passive or active elongation. Elasticity is the opposite of distensibility.
Smooth muscle and striated muscle can easily be differentiated from each other in a variety of ways, including appearance. For example, smooth muscle is uni-nucleated and contains sarcomeres (the functional units of muscle) that are arranged at oblique angles to each other; under a light microscope smooth muscle appears to be relatively featureless as a result of the orientation of its sarcomeres. On the other hand, striated muscle contains protein arrays called myofibrils that are parallel to each other and thus form striations or stripes. Cardiac muscle can be easily identified as distinct from skeletal muscle by appearance and differences in function, such as an intrinsic ability to contract. (We will not go into detail on smooth and cardiac muscle because though interesting, such discussion is not within the scope of this book.)
Skeletal muscle is found in many sizes and various shapes. The small muscles of the eye may contain only a few hundred cells, while the vastus lateralis may contain hundreds of thousands of muscle cells. The shape of muscle is dependent on its general architecture, which in turn helps to define the muscle’s function. Some muscles, such as the gluteal muscles, are quite thick; some, such as the sartorius, are long and relatively slender; and others, such as the extensors of the fingers, have very long tendons. These differences in muscle shape and architecture permit skeletal muscle to function effectively over a relatively wide range of tasks.
For example, thicker muscles with a large cross-sectional area can produce great amounts of force; longer muscles can contract over a greater distance and develop higher velocities of shortening; muscles with long tendons can form pulley arrangements that allow large external movement (e.g., grasping by the fingers) with relatively small movement of the muscles and tendons. Some long slender muscles such as the sartorius and biceps femoris are divided by transverse fibrous bands that form distinct sections or compartments (McComas 1996). Although fibers were previously believed to run the length of these muscles, because of these compartments the longest possible human muscle fiber is about 12 cm (4.7 in.) in length (McComas 1996). The individual compartments can have different fiber type distributions and different cross-sectional areas (English and Ledbetter 1982). Each compartment has a separate innervation; however, individual motor neurons often innervate muscle fibers in adjacent compartments. But the functional outcomes of compartmentalization are not completely understood. One possible consequence of compartmentalization is that it could ensure that contraction occurs relatively synchronously and rapidly along the muscle belly. However, it is also possible to recruit compartments separately (English 1984).
Muscle fibers can be arranged into two basic structural patterns, fusiform and pinnate (also spelled pennate). Most human muscles are fusiform, with the fibers largely arranged in parallel arrays along the muscle’s longitudinal axis. In many of the larger muscles the fibers are inserted obliquely into the tendon, and this arrangement resembles a feather (i.e., pinnation). The fibers in a pinnate muscle are typically shorter than those of a fusiform muscle. The arrangement of pinnate muscle fibers can be single or double, as in muscles of the forearm, or multipinnate, as in the gluteus maximus or deltoid (figure 2.1).