Muscle fibers in a skeletal muscle form bundles called fascicles.  The muscle fibers in a single fascicle are parallel, but the organization of fascicles in the skeletal muscle can vary, as can the relationship between the fascicles and the associated tendon. Four patterns of fascicle organization form parallel muscles, convergent muscles, pennate muscles, and circular muscles.

Parallel Muscles

In a parallel muscle, the fascicles are parallel to the long axis of the muscle. Most of the skeletal muscles in the body are parallel muscles. Some are flat bands with broad attachments (aponeuroses) at each end; others are plump and cylindrical with tendons at one or both ends. In the latter case, the muscle is spindle-shaped (Figure 11-1a), with a central body, also known as the belly, or gaster. The biceps brachii muscle of the arm is a parallel muscle with a central body. When a parallel muscle contracts, it gets shorter and larger in diameter. You can see the bulge of the contracting biceps brachii on the anterior surface of your arm when you flex your elbow.

A skeletal muscle cell can contract until it has shortened by roughly 30 percent. Because the fibers in a parallel muscle are parallel to the long axis of the muscle, when the fibers contract together, the entire muscle shortens by the same amount. If the muscle is 10 cm long, the end of the tendon will move 3 cm when the muscle contracts. The tension developed during this contraction depends on the total number of myofibrils the muscle contains. Because the myofibrils are distributed evenly through the sarcoplasm of each cell, we can use the cross-sectional area of the resting muscle to estimate the tension. A parallel muscle 6.25 cm² (1 in.²) in cross-sectional area can develop approximately 23 kg (50 lb) of tension.

Convergent Muscles

In a convergent muscle, the muscle fibers are spread over a broad area, but all the fibers converge at one common attachment site. They may pull on a tendon, an aponeurosis (tendinous sheet), or a slender band of collagen fibers known as a raphe. The muscle fibers typically spread out, like a fan or a broad triangle, with a tendon at the apex. The prominent chest muscles of the pectoralis group have this shape (Figure 11-1b). A convergent muscle has versatility, because the stimulation of only one portion of the muscle can change the direction of pull. However, when the entire muscle contracts, the muscle fibers do not pull as hard on the attachment site as would a parallel muscle of the same size. The reason is that the convergent muscle fibers pull in different directions rather than all pulling in the same direction.

Pennate Muscles

In a pennate muscle, the fascicles form a common angle with the tendon. Because the muscle cells pull at an angle, contracting pennate muscles do not move their tendons as far as parallel muscles do. But a pennate muscle contains more muscle fibers--and, as a result, produces more tension--than does a parallel muscle of the same size. (Tension production is proportional to the number of contracting sarcomeres; the more muscle fibers, the more myofibrils and sarcomeres.)

If all the muscle fibers are on the same side of the tendon, the pennate muscle is unipennate. The extensor digitorum muscle, a forearm muscle that extends the finger joints, is unipennate (Figure 11-1c). More commonly, a pennate muscle has fibers on both sides of the tendon. Such a muscle is called bipennate. The rectus femoris muscle, a prominent muscle that extends the knee, is bipennate (Figure 11-1d). If the tendon branches within a pennate muscle, the muscle is said to be multipennate. The triangular deltoid muscle of the shoulder is multipennate (Figure 11-1e).

Circular Muscles

In a circular muscle, or sphincter, the fibers are concentrically arranged around an opening or a recess. When the muscle contracts, the diameter of the opening decreases. Circular muscles guard entrances and exits of internal passageways such as the digestive and urinary tracts. An example is the orbicularis oris muscle of the mouth (Figure 11-1f).