Human Anatomy and Physiology - N. I. Fedyukovich 2003

Muscular System
Classification of Muscles

Human Muscles are classified by shape, position on the body, fiber direction, function, relationship to joints, and other criteria (Table 3).

Class="center">Table 3 Muscle shape depending on the arrangement of muscle fibers relative to the tendon

The shape of muscles can be highly diverse, depending on the arrangement of muscle fibers relative to the tendon (Fig. 54).

Fig. 54. Muscle shapes:

A — fusiform; B — biceps muscle; C — digastric muscle; D — muscle with tendinous intersections; E — bipennate muscle; F — unipennate muscle; 1 — muscle belly; 2, 3 — muscle tendons; 4 — tendinous intersection; 5 — intermediate tendon

Fusiform muscles are the most common. In these, the fiber bundles are oriented parallel to the long axis of the muscle, and the belly, gradually narrowing, transitions into a tendon. Muscles in which the muscle fibers attach to the tendon on only one side are called unipennate, and those attaching on both sides are bipennate. Muscles can have one or more heads, hence the names: biceps, triceps, and quadriceps. Some muscle fibers are arranged circularly, forming sphincter muscles that surround the oral and anal openings, among others.

A muscle's name may reflect its shape (rhomboid, trapezius, quadratus), size (long, short, major, minor), the direction of the muscle bundles or the muscle itself (oblique, transverse), or the function it performs (flexion, extension, rotation, elevation).

The arrangement of muscles relative to joints varies, determined by their Structure and function. Muscles that act on a single joint are called monoarticular; those that span two or more joints are biarticular and multiarticular. Some muscles may originate from and attach to bones without crossing any joints (for example, the hyoid, mylohyoid, Muscles of facial expression, Muscles of the floor of the Mouth, and perineal muscles).