Classification of the Muscle Fibers

Classification of the Muscle Fibers

There are seven ways in which skeletal muscles are classified:

■■ Location: Muscle names may indicate bones or body regions associated with the muscles being classified. For example, the intercostal muscles that run between the ribs (costals) and the temporalis muscle that overlies the temporal bone. The ­skeletal muscles of the body are listed in TABLES 10-1–10-15, which are organized by body area.

■■ Shape: Often, muscles have distinctive shapes. For example, the right and left trapezius muscles, which form a trapezoid, and the deltoid muscle, which is basically triangular (deltoid means “triangle”).

■■ Size: Muscle names may use words that mean long (longus), short (brevis), largest (maximus), or smallest (minimus). For example, the gluteus maximus and gluteus minimus, which are the large and small gluteus muscles, respectively.

■■ Direction of fibers: Muscle names may be related to directions in which their fibers and fascicles run compared to an imaginary line. This line may reference the body’s midline or a limb bone’s longitudinal axis. The term rectus refers to “straight,” meaning that the fibers are parallel to an imaginary line (or axis). The term transverse refers to fibers running at right angles to an axis. The term oblique refers to fibers that run obliquely to an axis, which means “neither parallel nor perpendicular.” For example, the transversus abdominis of the abdomen and the rectus femoris of the thigh.

■■ Number of origins: Muscles may have different amounts of heads attached to different origins. The term biceps means “two,” triceps means “three,” and quadriceps means “four.” One example is the biceps brachii of the arm, which has two origins.

■■ Location of attachments: Some muscle names are based on points of origin and insertion, with the origin always being named first. A good example is the sternocleidomastoid muscle of the neck. It has a dual origin, on the sternum (sterno) and clavicle (cleido). This muscle inserts on the mastoid process of the temporal bone.

■■ Actions: Muscles that are named for their actions may include terms such as adductor, pronator, extensor, or flexor in their names. One example is the adductor longus, in the medial thigh, which adducts the thigh. Also, some muscles are named because of specific movements. For example, the sartorius muscle is the longest muscle in the body and becomes active when the legs are crossed.

Several criteria are often combined in order to create a muscle name. While not all muscle names are highly descriptive, some of them are such as the extensor carpi radialis longus. It is easy to see that this muscle’s action is as an extensor. It acts on the carpi or wrist joint and is located close to the radius bone of the forearm (radialis). The term longus shows that it is long in comparison to other wrist extensor muscles.