The striated muscle cells, which comprise about 40% of the body weight, are voluntary. They are mostly attached to the bones to move the skeleton and are fast acting and powerful. The voluntary muscles are of three series: those more or less arranged around the axial skeleton (head, neck, and trunk), and those nonsegmentally arranged around the appendicular skeleton (arms and legs), and those associated with the visceral skeleton (brachiometric muscles). All muscles have basically the same structure. Each muscle has an attachment at both ends, called the origin and insertion, and a fleshy contractile part, called the muscle belly. The point of origin is the point of attachment where the muscle is anchored to the bone. The point of insertion is the attachment of the muscle to the bone it moves. These muscles are attached either directly or indirectly (via ~tendons~) to the bones, and work in opposing pairs (one muscle in the pair contracts, while the other relaxes) to produce body movements (The muscles work together to produce movement of a joint, to steady a joint, and to prevent movement in the direction opposite to those intended.). As a rule, only the insertion bone moves. The shortening of the muscle as it contracts pulls the insertion bone toward the origin bone. The origin bone stays put, holding firm while the insertion bone moves toward it. These muscles always tire with continued use and require rest.
Because of their cross-striped appearance under a microscope, these muscles are called striated. There are two types of striated muscle: dark |fibers| and light |fibers|. The dark |fibers| are a deep red color and predominantly produce slow, tonic movement. The light |fibers| are lighter in color and predominantly produce quick and contracted motions. Each muscle |fiber| is encased in a thin, transparent membrane called the sarcolemma. The |fibers| are subdivided longitudinally into minute fibrils and myofibrils encased in a fluid called sacroplasm. The muscle cells are elongated tubular structures with as many as several hundred nuclei and are actually fusions of cells (syncytia). The muscles are bound together in bundles of white |fibrous connective tissue| called perimysia. Striated muscles not directly under voluntary control include vocal cord muscles and the |diaphragm|.
