voluntary muscle (skeletal, striped, or striated muscle) Muscle that is under the control of the will and is generally attached to the skeleton. An individual muscle consists of bundles of long muscle fibres, each bounded by a sarcolemma and containing sarcoplasm, sarcoplasmic reticulum, and many nuclei. The whole muscle is covered with a strong connective tissue sheath (epimysium) and attached at each end to a bone by inextensible tendons. Running through each fibre are smaller fibres (myofibrils) having alternate light and dark bands, which contain protein filaments responsible for the muscle's contractile ability and give the muscle its typical striped appearance under the microscope. The functional unit of a myofibril is the sarcomere. See illustration.

The end of the muscle that is attached to a nonmoving bone is called the origin of the muscle; the end attached to a moving bone is the insertion. As a muscle contracts it becomes shorter and fatter, moving one bone closer to the other. Since a muscle cannot expand, another muscle (the extensor) is required to move the bone in the opposite direction and stretch the first muscle (known as the flexor). The flexor and extensor are described as antagonistic muscles. See illustration.

voluntary muscle (skeletal muscle) In human beings and other mammals, the most plentiful of the three types of muscle comprising the bulk of the body. It is under conscious control and has a striped appearance under a microscope. See also involuntary muscle

voluntary muscle n. see striated muscle.

voluntary muscle See STRIATED MUSCLE.