Muscles are identified as being voluntary muscles or involuntary muscles. The skeletal muscles are called voluntary muscles because the person controls their use. They are used to move the various parts of the body. Smooth muscles, found in the stomach and intestinal walls, vein and artery walls, and in various internal organs, are called involuntary muscles, because they are not generally controlled by the person. The cardiac muscles, or the heart muscles, are also involuntary muscles. The major functions of the muscular system are:

1. Body movement due to the contraction of skeletal muscles
2. Maintenance of posture also due to skeletal muscles
3. Respiration due to movements of the muscles of the thorax
4. Production of body heat, which is necessary for the maintenance of body temperature, as a by-product of muscle contraction
5. Communication, such as speaking and writing, which involve skeletal muscles
6. Constriction of organs and vessels, especially smoother muscles that can move solids and liquids in the digestive tract and other secretions, including urine, from organs
7. Heartbeat caused by the contraction of cardiac muscle that propels blood to all parts of the body