Muscle is the tough, elastic tissue that makes body parts move. All animals except the simplest kinds have some type of muscle.

People use muscles to make various movements, such as walking, jumping, or throwing. Muscles also help in performing activities necessary for growth and for maintaining a strong, healthy body. For example, people use muscles in the jaw to chew food. Other muscles help move food through the stomach and intestines, and aid in digestion. Muscles in the heart and blood vessels force the blood to circulate. Muscles in the chest make breathing possible.

Muscles are found throughout the body. As a person grows, the muscles also get bigger. Muscle makes up nearly half the body weight of an adult.

Kinds of Muscles

The human body has more than 600 major muscles. About 240 of them have specific names. There are two main types of muscles: (1) skeletal muscles and (2) smooth muscles. A third kind of muscle, called cardiac muscle, has characteristics of both skeletal and smooth muscles. It is found only in the heart.

Skeletal muscles help hold the bones of the skeleton together and give the body shape. They also make the body move. Skeletal muscles make up a large part of the legs, arms, abdomen, chest, neck, and face. These muscles vary greatly in size, depending on the type of job they do. For example, eye muscles are small and fairly weak, but the muscles of the thigh are large and strong.

All muscles are made up of cells called muscle fibers. Each skeletal muscle is composed of thousands of long, cylindrical muscle fibers. When viewed under a microscope, these fibers show alternating light and dark bands called striations. For this reason, skeletal muscles are also called striated muscles. The striations occur because thick and thin filaments (strands) repeatedly overlap each other. The thick filaments consist of a protein called myosin. The thin filaments are made up chiefly of the protein actin.

Muscle fibers have a variety of other specialized parts. Each skeletal muscle fiber has many elements called nuclei. These nuclei contain growth-producing substances that repair or remake various parts of the muscle fiber as they wear out. Each muscle fiber also has thousands of tiny sausage-shaped mitochondria. These structures produce the energy that the fiber needs in order to live and do its work.

Muscle fibers are held together by connective tissue. The ends of most skeletal muscles are joined to bones by a tough, flexible connective tissue called tendon. One end of the muscle is attached to a bone that does not move when the muscle contracts (draws together). This end of the muscle is called the origin. The other end, called the insertion, is attached to a bone that moves when the muscle contracts.

When a person stands erect, many skeletal muscles contract to make the body rigid. The skeletal muscles also can make one part of the body move while another part stays stiff. Skeletal muscles act both ways because they work in pairs. One muscle of each pair is called the flexor. It bends a joint and brings a limb closer to the body. The other muscle, the extensor, does the opposite. For example, the biceps muscle in the front of the upper arm is a flexor. When this muscle contracts, the elbow bends and the forearm and hand move toward the shoulder. The triceps muscle in the back of the upper arm is an extensor. When it contracts, the elbow straightens and the forearm and hand move away from the shoulder. At the same time, the biceps relaxes so the triceps can pull it back to its original length.

Skeletal muscles contract and pull on the bones they attach to when a nerve stimulates them. They usually move voluntarily (under conscious control) and are sometimes called voluntary muscles. But skeletal muscles also may move involuntarily (without conscious control). For example, involuntary movement occurs when a person jerks his or her hand away from a hot object before thinking about doing it.

Skeletal muscles adapt to exercise in special ways, depending on how they are required to work. For example, muscles grow larger and stronger if a person lifts heavy weights for a short period of time each day. Such exercise causes