There are three types of muscles:
Two of these kinds—skeletal and smooth—are part of the musculoskeletal system.
Skeletal muscle is what most people think of as muscle, the type that can be contracted to move the various parts of the body. Skeletal muscles are bundles of contractile fibers that are organized in a regular pattern, so that under a microscope they appear as stripes (hence, they are also called striped or striated muscles). Skeletal muscles vary in their speeds of contraction. Skeletal muscles, which are responsible for posture and movement, are attached to bones and arranged in opposing groups around joints. For example, muscles that bend the elbow (biceps) are countered by muscles that straighten it (triceps). These countering movements are balanced. The balance makes movements smooth, which helps prevent damage to the musculoskeletal system. Skeletal muscles are controlled by the brain and are considered voluntary muscles because they operate with a person's conscious control. The size and strength of skeletal muscles are maintained or increased by regular exercise. In addition, growth hormone and testosterone help muscles grow in childhood and maintain their size in adulthood.
Smooth muscles control certain bodily functions that are not readily under a person's control. Smooth muscle surrounds many arteries and contracts to adjust blood flow. It surrounds the intestines and contracts to move food and feces along the digestive tract. Smooth muscle also is controlled by the brain but not voluntarily. The triggers for contracting and relaxing smooth muscles are controlled by the body's needs, so smooth muscles are considered involuntary muscle because they operate without a person's conscious control.
Cardiac muscle forms the heart and is not part of the musculoskeletal system. Like skeletal muscle, cardiac muscle has a regular pattern of fibers that also appear as stripes under a microscope. However, cardiac muscle contracts and relaxes rhythmically without a person's awareness.