The muscular system consists of approximately 700 muscles that belong to one of three distinct categories: skeletal, smooth or cardiac. Skeletal muscles are attached to the body's skeleton and assist it in moving. Smooth muscles are found in the internal organs and help maintain normal functions such as bladder control. Cardiac muscle is only found in the heart.

Smooth muscle is also sometimes referred to as visceral muscle. In addition to the bladder, smooth muscle is also found in the lungs, blood vessels, stomach and intestines. Muscles in the human body can be further classified as voluntary or involuntary. Voluntary muscles move when a person initiates the action. Involuntary muscles move on their own and don't require the person to control the movement. Skeletal muscles are voluntary, while smooth and cardiac muscles are involuntary.

Muscles can move in two ways. They can either contract or pull. Many muscles are set up to work in pairs. One muscle pulls the bone in one direction while its complement pulls it in the opposite direction. The largest muscle in the human body is the gluteus maximus; it's the muscle located in the hip region of the body that humans use for sitting down.