The muscular system is the system that controls the movement of the bones in the body. There are about 630 muscles in the human body. There are three types of muscles in the body, smooth, skeletal, and cardiac. Muscles are either voluntary or involuntary. They work together to help the bones move. Muscles use energy that they get from food.

The jobs of the muscles are to contract and relax to move the bone. When two muscles work together to move a bone, one is the flexor and the other is the extensor. The extensor is the muscle that extends or straightens a bone in the body. The flexor is the muscle that bends a bone in the body. A contraction happens by the release of calcium from inside the cell. Muscles that have exercise react to stimuli quickly and powerfully.

One of the types of...