Contraction of a muscle produces heat which is transported to other parts of the body by the blood. This action is necessary for maintaining body temperature. There are three types of muscles in the human body. Skeletal muscle is the type that is attached to the bones of the skeleton by tendons providing for movement. Smooth muscle is found in the walls of hollow organs and tubes, such as the stomach and blood vessels.
Cardiac muscle forms the walls of the heart.
Muscle activation is controlled by the nervous system, hormones, and some factors within the muscles themselves. Bones and muscles become weakened from lack of use. Muscles will atrophy causing as then to be smaller and weaker. Exercise stretches muscles placing tension on bones. It is this tension that allows bones to remain strong.