Illustration showing the muscular system of the human body seen in a standing, adult male figure in classical pose (anterior view). The human body contains 630 active muscles. Muscle (red) is attached to bone by tendons (white) and other tissues. They are comprized of bundles of specialized cells that exert force by converting chemical energy into tension; movement is achieved via the contraction and relaxation of these bundles. Our bodies are equipped with three types of muscle: cardiac, smooth, and skeletal. Cardiac muscles, found in the heart, power the action that pumps blood throughout the body. "Smooth" muscles are part of the internal organs. Both cardiac and smooth muscles are involuntary muscles. The third type, "skeletal" muscles, are voluntary muscles because they are under conscious control within the nervous system. Skeletal muscles (shown here in red) comprise 23% of a woman's body weight and 40% of a man's body weight.