Muscle is attached to bone by tendons and other tissues, and exerts force by converting chemical energy into tension and contraction. Muscles move and make us capable of a variety of actions by simply contracting and becoming shorter. Muscles pull but they cannot push. Muscles are made up of millions of tiny protein filaments which work together to produce motion in the body. Each of more than 600 muscles is served by nerves which link the muscle to the brain and spinal cord. Our bodily needs demand that muscles accomplish different chores, so we are equipped with three types of muscles. Cardiac muscles, found only in the heart, power the action that pumps blood throughout the body. "Smooth" muscles surround or are part of the internal organs. Both cardiac and smooth muscles are called involuntary muscles, because they cannot be conciously controlled. The third type of muscles are called "skeletal" muscles. These carry out voluntary movements and are what ache after strenuous excercise. Skeletal muscles are the body's most abundant tissue, comprising aout 23% of a woman's body weight and about 40% of a man's body weight.