Each of more than 600 muscles is served by nerves. Linking muscles to the brain and spinal cord, a network of nerve circuits carries signals that direct the ebb and flow of muscular energy. Many muscles must work together to perform even the simplest jobs. Much of the muscular activity occurs outside the region of the conscious mind, as the body, through the neuromuscular network, manages its own motion. If the muscle fibers of a motor unit are stimulated enough by nerve impulses to contract at all, they contract to the maximum.
Athletes display some of the wonderful shows of force that the human body is capable of performing.
Yet such force is only possible through the arrangement of the muscles, bones and joints that make up the body's lever systems. Bones act as the levers, while joints perform as living fulcrums. Muscle, attached to bones by tendons and other connective tissue, exerts force by converting chemical energy into tension and contraction. When a muscle contracts, it shortens, in many cases pulling bone like a lever across its hinge. Muscles move and by their motions we move.