When ATP is used, it becomes adenosine diphosphate (ADP), and since muscles store little ATP, they must continuously replace the discharged ADP with ATP. The release of energy powers the swiveling of the myosin head. ATP binds to the cross-bridges between myosin heads and actin filaments. This process consumes large amounts of adenosine triphosphate (ATP), the energy source of the cell. This is called a ratchet type drive system. The myosin heads move in a coordinated style they swivel toward the center of the sarcomere, detach and then reattach to the nearest active site of the actin filament. Myosin filaments have club-shaped myosin heads that project toward the actin filaments, and provide attachment points on binding sites for the actin filaments. Within the sarcomere, actin and myosin fibers overlap in a contractile motion towards each other. The best proposed model for understanding contraction is the sliding filament model of muscle contraction. Upon stimulation by an action potential, skeletal muscles perform a coordinated contraction by shortening each sarcomere. Myofibrils are composed of sarcomeres, the basic building blocks of striated muscle tissue. Skeletal muscle, is a type of striated muscle, composed of muscle cells, called muscle fibers, which are in turn composed of myofibrils. See also: List of muscles of the human body
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