This process slides the actin filament along the myosin filament and is known as the sliding filament theory of muscle contraction. It was initially proposed by A.F. Huxley in 1957. Oddly enough, stopping the process of muscle contraction also requires energy. The saying 'it takes energy to relax', is certainly true for skeletal muscle. It is important to remember that the above scenario applies for groups of individual muscle fibers which, with their motor neuron, are called motor units.
Most movements require only a fraction of the total power available from an entire muscle.
Consequently, our motor system grades the intensity of muscle contraction by recruiting various numbers of motor units. Once muscle contraction starts the regeneration of ATP must occur rapidly. Energy from ATP derives from cleaving of the terminal phosphate of the ATP molecule. The resulting molecule is called ADP, adenosine diphosphate. However during short, high intensity contractions, CP serves as the major source of energy.