How is ATP used in muscles?

ATP provides the energy required for muscles to contract. 

Muscle contraction occurs when a thin actin filament slides over a thick myosin filament and attaches to a myosin head, which pulls the actin filament toward the center of the sarcomere, shortening the sarcomere and generating tension. ATP provides the energy that’s necessary for pulling back the myosin head, by hydrolyzing to ADP and phosphate. This interaction detaches the cross-bridge between actin and myosin, freeing the myosin for the next contraction. This detachment cannot occur without the energy provided by ATP and muscles would remain in a contracted state indefinitely.