Muscle cramps are caused when the muscle takes longer than normal to relax after a contraction. During contraction, the myosin heads attach on their binding sites on the actin filament and pulls on the filament hence shortening the sarcomeres. For the myosin head to release its binding site on the actin filament, ATP attaches to an active site on the myosin and is hydrolyzed to ADP and Pi. This causes relaxation after a contraction cycle. Cramps, therefore, may be due to depletion of ATP molecule in the muscles cells.
Another reason for cramps may be a high amount of calcium in the muscles that bind to troponin on the actin filaments. This exposes the myosin binding sites longer hence promotes the formation of cross-bridge even when the contraction of the muscle is not intended anymore.