Muscle contraction takes place with Adenosine Triphosphate (ATP) acting as the source of energy. The following reaction represents what exactly happens when muscle contraction takes place Actin + Myosin + ATP -> Actin + Myosin + ADP + (PI) + Force. The proteins actin and myosin are present as a bridge in the muscle fiber; this bridge is broken in the presence of ATP molecules. The bridge, free of actin, combines with actin again and ATP is hydrolyzed in the process. When actin recombines with myosin to form the bridge again, ADP and inorganic phosphate are released. The reaction also releases a amount of force that is responsible for the muscle fiber being able to contract.
The digestion of food produces ATP which is the power source for muscle fibers. Muscle contraction is not possible without ATP.