Muscle contraction is believed to occur through interactions of the myosin and actin.
During muscle contraction, actin-myosin crossbridges change such that sarcomere shortening occurs. Tropomyosin, a protein closely associated with actin, modulates actin-myosin interaction. Troponin, also associated with actin, functions to block actin-myosin crossbridge formation, thus inhibiting contraction. Energy for contraction is supplied by ATP hydrolysis. During the action potential of a myocardial cell, calcium ion rushes into the cell which causes the sarcoplasmic reticulum to release a large quantity of calcium intracellularly.