In the carrying out the myosin power stroke, myosin is acting as an ATPase. The free energy of ATP cleavage is used to change the conformation of the myosin head into a cocked position. When ADP and inorganic phosphate are released, the myosin head binds to actin. Think of actin and ATP as competitors for binding mysosin. When a new ATP binds the myosin head, it releases actin.