Many myosin molecules can make muscles get shorter using the energy released from breaking apart ATP molecules into ADP molecules and a phosphate group. The power stroke happens when a phosphate that n broken off gets released from the myosin. This makes the myosin change shape so that it pulls against the actin. When the ADP molecule is released and a new ATP molecule joins onto the myosin head, the head releases from the actin. The myosin breaks the new ATP and the cycle keeps going. The combined effect of all the power strokes from all the different myosin molecules makes the muscle get shorter.