Zoom in. Way way in. All of your skeletal muscle contractions come down to microscopic proteins changing shape, specifically myosin. As this protein binds and detaches from ATP it changes shape and is facilitated in moving another protein called actin. This movement synchronized across all sarcomere segments in a motor unit, or part of a muscle, is what causes your muscle to contract and ultimately your body to move. If you had no ATP, would your muscles be limp or rigid?