Cross bridge is the binding of myosin heads with exposed actin binding sites, and the power stroke occur by myosin heads to drag the actin filaments along with it. This happens when calcium ions move troponin-tropomysin complex from the actin binding sites. To release Myosin head fron actin binding site, an ATP molecule binds to myosin head and split in to ADP and a phosphate molecule. This releases energy causing myosin in the cocked crossbridge position, making it ready for a power stroke with actin to release ADP and a phosphate molecule. The cycle begins again when a new ATP molecule bind to myosin head.