Stimulation of the muscle causes calcium to be released into the cell. This binds with the actin filaments causing them to expose actives sites to the myosin cross bridges. The cross bridges bind to the active sites, forming a new molecular structure which causes the cross bridge to bend toward the center, pulling the actin filament with it. Energy from ATP is used to break the bond and allow the cross bridge to form a new bond with another active site further down the actin filament. This cycle continues until the msucle contraction is complete. Then ATP is used to cause active transport to move the calcium ions out of the muscle fiber causing relaxation of the muscle.