Muscle contraction happens when thin actin slide over thick myosin, causing the sarcomere to shorten. During contraction, myosin heads attach to actin forming cross bridges, then pull the actin filaments inward using energy from ATP. After each power stroke, the myosin heads detach, re-cock, and reattach further along the actin, repeating the cycle. As all sarcomeres shorten together, the entire muscle fiber contracts. When the stimulation stops, calcium ions are pumped back into the sarcoplasmic reticulum, the cross bridges break, and the muscle relaxes.