Final answer:
Muscle relaxation occurs when ATP binds to the myosin head, leading to detachment from actin, allowing the muscle to relax and resets the myosin for the next contraction.
Step-by-step explanation:
The release of myosin heads from actin during muscle relaxation is triggered by the binding of ATP to myosin. When a muscle relaxes, calcium ion levels drop, leading to tropomyosin re-covering the myosin binding sites on actin. ATP then binds to the myosin head, causing it to detach from the actin filament. This process is necessary for the relaxation of the muscle fibers and to prepare the myosin heads for the next contraction cycle.