ATP, or adenosine triphosphate, plays a crucial role in the sliding filament theory, which explains how muscles contract. When a muscle fiber is stimulated, ATP binds to the myosin heads of the thick filaments. This binding allows the myosin heads to detach from the actin filaments, effectively breaking the cross-bridge during muscle relaxation.
After detachment, the myosin heads hydrolyze ATP to ADP (adenosine diphosphate) and inorganic phosphate. This hydrolysis provides the energy needed for the myosin heads to reposition and reattach to the actin filaments at a new site, forming a new cross-bridge. As the myosin pulls the actin filaments toward the center of the sarcomere, the muscle contracts. The cycle repeats as long as ATP is available and calcium ions are present, allowing for persistent muscle contraction.