When do active sites on the actin become available for binding?

Active sites on actin become available for binding after calcium binds to troponin. This process is fundamental to muscle contraction. When calcium ions are released into the sarcoplasm, they bind to the troponin complex, which causes a conformational change in the troponin-tropomyosin complex.

This change shifts tropomyosin away from the actin filament's active sites, thus exposing them for binding with myosin heads. This interaction is crucial for muscle contraction to proceed, as the binding of myosin to actin allows for the cross-bridge cycle, which generates force and leads to muscle shortening.

While muscle fatigue, muscle relaxation, and events prior to calcium release may influence the overall contraction process, they do not directly facilitate the exposure of active sites on actin for the binding of myosin. It is the binding of calcium to troponin that specifically triggers this vital step in muscle contraction.