What defines the power stroke in muscle contraction?

The power stroke in muscle contraction is primarily characterized by the binding of myosin to actin, which initiates the cross-bridge cycle. This process begins when calcium ions are released from the sarcoplasmic reticulum and bind to troponin, causing a conformational change that moves tropomyosin away from the binding sites on actin. As a result, myosin heads can attach to these exposed binding sites, facilitating muscle contraction.

During the power stroke itself, the myosin head pivots, pulling the actin filament toward the center of the sarcomere. This action is propelled by the hydrolysis of ATP. When ATP is cleaved to ADP and inorganic phosphate, the energy released from this chemical reaction is what really drives the power stroke. Therefore, while the cleaving of ATP is an important element of muscle contraction, it is not what defines the power stroke itself; instead, it provides the energy necessary for the power stroke to occur.

Recognizing the role of myosin and actin interaction is fundamental to understanding the mechanics of muscle contraction.