Final answer:
The muscle action known as the power stroke is caused by the myosin heads forming cross-bridges with actin filaments, then bending and pulling the actin, leading to muscle contraction.
Step-by-step explanation:
The muscle that acts as a power stroke of closing against resistance is facilitated by the sliding action between actin and myosin molecules within the muscle fibers. The myosin head forms a cross-bridge with actin, which initiates the power stroke.
During this power stroke, the myosin head bends, releasing ADP and inorganic phosphate, and this movement pulls the actin filaments toward the center of the muscle sarcomere, leading to muscle contraction. Following the power stroke, ATP binds to the myosin head, causing the cross-bridge to detach and the myosin to return to its cocked position, allowing the cycle to repeat for further muscle contractions.