In the steps of skeletal muscle contraction, what best describes the power stroke?

• A. ATP binds to myosin, causing the myosin head to attach tightly to actin and form a cross-bridge.
• B. The myosin head pivots and pulls the actin filament toward the center of the sarcomere after releasing inorganic phosphate ($Pi$).
• C. Calcium ions bind to actin, causing tropomyosin to cover the myosin-binding sites on actin.
• D. ADP and ATP are hydrolyzed, causing the sarcomere to lengthen as actin slides away from the M line.

Answer: (B)

Verified step by step guidance

1. Understand that the power stroke is a key event in the sliding filament theory of muscle contraction, where the myosin head pulls the actin filament to shorten the sarcomere.
2. Recall that before the power stroke, the myosin head is energized by hydrolyzing ATP into ADP and inorganic phosphate (P\_i), which positions the myosin head in a 'cocked' state ready to bind to actin.
3. Recognize that the power stroke occurs after the myosin head binds tightly to actin and releases the inorganic phosphate (P\_i), triggering the pivoting motion of the myosin head.
4. Note that during the power stroke, the myosin head pivots and pulls the actin filament toward the center of the sarcomere (the M line), which shortens the muscle fiber.
5. Understand that after the power stroke, ADP is released from the myosin head, and a new ATP molecule must bind to detach the myosin head from actin and reset the cycle.