In the sliding filament theory of skeletal muscle contraction, what is the power stroke?

• A. The reuptake of ${\mathrm{Ca}}^2$ into the sarcoplasmic reticulum to end contraction.
• B. The hydrolysis of ATP that cocks the myosin head into a high-energy position without moving actin.
• C. The pivoting movement of the myosin head that pulls the actin filament toward the center of the sarcomere after the release of inorganic phosphate (Pi).
• D. The binding of ATP to the myosin head, causing myosin to detach from actin.

Answer: (C)

Verified step by step guidance

1. Understand that the sliding filament theory explains how muscle contraction occurs through interactions between actin and myosin filaments within the sarcomere.
2. Recognize that the myosin head undergoes several key steps during contraction: ATP binding, ATP hydrolysis, attachment to actin, the power stroke, and detachment.
3. Identify the power stroke as the specific step where the myosin head pivots, pulling the actin filament toward the center of the sarcomere, which shortens the muscle fiber.
4. Note that this pivoting movement happens after the myosin head releases inorganic phosphate (Pi), which triggers the conformational change necessary for the power stroke.
5. Distinguish the power stroke from other steps such as ATP hydrolysis (which cocks the myosin head), ATP binding (which causes detachment), and calcium reuptake (which ends contraction).