The event referred to as the power stroke occurs during skeletal muscle contraction when the myosin heads pull the actin filaments toward the center of the sarcomere. This action is critical for muscle shortening and force generation.

- The power stroke is initiated after the myosin head binds strongly to the actin filament, forming a cross-bridge.

- During the power stroke, the myosin head pivots, pulling the actin filament inward, which shortens the sarcomere and produces muscle contraction.

- This movement is powered by the release of inorganic phosphate (Pi) from the myosin head after ATP hydrolysis.

- The detachment of myosin from actin (Option 2) occurs after ATP binds to the myosin head and is not part of the power stroke itself.

- The binding of calcium ions to troponin (Option 3) triggers the exposure of actin binding sites but does not represent the mechanical event of the power stroke.

- Hydrolysis of ATP to ADP and Pi on the myosin head (Option 4) provides the energy to "cock" the myosin head but is a preparatory step rather than the power stroke.

In summary, the power stroke is the actual force-generating step where the myosin heads pull the actin filaments, resulting in muscle contraction.

Reference: Guyton and Hall Textbook of Medical Physiology, 14th Edition, Muscle Physiology, Chapter 12