The velocity of shortening in skeletal muscle follows the Hill force-velocity relationship: as load increases, velocity decreases. At zero load (Vmax), which structural event limits the maximum rate of cross-bridge cycling?

• A The rate of Ca²⁺ reuptake by SERCA into the SR, limiting the Ca²⁺ transient duration
• B The rate of actin-troponin-tropomyosin complex reformation, blocking re-binding until troponin is reset
• C The rate of ADP and Pi release from the myosin head (the power stroke step), which determines the intrinsic rate of cross-bridge cycling ✓
• D The rate of ATP synthesis by mitochondria, which must keep pace with ATPase demand at high velocities

Explanation

During the cross-bridge cycle, myosin-ADP-Pi binds actin strongly, ADP and Pi are released triggering the power stroke, and ATP then binds causing detachment. At zero load (Vmax), Pi and ADP release from the myosin head is the rate-limiting biochemical step. Fast-twitch fibres (type IIx) have a higher Vmax than slow-twitch (type I) fibres because their myosin heavy chain isoforms (MHC IIx/IIa) have faster ATPase activity and higher rates of ADP/Pi release. SERCA limits relaxation speed, not Vmax. ATP availability limits endurance performance but not instantaneous Vmax in aerobically supplied fibre.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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