The force-velocity relationship in skeletal muscle describes an inverse relationship between load and velocity of shortening. Which of the following BEST explains why maximum force is generated at zero velocity (isometric contraction)?

• A During isometric contraction, muscle fibers recruit only slow-twitch units with maximal cross-bridge force
• B At zero velocity, all cross-bridges are in the strongly bound, force-generating state simultaneously, because no time is wasted on detachment cycling ✓
• C At zero velocity, titin protein springs are maximally stretched, adding passive force to active cross-bridge force
• D Isometric contractions allow maximal calcium release from the SR compared to isotonic contractions

Explanation

During isometric (zero velocity) contraction, cross-bridges cycle slowly or not at all — the actomyosin bonds form and remain in the high-force state without rapid detachment. At high shortening velocities, cross-bridges must detach rapidly and reattach, spending proportionally less time in the force-generating (attached) state — hence lower force output. This explains why muscles generate maximum force (tension) at zero velocity. The force-velocity curve has practical implications for understanding muscle power output, which peaks at approximately one-third of maximum velocity.

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

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