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Force–Velocity Relationship

Shortening velocities are dependent on several independent factors. As discussed earlier, shortening velocity is proportional to fiber length. Shortening velocity also is dependent on the load being placed on the muscle.

Classic studies of muscle physiology have shown that the velocity of muscle shortening is related to the load being moved by the muscle. The rate of myosin ATPase induced cross-bridge flexion between thick and thin filaments is reduced as the stress placed on the cross-bridges increases. Heavier loads increase the amount of time that is required for cross-bridge cycling, thereby reducing the shortening velocity of the muscle.

As the load increases, the velocity of shortening approaches zero, and the isometric force being generated approaches a maximum. Conversely, an unloaded muscle can generate maximum shortening velocity. Again, the maximum shortening velocity of any given muscle is dependent on the fiber type and myosin ATPase that are unique to that muscle.

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