How does increasing the load affect the velocity of a muscle contraction?

• A. Increased load speeds contraction and increases shortening velocity.
• B. Increased load slows contraction or may stop it; lighter loads permit faster shortening.
• C. Load has no effect on contraction velocity.
• D. Contraction velocity increases with load until maximum speed is achieved.

Answer: (B)

The central idea is that a muscle’s shortening speed depends on the external load it has to overcome. For a contraction to shorten, the muscle’s internal force must exceed the opposing load. As the load increases, the cross-bridge cycling that generates force has to work harder to balance that load, so the rate of shortening slows down. If the load reaches the muscle’s maximum isometric force, the muscle can generate that force without shortening at all, so the velocity is essentially zero. With lighter loads, fewer cross-bridges need to stay engaged to hold and move, detachment happens more quickly, and the muscle shortens more rapidly. In short, heavier loads slow or even halt shortening, while lighter loads allow faster shortening.