Learning Objective

Predict how changes in afterload affect muscle shortening velocity, distance shortened, and force production, and explain the roles of ATPase activity, preload, and muscle size in determining Vmax and maximal force.

Relationship Between Velocity and Load

The force–velocity relationship describes how the speed of muscle shortening changes with load.

• Maximum velocity (Vmax): Occurs when there is no afterload on the muscle.
• Increasing afterload: Decreases shortening velocity.
• Isometric contraction: Occurs when afterload exceeds the maximum force the muscle can generate; the muscle cannot shorten.

Key points:

• Vmax depends on ATPase activity: Muscles with higher ATPase activity shorten faster (fast vs. slow fibers).
• Maximum force: Achieved with complete summation and full motor unit recruitment.
• Absolute force: Directly proportional to muscle mass and preload, with maximal force at optimal length (Lo).

Example

• Muscle A (red) = smaller, slower.
• Muscle B (white) = larger, faster.
• As load increases, both shortening velocity and distance shortened decrease.

Activity