What mechanism increases the total force produced by a muscle during graded contraction?

• A. Increasing firing rate of a single motor neuron.
• B. Recruitment of additional motor units to activate more muscle fibers.
• C. Decreasing calcium release in the sarcoplasmic reticulum.
• D. Isometric contraction only.

Answer: (B)

The amount of force a muscle can produce in a graded contraction mainly rises by bringing more muscle fibers into the contraction through recruiting additional motor units. Each motor unit (a motor neuron plus all the fibers it innervates) contributes a certain amount of force when it fires. The total muscle force is the sum of the forces from all active motor units, so activating more units increases the overall tension smoothly as needed. The nervous system follows the size principle, first recruiting smaller, weaker units and then larger ones as more force is required, which allows fine control and a graded increase in force. While increasing the firing rate of a single motor neuron can boost force from its own fibers (rate coding), it doesn’t raise the total force as effectively as adding more motor units. Decreasing calcium release would weaken contraction, and isometric contraction is a type of contraction, not a mechanism for increasing force during graded contraction.