A researcher studies force gradation in a motor unit. Beyond recruiting additional motor units, the nervous system increases force output from an already recruited motor unit by modulating which of the following?

• A Increasing the amplitude of individual muscle fiber action potentials
• B Recruiting additional muscle fibers within the same motor unit
• C Increasing the firing frequency of the alpha motor neuron, causing temporal summation and then tetanus ✓
• D Increasing the conduction velocity of the alpha motor neuron axon

Explanation

Force gradation within a single motor unit occurs primarily through rate coding, which is increasing the firing frequency of the alpha motor neuron. At low frequencies, individual twitches are seen. As frequency increases, temporal summation occurs, followed by fused tetanus and maximal tension. Option B is incorrect because all fibers within a motor unit are innervated by the same neuron and fire together; additional fibers cannot be independently recruited within a unit.

Reference: Ganong's Review of Medical Physiology, 26th ed.

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