Which statement best describes why NMES can be more fatiguing than voluntary contraction?

• A. It recruits only sensory fibers.
• B. It recruits slow-twitch fibers first.
• C. It may fire fibers in a non-physiologic order, rapidly fatiguing large, fast-fatigable motor units.
• D. It reduces fatigue by recruiting only slow-twitch fibers.

Answer: (C)

Electrical stimulation can activate motor units in a non-physiologic order, which makes NMES contractions more fatiguing than voluntary contractions. In voluntary effort, the nervous system follows the size principle, recruiting small, fatigue-resistant units first and only later engaging larger, fast-fatigable ones as needed. NMES, however, tends to depolarize larger-diameter axons more readily, so fast-fatigable motor units are recruited early and often in a synchronized manner. These fibers generate high force quickly but fatigue quickly, leading to faster overall fatigue during NMES.

Other statements don’t fit because NMES mainly stimulates motor fibers, not just sensory ones; it does not preferentially recruit slow-twitch fibers first; and it does not reduce fatigue by recruiting only slow-twitch fibers.