Which statement about NMES-related fatigue is true?

• A. NMES fatigues faster due to synchronous motor unit recruitment
• B. NMES fatigues slower than volitional contraction
• C. Fatigue levels are identical between NMES and volitional contraction
• D. NMES never causes fatigue

Answer: (A)

NMES fatigue occurs faster because electrical stimulation activates many motor units at the same time, rather than following the natural, asynchronous recruitment pattern used during voluntary effort. This synchronous activation recruits a larger proportion of fast-twitch, fatigue-prone fibers and imposes a high metabolic load with limited local blood flow during the stimulus. In contrast, voluntary contractions recruit motor units progressively and asynchronously, letting some fibers rest while others work, which distributes the workload and slows fatigue. So, the idea that NMES fatigues faster due to synchronous motor unit recruitment is the best explanation. The other statements don’t fit with how NMES engages muscle fibers and energy systems, and NMES can indeed cause fatigue under typical use.