What is the typical recruitment order during NMES compared to voluntary contraction?

• A. NMES recruits smaller fatigue-resistant units first; voluntary recruits larger fast-fatigable units
• B. NMES recruits motor nerves first; voluntary recruits sensory nerves first
• C. NMES and voluntary recruit in the same order
• D. NMES recruits larger fast-fatigable units first; voluntary recruits smaller fatigue-resistant units

Answer: (D)

The key idea is how motor units are activated. In voluntary effort, the nervous system follows the size principle: small, fatigue-resistant motor units (slow-twitch) are recruited first, and only as more force is needed are larger, fast-fatigable units recruited. Electrical stimulation changes that order. When NMES is used, the external current tends to activate larger-diameter axons more readily, so larger, fast-fatigable motor units are recruited first. They produce more force quickly but fatigue faster, which is why NMES often leads to quicker fatigue than voluntary contractions. So the typical recruitment pattern is larger fast-fatigable units first with NMES, while voluntary contraction starts with smaller fatigue-resistant units.