Why may NMES be more fatiguing than voluntary contraction?

• A. Electrical stimulation recruits large, fast-fatigable motor units quickly and may fire fibers in a non-physiologic order with less central pacing, leading to faster fatigue.
• B. NMES recruits slow-twitch fibers only, avoiding fatigue.
• C. NMES uses lower intensity than voluntary; fatigue is unlikely.
• D. Fatigue is worse due to higher blood flow.

Answer: (B)

The key idea is that NMES often creates contractions in a way the body doesn’t do naturally. Electrical stimulation tends to recruit motor units in a non-physiologic order, typically pulling in large-diameter, fast-fatigable fibers early and firing many of them together. This is different from voluntary effort, where the nervous system follows the size principle and distributes the work across fatigue-resistant, smaller units first, with asynchronous firing that spreads the load over time.

Because NMES activates these fast-fatigable fibers and does so more synchronously and at higher intensities, they fatigue quickly and metabolic byproducts build up faster, leading to quicker fatigue of the muscle. It’s not about recruiting only slow-twitch fibers; NMES does not selectively spare fatigue-prone fibers—the opposite tends to happen.

The idea that higher blood flow would worsen fatigue isn’t how fatigue from NMES arises; better perfusion actually helps sustain muscle work, so that factor wouldn’t explain greater fatigue with NMES.