Which fiber type is recruited for volitional contraction versus NMES?

Which fiber type is recruited for volitional contraction versus NMES?

• A. Volitional: Type I; NMES: Type II
• B. Volitional: Type II; NMES: Type I
• C. Volitional: Type II; NMES: Type II
• D. Volitional: Type I; NMES: Type I

Answer: (A)

Voluntary contractions use the size principle: the smallest, most fatigue-resistant motor units are recruited first. That means Type I slow-twitch fibers are activated first to produce smooth, sustained force, with Type II fibers added only as more force is needed. In contrast, NMES tends to depolarize nerves in a way that favors larger-diameter fibers, so Type II motor units are recruited first (often in a synchronized burst), producing stronger but less fatigue-resistant contractions. This makes the best pairing: voluntary recruitment of Type I fibers and NMES recruitment of Type II fibers.

Voluntary contractions use the size principle: the smallest, most fatigue-resistant motor units are recruited first. That means Type I slow-twitch fibers are activated first to produce smooth, sustained force, with Type II fibers added only as more force is needed. In contrast, NMES tends to depolarize nerves in a way that favors larger-diameter fibers, so Type II motor units are recruited first (often in a synchronized burst), producing stronger but less fatigue-resistant contractions. This makes the best pairing: voluntary recruitment of Type I fibers and NMES recruitment of Type II fibers.