When using electrical stimulation to contract innervated muscles, which nerve fibers are activated FIRST?

• A. Small diameter nerve fibers
• B. Large diameter nerve fibers
• C. Nerve fibers that innervate the slow-twitch muscle fibers
• D. Nerve fibers that innervate the type I muscle fibers

Answer: (D)

When electrical stimulation is applied to contract innervated muscles, the fibers that have the largest diameter are activated first. These large-diameter, myelinated motor fibers reach the activation threshold with the applied current more readily than smaller fibers, so they respond earlier. They innervate fast-twitch (Type II) muscle fibers, which tend to fatigue faster. This recruitment pattern contrasts with voluntary contraction, where smaller, slow-twitch (Type I) units are typically recruited first. So the first nerve fibers to respond to electrical stimulation are the large diameter ones.