To stimulate denervated muscle, which parameter must be increased?

• A. Phase duration longer than 1 ms
• B. Frequency
• C. Amplitude
• D. Pulse width shorter

Answer: (A)

When a muscle is denervated, there is no functioning motor nerve to trigger contraction, so the stimulus must directly excite the muscle membrane. Direct excitation of denervated muscle fibers requires delivering enough charge over a longer period, i.e., a longer pulse width. Increasing the phase duration to longer than about 1 ms provides that longer, more penetrating stimulus that can depolarize the muscle fiber itself and produce a contraction. Shorter pulses won’t reach the muscle membrane threshold, and simply increasing frequency won’t compensate because there’s no nerve impulse to tetanize. Amplitude can help, but without a long enough pulse duration, the muscle still won’t respond.