Monophasic or DC waveform is generally avoided for muscle contraction except for which condition?

• A. Denervated muscle
• B. Acute fracture
• C. Superficial skin lesions
• D. Hypertonia

Answer: (D)

Monophasic or DC stimulation is avoided for muscle contractions because the constant unidirectional current can cause strong skin irritation, electrochemical changes at the electrode–skin interface, and patient discomfort. It also tends to produce less comfortable, more fatigue-prone contractions when used over intact nerves.

The one exception is when the muscle is denervated. In denervated muscle, there is no functioning motor nerve to depolarize, so you must directly excite the muscle fibers themselves. Direct muscle fiber excitability requires longer pulse durations and typically a monophasic (DC-like) waveform to deliver sufficient charge to reach the muscle membrane threshold. Under these conditions, the goal is direct muscle stimulation rather than nerve-driven contraction, so the use of monophasic DC is appropriate despite the usual cautions.

So, the reason this waveform is special is that, with denervation, the pathway to contraction changes from the nerve to the muscle fiber itself, demanding a different electrical approach than is used for innervated muscles.