In NMES, which parameter primarily controls contraction strength?

• A. Pulse duration
• B. Amplitude (current intensity)
• C. Carrier frequency
• D. Electrode size

Answer: (D)

In NMES, contraction strength is driven mainly by how much current is delivered to the nerve. Increasing the amplitude (current intensity) recruits more motor units and produces a stronger contraction, because more fibers reach their activation threshold and generate force. Pulse duration also affects strength to some extent since longer pulses deliver more charge per pulse and can recruit additional fibers, but the primary lever for max force is the current amplitude. Carrier frequency mainly influences comfort and fatigue rather than the amount of force produced. Electrode size changes current density and where the stimulation occurs (a smaller electrode makes activation more focal and can feel stronger in the targeted area), but it doesn’t inherently increase the muscle’s maximal force as much as increasing the stimulus amplitude does.