In NMES, which parameter besides amplitude influences recruitment of motor units?

• A. Pulse duration
• B. Carrier frequency
• C. Electrode size
• D. Treatment duration

Answer: (A)

The strength–duration relationship governs how many motor units NMES recruits at a given amplitude. The threshold to depolarize a motor axon depends on the charge delivered per pulse, which is the product of current and pulse duration. A longer pulse duration delivers more charge at the same amplitude, making it easier for more axons to reach their threshold and be recruited. This means increasing pulse duration lowers the current you need and can recruit additional motor units, matching the observed effect described by rheobase and chronaxie in the strength–duration curve. Carrier frequency mainly affects comfort and fatigue rather than the immediate threshold for recruitment, electrode size changes current density and spatial distribution rather than the threshold itself, and treatment duration influences total exposure time rather than which motor units are activated.