Muscle Fatigability Responses to Pulsed and Kilohertz-Frequency Currents With Different Phase Durations: A Randomized Crossover Trial

Purpose To determine whether current type (pulsed current [PC] vs kilohertz-frequency alternating current [KFAC]) or phase duration (200 vs 500 µs) primarily influences contraction fatigability, neuromuscular electrical stimulation (NMES) efficiency, sensory discomfort, and neurophysiological responses of the triceps surae. Methods In this randomized, participant- and assessor-blinded crossover trial, 44 healthy adults (24.4 ± 4.9 years) completed four NMES conditions separated by at least 7 days: PC 200, PC 500, 2.5-kHz KFAC 200, and 1-kHz KFAC 500. Each fatigability protocol comprised 20 evoked contractions initiated at 20% of maximal voluntary contraction. Outcomes included maximal voluntary and evoked torque, torque-time integral (TTI), TTI decline, NMES efficiency, sensory discomfort, electromyographic activity, spinal excitability, and central activation. Repeated-measures analyses of variance assessed current type, phase duration, and time. Results The 500-µs conditions produced greater total TTI and higher NMES efficiency than the 200-µs conditions (P ≤ 0.001). Voluntary torque, evoked torque, and electromyographic root mean square amplitude decreased after the fatigability protocol (P < 0.001). Current type did not affect fatigability or sensory discomfort, and no current type × time interaction was detected for spinal excitability or central activation. The absence of significant current-type effects should not be interpreted as equivalence because equivalence testing was not performed. Conclusion Phase duration was more influential than current type under the tested conditions. A 500-µs phase duration improved evoked work and NMES efficiency without increasing sensory discomfort, highlighting phase duration as a physiologically relevant parameter when NMES protocols are designed to sustain repeated contractions. Clinical Trial: NCT05894044 on 2023-05-23.