Objectives: Determine the effects of varying frequency, phase duration, and interphase interval (IPI) of symmetrical, biphasic pulsed current (SBPC) on quadriceps femoris muscle torque production, perceived discomfort, and muscle fatigue when using neuromuscular electrical stimulation (NMES). Methods: Ten recreationally active able-bodied participants completed this study. Muscle torque was measured during a series of laboratory based, NMES-induced muscle contractions with varying combinations of phase durations (50, 100, 150, or 200 mu s) and IPIs (50, 100, 150, or 200 mu s) at 25 and 50 Hz. A three-factorial repeated measures experimental design was used. After each contraction, participants rated perceived discomfort on a 10-cm visual analog scale. Subsequently, fatigue tests were conducted using different frequency-IPI combinations. Results: Results of within-subjects ANOVAs revealed no interaction among the three factors: frequency, phase duration and IPI. Significant main effects of phase duration and IPI, but not frequency, on torque production and perceived discomfort were observed. Specifically, significant linear and quadratic trends were observed for the effect of phase duration and IPI on torque production, and the effect of IPI on discomfort, and only a significant linear trend for the effect of phase duration on discomfort. The shorter IPI combined with the longer phase duration produced greater torque with tolerable discomfort. There was no significant effect of IPI on muscle fatigue. Conclusions: Study findings indicated that when administering SBPC on the quadriceps femoris muscle, clinicians should select an IPI of 50 mu s and a longer phase duration (e.g. 200 mu s) to maximize torque without imposing intolerable discomfort.
