Effects of amplitude and phase-duration modification on electrically induced contraction force and discomfort

Background: Neuromuscular electrical stimulation (NMES) is commonly used in rehabilitation. However, the optimal combination of phase-duration and amplitude for enhancing motor output is not yet resolved. Objective: To test the effects of increasing phase-duration and amplitude on isometric knee extension force and discomfort, while controlling the effects of electrode-skin resistance and body mass index (BMI). Methods: Twenty-one healthy volunteers participated in the study. Stimulation was set at 250 μsec phase-duration and 45 Hz to evoke 10% of maximal voluntary isometric contraction of the quadriceps. Electrode-skin resistance was measured. Then, electrically induced contraction (EIC) forces and discomfort level were measured under four conditions: Moderate (25%) or substantial increase (50%) from baseline amplitude with constant phase-duration and moderate (25%) or substantial increase (50%) in phase-duration with amplitude constant. Results: Compared with baseline, EIC force was significantly higher in all intensification conditions, while discomfort was significantly greater in all conditions except for moderate increase in phase-duration (p = 0.44). Amplitude intensification produced significantly higher force and greater discomfort than phase-duration. Electrode-skin resistance and BMI were not significant covariates. Conclusions: Greater force is elicited by increasing amplitude than by similar increase in phase-duration; however, the associated discomfort is also higher. Clinicians may use phase-duration while conditioning for NMES.