Quadriceps femoris muscle torques and fatigue generated by neuromuscular electrical stimulation with three different waveforms

Background and Purpose. Neuromuscular electrical stimulation is used by phy sical therapists to improve muscle performance. Optimal forms of stimulatio n settings are yet to be determined, as are possible sex-related difference s in responsiveness to electrical stimulation. The objectives of the study were: (1) to compare tile ability of 3 different waveforms to generate isom etric contractions of the quadriceps femoris muscles of individuals without known impairments, (2) to compare muscle fatigue caused by repeated contra ctions induced by these same waveforms, and (3) to examine the effect of se x on muscle force production and fatigue induced by electrical stimulation. Subjects. Fifteen women and 15 men (mean age = 29.5 years, SD = 5.4, range = 22-38) participated in die study. Methods. A portable battery-operated s timulator was used to generate either a monophasic or biphasic rectangular waveform. A stimulator that was plugged into art electrical outlet was used to generate a 2,500-Hz alternating current. Phase duration, frequency, and on-off ratios were kept identical for both stimulators. Participants did n ot know die type of waveform being used. Torque was measured using a comput erized dynamometer: a maximal voluntary isometric contraction (MVIC) of the right quadriceps femoris muscle set at GD degrees of knee flexion was dete rmined during tire first session. In each of rile 3 testing sessions, torqu e of contraction and fatigue elicited by one waveform were measured. Order of testing was randomized. Torque elicited by electrical stimulation was ex pressed as a percentage of average MVIC. A mixed-model analysis of variance was used to determine tile effect of stimulation and sex on strength of co n traction and fatigue. Bonferroni-corrected post hoc tests were used to fu rther distinguish between tire effects of tile 3 stimulus waveforms. Result s. The results indicated that the monophasic and biphasic waveforms generat ed contractions with greater torque than the polyphasic waveform. These 2 w aveforms also were less fatiguing. Tile torques from the maximally tolerate d electrically elicited contractions were greater for the male subjects tha n for the female subjects. Discussion and Conclusion. Muscle torque and fat igue of electrically induced contractions depend on the waveform used to st imulate the contraction, with monophasic and biphasic waveforms having an a dvantage over the polyphasic waveform, All tested waveforms elicited, oil a verage, stronger contractions in male subjects than in female subjects when measured as a percentage of MVIC. [Laufer Y Ries JD, Leininger PM, Alon G. Quadriceps femoris muscle torques and fatigue generated by neuromuscular e lectrical stimulation, with three different waveforms.