Comparative comfort of three waveforms used in electrically eliciting quadriceps femoris muscle contractions

The purpose of this study was to compare the relative comfort levels of electrical stimulation having different waveforms, but otherwise identical current characteristics, delivered percutaneously to normal quadriceps femoris muscles contracting at the same intensity level. The quadriceps femoris muscles of 20 healthy subjects were stimulated to a torque level 60% of that obtained in a maximal voluntary isometric contraction, using pulsed current with a carrier frequency of 2,500 Hz, at 50 pulses per second of 10-msec pulse duration. Three different waveforms were used: sinusoidal, sawtooth (triangular), and square. The relative comfort level of each electrically elicited contraction for each waveform was determined for each subject using a 20-cm-long visual analog scale. The results showed that no one waveform was most comfortable (least uncomfortable) and the difference was significant in what the subjects perceived to be the most comfortable contraction, regardless of waveform (p less than .01). These results indicate that a subject's perception of discomfort changes as the waveform of stimulation varies and that individual preferences exist for different waveforms. Selection of the most comfortable waveform could prove beneficial when the intensity of muscle stimulation is increased.