Effect of random interpulse interval modulation on neuromuscular fatigue.

Neuromuscular endurance during electrical stimulation may be enhanced if naturally occurring motor unit firing patterns are used. Variability in the interpulse interval (IPI) distribution may enable brief periods of rest and optimization of force output. Nine individuals participated in three 3-minute fatigue protocols of the thenar muscles elicited by supramaximal stimulation of the median nerve. All protocols consisted of a mean IPI of 33.3 ms and differed only in the type of IPI modulation, which was constant (0%), random (+/-20%), or ramped from 0% to +/-20%. M-wave amplitude declined following all protocols and the reduction was smallest following the ramp protocol. There was no significant difference among the starting or final forces or between the overall force-time integrals for the three protocols. Thus, IPI variability did not improve endurance time during electrical stimulation and the M-wave amplitude was not a reliable indicator of muscle force output.