Mechanisms underlying the training effects associated with neuromuscular electrical stimulation.

Although neuromuscular electrical stimulation (NMES) can increase the ability of muscle to exert force, the means by which this is accomplished seem to be different from those associated with voluntary exercise. The aim of the study was to determine whether the recruitment order of motor units elicited by over-the-muscle electrical stimulation is different from that achieved with voluntary activation of muscle. This difference was tested by comparing muscle twitch responses that were elicited by Hoffmann reflexes (H-reflexes) and direct motor responses (M-responses) and by examining the effect of submotor NMES on the twitch force associated with H-reflexes. Because H-reflexes represent the summed activity of many motor units, in a manner that is consistent with volitional activation, variation in the time to peak twitch force indicates changes in the population of motor units that contribute to the response. The results demonstrated that the percutaneous application of submotor NMES to the limbs of human subjects causes a faster-contracting population of motor units to be activated during a test H-reflex. Consequently, it seems that the application of NMES preferentially activates faster-contracting motor units, perhaps those that are normally only active at high exercise intensities under voluntary conditions.