Minimizing discomfort with surface neuromuscular stimulation.

The purpose of this study was to evaluate the effects of stimulus parameters, electrode types, and electrode positions on the perception of discomfort during lower extremity surface neuromuscular stimulation. Ten normal and eight neurologically impaired (four incomplete spinal cord and four stroke) subjects were enrolled. Neurologically impaired subjects had some sensation, although it was often reduced. Parameters of the stimulation were varied in a way that produced the same level of ankle dorsiflexion, as measured with a goniometer. Discomfort was assessed after each stimulation with a 0-10 verbal scale (0, no discomfort; 10, worst pain). Increasing the pulse frequency was associated with increased discomfort for subjects in both groups (p > 0.05). Increasing the pulse duration was associated with increased discomfort in the neurologically impaired subjects (p > 0.05), but not in the normal subjects (p > 0.05). The electrode size and type had no effects on discomfort (p > 0.05). Stimulation of the peroneal nerve over the fibular head was better tolerated than the direct motor point stimulation of the tibialis anterior motor point (p < 0.05). The data suggest that to minimize discomfort, surface stimulation should be applied over nerves rather than motor points, and frequency and pulse duration should be set as low as possible for a given degree of contraction.