Interaction of artificial and physiological activation of the gastrocnemius during gait.

Objectives.  The purpose of this research was to understand the effects of surface functional electrical stimulation (FES) of the tibial nerve on the activation of the gastrocnemius medialis of the stimulated side. Methods.  FES was carried out on six healthy subjects, initiated at three different times during gait: early, mid, and late stance. Each stimulation burst consisted of 15 pulses, applied for 300 msec, at 50 Hz stimulation frequency. Mixed model statistical analysis was carried out on the median onset and offset times of the gastrocnemius medialis and the root mean square of the interpulse interval responses. Results.  Results indicate that the electromyography response to FES is dependent on the time of application. The most prominent effects found in the intervals between the stimulation pulses (interpulse intervals) were found when stimulation was applied early in the stance phase. This study revealed that the only statistically significant effect on burst timing was a delay in offset timing due to mid-timed stimulation. Conclusions.  We conclude that additional activation may have been compensated, at least in part, by blocking of the physiological activation during the stimulation burst.