Extra Forces induced by wide-pulse, high-frequency electrical stimulation: Occurrence, magnitude, variability and underlying mechanisms

•The percentage of responders showing “Extra Forces” to wide-pulse, high-frequency (WPHF) neuromuscular electrical stimulation has been previously over-estimated.•Force output evoked by WPHF shows markedly high inter- and intra-individual variability.•In the responder group only, H-reflex is depressed immediately after WPHF indicating a significant central contribution to “Extra Forces”.

ObjectiveIn contrast to conventional (CONV) neuromuscular electrical stimulation (NMES), the use of “wide-pulse, high-frequencies” (WPHF) can generate higher forces than expected by the direct activation of motor axons alone. We aimed at investigating the occurrence, magnitude, variability and underlying neuromuscular mechanisms of these “Extra Forces” (EF).MethodsElectrically-evoked isometric plantar flexion force was recorded in 42 healthy subjects. Additionally, twitch potentiation, H-reflex and M-wave responses were assessed in 13 participants. CONV (25 Hz, 0.05 ms) and WPHF (100 Hz, 1 ms) NMES consisted of five stimulation trains (20 s on-90 s off).ResultsK-means clustering analysis disclosed a responder rate of almost 60%. Within this group of responders, force significantly increased from 4% to 16% of the maximal voluntary contraction force and H-reflexes were depressed after WPHF NMES. In contrast, non-responders showed neither EF nor H-reflex depression. Twitch potentiation and resting EMG data were similar between groups. Interestingly, a large inter- and intrasubject variability of EF was observed.ConclusionThe responder percentage was overestimated in previous studies.SignificanceThis study proposes a novel methodological framework for unraveling the neurophysiological mechanisms involved in EF and provides further evidence for a central contribution to EF in responders.