Effects of pulse width and frequency on evoked responses in electrostimulation: comparison between three muscle groups

Introduction Neuromuscular electrical stimulation (NMES) is an innovative and effective (re)training strategy to improve or restore neuromuscular function (Maffiuletti et al., 2018). Contractions induced by NMES differ in many aspects from voluntary contractions, as motor unit (MU) recruitment is random, synchronous and spatially fixed (mostly superficial; Maffiuletti, 2010). Consequently, several limitations, such as higher fatigability (Vanderthommen et al., 1999) and discomfort (Delitto et al., 1992) might restrain its clinical implementation. The use of specific stimulation parameters may partly overcome these limitations. Indeed, the use of wide pulses (≥ 1 ms) delivered at low stimulation intensity leads to a preferential recruitment of Ia sensory axons (Veale et al., 1973) which may promote MU central (reflexive) recruitment. Furthermore, the high stimulation frequencies (> 80 Hz) would facilitate the temporal summation of post-synaptic excitatory potentials and reflexively activate spinal motoneurons through Ia afferents (Dideriksen et al., 2015), which may increase force production. Another potential advantage of wide pulse high frequency (WPHF) NMES is that low stimulation intensities are required to limit antidromic collision, and these lower intensities are associated with less discomfort (Delitto et al., 1992). Therefore, by stimulating at intensities expected to generate ~10% of the maximal voluntary contraction (MVC) force, WPHF NMES induces, in some individuals, a progressive increase in force during the stimulation, called ‘extra force’. It can reach up to 80% of the MVC force in plantar flexors (Neyroud et al., 2018) but the response to WPHF NMES in other muscle groups is less documented. Extra force is usually accompanied by a prolongation of the surface electromyographic (EMG) activity after cessation of the stimulation, also called ‘sustained EMG activity’ which is interpreted as MU recruited through the central pathway (Neyroud et al, 2018). The main aim of the present study was to explore the effect of varying stimulation parameters on the NMES-evoked force and sustained EMG activity in the plantar flexors, knee extensors and elbow flexors. It was hypothesized that the plantar flexors would show higher centrally-mediated responses to NMES than knee extensors and elbow flexors, especially with large pulse duration. Methods Sixteen volunteers, 2 women and 14 men (29 ± 6 yr, 177 ± 6 cm, 74 ± 11 kg) participated to three experimental ses