Torque gains and neural adaptations following low-intensity motor nerve electrical stimulation training

The purpose of the study was to assess neural adaptations of the plantar-flexors induced by an electrical stimulation training applied over the motor nerve at low intensity using two different stimulation frequencies. Thirty subjects were randomly assigned into 3 groups: 20 Hz. 100 Hz, and control group. The training consisted of 15 sessions of 25 stimulation trains applied over the tibial nerve and delivered at an intensity evoking 10% maximal voluntary isometric contraction (MVIC). Before and after training, MVIC was assessed and neural adaptations were evaluated by the voluntary activation level (VAL) and the V-wave (normalized by the superimposed muscle compound action potential, V/M-SUP). H-reflex and motor-evoked potential (MEP) recorded during MVIC were studied to assess spinal and corticospinal excitabilities [i.e., maximal H-reflex during maximal voluntary isometric contraction (H-SUP)/M-SUP and maximal motorevoked potential during maximal voluntary isometric contraction (MEPSUP)/M-SUP]. MVIC significantly increased after training only for the two training groups (P = 0.017). This increase was accompanied by a significant increase of VAL only for these groups (P = 0.014). whereas statistical analysis revealed a time effect for V/M-SUP (P = 0.022). H-SUP/M-SUP and MEPSUP/M-SUP were significantly increased at post conditions only for the 100 Hz group (P = 0.021 and P = 0.029). Results show that low-intensity electrical stimulation training applied over the motor nerve can induce torque gains, accompanied by neural adaptations. Stimulation frequency differentially affected spinal and corticospinal excitabilities, indicating that neural adaptations could have a supraspinal origin for the 20-Hz protocol. whereas spinal and supraspinal mechanisms were implicated in the torque increases after the 100-Hz training. NEW & NOTEWORTHY This study brings new insights into the neurophysiological mechanisms responsible for torque gains after electrical stimulation training using wide pulse duration and low stimulation intensity applied over the motor nerve. Stimulation frequency had a distinct impact on spinal and/or supraspinal origins of the observed neural adaptations. The use of the aforementioned stimulation parameters in rehabilitation settings can be proved beneficial in terms of strength gains while avoiding any serious discomfort because of stimulation.