Effects of submotor-threshold tibial nerve stimulation on plantarflexion torque in healthy adults and people with multiple sclerosis

Background: Submotor-threshold electrical stimulation (subES) can enhance motoneuron excitability, presenting a potential strategy for improving exercise efficiency in people with multiple sclerosis (PwMS). Objective: This study aimed to investigate the effects of superimposed subES of the tibial nerve during voluntary isometric contraction on ankle plantarflexor torque production. Methods: A total of 48 participants, comprising both PwMS and healthy participants, performed plantarflexion at three voluntary isometric contraction (MVIC) levels: 20, 60, and 100 %. During the plantarflexion, the tibial nerve was stimulated with one-second trains of subES at frequency of 80 Hz. Results: Plantarflexor torque decreased over one-second time intervals (from before to during and from during to after a train of subES) in both groups at 60 and 100 % MVIC (p < 0.01). While subES did not affect EMG signal amplitude (p > 0.05), it did lead to a statistically significant increase in median EMG frequency (p < 0.05). No differences in the effects of subES were observed between the two groups regarding torque, median EMG frequency, or EMG amplitude of the soleus muscle (p > 0.05). Conclusions: The findings indicate that the subES of the tibial nerve, as presented in this study, has limited potential for acutely enhancing ankle plantarflexor torque in PwMS.