Concurrent effects of EMG-triggered NMES and anodal tDCS on cortical and behavioural outcome measures: The role of ipsilateral versus contralateral anodal stimulation

Purpose Neuromodulatory interventions are increasingly used to enhance neuroplasticity and motor performance. Electromyography-triggered neuromuscular electrical stimulation (EMG-triggered NMES) and anodal transcranial direct current stimulation (a-tDCS) target peripheral and cortical components of the motor system; however, the effects of their concurrent application remain unclear. This study examined whether concurrent EMG-triggered NMES and a-tDCS applied over the ipsilateral or contralateral primary motor cortex differentially modulate corticospinal excitability (CSE) and motor performance. Methods Twenty healthy adults participated in a randomized, counterbalanced, crossover study with three sessions: (1) EMG-triggered NMES + contralateral a-tDCS, (2) EMG-triggered NMES + ipsilateral a-tDCS, and (3) EMG-triggered NMES + sham a-tDCS. Primary outcomes were CSE, and secondary outcomes included short-interval intracortical inhibition (SICI), long-interval intracortical inhibition (LICI), intracortical facilitation (ICF), maximal voluntary contraction (MVC EMG ), and performance on a choice reaction time task (CRTT). Results CSE increased significantly following EMG-triggered NMES + contralateral a-tDCS ( p < 0.001and sham a-tDCS ( p < 0.001), but not after ipsilateral a-tDCS (P = 0.806). EMG-triggered NMES + contralateral a-tDCS increased ICF (P = 0.038) and reduced SICI (P = 0.029) and LICI (P = 0.05). Conversely, MVC (P = 0.021) and CRTT (P = 0.035) increased following EMG-triggered NMES + ipsilateral a-tDCS, with MVC differing significantly from the other conditions (P = 0.001). Conclusion EMG-triggered NMES combined with contralateral stimulation enhances CSE, whereas ipsilateral stimulation improves behavioural outcomes without altering contralateral CSE. These findings indicate a dissociation between changes in CSE and behavioural performance, suggesting that behavioural improvements may reflect neural processes not captured by the measures used in this study.