Transcutaneous electrical nerve stimulation modulates corticospinal excitability while preserving motor unit discharge properties during isometric contractions

Aim Transcutaneous electrical nerve stimulation (TENS) aims to supplement sensory feedback to improve force steadiness or motor function. In this study, we directly assessed potential changes in corticospinal excitability and motor unit discharge characteristics from the first dorsal interosseous (FDI) muscle due to TENS by using transcranial magnetic stimulation (TMS) and high-density surface electromyography (HDsEMG). Methods Eleven healthy young adults performed a series of submaximal isometric index abductions. We estimated i) motor evoked potential (MEP) amplitudes, ii) persistent inward current amplitudes (PIC, i.e., delta F), iii) motor unit recruitment thresholds and discharge rates, and iv) common synaptic input to motor units before and after TENS. Results TENS did not affect force steadiness (2.5 ± 0.9% and 3.3 ± 1.9% ( p = 0.010)). MEP amplitudes decreased at 110% of the resting motor threshold (rMT; 0.72 ± 0.66 mV vs . 0.59 ± 0.63 mV; p < 0.001), increased at 130% rMT (1.18 ± 1.10 mV vs . 1.41 ± 1.29 mV; p < 0.001). Delta F increased after TENS (3.7 ± 2.2 pps vs . 4.5 ± 2.6 pps; p = 0.010). We did not find a change in the level of common synaptic input or in the temporal variability of motor unit discharge rates after the session of TENS. Conclusion These results suggest that TENS can modulate corticospinal excitability through supraspinal and spinal processes and, thus act as a priming technique. At the same time, TENS does not generate short-term changes in the neural control of force in young, healthy adults. ### Competing Interest Statement The authors have declared no competing interest.