Data analysis
A custom MATLAB (MathWorks Inc., Natick, MA) program was used to analyze the EMG data offline. To measure the soleus and TA pre-stimulus (i.e., background: BG) EMG, the EMG signal was full-wave rectified and the mean absolute EMG amplitude was calculated for the 50 ms immediately before stimulation. Soleus H-reflex and M-wave amplitudes were measured as peak-to-peak EMG amplitude, typically in the windows of 32–44 ms post-stimulus for the H-reflex and 6–21 ms post-stimulus for the M-wave.For analysis of submaximal H-reflexes, the trials with too large or too small M-waves, soleus BG, or TA BG were removed from the statistical analysis to perform adequate comparison of H-reflexes across multiple timepoints. Across all participants, 86 ± 13 SD% of H-reflex trials were included in the final analysis. For each session, soleus, TA, and M-wave values were then normalized to the individual’s pre-tDCS MThe pre-tDCS and post-tDCS H