Changes in neural drive after strength training are better estimated from absolute than normalized EMG amplitude

In this study, we aimed to determine the association between changes in estimates of neural drive and global measures of electromyographic (EMG) amplitude elicited by short-term strength training. A cohort of 13 individuals performed 4 wk of strength training, which increased the maximal voluntary force (MVF) of the ankle dorsiflexors by approximately 14%, maximal root mean square (RMS) EMG amplitude for the tibialis anterior by approximately 42%, motor unit discharge rate by approximately 11%, and decreased motor unit recruitment threshold by approximately 10%. The increase in EMG amplitude during the submaximal contractions was observed at 50 and 70% of MVF (P < 0.05) but only for the absolute (lV) and not the normalized (% of MVF) root mean square (RMS) values. At the level of individual participants, it was possible to predict with moderate strength the changes in recruitment threshold and discharge rate after training (recruitment threshold vs. RMS, r = -0.55, P = 0.041; discharge rate vs. RMS, r = 0.56, P = 0.037, repeated measures correlations). These associations were not statistically significant when the EMG amplitude was normalized by the RMS values during the MVF contractions. Moreover, modeling the EMG with only the tracked motor units produced a strong correlation between the changes after training for both the reconstructed and measured EMG (r = 0.86, P < 0.001). These results demonstrate that the adaptations in neural drive experienced by individual participants after short-term (<1 mo) training interventions can be estimated from the absolute amplitude of multichannel EMG signals.