Relationships between surface EMG variables and motor unit firing rates

Although surface electromyography (sEMG) is a widely used electrophysiological technique, its physiological interpretation remains somewhat controversial. This study examined the relationship between motor unit firing rates (MUFR) and the root mean square (RMS) amplitude and mean power frequency (MPF) of the sEMG signal in the biceps brachii. Eleven subjects performed maximal isometric elbow flexion while indwelling and sEMG recordings were obtained from the biceps. The RMS amplitude and MPF of the surface signal, and the mean MUFR from the indwelling signal, were calculated over 500 ms epochs. Group means showed a strong MUFR–RMS amplitude relationship (r ² = 0.91), but a weak MUFR–MPF relationship (r ² = 0.20). Using all trials, the MUFR–RMS amplitude (r ² = 0.19) and MUFR–MPF (r ² = 0.0037) relationships were much weaker. Within individual subjects, the MUFR–RMS amplitude (mean r ² = 0.13 ± 0.17) and the MUFR–MPF (mean r ² = 0.040 ± 0.041) relationships were also weak. These results suggest that MUFR cannot be predicted from the characteristics of the sEMG signal.