Motor unit activity and surface electromyogram power spectrum during increasing force of contraction

Summary Twelve male subjects were tested to determine the relationship between motor unit (MU) activities and surface electromyogram (EMG) power spectral parameters with contractions increasing linearly from zero to 80% of maximal voluntary contraction (MVC). Intramuscular spike and surface EMG signals recorded simultaneously from biceps brachii were analyzed by means of a computer-aided intramuscular MU spike amplitude-frequency (ISAF) histogram and an EMG frequency power spectral analysis. All measurements were made in triplicate and averaged. Results indicate that there were highly significant increases in surface EMG amplitude (71±31.3 to 505±188 V,p<0.01) and mean power frequency (89±13.3 to 123±23.5 Hz,p<0.01) with increasing force. These changes were accompanied by progressive increases in the firing frequency of MU's initially recruited, and of newly recruited MU's with relatively larger spike ampltitudes. The group data in the ISAF histograms revealed significant increases in mean spike amplitude (412±79 to 972±117 V,p<0.01) and mean firing frequency (17.8±5.4 to 24.7±4.1 Hz,p<0.01). These data suggest that surface EMG spectral analysis can provide a sensitive measure of the relative changes in MU activity during increasing force output.