Surface EMG shows distinct populations of muscle activity when measured during sustained sub-maximal exercise.

Muscle activity patterns were measured from the rectus femoris, biceps femoris, tibialis anterior and gastrocnemius medialis muscles using electromyography (EMG). Recordings were made from six runners while they ran for 30 mm around a track at a constant, self-selected pace. They were made from three sites along each muscle belly, and for ten consecutive steps on each 450 m lap completed. The intensity of the myoelectric signals was resolved into components in time and frequency space using wavelet analysis. Distinct populations of high- (170-220 Hz) and low-frequency (40-60 Hz) components could be seen in the frequency spectra. There was no significant effect of the electrode position on the rates of change of EMG intensity for any of the muscles, and there was no significant difference in these rates between a 150-ms window at the end of the swing phase and a 150-ms window during the stride phase of running. There were significant differences in the way in which each subject responded to the task, and the way in which the FMG intensities changed at the different frequency bands. There was a significant reduction in EMG intensity at low frequencies and a significant increase at high frequencies, and these changes were ubiquitous for all four muscles tested. The frequencies that showed the greatest changes coincided with the frequencies where distinct populations of activity occurred within the signal. These changes in muscle activity are different from those seen for maximal fatiguing contractions. However, they suggest that the patterns of muscle recruitment may change during sustained sub-maximal exercise.