EMG power spectrum as a measure of muscular fatigue at different levels of contraction.

The shift in the power spectrum resulting from a 5-7 min fatigue-inducing effort followed by a 1-2 min recovery period of two elbow flexors, the biceps brachii (BB) and the brachio-radialis (BR), was assessed using two variables, the mean frequency Fm and the median or central frequency Fmd. These two variables were calculated in pre- and post-fatigue conditions and following a brief recovery, at four levels, namely 20, 40, 60 and 80 per cent of maximum voluntary contraction (MVC). These were taken from a ramped isometric effort that is from 0 to 100 per cent MVC. The EMG activity of the two flexors was recorded with bipolar surface electrodes from a group of ten volunteers. Following muscle fatigue, induced with a maintained 60 per cent MVC isometric contraction, a statistically significant (p less than 0.05) shift towards the lower frequencies was observed for both Fm and Fmd for both muscles. Following a brief recovery, a shift towards the pre-fatigue higher frequencies was statistically significant (p less than 0.05). These two synergists responded to muscle fatigue and recovery similarly, as they both demonstrated parallel shifts in power spectrum. The power spectrum is consequently a reliable measure of muscular fatigue. It is also complementary to the net articular moment results.