Correlation between surface electromyograms and the susceptibility to fatigue of the human masseter muscle.

Seventeen healthy male subjects were asked to clench as long as possible in the intercuspal position while keeping the electromyographic activity (EMG) of the masseter muscle at 50 per cent of the maximum activity. The endurance time was used as an indicator for the muscular resistance to fatigue. At the 95 per cent probability level, there was a statistically-significant correlation between the endurance time and the root mean square value (RMS) of the EMG signal, the mean power frequency value (MPF) of the signal and the parameter MPF 100 per cent/MPF10 per cent used as an indicator for the contraction-level dependency of the mean power frequency. There was a strong correlation (p less than 0.001) between the endurance time and the rate of EMG frequency shift to lower frequencies measured during the first 30 s.