The mean frequency of the EMG of the knee extensors is torque dependent both in the unfatigued and the fatigued states.

The present study aimed to investigate the shift in the mean frequency (fmean) of the power density spectrum of the electromyogram (EMG) during endurance tests of the knee extensors at three different torque levels (70% MVC, 25% MVC and 10% MVC). Fourteen clinically healthy men performed the three endurance tests until exhaustion. Surface EMG(s) were obtained from the vastus medialis, the vastus lateralis and the rectus femoris muscles and fmean and signal amplitude (RMS) were computed. The subjects rated the perception of fatigue in the knee extensor muscles throughout the three tests using a 10-point graded scale. The endurance time increased with decreasing torque level. A significantly higher perception of fatigue was found at the low torque level (10% MVC) at the end of the endurance time. Individual time series analysis (using linear regression) was performed. It was found that linear models described well the behaviour of fmean throughout the endurance time at the three torque levels. When the endurance times were normalized significant differences in the rate of decrease in fmean were found for vastus lateralis and rectus femoris; the rate decreased with decreasing torque level. For these two muscles significant differences were also found in fmean in the unfatigued state; at 10% MVC significantly lower fmean was found. The present study demonstrated both that the unfatigued fmean and the rate of decrease in fmean (with normalized endurance time) were torque dependent. The latter finding, combined with our result that the subjective fatigue was highest at the lowest torque level, calls into question the use of fmean as a valid indicator of peripheral fatigue.