The temporal occurrence of the mean power frequency shift of the electromyogram during maximum prolonged dynamic and static working cycles.

The study investigated the temporal occurrence of the mean power frequency shift (MPF) of the EMG during prolonged (dynamic and static) working cycles. Ten healthy subjects took part in the experiment. The experimental protocol required each subject to first complete 70 maximum isokinetic knee extensions at 90 degrees s-1). Without any rest, the subject then performed a maximum static knee extension with the dynamometer positioned at 45 degrees of flexion until subjective exhaustion. This sequence of dynamic and static exercise was repeated two more times with a 5-min rest interval preceding each sequence. The EMG signal of the rectus femoris was obtained simultaneously with the torque signal (PT) from the dynamometer. Dynamic contractions PT and MPF decreased throughout the three dynamic periods. Patterns composed of two phases were found; first an initial steep decrease during the first 40 contractions followed by a less steep decrease. Static contractions PT decreased significantly during the three static contractions. Significant (small) decreases in MPF occurred only during the second and third static contractions. Since the major part of the MPF shift occurred early in the exhaustive performance, it is questioned whether the MPF shift detects peripheral fatigue of the type 1 fibers.