Prolonged muscle activity leads to a decline in force production and the fatigue process begins immediately after the onset of activity. The changes in the surface electromyogram (EMG) have been used extensively as indices of muscle fatigue. A special attention has been paid to frequency analysis of EMG. The reduction of muscle fibre conduction velocity (CV) was accepted as one of the main factors causing myoelectric signal spectral compression. The changes of CV reflect mainly peripheral mechanisms. However, some findings supporting the thesis that central factors contribute to the spectral shift have been also reported. The purpose of present study was to take advantage of the surface branched electrodes and the level-trigger averaging technique to assess the myoelectrical parameters and its changes during sustained isometric maximal voluntary contraction efforts.