EMG power spectrum and maximal M-wave during eccentric and concentric actions at different force levels.

The aim of the present study was to examine changes in the EMG power spectrum and in the maximal M-wave due to different activation levels of eccentric and concentric actions. Average EMG (aEMG) increased as the force increased but no consistent differences were observed in the amplitude of the maximal M-wave in any of the conditions. However, as the force level increased the duration of the M-wave decreased significantly (P<0.01) both in eccentric and concentric actions while no differences were observed between the action types. On the average, no major differences were observed in median frequency (MF) between any of the conditions, although rather large variations were observed throughout the motions. Increased muscle fiber conduction velocity (CV), as indicated by the decreased duration of the M-wave, could be due to recruitment of fast motor units with higher muscle fiber conduction velocity and increased firing rate of the active units. Also the changes in the muscle fiber length between the force levels may have affected CV.