Median frequency of the electromyographic signal: effect of time-window location on brief step contractions

The purpose of this study was to determine, for different back muscles, if the median frequency (MF) of the electromyographic (EMG) power spectrum cha nges according to the position of the time window during a 5 s step contrac tion. Twenty males with no known back problems were standing upright in a d ynamometer allowing lower limb and pelvis stabilization. Trunk extension ef forts were performed by pushing on a force platform positioned at the T4 le vel while the extension moment at L5/S1 was displayed as visual feedback. T he EMG signals from four homologous back muscles (multifidus at L5, ilicost alis lumborum at L3, and longissimus at L1 and T10) were collected using ac tive surface electrodes during two 5 s static step contractions performed a t five force levels (10, 20, 40, 60 and 80% of the maximal voluntary contra ction). The root mean square (RMS) and MF values of the EMG signals corresp onding to three 250 ms time windows (beginning, middle and end of each step contraction) were computed. The RMS values of several back muscles increas ed from the first to the third time window for contractions performed at hi gh force levels only. However, a concomitant decrease in the MF values was observed only for the left multifidus muscle. It was concluded that muscle fatigue does not generally manifest itself during 5 s step contractions thr ough the EMG signal. However, it is recommended to use step contractions la sting less than 5 s and to choose a time window located in the first 1-3 s to completely eliminate the possible effects of fatigue. (C) 2001 Elsevier Science Ltd. All rights reserved.