EVALUATION OF THE MEAN POWER FREQUENCY OF THE EMG SIGNAL POWER SPECTRUM AT ENDURANCE LEVELS DURING FATIGUING ISOMETRIC MUSCLE CONTRACTIONS

Surface electromyographic (EMG) signals of the triceps brachii muscle during three consecutive long-lasting isometric muscle contractions at a maximum effort level were recorded and then analyzed. The decrease of the mean frequency of an EMG power spectrum (MNF) as a function of time during sustained isometric contraction was efficiently modelled using the exponential model. Criteria to decide whether the sustained isometric contraction was sufficiently long for the reliable determination of the so called endurance level (MNF) were tested. It was shown that if the maximal voluntary muscle contraction is sustained for long enough so that the fitted exponential curve fulfils certain predefined derivative criteria as described in the text, the data could be used for reliable estimation of the MNF p without the experimental MNF data actually reaching the plateau level. Using these criteria, it was found that there are no differences in the MNF p values of the two consecutive series of maximal voluntary isometric muscle contractions. This might also indicate that MNF p could be used as the normalization criteria for the extent of muscle fatigue in frequency domain.