CHANGES IN THE ELECTROMYOGRAPHIC SPECTRUM POWER DISTRIBUTION CAUSED BY A PROGRESSIVE INCREASE IN THE FORCE LEVEL

The purpose of the present study was to determine the specific changes occurring in the power spectrum with an increasing force level during isometric contractions. Surface electromyographic signals of the tric eps brachii (TB) and the anconeus (AN) of 29 normal subjects were reco rded during isometric ramp contractions performed from 0 to 100% of th e maximum voluntary contraction (MVC) in a 5-s period. Power spectra w ere obtained at 10, 20, 30, 40, 50, 60, 70, 80 and 90% MVC. Changes in the shape of these spectra were evaluated visually and with the calcu lation of several statistical parameters related to the distribution o f power along the frequency axis, such as median frequency and mean po wer frequency, standard deviation, skewness, first and third quartiles and half-power range. For the AN, the behaviour of the spectrum was r elatively similar across subjects, presenting a shift toward higher fr equencies without any major change in the shape of the spectrum. For t he TB, subjects with a thin skinfold thickness presented similar behav iours. In subjects with a thicker skinfold, however, a loss of power i n the high frequency region paralleled the increase in the force level . Significant correlations were obtained between the extent of the cha nge in the value of higher order statistical parameters across force a nd the thickness of the skin. This points out the importance of the sk infold layer when recording with surface electrodes. Furthermore, the use of a combination of several parameters appears to provide a better appreciation of the changes occurring in the spectrum than any single parameter taken alone.