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 triceps brachii (TB) and the anconeus (AN) of 29 normal subjects were recorded during isometric ramp contractions performed from 0 to 100% of the maximum voluntary contraction (MVC) in a 5-s period. Power spectra were 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 calculation of several statistical parameters related to the distribution of power along the frequency axis, such as median frequency and mean power frequency, standard deviation, skewness, first and third quartiles and half-power range. For the AN, the behaviour of the spectrum was relatively similar across subjects, presenting a shift toward higher frequencies without any major change in the shape of the spectrum. For the TB, subjects with a thin skinfold thickness presented similar behaviours. In subjects with a thicker skinfold, however, a loss of power in the high frequency region paralleled the increase in the force level. Significant correlations were obtained between the extent of the change in the value of higher order statistical parameters across force and the thickness of the skin. This points out the importance of the skinfold 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.