Influence of electrode position on changes in electromyograph parameters of the upper trapezius muscle during submaximal sustained contractions

The purpose of the present study was to evaluate whether differences could be found in the changes in mean power frequency (MPF) and root-mean-square (rms) due to electrode positions on the upper trapezius muscle during a sustained submaximal task. A group of 25 healthy subjects performed a continuous forward flexion of the right arm at 20% of their maximal voluntary contraction (MVC). Three pairs of bipolar surface electrodes were positioned on a straight line between the spine of the seventh cervical vertebra (C7) and the lateral edge of the acromion. The fourth pair (the caudal position) was placed 2 cm below the midpoint of this line. The analysis of variance revealed significant differences between the electrode positions during the short 20% MVC and during MVC (P ≤ 0.01). Furthermore, lower rms values were found when the electrodes were placed symmetrically around the midpoint of the reference line, which confirms previous studies. A statistical model has been developed to discriminate additionally the very small changes in the electromyogram parameters due to electrode position during the sustained 20% MVC. It was found that the slope coefficients of rms and MPF were significantly different from 0 (P ≤ 0.01), although to a lesser degree for rms and more clearly for MPF of the caudal position (P ≤ 0.05). Furthermore, significant differences were found between several combinations of the uppermost electrodes and the caudal position (P ≤ 0.01). From this study, it is concluded that it is important to evaluate several electrode positions on the upper trapezius muscle to be able to represent the behaviour of this muscle accurately during a sustained contraction.