Functional subdivision of the upper trapezius muscle during low-level activation.

The electromyographic (EMG) amplitude was recorded using bipolar surface electrodes placed at different positions above the upper trapezius muscle of 16 healthy subjects. One of the aims of this study was to investigate the variation in EMG activity between electrode positions. For this purpose three tasks were performed: a mental activation test, a dynamic movement test and 90 degrees arm abduction. The EMG signals were full-wave rectified and averaged within windows that were 0.2 s in length. Normalized EMG activity showed significantly different EMG amplitudes at different electrode positions for two of the three tasks. The second aim of this study was to investigate whether the upper trapezius muscle may be functionally subdivided. For this purpose the normalized EMG amplitudes of each task were compared with the EMG amplitude recorded during submaximal shoulder elevation. While the EMG level was similar at one electrode position, significant differences were found at some of the other electrode positions, indicating a functional subdivision of the muscle. The present results indicate that for comparisons of upper trapezius EMG activity levels between some tasks or between subjects, it is worthwhile to make EMG recordings at several electrode positions.