Sustained trapezius muscle activity has been shown to increase the risk of neck and shoulder pain: number of eight-minute-periods with sustained activity >0.5% EMGmax (Østensvik 2009), and more than 70% of burst time, i.e. working time above 2% EMGmax (Mork 2006). Both studies consider the occurrence of continuous muscle activity above a certain discrimination level. The aim of this study was to evaluate the effect of data processing procedures on similar measures of sustained trapezius muscle activity.

Full-day bilateral EMG from upper trapezius muscles were collected from forty subjects (17 men, 23 women, median age 22 years, range 22-26), who had just left technical school and started mainly as hairdressers, electricians and students. Number of periods and total duration of activity above discrimination levels 0.5, 1 and 2% EMGmax were retrieved from data processed by each of six Root Mean Square (RMS) window lengths. Several analyses of patterns of activity and rest were made, and this paper presents some basic descriptives concerning total duration of muscle activity.

The static muscle activity level (APDF) was 0.3% EMGmax and the mean burst time was 65%. The total time of muscle activity depended largely on the discrimination level (Figure: panel A compared to B). For all discrimination levels, the total activity time increased with an increasing RMS length, and became larger when the demands for a minimum period duration were relaxed (figures A and B).

Thus, muscle activity patterns extracted from EMG are highly sensitive to the data processing procedure and we suggest that a standardized approach should be developed so that future studies will be comparable.