Figure 11.4 Distribution of EMG levels recorded bilaterally on the upper trapezius muscles during the performance of computer-aided-design work with a mouse. The percentage time is shown, where the EMG level was recorded within level classes according to the exposure variation analysis (EVA).

During the performance of low-level work tasks the difference between the static and peak EMG levels are often small due to relatively sustained muscle activity patterns. Improvements of such work tasks may not be achieved through reductions in exposure levels and focus should rather be directed towards the number of rest pauses and the variation in the exposure pattern which may be quantified by gap or EVA analyses of EMG signals. For instance, EVA analyses of the EMG recording from the left and right upper trapezius muscles of a computer-aided design worker showed no periods of muscular rest on the right side, which almost continuously operated a computer mouse (Jensen et al., 1997) (see Figure 11.4). On the left side, which performed occasional keyboard operations, periods of very low or no activity at all (< 1 per cent EMGmax) were recorded for about 13 per cent of the recording time. Similarly, gap analyses showed 0.1 gaps/min in the recordings from the right m. trapezius and 7.3 gaps/min in the recordings from the left m. trapezius.