Standardized and machine-paced work tasks at a packing machine were examined to evaluate interindividual variability of muscle activity patterns. Ten trained female workers, without musculo-skeletal complaints at the time of the recording, performed the work tasks while electromyographic (EMG) recordings were obtained from both upper trapezius muscles Static muscle activity and periods of between 0 2 and 2 S duration with low muscle activity, EMG gaps, were analysed Complaints of muscular fatigue, soreness or pain in the neck and shoulders during the last 12 months were recorded The level of static muscle activity was 1 6 (range 0 4 to 2 5) per cent of maximal voluntary contraction and median number of EMG gaps was 4 8 (range 0 8 to 20) per minute Workers with previous episodes of complaints (five subjects) had higher levels of static muscle activity and fewer EMG gaps than workers without such episodes (p < 0.05, Wilcoxon 2-sample test, one-tailed) Considerable interindividual variability of muscle activity patterns was found in spite of stereotyped work No causal relations may be inferred from the correlation between the level of trapezius activity and complaints, though it indicates that individual, inexpedient muscle activity patterns may constitute an important risk factor for development of musculo-skeletal complaints.