Optimisation of the temporal aspects of the task design requires a better understanding of the development of muscle fatigue in the neck and shoulder region over time. Our objective was to investigate the development of muscle fatigue and to determine the relationship between objective and subjective estimates of fatigue in two production companies. Estimates were obtained through electromyography during test contractions and ratings of perceived discomfort. EMG amplitude increased during the day in both cases while MPF decreased only in one case. In both cases, a more detailed frequency analysis of the EMG signals showed an increase in lower frequency power accompanied by a decrease of the higher frequency power. Local perceived discomfort increased over the course of the day in both cases. However, no clear relationship between perceived discomfort and objective indicators of fatigue was found.