Abstract
To investigate the effects of MMH task conditions on the activity and fatigue of the trunk musculature, EMG signals from eight major trunk muscles were analyzed during 120 minutes of repetitive dynamic lifting tasks. Two independent task variables were the work strategy of weight and lifting frequency combination and the body posture. The dependent variables were the amplitude of EMG signals and the amount of median power frequency (MPF) decrease over time for the eight trunk muscles. The results of the study indicated that the recruitment and the level of force exertions of the trunk muscles during manual lifting tasks are a function of the direction and the weight of the external load. The different activities of the muscles due to given task conditions also influence the fatiguing process of the individual muscle. The muscles in the dorsal part of trunk were activated during the symmetric task conditions, while the muscles on the contralateral side to the workload were more strongly activated during the asymmetric task conditions. The decreasing trends of MPF were found in some activated muscles, and they were more pronounced for the asymmetric posture than for the symmetric posture. It was also seen that the muscles became fatigued faster for light load-high frequency conditions than for heavy load-low frequency conditions.
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