The objective of this study was to investigate the effects of fatigue, caused by frequent manual lifting. on lifting velocity and lifting acceleration. Ten male volunteers performed lifting at a rate of 4 times per minute, continuously, for two hours using the free-style posture A box($30cm{ imes}30cm{ imes}20$) with a fixed weight (15.9 Kg) was used as the load for lifting, Heart rate, oxygen consumption, and EMG were also measured to estimate the level of fatigue, The posture as well as acceleration was recorded. The results showed that the lifting acceleration at the end of two hour increased significantly (20%, p<0.001) compared to the acceleration after fifteen minutes of lifting. It was also found that subjects changed their lifting postures as the result of fatigue. All subjects also indicated pain in their upper legs and the lower back at the conclusion of the experiment.