Changes in lifting dynamics after localized arm fatigue

The purposes of this study were (1) to compare the lifting strategies during arm fatigue and non-fatigue conditions and (2) to evaluate the effects of localized arm fatigue on L5/S1 compressive forces during lifting. The hypothesis was that isometrically induced arm fatigue can alter the lifting strategy selection resulting in an increase in the initial acceleration and leading to an increase in lower back stress. Biomechanical analyses of lifting were done before and after the performance of holding activity to induce arm muscle fatigue, Differences in the lifting strategies used including the accelerated effect, pre-lifting technique, and stiffening of the arms were monitored to determine their influence on L5/S1 compressive forces under various load and range conditions. The results show that lifting strategy changed significantly after arm fatigue, especially when the load was less than 20 kg. These changes included the use of increasingly stooped and accelerated techniques adopted at the beginning of the lift and stiffening of the arms at the end of the lift. Arm fatigue resulted in increased compressive forces at the L5/S1 disc due to the use of accelerated techniques and the inherent disadvantage of these techniques in the pre-lifting posture. In this study, lifting strategies changed as a function of arm fatigue, resulting in increased lower back loading. These findings suggest that whole-body lifting should be avoided after localized arm fatigue in order to decrease the risk of injury to the lower back.