Introduction. Significant variability in spinal load during lifting trials under identical conditions cannot be explained by a traditional lifting injury model. In addition to the environmental factors, such as weight and distance, the operational factors, such as lifting speed, need to be considered. Few studies have examined the lifting speed and its function on biomechanical demand. The aim of this study were: (1) to test the speed-accuracy trade-off phenomenon in the manual lifting; and (2) to investigate the effect of weight and precision demand at the lifting destination on selected trunk muscles at varied lifting speed.

Methods. Thirty nine healthy participants performed a total of 24 lifts (3 lifting trials per condition, 2 target sizes, 2 box weights, and 2 requested paces), from waist to shoulder level. Kinematical data of trunk, upper arm, forearm and the box and electromyography (EMG) of nine muscles were collected. Based on kinematical data, phases of lifting were defined and EMG data were analyzed for each phase. The effect of lifting pace, box weight and target size on lifting velocity, box control, upper body kinematics, and other lifting related outcome variables was analyzed using repeated-measures MANOVA.

Results. Both mean and peak velocity of the box significantly (p<0.001) affected by lifting pace, box weight, and target size but there were significant interactions. The temporal recruitment pattern of trunk muscles was significantly affected by lifting velocity. The recruitment of Lumbar Erector Spinae (LES) increased significantly at the beginning phase of faster lifts. The proportion of variance in the LES, explained by the lifting duration, was greater than that by weight or target size.

Conclusions. The Fitts' speed-accuracy trade-off was not applicable to the manual lifting in this study. The temporal recruitment pattern of the trunk muscles and the consequential mechanical demands on the lumbar spine can be independently influenced by the lifting velocity and by the weight of the load. This suggests that different motor strategies may be endorsed for different lifting conditions. These findings underscore the importance of proper lifting technique and the training programs beyond optimizing the environmental conditions for lifting.