Analysis of internal torso loading in asymmetric and dynamic lifting tasks

BACKGROUND: Asymmetric and dynamic lifting is known to be one of the leading causes of occupational lower back disorders (LBDs). Biomechanical modeling has been utilized to investigate lifting task characteristics so that the task demands can be kept within a limit, and internal muscles and joints are not injured. OBJECTIVE: This study implemented AnyBodyTM to analyze internal torso loading in asymmetric and dynamic lifting tasks. METHODS: A six-camera motion capture (mocap) system collected dynamic motion data of lifting 30 lb (13.6 kg) weight at 0?, 30? and 60? asymmetry. The mocap data drove the AnyBodyTM model, and the study investigated the effect of the asymmetry. RESULTS: Erector spinae was the most activated muscle for both symmetric and asymmetric lifting. When lifting origin became more asymmetric toward right, erector spinae activity was reduced, but oblique muscles increased their share of activity to counter the external moment. Most muscle tensions peaked at the lift initiation phase except left external oblique and right internal oblique. Left external oblique played a minor role in the right asymmetric lifting task, and the difference of activation for right internal oblique may be due to variance of the motion. Surprisingly the lift asymmetry decreased both compression and shear forces at the L5/S1 joint. CONCLUSIONS: This finding contradicted the results obtained from other research studies. The reduction in spine forces is postulated to have resulted from the increased oblique muscles' share in the production of back extensor moment. Since these muscles have longer moment arms, they generated lesser spine force to counteract the external moment. The subject also tended to squat as lifting origin became asymmetric, which effectively reduced the load moment on the spine.