Spinal loads during two-person team lifting: effect of load mass distribution

Abstract

The purpose of this study was to examine the relationship between load mass distribution (LMD) and spinal load during team lifting tasks. Two-person lifting teams were required to lift a box containing a mass of 30 or 60 kg from the floor to standing knuckle height. Adjusting the position of the centre of mass within the box by ᱵ and ᷮ5 cm relative to the evenly distributed position (0 cm) yielded three LMD ratios (69:31, 59:41 and 50:50), which represented the percentage of the total mass lifted by each team member. The external force acting on the hands and sagittal plane segmental kinematics were measured and used in a simple dynamic biomechanical model to calculate the torque, compression and shear forces experienced at L4/L5. Spinal load estimates (i.e. maximum and average L4/L5 torque, compression force and shear force) significantly increased with load mass and were positively correlated with LMD (r=0.86-0.99, p<0.05), indicating that the person at the heavier end of the asymmetrical load experienced higher spinal loads than their lifting partner. However, the asymmetry in spinal loads between the two team members was found to be significantly lower than the asymmetry in the LMD ratios. This result was primarily due to: (i) a significant positive correlation between LMD and the horizontal "pulling" force acting on the hand and (ii) a significant negative correlation between LMD and the moment arm of the vertical force acting through the hand relative to L4/L5. Thus, when lifting an unevenly balanced load a two-person lifting team seems to adopt a lifting strategy that partially alleviates the larger spinal loads experienced by the team member at the heavier end of the load.