Biomechanical differences between best and worst performances in repeated free asymmetrical lifts

Abstract

The purpose of the study was to evaluate, in 10 novice workers, the effect of the free practice of asymmetrical lifts with 3 different 15-kg containers. Practice effects were evaluated during one session (trials 1, 25 and 49), and also one month later (trial 50); containers effects were evaluated for homogeneity (2 boxes: homogeneous vs. heterogeneous) and for format (homogeneous box and cylinder). Each subject performed 150 lifts (50 practice trials and 3 containers per trial) but only 12 lifts were first analyzed. The data were obtained from 4 video cameras and a large force plate. A 3D dynamic rigid body model was used to evaluate low-back kinetics and kinematics. There were not any significant differences between the practice trials nor the boxes but there was high variability of performance within and between subjects. The main objective was then to evaluate the variability of these trials by contrasting the worst and best trials using three safety criteria: mechanical work, back efforts and asymmetry. The best strategies of reduced mechanical work (mean difference: 31%) was mainly associated with reduced knee flexion at take-off; strategies of reduced low-back moments at take-off (27%) were associated with smaller knee flexion and asymmetrical trunk efforts but larger feetspacings; finally, strategies of reduced back asymmetry at take-off (155%) were associated with reduced asymmetry of posture i.e. a better parallelism between shoulders/pelvis/handgrips and shoulders more parallel to the ground. Conclusions based on analyses of single trials or even means may be misleading. Relevance to industry Analyses of handling strategies including best and worst performances on the basis of specific safety criteria may help to better understand risk factors and suggest potential solutions for safer handling.