Variation in spinal load and trunk dynamics during repeated lifting exertions

Objectives. To quantify the variability in lifting motions, trunk moments, and spinal loads associated with repeated lifting exertions and to identify workplace factors that influence the biomechanical variability. Design. Measurement of trunk dynamics, moments and muscle activities were u sed as inputs into EMG assisted model of spinal loading. Background. Traditional biomechanical models assume repeated performance of a lifting task produces little variability in spinal load because the asse ssments overlook variability in lifting dynamics and muscle coactivity. Methods. Five experienced and seven inexperienced manual materials handlers performed 10 repeated lifts at each combination of load weight, task asymm etry and lifting velocity. Results. Box weight, task asymmetry and job experience influenced the magni tude and variability of spinal load during repeated lifting exertions. Surp risingly, experienced subjects demonstrated significantly greater spinal lo ads and within-subject variability in spinal load than inexperienced subjec ts. Trial-to-trial variability accounted for 14% of the total variation in compression overall and 32% in lateral shear load. Although the mean spinal load was safely below the NIOSH recommended limit; due to variability abou t the mean, more than 20% of the lifts exceeded the recommended limit. Conclusion. Spinal load changed markedly from one exertion to the next desp ite identical task requirements. Trial-to-trial variability in kinematics, kinetics, and spinal load were influenced by workplace factors, and may pla y a role in the risk of low-back pain. Relevance Ergonomic assessments considering only the mean value of spinal l oad overlook the fact that a large fraction of the lifts may exceed recomme nded levels. (C) 1999 Elsevier Science Ltd. All rights reserved.