The effects of lifting height and asymmetry on maximum acceptable weight of lift, average heart rate, and estimated biomechanical loading to the lumbar spine.

Epidemiological studies have indicated that work involving lifting heavy objects or lifting and twisting with moderate weights can impose increased compressive, shear, and torsional stresses on a worker\u27s lumbar spine. Current statistics indicate that musculoskeletal sprains and strains account for more than 40% of the 2.0 million lost-time work-related injuries that occurred in 1995. Considering both direct and indirect costs, lost-time musculoskeletal injuries can cost U.S. industries {dollar}70 billion to \\{dollar}80 billion annually. The objective of this study was to investigate the effects of lifting symmetrically and asymmetrically from knee height to chest or eye height on the (a) psychophysically selected weights lifted, (b) average heart rate over two 30-min lifting sessions for four test conditions, and (c) estimated compressive loading to the L5/S1 intervertebral disc for each test condition. The weights lifted were compared to the Recommended Weight Limits that result from an analysis of the task characteristics using the revised NIOSH lifting equation. Eight West Virginia University engineering students volunteered as subjects. Paired comparison t-tests indicate that there was a significant difference in average weight lifted between chest and eye height for symmetrical lifts {dollar}(p\u3c.01){dollar} and asymmetrical lifts {dollar}(p\u3c.02).{dollar} There was no significant difference in weight lifted for twisting versus no twisting at either chest or eye height. There was no significant difference in average heart rate or average estimated biomechanical loading to the lumbar spine for: (i) symmetric versus asymmetric lifting at chest height, or (ii) at eye height, (iii) all symmetric lifts (chest and eye together) versus all asymmetric lifts, or (iv) all chest lifts (both lift types) versus all eye height lifts. There was an indication {dollar}(p\\le.10){dollar} that average heart rate was higher for symmetrical lifts at eye height than for asymmetrical lifts at eye height. There was also an indication {dollar}(p\\le.10){dollar} that average back compressive force when lifting asymmetrically at eye height was less than average back compressive force when lifting symmetrically at eye height. When using the revised NIOSH lifting equation to analyze the requirements of the experiment and evaluate the weights lifted by the subjects, there was a significant increase {dollar}(p\u3c.001){dollar} in the weights lifted versus the Recommended Weight Limits calculated by the revised NIOSH equation