A Biomechanical Investigation of the Asymmetric Multiplier in the Revised NIOSH Lifting Equation

There is growing evidence, from epidemiological and biomechanical sources, that lifting performed in asymmetric postures is a risk factor for the development of a musculoskeletal injury. In the recent update of the NIOSH Lifting Guide, a linear Asymmetric Multiplier was added to account for this type of risk. The present study addresses the form of this Multiplier through analysis of several asymmetric lifting tasks. Both spinal loading and a derived metric of muscle injury risk were calculated as a function of asymmetry angle. The results suggest that there is a non-linear increase in injury risk with respect to asymmetry. Only moderate increases in risk were predicted for asymmetry of 0°–30°, and sharply increasing risk as asymmetry reaches 90°, implying that ergonomic intervention should be concentrated on tasks with the highest asymmetries.