Evaluation of the Revised NIOSH Lifting Equation (1991): On the Asymmetric Multiplier

The NIOSH 1981 lifting equation (NIOSH, 1981) [1] is based on biomechanical, physiological, psychophysical, and epidemiological criteria in order to identify the risk of low back injuries in bi-manual lifting. The 1991 revised lifting equation [2] expands the application scope of the 1981 equation for evaluating asymmetrical lifting tasks, lifts of objects with less than optimal hand-container couplings, and a larger range of work duration and lifting frequencies; also, the load constant is reduced from 40 kg to 23 kg; the Lifting Index (LI) and the Recommended Weight Limit (RWL) replace the Maximal Permissible Limit (MPL) and Action Limit to indicate the potential risk. LI is an actual lifted weight and uses RWL as its unit. If LI < 1, it assumes that more than 99% of males and 75% of females are capable of performing the lifting task and the risk of causing low back injury is nominal. If LI < 3, it assumes that less than 25% of males and 1% of females are capable of performing the lifting task, and a greater percentage of manual materials handling workers is likely to be at risk of developing low back pain. Waters et al. (1993) [3] noted that although the 1991 equation has not been fully validated, the recommended weight limits derived from the revised equation are consistent with or lower than those generally reported in the literature. Hidalgo et al. (1995) [4] showed that the results of cross-validation of the NIOSH limits for psychophysical criterion confirmed the validity of assumptions made in the 1991 NIOSH revised lifting equation and for the biomechanical and physiological criteria which were not in total agreement. This validation was based on the psychophysical, biomechanical and physiological data recently published in the subject literature (Genaidy et al. 1990, Snook and Ciriello 1991, Genaidy et al. 1993, Asfour et al. 1991) [2]. Most of the validation limit in the sagittal plane lifting and 3.4 KN compressive force on the spine is the standard line in biomechanical criteria. The 1991 revised NIOSH lifting equation recommended that the allowable weight of lift be reduced by about 30% for lifts involving asymmetric twists of 90 degrees (Water et al., 1993), which considers asymmetric multiplier in the sagittal lifts as 1 and 90 degrees lifts as 0.7, and linearly reduced between the sagittal lifts and 90 degrees lifts. This maximum acceptable lifting capacity is applied not only in the heavier loads but also in the lighter loads. The hypothesis in evaluating NIOSH lifting equation on the asymmetric multiplier is that asymmetric multiplier may be appropriate adopted in the heavier loads (LI = 3) but will be improper applied in the lighter loads (LI = 1). Several types of data, static and dynamic data of the L5/S1 forces, moments, and compressive forces, are observed.