Comparing the results of five lifting analysis tools

Abstract

The objective of this study was to compare the results of the NIOSH, ACGIH TLV, Snook, 3DSSPP and WA L&I lifting assessment instruments when applied to a uniform task (lifting and lowering milk cases with capacities of 15 and 23 l). To enable comparisons between the various lifting assessment instruments, the outputs of each method were converted to an exposure index similar to the NIOSH Lifting Index. All instruments showed higher exposures associated with lifting the 23 l cases versus the 15 l cases. The NIOSH, ACGIH TLV and Snook methods were similar in their results with respect to the pattern of exposure over various height levels and the differences in exposures associated with lifting 15 and 23 l cases. However, the WA L&I and 3DSSPP predicted substantially lower exposures. The reasons for instrument differences are presented so that practitioners can better select the methods they need and interpret the results appropriately.

Introduction

The prevalence and severity of work-related back injuries is a significant problem; in the State of Washington, self-insured worker's compensation claims data for the period of 1992–2000 shows that 51.5% of compensable closed claims were for back disorders (Safety and Health Assessment and Research for Prevention (SHARP), 2002). Heavy lifting has been identified as a major risk factor for the development of back injuries (Marras et al., 1993; Hildebrandt, 1987). Ergonomists have long sought ways to objectively quantify the exposures associated with heavy lifting in order to more accurately anticipate high risk activities and prescribe appropriate interventions (Dempsey, 1998). Consequently, numerous lifting analysis tools have emerged with each one having different inputs, outputs, and subsequent interpretive capacities. These tools, while advancing the ability to characterize, quantify and predict the exposures, nevertheless retain a degree of uncertainty, incompleteness, and differing attributes that practitioners need to be aware of when using and interpreting results (Fallentin et al., 2001; Janowitz et al., 2005).

Five lifting analysis methods were chosen for comparison: the revised 1991 National Institute of Occupational Safety and Health lifting equation (NIOSH) (Waters et al., 1993), the American Conference of Governmental Industrial Hygienists lifting threshold limit values (ACGIH TLV) (ACGIH, 2005), the Liberty Mutual “Snook” Lifting Tables (Snook)(Snook and Ciriello, 1991), the University of Michigan 3D static strength prediction program (3DSSPP) (University of Michigan, 2001) and the Washington State ergonomics rule lifting calculator (WA L&I) (WAC 296-62-051, 2000a, WAC 296-62-051, 2000b).

Each of the instruments has unique attributes that make it desirable for this study. The NIOSH instrument was selected because it is universally recognized and widely used throughout the world. The ACGIH TLV tool is largely based on the NIOSH instrument, but is designed to be more expedient than NIOSH. It is the most recently developed tool and used mostly within the US. The Snook, a psychophysical-based tool, is used by US practitioners to obtain design guidelines. The 3DSSPP was chosen because of its unique low back compression force predictive capabilities. Finally, the WA L&I, which is based on the NIOSH methodology, was designed for use as a regulatory instrument in Washington state and was designed to identify the highest risk jobs.

The specificity and resolution of inputs (i.e., angles, distances) varies across these tools (Table 1). All methods derive an outcome that attempts to predict the relative safety and/or risk of a lift for given populations, but use different terminology and parameters to do so. All instruments are limited in their capacity to incorporate the degree of acceleration/deceleration, shear forces, ambient temperature extremes, one-handed lifts, the lifting of unstable and odd-shaped objects, or the continuous three-dimensional (3D) forces and moments about the spine into their calculations (Fathallah, 1997).

While the literature is replete with descriptions of individual instruments, quantitative comparisons between the respective methods is less complete (Marklin and Wilzbacher, 1999; Marras et al., 1999; Waters et al., 1998). The aim of our study was to compare the results of the five lifting analyses of a common lifting task in order to clarify the similarities and differences between instruments and provide guidance to ergonomics practitioners.