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own psychophysical study for their specific MMH conditions, but doing so would be costly, time
consuming, and require expertise, so conducting a study is usually not feasible by practitioners.
When using the tables, some limitations of the approach must be considered. During the testing, the
load limits were based on subjective assessments by the test subject. So, although the limits may provide a
realistic assessment of what a worker feels he or she can tolerate, it is questionable whether a subject can
anticipate how much can be tolerated over the long term without incurring an injury. Marras et al.
(1999), in an investigation of measurement effectiveness of different MMH assessment tools, found
that in nearly two thirds of jobs that could be classified as high and medium risk the psychophysical
criteria would indicate that those jobs are acceptable. So, a psychophysical approach appears to under-
estimate the actual level of risk.
The user should also keep in mind that the model also assumes good coupling (between the load and
the hands and between the feet and the floor), two-handed symmetrical material handling, moderate
load lifts, unrestricted working postures, and a favorable physical environment.
34.4.1.2 NIOSH Lifting Equation
One of the tools used most often by the practitioner, and also one of the most studied, is the Revised
NIOSH Lifting Equation developed in 1993 and described by Waters et al. (1993). The equation is
used to determine a lifting index (LI), an index of relative physical stress associated with MMH tasks.
Inputs to the model are: (1) horizontal distance of the load from the spine, (2) vertical location of the
lift, (3) vertical travel distance of the lift, (4) frequency of the lift, (5) angle of asymmetry of the lift,
(6) lifting frequency, (7) quality of the coupling, and (8) weight of the actual load being lifted.
A comprehensive guide to applying the equation in industry can be found in NIOSH (1994).
The revised equation built upon an equation originally released in 1981 (NIOSH, 1981). The 1993
equation was developed to reflect new findings, allow for asymmetric lifting tasks, and to account for
objects with suboptimal hand-container couplings. Psychophysical, biomechanical, and physiologic
criteria were used to determine the parameters of the equation, thus helping the tool to be more
robust and usable in a variety of situations.
Several studies have been conducted to determine the validity of the model in identifying tasks associ-
ated with a higher risk of work-related low back pain. In the investigation by Marras et al. (1999) of the
effectiveness of several methods of MMH assessment, it was found that both the 1981 and the 1991
equations had predictive power to identify jobs associated with high-risk of work-related low back dis-
orders. Although both equations were about equally predictive, the 1981 equation achieved high speci-
ficity, meaning that it “was liberal in the assessment of risk by misidentifying most jobs as safe,” while the
1993 equation, conversely, achieved high sensitivity “because it is more conservative and identified most
jobs as being risky.”
Waters et al. (1999) also evaluated the correlation between the LI and epidemiologic data; the data set
included 50 jobs from four industrial sites. Based upon the findings that workers who perform jobs with
an LI greater than 2.0 are at a significantly risk of having low back pain, the authors concluded, “The LI
may be a useful indicator of risk of LBP caused by manual lifting.” Interestingly, the data also showed that
the risk of those who are exposed to an LI greater that 3.0, the highest exposure group, experienced lower
risk than those in the group performing tasks where the LI was between 2.0 and 3.0. The authors theorize
that this effect is due to a combination of worker selection and the survivor effect. That is, workers who
are not able to perform MMH tasks where the LI is greater than 3.0 are injured or leave the jobs, thus
leaving only those individuals who may have a high tolerance for manual lifting.
One attribute of using the NIOSH equation that is particularly pertinent to the practitioner concerns
accuracy of the measures of the variables used as model inputs. Waters et al. (1998) investigated the accu-
racy of input measurements as recorded by highly educated nonergonomists with experience in data
measuring and who participated in a 1-day training class. They found that in general, the students
could accurately make the required measurements. However, the students did have trouble with three
of the required measurements: (1) measurement of asymmetry angles, (2) the coupling factor, and (3)
locating the reference point that is midpoint between the angles. The measurement of asymmetry
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