Civil Engineering Reference
In-Depth Information
level, the task is deemed acceptable. If the actual weight exceeds the MPL (calculated as three times the
AL), significant risk exists and the task should be carefully evaluated and redesigned.
After over a decade of using this assessment tool, researchers at NIOSH recognized the limitations of
this tool in assessing the variety of realistic working conditions faced by those performing manual
materials handling tasks. Specifically, they identified the inability of the 1981 equation to assess the
risk of asymmetric work postures and varied kinds of coupling between the lifter and the load being
lifted. So in 1993 they published a Revised NIOSH Lifting Equation (Waters et al., 1993). The workplace
variables considered in this revised equation include: vertical position of load, horizontal distance
between the load and the spine, frequency of lifting, vertical travel distance of the load, asymmetric
posture of torso, and coupling quality between the lifter and the object being lifted. As in the 1981
equation, these measures are then combined in a multiplicative model but instead of calculating an
AL this model calculated a value called the recommended weight limit, or RWL, which describes a
weight that can be lifted safely by a majority of the working population. The ratio of the actual
weight being lifted in the job to this RWL is a value called the lifting index (LI). LI values greater
than 1 are said to place some workers at increased risk, while values greater than 3 are said to be a poten-
tial problem for a majority of healthy industrial workers (Waters et al., 1994). There are a number of
stated assumptions that should be considered when applying this assessment technique. Among these
assumptions are that the workers perform only two-handed lifts, they work for no more than 8 h, they
are not lifting or lowering objects faster than 75 cm
/
sec, and they are lifting in a relatively unrestricted
work environment.
There have been several studies that have been conducted to evaluate the effectiveness of the NIOSH
method in predicting the reporting of low back pain
discomfort. In a study of 97 MMH jobs, Wang
et al. (1998) report a monotonically increasing relationship between the severity ratings of low back dis-
comfort and the NIOSH LI. Their results showed that for jobs with a severity rating of 0 (on a 0-5 point
scale) the mean LI was 0.8 while for jobs with a mean severity rating of 5 the mean LI was 4.1 with
intermediate points following the trend. Another significant result from this study was that 42 of the
97 evaluated jobs had an RWL
/
0, a result that the authors attribute to having tasks wherein lifting
¼
frequencies and
or horizontal distances exceeded those allowed by the NIOSH modeling methodology.
In another study (Waters et al., 1999), 50 jobs in four different industrial facilities, were evaluated and
the authors report that the unadjusted prevalence odds ratio for reported low back pain were 1.14, 1.54,
and 2.54 for LIs of 0-1, 1-2, and 2-3, respectively. Interestingly, they report an unadjusted prevalence
odds ratio of 1.63 for jobs with an LI of
/
3, and note potential selection and survivor effects may have
influenced the results of their analysis. In addition to these studies that have considered the relationships
between the NIOSH assessments and reporting of discomfort there have also been several studies that
have considered “usability” aspects of the assessment tools. The reader is referred to the following
articles for further information: Dempsey (2002), Dempsey and Fathallah (1999), and Waters et al.
(1998).
While the NIOSH method is straightforward in application and has great utility in many industrial
environments the static representation of the workplace does not take into account some of the
human performance issues that have been implicated in the low back injury process. Specifically, in per-
forming an MMH task, the three-dimensional postures, velocities and accelerations have been shown to
play a role in the development of low back injuries (Marras et al., 1995). It should be noted that the
NIOSH modeling approach did consider dynamics from physiological and psychophysical perspectives,
but in many cases trunk motion plays a direct role in biomechanical loading and therefore is a facet of
risk that is not directly addressed in the NIOSH approach.
.
35.3 LMM Risk Assessment Model
A risk assessment tool developed by Marras et al. (1993) recognized the importance of these lifting
dynamics in the development of a low back injury. This assessment tool made use of a device called
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