Civil Engineering Reference
In-Depth Information
46.6 Applying the Equations
46.6.1 Using the RWL and LI to Guide Ergonomics Design
The RWL and LI can be used to guide ergonomics design in several ways:
1. The individual multipliers can be used to identify specific job-related problems. The relative mag-
nitude of each multiplier indicates the relative contribution of each task factor (e.g., horizontal,
vertical, frequency, etc.).
2. The RWL can be used to guide the redesign of existing manual lifting jobs or to design new
manual lifting jobs. For example, if the task variables are fixed, then the maximum weight of
the load could be selected so as not to exceed the RWL; if the weight is fixed, then the task variables
could be optimized so as not to exceed the RWL.
3. The LI can be used to estimate the relative magnitude of physical stress for a task or job. The
greater the LI, the smaller the fraction of workers capable of safely sustaining the level of activity.
Thus, two or more job designs could be compared.
4. The LI can be used to prioritize ergonomics redesign. For example, a series of suspected hazardous
jobs could be rank ordered according to the LI and a control strategy could be developed accord-
ing to the rank ordering (i.e., jobs with lifting indices above 1.0 or higher would benefit the most
from redesign).
46.6.2 Rationale and Limitations for LI
The NIOSH RWL equation and LI are based on the concept that the risk of lifting-related LBP increases
as the demands of the lifting task increase. In other words, as the magnitude of the LI increases, (1) the
level of the risk for a given worker would be increased and (2) a greater percentage of the workforce is
likely to be at risk for developing lifting-related LBP. The shape of the risk function, however, is not
known. Without additional data showing the relationship between LBP and the LI, it is impossible to
predict the magnitude of the risk for a given individual or the exact percentage of the work population
who would be at an elevated risk for LBP.
To gain a better understanding of the rationale for the development of RWL and LI, consult the paper
Waters et al. (1993). This article provides a discussion of the criteria underlying the lifting equation and
of the individual multipliers. This article also identifies both the assumptions and uncertainties in the
scientific studies that associate manual lifting and low back injuries.
46.6.3 Job-Related Intervention Strategy
The LI may be used to identify potentially hazardous lifting jobs or to compare the relative severity of two
jobs for the purpose of evaluating and redesigning them. From the NIOSH perspective, it is likely that
lifting tasks with an LI
1.0 pose an increased risk for lifting-related LBP for some fraction of the work-
force (Waters et al., 1993). Hence, to the extent possible, lifting jobs should be designed to achieve an LI
of 1.0 or less.
Some experts believe, however, that worker selection criteria may be used to identify workers who
can perform potentially stressful lifting tasks (i.e., lifting tasks that would exceed an LI of 1.0) without
significantly increasing their risk of work-related injury above the baseline level. Those who endorse
the use of selection criteria believe that the criteria must be based on research studies, empirical obser-
vations, or theoretical considerations that include job-related strength testing or aerobic capacity testing.
Even these experts agree, however, that many workers will be at a significant risk of a work-related injury
when performing highly stressful lifting tasks (i.e., lifting tasks that would exceed an LI of 3.0). Also,
“informal” or “natural” selection of workers may occur in many jobs that require repetitive lifting
tasks. According to some experts, this may result in a unique workforce that may be able to work
.
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