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1.6.3.1.
Physical Ergonomic Assessement Tools
In ergonomics, lifting and lowering tasks can be categorized according to their risk level of suffering
WRLBD. Physical demands assessment tools were used proactively or reactively to make useful
inferences about workers behaviour on manual material lifting or lowering tasks in manufacturing
environments in order to identify potential precursors of developing WRLBD before workers
develop symptoms severe enough to require medical treatment and to lead to work absenteeism
and work disability (Cohen et al., 1997). Frequently ergonomic assessment tools for lifting tasks
are: (I) the revised NIOSH lifting equation (Waters et al., 1993; Waters, 2006) and its modifications
(Grieco et al., 1997; Hidalgo et al., 1997; Shoaf et al., 1997) and standards (EN 1005-2; ISO
11228-1) that are used to estimate relative magnitude of physical stresses for lifting tasks3 , (II) the
ACGIH TLV guideline (Marras and Hamrick, 2006), which estimates threshold limit values under
which workers may be repeatedly exposed without developing WRLBD associated with repetitive
lifting tasks and the OSU/BWC guideline (Hamrick, 2006) that extent the threshold limit values for
workers currently experiencing LBP, (III) the low back disorder model (Marras et al., 1993, 2000),
which uses trunk kinematics data recorded by the iLMM to estimate the likehood of a repetitive
lifting task without job rotation to be considered high or low risk for development of WRLBD,
and (IV) the University of Michigan 3D Static Strength Prediction Program (3DSSPP) that predicts
the percentage of workers who have sufficient strength to perform a lifting task, together with
an estimation of spine load (compression and shear forces) (Chaffin, 1997). Each of the above
outlined models have strengths and limitations, though they share complementary information
(Mirka et al., 2000). Differences have been found between risk assessment tools (Lavender et al.,
1999; Marras et al., 1993; Russell et al., 2007). The main difference is their ability to quantify
acute or cumulative risk level.
The NIOSH lifting equation (Fig. 1.9) was developed to assist safety and health practitioners
evaluate lifting demands in order to prevent or reduce the occurrence of occupational LBDs (NIOSH,
1981, 1994; Waters et al., 1993). It is used in the decision-making process of (re)designing a
job that contains repeated lifting (or lowering) tasks. An additional benefit of this equation is
also the potential to reduce other WRMSD associated with some lifting tasks such as shoulder
or arm pain (Waters et al., 1993). The 1991 revised lifting equation (Waters et al., 1993) has
been introduced to upgrade the 1981 Work Practices Guide for Manual Lifting (NIOSH, 1981) by
including the possibility to evaluate those jobs that are violating the sagitally symmetric lifting
assumption of the old version and by introducing corrections for the reduced physical strength of
the disabled/rehabilitated worker.
The equation estimates the recommended weight limit (RWL) by taking into consideration the
weight of the load plus several other factors of the lifting tasks that contribute to the risk of injury.
These factors are ‘weighted’ according to biomechanical, physiological and psychophysical criteria
(the most conservative load limit allowed by any individual criterion is chosen) and then are used
as multiplicative factors in the Equation (1.1) (e.g., HM is the horizontal multiplier, etc.) (NIOSH,
1994; Waters et al., 1993):
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NIOSH equation and its modications cannot predict the magnitude of the risk for a given individual and the exact
percentage of the work population who would be at an elevated risk for development of WRLBD.
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