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LBP and to study the validity and feasibility of using the NIOSH lifting equation in China. The NIOSH
equation was used to evaluate lifting risk for 69 workers mainly involved in manual materials handling
(MMH) (job A) and 51 machinery workers who worked in less demanding MMH tasks (job B). The
prevalence of LBP lasting for more than a week due to lifting were 26.09 and 5.88% for jobs A and B,
respectively. The NIOSH LI was estimated to be 2.4 for job A, and 0
1 for job B. The authors
concluded that the NIOSH equation is an important tool in assessing characteristics and risk factors
of LBP for MMH tasks.
In another study, Marras et al. (1999) examined the relationship between low-back disorders and
various risk factors for LBP due to manual lifting. One of the objectives of the study was to evaluate
the validity and effectiveness of the revised NIOSH lifting equation to correctly identify jobs with
varying levels of risk of low back disorder, where job risk was defined according to historical records
of low back disorder injuries. High-risk jobs were defined as jobs in which more than 12 low back dis-
orders were recorded per 100 exposed workers (mean of 22 injuries
LI
,
,
100 exposed workers), medium-risk
jobs were defined as jobs in which between 1 and 12 low back disorders were recorded per 100 exposed
workers, and low-risk jobs were defined as jobs in which no low back disorder was recorded per 100
exposed workers. The results indicated that the when the average horizontal distance was used, the
NIOSH equation was predictive of risk of low back disorder, resulting in an odds ratios of 3.1 (95%
CI, 2.6 to 3.8) for high-risk jobs compared to low-risk jobs. When the maximum horizontal distance
was used, the odds ratio was increased to 4.3 for high-risk jobs compared to low-risk jobs.
In another study, conducted by Lee et al. (1996), researchers examined whether the NIOSH lifting
equation was applicable for an Asian population. The application of the NIOSH equation for establishing
weight limits for Korean workers was examined using the psychophysical method. The study population
consisted of 53 male college students and 16 male field workers. The subjects were required to perform six
different lifting tasks in the sagittal plane, at various lifting frequencies and heights, for 8 h. The RWLs for
each lift were calculated using the NIOSH equation. The psychophysical method, in which subjects were
allowed to adjust the weight of lift during a 20-min period, was also used to estimate the maximum
acceptable weight of lift (MAWL) for each lifting task. While students generally had larger body sizes
than the worker population, workers were generally stronger than the students. Within each group,
neither the frequency nor the vertical height of lift was significantly related to the differences between
the NIOSH Lifting Equation (NLE) and psychophysically-based weight limits. The MAWLs of the
workers were significantly higher than those of the students. While this difference increased with increas-
ing lift frequency, it was not sensitive to lift height. When the data were adjusted to represent the entire
Korean young male population, no significant differences were observed between the NIOSH rec-
ommended weights of load and the adjusted MAWLs. Although the load constant of the NIOSH
equation was 23 kg, that of the students was 20.24 kg and that of the workers was 25.05 kg. The
authors conclude that the NIOSH weight limit equation is well suited for young, healthy Korean males.
Recently, Hidalgo et al. (1995, 1997) conducted a study designed to evaluate the validity of the psy-
chophysical, biomechanical, and physiological criteria used in establishing the NIOSH lifting equation
(Waters et al., 1991). The criteria used to develop the equation were cross-validated against the data
published by different researchers in the scientific literature. Assessment of the 1991 NIOSH lifting
equation indicated that there are differences between the NIOSH equation values and the psychophysical
limits for some types of lifts and that the RWL likely would protect about 85% of the female population
and 95% of the male population. The authors, however, noted that the 3.4 kN limit for compression
on the lumbosacral joint may be too high to protect all workers and that the energy expenditure
limits used in development of the RWL index can be sustained by 57 to 99% of worker population
when compared to the physiological limits based on previous fatigue studies. The authors concluded
that the results of the cross-validation for psychophysical criterion confirmed the validity of assump-
tions made in the 1991 NIOSH revised lifting equation, but that the results of cross-validation for
the biomechanical and physiological criteria were not in total agreement with the 1991 NIOSH
model. They did not, however, actually evaluate whether the equation would protect workers or not
in the study.
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