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twisting velocity, and (V) maximum trunk sagittal angle. A multivariate logistic model (the
industrial Lumbar Motion Monitor (iLMM) risk assessment model) consisting of the above five risk
factors were able to predict the probability of high-risk group membership (odds ratio of 10.7).
Another model that is used to rate lifting tasks in terms of risk for development of LBD is the NIOSH
lifting equation (Waters et al., 1993), which implies the following factors: (I) lifting frequency,
(II) box weight, and (III) horizontal distance, (IV) vertical height and (V) vertical displacement of
the box’s center of gravity (COGB ) with respect to the mid-point between the inner-ankle bones.
A comparison between iLMM and NIOSH risk assessment models showed that the multivariate
logistic model consisting of five risk factors implied by the NIOSH lifting equation yielded less odds
ratios than the iLMM dynamic model (Marras et al., 1993)—odds ratios of 3.5 when the average
values were used and 4.6 when the maximum factors of the workplace factors were used. It was
postulated that NIOSH lifting equation cannot identify low- and medium-risk jobs well and may
require the changing of jobs that do not necessarily place workers at risk (Marras et al., 1999).
NIOSH revised equation relied principally on biomechanical studies of static lifting tasks, whereas
the contribution of lifting load dynamics to injury risk was relied mostly on physiological and
psychophysical studies. Therefore, NIOSH equation was limited to assess mainly smooth and slow
lifting tasks. It can be said, that with the development of the iLMM risk assessment model (Marras
et al., 1993) it was recognized the importance of trunk kinematics as a conditioning parameter
that increases the risk factor for development of WRLBD during lifting tasks.
A common limitation of the above settings is that they cannot be applied for handling unstable
(liquid) loads. They are limited mainly for manufacturing or industrial settings where loads do
not present any instability. Although there is some epidemiological evidence that workers who
handle liquid loads have higher incidence of LBD compared to others with similar lifting demands
(McGlothlin, 1996; Personick and Harthun, 1992), and moreover the unstable factor is considered
by the agencies a parameter that increases the risk for the development of WRLBD (NIOSH, 1981,
1994, 1997), the workers who handle liquid loads are assessed with the same parameters that are
used for assessing stable loads, however, with an additional but subjective stressor added by the
specialist.
1.5.2.
Low Back Pain Causality
As showed the conceptual model of Fig. 1.6, at the pathogenesis period, which comprises the
pathophysiology and the outcomes, the mechanical, physiological, and psychological risk factors
influence the causal factor for development of WRLBP – the internal loading to the tissues –
which in turns provoke biological reactions. The pathophysiological mechanisms underlying these
pathways may have not been known yet. However, different biological hypotheses have been
put forward with respect to the pathways structure in order to associate the responses of the
tissues and cells to an etiologic agent coherent with occupational exposure (Forde, Punnett, and
Wegnar, 2002). Several models and hypotheses have been presented in order to describe the
assumed causal pathway(s) between putative work environment risk factors and symptoms of
MSD (Armstrong et al., 1993; Hagberg et al., 1995; Marras et al., 2009; NRC-IOM, 2001; Op de
Beek and Hermans, 2000; WHO, 1985). However, the diagnosis of LBP is challenging.
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