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stability. The third is the cumulative load theory, which states that because all biological tissues
have viscoelastic properties with its individual characteristics, repeated and prolonged usage leads
to slow mechanical degradation and reduction of their stress-bearing capacity, rendering the tissues
more vulnerable to injuries at lower tolerance levels, which are product of load magnitude and
frequency. Herein, localized muscle fatigue favors biological safety by preventing cumulative load
from rising rapidly as the maximum voluntary contraction for level can neither be held for a long
time, and nor can it be repeated in quick succession. The fourth is the overexertion theory, which
is similar to the cumulative load theory except that the injury is acute or rather momentary in time
because the exertion exceeds the tolerance limits of the system, or the ability of the structure to
withstand the load, or it exceeds the tolerable strain rate. An overexertion injury precipitation
can also occur in a situation when the combination of exertion and repetition does not allow
adequate recovery and leads to overexertion. Herein, localized muscle fatigue in combination
with inadequate recovery can leads to overexertion and muscle fibers damage. Overexertion by
definition is a function of internal stresses magnitude, duration, frequency, adopted posture and
motion and it addresses only the internal physical factors. Direct trauma is excluded.
1.6.
1.6.1.
Prevention of Work-Related Low Back Disorders
Ergonomic Intervention
In seeking to cope with WRMSD and to foster safer work environments, an ergonomic intervention is
required to eliminate the occupational risk factors for MSD (WHO, 1985). In 1962, the International
Labour Organization (ILO) (cited by NIOSH, 1981) suggested limits for occasional weight lifting
based on inspection of injury and illness statistics, which depicted manual material handling
(MMH) as contributing to about threefold of increase in spinal injuries among others. From a
biomechanical standpoint, if MMH activities cannot be avoided in the workplace, at least, they
should be ergonomically well designed as they are a potential precursor of LBP (WHO, 1985).
Based on systematic review of the epidemiologic evidence, the European Guidelines for Prevention in LBP does not recommend standalone physical ergonomics interventions programs, but
multidimensional interventions that include physical ergonomics interventions for reduction of
the prevalence and severity of occupational LBP. However, it recognizes the fundamental role of
physical ergonomics in order to redesign the workspace for early return to work (Burton, 2005).
In many circumstances where manual lifting and lowering tasks are unavoidable, work redesign
may carry on in order to decrease the mechanical loads on the joints at acceptable individual levels
for each worker by reducing the exposure to dangerous loading conditions and/or to stressful
body movements and thus to ensure that musculoskeletal system cannot be overloaded and fail
when workers perform various MMH activities in the workplace.
An optimal ergonomic intervention is achieved by an engineering control approach and preferably
through task automation or mechanization. However, in some cases where such mechanical aids
are not feasible and consequently manual lifting tasks cannot be avoided, handling devices can
be used to simplify the problem of handling an object. Engineering controls are the preferred
method of risk control because they permanently reduce or eliminate the biomechanical risk
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