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Background and Literature Review
1.1.
Background
Manual material lifting and lowering tasks are the primarily risk factors for development of workrelated low back disorders (WRLBD) (Op de Beek and Hermans, 2000). WRLBD causation is
primarily based on the mechanical disruption of spinal support structures, where the integrity of
the connective tissue is violated and its mechanical order perturbed due to spine loading (Adams
and Roughley, 2006; Marras, 2008). Since spine loading is mainly imposed by trunk muscles
(co)activity in response to external loading, the magnitude of the myoelectric activity of trunk
musculature during lifting and lowering tasks has been investigated extensively. However, there is
lack of investigation regarding the ability of the “neural controller” to coordinate the many trunk
muscles necessary to stabilize the spine during lifting or lowering tasks and whether this ability
is stressed by work-design factors. Previous studies have shown that the constraints imposed by
work-design factors influence not only the (I) magnitude of kinematic, kinetic, and myoelectric
variability among participants who perform identical lifting tasks in terms of load weight and origindestination state in both realistic condition and in control laboratory environments (isokinetic,
isometric, isoinertial) (Granata, Marras, and Davis, 1999; Mirka and Marras, 1993; Mirka and
Baker, 1996), but also (II) their spatiotemporal interjoint coordination and local dynamic stability
of their coordinative movements (Burgess-Limerick et al., 2001; Graham and Brown, 2012;
Graham, Sadler, and Stevenson, 2012; Graham et al., 2011; Scholz, 1993a,b). Regarding the
former, variability was associated with a high probability of identifying a risky lifting task healthy,
since ergonomics assessment tools are based on mean values; regarding the latter, variability
was associated with an increased risk for development of low back disorders (LBD) due to trunk
accelerations following sudden biomechanical and neuromuscular perturbations.
LBD are associated with accelerations that undergoes the spine as a result of trunk muscles activity
in response to the loading or in response of postural reactions to balance disturbances (Butler
et al., 1993; Commissaris and Toussaint, 1997; De Looze et al., 2000; Van der Burg, Kingma,
and Van Dieën, 2003, 2004; Van der Burg and Van Dieën, 2001b; Van der Burg, Van Dieën, and
Toussaint, 2000; Van Dieën and Looze, 1999). Previous studies postulated that certain combinations
of postural perturbations and voluntary movements during lifting tasks could require different
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