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Figure 1.7 Load-tolerance capacity and LBP. When tissue biological adaptation is disturbed it is
lowering its stress bearing capacity and raising the injury potential (adapted from Kumar, 2001).
healing process is interrupted it results to MSD of cumulative origin and functional disability
(Barbe and Barr, 2006; Barr and Barbe, 2004; Kumar, 2001; Marras, 2006; Pećina and Bojanić,
Tolerance limit is further influenced by individuals factors (anthropometric characteristics, strength,
endurance, age, leisure time, genetic makeup, etc) (see Fig. 1.6) and improper muscle rectruitment
patterns due to (I) muscle fatigue, (II) co-contractions, or (III) motor control errors . Co-contraction
of antagonist muscles increase the mechanical loads (compression, shear and torsion) or change
the nature of the mechanical loads placed on the body’s articulations and tissues during an exertion
or motion (Radwin, Marras, and Lavender, 2002). Localized muscle fatigue is failure of the fiber
to contract in response to continuing a motor neuron stimulation. The EU-OSHA defines localized
muscle fatigue as a potential precursor to WRMSD (Buckle and Devereux, 1999; Op de Beek and
Hermans, 2000). Localized muscle fatigue changes the loadings experienced by the supported
spinal structures. While the fatigued fibers are not themselves permanently damaged, they can
put other motor units at risk of structural damage due to inappropriate recruitment or excessive
strain from external loads (NRC-IOM, 2001; Radwin, Marras, and Lavender, 2002). Moreover,
localized muscle fatigue may result in ballistic motions and exertions in which loads are poorly
controlled and rapidly accelerated indicating that there are large impulse forces within muscles
and connective tissues (NRC-IOM, 2001; Radwin, Marras, and Lavender, 2002).
McGill (1997), reported that improper muscle activation patterns or trunk force magnitudes
provoked single vertabra rotation resulting in pain. As joint stiffness that is needed to maintain
mechanical stability of spine depends on the relative activation of the muscles spanning the
joints and muscle strength capability, inappropriate muscle activation patterns may reduce the
stiffness needed for spine mechanical instability provoking spine buckling. The in vitro buckling
of ligamentous lumbar spine under compressive forces lesser than bodyweight (80 N) (Crisco
and Panjabi, 1992; Crisco et al., 1992), highlights the importance of the “neural controller” to
control the forces of trunk musculature that stiffen the spine. NIOSH (1981), pointed out the
difficulty that is supposed to be for the worker to control dynamic actions that result to high inertial
forces. Furthermore, NIOSH (1981) is aware that in the fast movements, the ability of the “neural