Article

Experimental studies suggest that flexed working postures reduce
passive support of the spine, which could represent a significant risk factor for
the development of occupational low back disorders. Neuromuscular compensations
to reduced passive stiffness include increases in baseline activity or reflexive
activation of trunk muscles. Yet, alterations and recovery of the synergy between
active and passive tissues following prolonged flexion in humans are currently
unknown. METHODS: Twelve healthy participants were exposed to all combinations of
two trunk flexion durations (2 and 16 min) and three flexion angles (33, 66, and
100% of individual flexion-relaxation angle). Load relaxation was recorded
throughout exposures, whereas trunk stiffness and reflexive behaviors of the
lumbar extensor muscles were investigated during dynamic responses to sudden
perturbations. FINDINGS: The magnitude of load relaxation increased with
increasing flexion angle. Trunk stiffness decreased and reflex gains increased
following flexion exposures; for both outcomes, acute changes were larger
following exposure to increasing flexion angle. Reflex gains remained elevated
one hour after exposure to maximum flexion. INTERPRETATION: Exposure to prolonged
trunk flexion changed trunk stiffness and reflex behavior in patterns consistent
with epidemiological evidence linking such exposure with the risk of occupational
low back disorders. Observed increases in reflex gains, at least among healthy
individuals, may be a compensation for decreases in passive trunk stiffness
following acute exposure to flexed postures. It remains to be determined whether
the neuromuscular system can similarly respond to accumulated disturbances in
passive structures following exposure to repeated flexion tasks.
CI - Copyright (c) 2010 Elsevier Ltd. All rights reserved.

Langue : ANGLAIS