Abstract
One important mechanical function of the lumbar spine
is to support the upper body by transmitting
compressive and shearing forces to the lower body
during the performance of everyday activities. To
enable the successful transmission of these forces,
mechanical stability of the spinal system must be
assured. The purpose of this study was to develop a
method and to quantify the mechanical stability of the
lumbar spine in vivo during various three-dimensional
dynamic tasks. A lumbar spine model, one that is
sensitive to the various ways that individuals utilize
their muscles and ligaments, was used to estimate the
lumbar spine stability index three times per second
throughout the duration of each trial. Anatomically,
this model included a rigid pelvis, ribcage, five
vertebrae, 90 muscle fascicles and lumped parameter
discs, ligaments and facets. The method consisted of
three sub-models: a cross-bridge bond
distribution-moment muscle model for estimating muscle
force and stiffness from the electromyogram, a rigid
link segment body model for estimating external forces
and moments acting on the lumbar vertebrae, and an 18
degrees of freedom lumbar spine model for estimating
moments produced by 90 muscle fascicles and lumped
passive tissues. Individual muscle forces and their
associated stiffness estimated from the EMG-assisted
optimization algorithm, along with external forces were
used for calculating the relative stability index of
the lumbar spine for three subjects. It appears that
there is an ample stability safety margin during tasks
that demand a high muscular effort. However, lighter
tasks present a potential hazard of spine buckling,
especially if some reduction in passive joint stiffness
is present. Several hypotheses on the mechanism of
injury associated with low loads and aetiology of
chronic back pain are presented in the context of
lumbar spine stability.
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