The Relationship between Lordosis and the Position of the Centre of Reaction of the Spinal Disc
Gracovetsky S, Zeman V, Carbone A.
Journal of Biomedical Engineering
9(3) : 237-248, 1987b.
Abstract
Whenever we stand upright in apparent static equilibrium, we are, in fact,
continuously making minor adjustments in order to maintain our balance. For each
intervertebral joint, a point must exist at which all moments applied to the joint are
continually and dynamically balanced. This point, known as the centre of reaction, is a
mathematical invention necessary to perform analyses of spinal motion, but which need not
be associated with any real anatomical structure. As the spine flexes and extends, this
centre is expected to move; where it moves and the rationale for its motion is worthy of
enquiry. In this paper, we propose that the centre of reaction remains confined to the
nucleus of the disc, but only if one simple but crucial assumption is made; that the
motion of the spine and control of its musculature maintain a minimum and equal stress at
each intervertebral joint. This is a simple hypothesis which imposes a very specific
relationship between lordosis and the centre of reaction. We investigate this relationship
and its consequences on the teaching of lifting. The impossibility of designing
experiments adequately to verify this hypothesis prevents in vivo verification, and,
therefore, the model's verification must be made through the inferred consequences. For
example, the capability of correctly predicting physiological responses of the
musculoskeletal system is an indication of the validity of a model; however, this is not
universally accepted. In this particular case, additional experimental data are available
in the form of the locus of the centre of rotation of a vertebra vis-a-vis its lower
neighbour as the spine flexes and extends. It is instructive to compare the predicted
locus of the centre of reaction to experimentally determined locus of the centre of
rotation; more than just coincidental similarities are found.
