Biomechanics of back extension torque production about the lumbar spine

Abstract

The aim of this thesis has been to develop a realistic mechanical description of lumbar back extension torque production. Such a description is desirable for understanding how back extension torque is generated as well as how the spine and surrounding structures are loaded during back extension tasks.

Investigations utilising anatomical images from the Visible Human Project revealed that many of the erector spinae fascicles originating from the lumbar levels attached to the erector spinae aponeurosis covering the dorsal part of the muscles. This observation was contradicting the descriptions used in previous detailed back extension models. The different interpretation of the muscle geometry will have implications for the modelling of both lumbar torque production and lumbar spinal loading.

The Visible Human data were also used to map the detailed anatomy of the multifidus and quadratus lumborum Except for the thoracic part of multifidus, which has not been investigated in detail before, these muscles did not show any major deviations from earlier studies. A model of the mechanisms involved in intra-abdominal pressure (IAP) induced unloading of the lumbar spine was generated in order to clarify some controversies from the literature. It was shown that the unloading mechanism could be viewed as a pressurised column, with the maximally possible cross-sectional area restricted in size by the smallest abdominal transverse cross-sectional area, pushing the rib cage and pelvis apart.

An abdominal form with zero longitudinal curvature was found to have some important mechanical benefits for the generation of IAP induced alleviation of compressive loading of the lumbar spine. In combination with physiological measurements of back extensor torque, IAP and abdominal geometry, the model- calculation showed a possibility for contributions from IAP to lumbar extensor torque production of about 10% of the total maximal voluntary back extensor torque, and reductions of spinal compression of up to 40% as compared to torque creation without LAP.

Magnetic resonance imaging was used to study the effect of mechanical changes due to varying flexion-extension in the lumbar spine. It was observed that the back extensor muscles tended to increase their lever arm lengths when the spine was extended. This would imply less need for muscle force in order to create a given torque in the extended as compared to the flexed position. Contrary to this the spinal unloading effect from the IAP was greatest with the spine held in a flexed position. Since these two opposing effects were of the same magnitude it is not evident which posture will reduce mechanical loading for a given torque production. The model was tested in different ways. By generating LAP through stimulation of the phrenic nerve it was shown that LAP could generate extensor torque about the lumbar spine reasonably well according to model predictions. The specific muscle tensions needed to generate measured maximal voluntary back extension torques agreed well with in vitro measurements of maximal tension from the literature. The model could generate close to simultaneous equilibrium about all the lumbar discs simply by a uniform muscle activation of all back extensor muscles.