Previous in-vivo studies showed that compressive force acting on the spine may exceed 2600 N. However, the ligamentous lumbar spine becomes unstable when subjected to compressive loads less than 100 N. It is generally accepted that the ligamentous spine itself is unstable but can be stabilized by muscle forces (MFs) in vivo. Nevertheless, normal spinal muscle contraction patterns remain unknown. In recent in vitro studies, when the direction of the applied load was controlled along the spinal curvature so that the internal spinal load became perfect compressive follower loads (CFLs) at all lumbar levels, the ligamentous lumbar spine was found to withstand large compressive load (up to 1200 N) without buckling while maintaining its flexibility in neutral or flexed postures. The results of in-vivo animal studies also have shown that shear stress has a more detrimental effect on the rate of disc degeneration compared to compressive stress. These results suggest CFLs in the lumbar spine would be a normal spinal load whereas the transverse (or shear) load abnormal. An initial test of this postulation would be to investigate whether the spinal muscles can create perfect internal CFLs in the lumbar spine in all 3-D postures. In addition, small intrinsic muscles (SIMs) are crucial for better control of the direction of the internal spinal load along the spinal axis was also proposed. A finite element (FE) model together with an optimization model were used for this study. Both models consist of the trunk, sacrolumbar spine and 244 spinal muscles. Different from other studies, 54 SIMs were also included in the models. The FE model was validated by comparing the ROM of the spine with the literature data. Minimization of the summation of the spinal loads and moments was used as the cost function for the optimization model. The geometrical data obtained from the FE model was used as the input for the optimization model; it was then used to calculate the MFs required for creating the CFLs at all lumbar spine levels. The MFs determined in the optimization model were then imported back to the FE model as input loads to check the stability of the spine under this loading condition. Five different postures were studied: neutral, flexion 40°, extension 5°, lateral bending 30° and axial rotation 10°. Many optimization solutions for spinal muscle force combinations creating pure CFLs in the lumbar spine were found available in each posture. However, FE analyses showed that only muscle forces and patterns solved at FLPs along the curve in the vicinity of the baseline curve stabilized the lumbar spine. Stability was determined by small displacement of the trunk (less or equal to 5mm) due to small deformation of the lumbar spine. The magnitudes of joint reaction forces (JRFs) predicted from the optimization model were comparable to those reported in the literature. When the SIMs were removed, optimization solutions were still feasible in all five postures, but JRFs and trunk displacement were increased. This suggests the need of SIM inclusion in future spine biomechanics studies and clinically, damages to the SIMs may have a high risk of future spinal problems, such as spinal instability, early disc degeneration, deformity and/or early failure of spinal fixation devices. The results from this study supported the hypothesis that the perfect CFLs at all lumbar levels could be the normal physiological load under which the lumbar spinal column could support large load without buckling while allowing flexibility. SIMs played an important role in creating CFLs as by including SIMs in the models, the JRFs at all lumbar spine levels were lowered and the stability of the spine was increased.

La lombalgie constitue une affection fréquente de la population adulte des pays industrialisés. Les médias parlent souvent de « mal de dos, mal du siècle ». Les études épidémiologiques révèlent en effet que la majorité des individus souffriront un jour du dos. La lombalgie présente généralement une évolution naturelle favorable. Les répercussions socio-économiques majeures associées aux douleurs lombaires, et qui conduisent certains à employer le terme de « fléau socioéconomique », résultent des lombalgies persistant plus de 3 mois (lombalgies chroniques) qui affectent pourtant moins de 10% des lombalgiques mais dont le traitement constitue un défi considérable pour le monde médical et la société. Ces observations ont motivé notre recherche relative à lévaluation et la revalidation de la lombalgie chronique. En effet, malgré une littérature scientifique abondante, ce sujet reste méconnu et parfois même controversé. Après avoir précisé létat de la question (chapitre II), notre contribution personnelle (chapitre III) concerne lévaluation et la rééducation musculaire du lombalgique chronique. Notre contribution comporte trois parties : La PREMIERE PARTIE déterminera les tests les plus adaptés pour évaluer les muscles extenseurs du tronc de lombalgiques chroniques. A cette fin, nous avons appliqué la procédure suivante : Etape 1 : revue de la littérature, analyse critique et sélection des évaluations disponibles. Etape 2 : analyse comparative des tests sélectionnés sur des sujets sains afin dapprécier leur faisabilité et leur reproductibilité. Etape 3 : étude de la reproductibilité des tests sélectionnés sur des patients lombalgiques chroniques. Etape 4 : évaluation de la sollicitation cardio-vasculaire des épreuves. Etape 5 : validation des épreuves en examinant la sollicitation musculaire. La SECONDE PARTIE appréciera lampleur du déconditionnement physique du sujet lombalgique chronique tout en précisant son caractère global ou local. Cette étude compare, chez des sujets lombalgiques et sains appariés, les performances des extenseurs du tronc mais également dautres muscles du tronc et des membres inférieurs ainsi que du système cardio-respiratoire. La TROISIEME PARTIE examinera lefficacité lefficacité de la nouvelle prise en charge multidisciplinaire (octobre 2004) proposée par le service de Médecine Physique du CHU de Liège aux patients lombalgiques chroniques. Lors de la discussion générale (chapitre IV), nous établirons des recommandations relatives à lévaluation et au traitement des lombalgiques chroniques.