Using two nonlinear finite element models of the lumbar spine, the concept of optimal posture is explored by minimizing the segmental sagittal moments required for the equilibrium of the passive lumbar spine under a total of 2800 N axial compression while varying the pelvic tilt and lumbar lordosis. The redundant active-passive system is subsequently solved for this posture using a novel kinematics-based muscle calculation algorithm along with minimization approach. Some flattening in the lumbar spine substantially reduces the required moments and internal passive shear forces under 2800 N axial compression force. Small muscle forces are calculated for this optimal posture. The role of flattening in the lumbar lordosis and posterior pelvic tilt in diminishing the lumbar muscle activities in neutral postures is demonstrated. Without such changes in posture, the required moments probably exceed the moment-generating capability of local lumbar muscles. Consideration of such active-passive synergy and lack of its restriction may prove crucial in many activities. Moreover, a kinematics-based algorithm is proposed for the solution of spinal redundancy that fully accounts for the existing passive-active synergy while simultaneously satisfying all kinematics and equilibrium conditions along the length of the spine.

The coupled load sharing-posture response of the human lumbar spine under relatively large compression loads with and without sagittal rotations remains yet to be adequately understood. Such investigations should account for the observed changes in the pelvic tilt and lumbar lordosis and influences thereof on the passive-active equilibrium and stability of the human spine. In pursuit of these goals, the objectives of the current work, using a nonlinear finite element model of the lumbosacral spine, are set as follows: - Study of the synergy of the active-passive lumbar spine under an axial compression load of 2800 N in quasi-neutral position; - Development of an optimal posture accounting for the postural adaptation of the spine in order to withstand the applied 2800 N load in quasi-neutral posture; and - Evaluation of local lumbar muscle forces in order to maintain the equilibrium of the system in such optimal posture.