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.

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The kinetic redundancy in human musculoskeletal trunk system allows for the active control of posture during a specific task while satisfying equilibrium and stability requirements. Such control could aim, for example, to minimise forces in active musculature and stresses in passive tissues. It results in a superfluous system of equations the solution of which not yet satisfactorily achieved despite its importance in evaluation and treatment of spinal disorders. Due to shortcomings in existing reduction, optimisation and EMG-driven models, and combination thereof, a novel kinematics-based finite element approach [1,2] is employed that fully accounts for the synergy between passive and active trunk sub-systems. In this study, the kinematics-based model is applied to determine spinal muscle forces and internal ligamentous passive loads at different disc levels in standing postures under gravity loads with and without 200N weights carried in both hands (positioned symmetrically close to the body, either anteriorly or on sides).