Muscle force evaluation and the role of posture in human lumbar spine under compression.

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.