For stability analysis of the lumbar spine, a hypothesis is presented that the disc has stress sensors driving a feedback mechanism which can react to the imposed loads by adjusting the contraction of muscles. A three dimensional model of the musculoskeletal system with a detailed lumbar spine finite element model was combined with an optimization technique to calculate muscle forces. The musculoskeletal model consisted of a detailed whole lumbar spine, pelvis, and simplified trunk model. For computational efficiency, the vertebral body and pelvis were modeled as a rigid body and rigid truss elements were used for the rib cage construction. Minimization for deviation of nucleus pressure or Tresca stress in the nucleus was chosen for muscle force calculations. The results indicate that the originally C-shaped lumbar spine was flattened at the upper level, while the more lordotic curvature was generated at the lower level. Muscle forces generated not only in deep muscles, but also in extensor muscles, play an important role in sustaining the lumbar spine to the external load. The resultant forces acting in each vertebra show somewhat different magnitudes and directions compared to the follower load that mimics the deep muscle contraction in in-vitro experiments.