The mechanical stability of the lumbar spine provided by the muscle (active system) and corresponding reaction forces produced in the passive subsystem (ligaments and facet joint) was examined during isometric forward flexed and erect standing postures. Paraspinal muscle forces were derived based on the hypothesis that the disc has a mechanoreceptor function and the paraspinal muscle would be activated according to a sensor driving feedback mechanism. A three dimensional musculoskeletal model of the body including a detailed lumbar spine finite element model was developed. Optimization technique was adapted to calculate muscle forces. The results indicated that all the muscles were properly combined to maintain posture and stabilize the lumbar spine. Deep muscles, which directly connected to each vertebra, mainly produced compressive or tensile force on each motion segment, and the passive elements were proven to mainly resist the shear force.