Estimation of muscle forces in the lumbar spine during upper-body inclination

Objective. To estimate the muscle forces during upper-body inclination and to determine their influence on stress distribution in the annulus fibrosus of the lumbar spine discs. Design. The muscle forces and stresses were calculated using a non-linear f inite element model of the lumbar spine. Background. Little is known about the influence of muscle forces on the def ormation of, and stresses in, the lumbar spine. In most studies, muscle for ces are neglected. Methods. Three-dimensional non-linear finite element models of the ligament ous lumbar spine, with and without internal spinal fixators, were created. They were validated by use of experimental data From in vitro measurements on cadaver specimens. In a second step, the influence of muscle forces on s tresses in the annulus fibrosus of the lumbar spine discs was investigated in a parameter study. This was done for different inclination angles of the upper-body. Results. Good agreement between analytical and experimental results proved achievable when loading with pure moments in the three main planes of the l umbar spine. For inclination of the upper-body, the flexion angle clearly h as a strong influence on the stresses in the lumbar spine while the influen ce of local muscles was small. The stress distribution in the discs differe d considerably when the muscle forces are neglected and only a pure moment is applied. Conclusions. This study confirmed earlier ones that have shown that muscle forces should not be neglected when studying the stresses in the lumbar spi ne. The local dorsal muscles, however, have only a small influence on the s tresses in the discs.