Paraspinal muscle activation in accordance with mechanoreceptors in the intervertebral discs.

Paraspinal muscle forces were derived computationally based on the hypothesis that the intervertebral disc has a transducer function and the muscle is activated according to a sensor-driving control mechanism. A three-dimensional finite element model of the musculoskeletal system, which consisted of a detailed whole lumbar spine, pelvis, simplified trunk model, and muscles, was developed and combined with an optimization technique to calculate muscle forces in isometric forward flexed and erect standing postures. Minimization of deviations in the nucleus pressure and averaged tensile stress in the annulus fibers at five discs was used for muscle force calculations. The results indicated that all the muscles were properly activated to maintain posture and stabilize the lumbar spine. The nucleus pressure difference was decreased during the iterative calculations and its resulting value at the L4/L5 level was consistent with in vivo measurements. Muscle activation produced vertebra motion, which resulted in a relatively uniform stress distribution in the intervertebral discs. This can minimize the risk of injury at a specific level and increase the ability of the spine to sustain a load.