A follower load as a muscle control mechanism to stabilize the lumbar spine

The spine is a long and slender column which supports the upper body. In the 1960s, the compressive force acting on the spine estimated by the intradiscal pressure measured in vivo was found to exceed 2600N. However the ligamentous lumbar spine is known to be unstable when subjected to compressive loads of 88 N. It is generally agreed that the ligamentous spine itself is unstable but can be stabilized by the muscle forces (MFs) in vivo. Biomechanical loads are known to be closely associated with spinal disorders. Normal spinal loads, however, remains poorly understood due to the lack of knowledge of the MF control mechanism for normal biomechanical functions. To determine the spinal MFs creating compressive follower loads (CFL) in the lumbar spine in various sagittal postures and to investigate if such MFs can maintain the spinal stability, 3-D optimization and FE models of the spinal system (trunk, lumbar spine, sacrum, pelvis, and 232 muscles) were developed and validated using reported experimental data. Optimization models were used to determine the MFs creating CFLs in the lumbar spine in various sagittal postures from 10° extension to 40° flexion. The deformation of the lumbar spine under these MFs and trunk weight was predicted from FE models. The stable lumbar spine deformation was determined by the resultant trunk sway less than 10 mm. Optimization solutions of MFs, CFLs, and follower load path (FLP) location were feasible for all studied postures. The FE predictions clearly demonstrated that MFs creating CFLs along the base spinal curve connecting the geometrical centers(GCs) or along a curve in its vicinity (within anterior or posterior shift by 2 mm) produce the stable deformation of the lumbar spine in the neutral standing and flexed postures, whereas the MFs creating the smallest CFLs resulted in the unstable deformation. In case of extended postures, however, it was not possible to find the CFL creating MFs that produce stable