We presented a novel model of level-specific, nonlinear functional spinal stiffness and integrated this model in an existing rigid-body model of the thoracolumbar spine (https://simtk.org/projects/spine_ribcage). Also stiffening effects of compressive loading and thoracic spine interaction were successfully integrated. All 6-DoF of the intervertebral joints spanning from T1 to S1 were enabled.
The stiffness formulations are comprehensive enough to capture the physiological response of spinal kinematics under various loading conditions, and are also simple and generic enough to be used for various spine modelling studies.

For more information, please see our paper:
Implementation of physiological functional spinal units in a rigid-body model of the thoracolumbar spine. Journal of Biomechanics. https://doi.org/10.1016/j.jbiomech.2019.109437