An enhanced and validated generic thoraco-lumbar spine model for prediction of muscle forces

A direct measurement of the complete loads in the spine continues to remain elusive. Analytical musculoskeletal models to predict the internal loading conditions generally neglect or strongly simplify passive soft tissue structures. However, during large intervertebral motions, passive structures such as ligaments and the stiffness of the intervertebral discs are thought to play a critical role on the muscle forces required for equilibrium. The objective of the present study was to add the short segmental muscles, lumbar ligaments and disc stiffnesses to an existing base musculoskeletal model of the spine in order to establish what role passive soft tissue structures play in spinal loading, but also validate these results against experimentally determined load data. The long trunk muscles not included in previous models, short segmental muscles, lumbar ligaments and disc stiffnesses were implemented into a commercially available musculoskeletal spine model construct. For several activities of daily living, the loads acting on the vertebral bodies were then calculated relative to the value for standing, and then compared to the corresponding values measured in vivo. Good agreement between calculated and measured results could be achieved in all cases, with a maximum difference of 9%. The highest muscle forces were predicted in the m. longissimus (146N) for flexion, in the m. rectus abdominis (363N) for extension, and in the m. psoas major (144N and 81N) for lateral bending and axial rotation. This study has demonstrated that the inclusion of the complete set of muscle and ligament structures into musculoskeletal models of the spine is essential before accurate spinal forces can be determined. For the first time, trend validation of spinal loading has been achieved, thus allowing confidence in the precise prediction of muscle forces for a range of activities of daily living.