Effect of ligament stiffness on spinal loads and muscle forces in flexed positions

Abstract

Ligaments assist muscles in stabilizing the spine within physiological ranges of motion by limiting the displacements, but the role of ligaments in spinal loads and muscle force distribution remains unknown. The purpose of this study was to investigate the effect of different stiffness on joint resultant forces and muscle forces in different flexed positions. For this study, five ligament stiffness sets were determined from the literature and applied to a musculoskeletal spine model. The dimensions of the model were adjusted according to subjects in the in vivo experiments used for validation, and spinal loads and muscle forces were determined during flexed positions. The differences between the spinal loads due to different ligament stiffnesses were insignificant (maximum difference 12%). However, the different ligament stiffnesses showed a strong effect on individual muscle forces. Among the short muscles, lumbar multifidi exerted only 65 N without ligaments but the force increased up to 254 N due to adding the maximum ligament stiffness. However, the load in the erector spinae was significantly decreased (30%). The results of this study showed that in addition to long and superficial muscles, ligaments also played an important role in stabilizing the spine in flexed positions.