Effects of muscle dysfunction on lumbar spine mechanics: A finite element study based on a two motion segments model

Study Design. A combined finite element and optimization approach was developed to investigate the clinically relevant biomechanical parameters of the muscular lumbar spine under five quasistatic back-lifting conditions. Objectives. To quantify the effects of muscle 'dysfunction' on the mechanical behavior of the lumbar spine. Summary of Background Data. Trunk muscles have been proven to play an important role in the normal functioning of the spine. Although passive structures of the spine are believed to be subjected increasingly to mechanical stresses when muscular support is inadequate, supportive quantitative data have been lacking. Methods. External loads at L3-L4 for various lifting tasks were estimated experimentally and partitioned to the disc and muscles across the L3-L4 segment using an optimization scheme. These forces were incorporated into a finite element model of the ligamentous L3-L5 lumbar spine. Muscle 'dysfunction' was simulated by decreasing the computed muscle forces. Results. The range of motion, intradiscal pressure, forces in ligaments, and load across facets increased nonlinearly with the increases in trunk flexion and the load held in hands. At higher loads or at larger flexed postures, muscles were found to play a more crucial role in stabilizing the spine compared with the passive structures. Muscle 'dysfunction' destabilized the spine, reduced the role of facet joints in transmitting load, and shifted loads to the discs and ligaments. Conclusions. Muscle dysfunction disturbs the normal functioning of other spinal components and may cause spinal disorders.