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The objective of this study was to quantitatively analyze the effect of lumbar spinal muscle atrophy on the compressive (perpendicular to the upper surface of the disc) and shear (parallel to the upper surface of the disc in the anterior-posterior direction) forces change on lumbar intervertebral discs using a full body musculoskeletal modeling approach. Muscles atrophy was modeled with reduction of the functional cross-sectional area (FCSA) of the muscles. Compressive and shear forces under two levels of lumbar muscle atrophy (20% and 40%) at eight daily postures (lying on back, seating slouched, seating straight, standing, standing flexed (36°), standing lift a 20 kg weight close to chest, standing lift a 20 kg weight flexed (38°), and standing lift a 20 kg weight with arm stretched) were analyzed. There was small increase in compressive forces on lumbar discs with muscle atrophy at most postures except lying and sitting straight. The maximum increase of compressive forces on lumbar discs were 23 N (6%), 28 N (5%), 34 N (2%), 71 N (6%), 89 N (4%), and 190 N (10%) with 20% atrophy, and 66 N (19%), 77 N (12%), 98 N (6%), 169 N (14%), 256 N (12%), 501 N (24%) with 40% atrophy at seating slouched, standing, standing flexed, standing lift close, standing lift flexed, and standing stretched arm, respectively. The shear force did not change significantly on lumbar discs with muscle atrophy. This study is important for understanding the biomechanical mechanisms of how lumbar muscle atrophy may affect the lumbar IVD health.

The fixed charge density (FCD) in the intervertebral disc (IVD) matrix is essential for its capacity of absorbing water, particularly during overnight bed rest. However, the FCD decreases with IVD degeneration, reducing the disc propensity to swell and the related convective transport of molecules across the IVDs. The objective of this study was to investigate the effects of the FCD on water intake in the IVD during bed rest. A multibody musculoskeletal model was extended to include the osmotic properties of the IVDs, and used for the analysis of IVD swelling and its water content in a human subject during bed rest. The simulations were conducted with both healthy lumbar IVDs and lumbar IVDs with a reduced FCD. It was predicted that a decrease in the FCD had a considerable impact on the IVDs swelling during bed rest. A 20% and a 45% reduction in the FCD resulted respectively in an average 25% and 55% reduction of disc water intake overnight. This study provided an additional, quantitative information on IVD swelling in human subjects during bed rest. The computational model presented in this paper may be a useful tool for estimating disc hydration at different loading and pathological conditions.