Effect of lumbar muscle atrophy on the mechanical loading change on lumbar intervertebral discs.

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.