Effect of hypergravity on the biomechanics of the musculoskeletal system in human lumbar spine: a numerical study

The aim of this study was to investigate the effects of hypergravity on the biomechanics of the human lumbar spinal musculoskeletal system. We quantitatively analyzed the biomechanics of the intervertebral disc (IVD), lumbar spinal muscles, and ligament biomechanics changes in jet pilots under various hypergravity conditions, namely, 1G (represents the normal gravity on the Earth), 3G, 6G and 9G (represents various hypergravity), strengthened 10%-3G/6G/9G (various hypergravity after muscle strengthened by 10%), strengthened 20%-3G/6G/9G (various hypergravity after muscle strengthened by 20%) using a full body musculoskeletal modeling approach, and whether these biomechanical changes lead to lumbar IVD damages with a finite element method. The compressive and shear forces on the lumbar IVD, the height and water content of the lumbar IVD, muscle activation, and ligament force, as well as the mechanical damage in the IVD under the above various hypergravity were quantitatively compared and analyzed. Compared with 1G, hypergravity produced dose-dependent changes in lumbar spine biomechanics and IVD integrity. Under 3G, 6G, and 9G, average lumbar IVD compressive forces increased by 68%, 190%, and 311%, while shear forces increased by 53%, 140%, and 212%, respectively. IVD height decreased by 3.0%, 8.0%, and 12.4%, and water content decreased by 2.4%, 7.2%, and 12.4%. Most lumbar muscle activations increased by 3.0%, 7.0%, and 13.0%, whereas most ligament forces decreased by 25.0%, 34.0%, and 40.0%. AF damaged volume fraction increased by 0.5%, 3.7%, and 7.3%. IVD stress also increased markedly, with AF stress rising by 73%, 215%, and 358%, and NP stress by 58%, 163%, and 267% under 3G, 6G, and 9G, respectively. The muscle strengthening after training showed no significant change on the loading distribution among different musculoskeletal segments under hypergravity. Our results indicated that loading on the IVD significantly increased, biochemical environment and morphology within the IVD changed, and the risk of annulus fibrosus damage increased; muscle strengthening did not have a significant impact on the load on various tissues under hypergravity. Our findings were important for understanding the hypergravity-induced effect on the health of human lumbar spine.