Simulation Analysis and Prediction of Lumbar Spine Injury Biomechanics in Upright, Reclining, and Lying Postures Under Lunar Reentry Loads

This study aims to investigate the biomechanical impact of lying, reclining, and upright seating postures on lumbar spine injuries under lunar reentry loads. According to the aerospace medicine selection standard, a detailed three-dimensional finite element model of the lumbar spine including vertebrae, intervertebral discs, and ligaments was established. The model was verified by the range of motion test and the spinal experiments test. Subsequently, the biomechanical effects of astronauts’ lumbar injuries in lying, 50° reclined, and upright seating postures were analyzed for the two kinds of reentry loads of Chang’e-5 T1 (CE-5T1) vehicle and the Apollo 10 spacecraft, and the evaluation and prediction of lumbar spine injuries were conducted in conjunction with the biological tissue injury criteria. The simulation results indicated that, under both loads, the stress values of vertebrae in lying, 50° reclined, and upright seating postures increased, and the rate of increase rose. When the peak acceleration of CE-5T1 approached 9 G, the vertebral stress reached the maximum with 104.85 MPa, 99.32 MPa, and 80.21 MPa in the lying, 50° reclined, and upright seating postures, respectively. The vertebral displacement also increased from 11.13 mm to 13.77 mm and 15.22 mm, which increased by approximately 23.2% and 36.7% compared to the lying posture. Under both reentry loads, the maximum stress on the intervertebral discs was located at the L4–5 segment. Combined with the Axis Dynamic Response Criteria for spinal injury (MDRC), the MDRC values in the upright seating posture were 0.71 and 0.58 for CE-5T1 and Apollo 10 reentry loads, respectively. While these values did not reach the spinal injury threshold of 1, they were higher than those for the lying and 50° reclined seating postures, indicating that reentry load in upright seating posture may cause damage to the lumbar and pose a risk to astronauts’ health and safety. This study provides scientific evidence for the design of return capsule seats, recommending that the sitting posture angles be carefully considered for lumbar biomechanics to reduce the risk of lumbar injury under high reentry loads and ensure the health and safety of astronauts.