Level-Specific Lumbar Spine Injury Tolerance Under High-Rate Vertical Loading Relevant to Underbelly Blast Environments: A Combined Cadaveric Experimental and Finite Element Study of Three-Vertebra Spinal Segments

PURPOSE: This study aimed to investigate lumbar vertebral level-specific fracture susceptibility under high‑rate vertical loading using a combined experimental-computational approach. METHODS: = 0.93-0.98) and fracture loads (within 10-34% of experimental data). Parametric simulations were then performed to assess the effects of spinal posture (flexion, extension, lateral bending), vertebral geometry (height and cross-sectional area), and areal bone mineral density on tolerance of the lumbar spine to compression-type fractures. RESULTS: Spinal posture exerted a strong, level‑dependent influence on fracture risk. Flexion generally reduced injury likelihood at L1, L2, and L4, whilst extension increased the risk, with L3 and L5 showing inverse or unique behaviour. Lateral bending increased fracture susceptibility, particularly at L5. Increased vertebral height reduced axial stiffness and fracture load, except at L5, whilst larger cross-sectional area consistently enhanced mechanical resistance. Areal bone mineral density was a strong positive predictor of compressive fracture tolerance across all levels. Level-specific injury prediction curves were developed using survival analysis and showed position-dependent trends, with inferior vertebrae generally more resistant, except L5, which remained highly vulnerable. CONCLUSION: This study provides the first vertebral level-specific injury prediction curves under dynamic axial compression and highlights the roles of posture, geometry, and bone quality in lumbar fracture tolerance. These findings offer insights for injury prediction and protective system design in defence contexts.