Effect of Postural Differences on Biomechanical Injury Probability in Aerospace Vehicle Occupants

Spacecraft occupants may be exposed to potentially injurious dynamic loads during missions, particularly during land and water landings. Crewmembers ingress their seats in microgravity with few positioning cues to conform to their seats, and may experience asymmetric or inadequate restraint tensioning, thus varying their posture during landing. The effect of an occupant's spinal posture on their head-neck-spine complex injury probability from experiencing dynamic loads is investigated in this computational study. The Global Human Body Model Consortium's 50th anthropometric percentile male (simplified occupant) model was used as the human surrogate. Three postures were examined - an upright posture, a head-extended posture with a kyphotic spine ('bent-forward'), and a quasi-submarined posture with the upper back leaning into the test seat ('slouched-back'). Each posture was subjected to vertical (+Gz inertial), supine (+Gx inertial) and oblique-sagittal loads (+Gxz inertial) peaking at 15g in a rise time of 65 milliseconds. Based on these simulations, head injury criterion (HIC-15), neck injury criterion (Nij) and peak resultant intervertebral thoracolumbar forces and corresponding injury probabilities were calculated. When experiencing vertical (+Gz) loading, the bent-forward posture registered the highest peak vertebral load of 6100 N at T2-T3 joint, and a total injury probability (TIP) of 99.96%. When subjected to supine (+Gx) loading the slouched-back posture elicited a peak of 4140 N (L4-L5). The head contacted the headrest in all postures in supine loading, recording a minimum HIC-15 value of 951 (slouched-back), resulting in a 100% TIP for all analyzed postures. The oblique-sagittal loading condition yielded the least TIPs of 12.26%, 47.0% and 12.43% for bent-forward, upright and slouched-back respectively. The neck was most prone to injury in upright posture, with an average Nij of 0.97 across all loading profiles, compared to 0.133 in slouched-back and 0.07 in bent-forward postures. The differences in thoracolumbar intervertebral forces and neck injury criteria across postures are significant. The lower back (L4-L5 and T10-T11) experienced the highest forces in the majority of analyzed cases. Based on the loading profiles examined in this study, an occupant's posture is correlated with peak magnitude of force and injury probability. An occupant head-neck-spine complex is least likely to injure in the slouched-back posture or in oblique-sagittal loading.