Modification and Validation of the Neck Soft Tissue Models in the SAFER HBM

Abstract

Cervical spine injuries are common in motor vehicle collisions. Human body models (HBMs) are used in virtual crash simulations to estimate injury risks. The SAFER (Vehicle and Traffic Safety Center at Chalmers) HBM (SHBM) is one such model capable of simulating occupant kinematics and evaluating injury risks. Within the SAFER research initiative, the cervical spine model is being updated under the project “Improving Neck Injury Prediction in Car Crashes Using SAFER HBM.” Neck response is influenced not only by the cervical spine but also by surrounding soft tissues such as muscle, fat, and skin. The aim of this thesis is to investigate and modify the material model and the model geometry of the SHBM neck soft tissue, then validate the biofidelity of the updated neck soft tissue model under both low and high acceleration crash scenarios. Head-neck range of motion (ROM) experiments conducted with human volunteers were replicated using SHBM, and the resulting neck responses were analyzed to understand the effect of the neck soft tissue to neck ROM. A parameter study was carried out to adjust the soft tissue parameters and achieve better agreement between the SHBM neck response and the experimental ROM data. The updated parameters obtained from the parameter study were further calibrated by simulating post-mortem human subject (PMHS) tests using the SHBM under high-acceleration loading conditions. Design modifications such as refining the neck soft tissue mesh and incorporating the nuchal ligament were subsequently implemented to enhance model biofidelity. Finally, a validation study was conducted by simulating both low- and high-speed PMHS crash tests using the original SHBM model geometry with the most promising parameter configurations. ROM simulations revealed that the SHBM neck exhibited higher resistance to extension and axial rotation compared to experimental results. Following parameter tuning and calibration, a notable reduction in neck moments was achieved. Validation simulations using the updated parameters yielded responses that closely matched experimental reference values. Although material updates improved neck response, further refinement particularly of the strain-rate-dependent skin model could enhance biofidelity across varying loading rates. Exploring alternative neck flesh material models and refining nuchal ligament design are also promising directions. A key limitation is that SHBM represents only 50th percentile male, limiting generalizability. Extending future investigations to account for population diversity would improve model applicability. In conclusion, the current SHBM version (v11.1.0) exhibits neck responses under low acceleration conditions that are stiffer than reference values, and softer responses under high acceleration conditions than reference values. The calibrated material parameters resulted in response that was better aligned with experimental data. Minor influence on head forward excursion in validation simulations. The addition of the nuchal ligament improved flexion behavior and is recommended for future versions.