Optimization of chest deflection definition in human body models: Implication for pedestrian injury assessment

Pedestrian chest injuries remain a significant global concern, ranking as the second leading cause of death in traffic accidents. Although human body model (HBM) provides a strain-based methodology for predicting pedestrian chest injuries, conventional deflection-based injury metrics remain valuable due to their hard validation of HBM biofidelity at the tissue response level. Consequently, achieving accurate chest injury assessment through HBM technology represents a critical biomechanical research challenge. This study aims to identify the optimal method for defining thorax deflection in HBM to improve injury assessment in vehicle-pedestrian collisions.

A simulation matrix was designed, incorporating 5 different vehicle front-end types, 3 impact angles, 4 collision speeds, and 2 pedestrian postures, resulting in a total of 120 pedestrian-vehicle collision simulations. The simulations utilized the Total Human Model for Safety version 4.02 pedestrian model and predefined chest bands spanning from the upper to the lower chest. A total of 4 distinct methods for defining chest deflection were tested across the simulations. Principal component analysis was applied for dimensionality reduction to further refine the analysis. Two biomechanically distinct injury risk functions were employed to assess chest injury: the strain-based method, incorporating principal strain distributions across 12 ribs, and the deflection-based method, based on our pre-defined 6 chest band measurements.

Our experimental findings reveal that injury risk decreases initially as the bonnet leading edge height increases, reaching a minimum around 950 mm with abbreviated injury scale 3+ and the number of fractured ribs (NFR) 3+ risk below 0.2, and then increasing thereafter. The study also demonstrated minimal chest injury risk acrossNFR2+/NFR3+ and NFR7+ conditions, with NFR risk below 0.2, despite localized high rib strain observed in certain individual ribs up to 8.28 × 10^-3. Meanwhile, A newly developed index termed PC_deflection, which is derived from the sum of the products of the weighting factors corresponding to 2 optimal thorax deflection measurement methods with NFR3+ correlation coefficients of 0.743 and 0.753, was identified as an effective predictor of injury risk in HBM simulations.

This study successfully determined the optimal method for measuring thorax deflection for HBM chest injury prediction, which could serve as a complementary metric for pedestrian chest injury assessment.