## 2 Materials and Methods  
Validation of the Finite Element Model of Occupant Restraint Systems with a Regular Seat Model and a Three-Point Seat Belt System  
The validity of the finite element model of occupant restraint systems directly affects the prediction accuracy of occupant kinematics and subsequent injury risk assessment during collision events. The Total Human Model for Safety (THUMS ver6.1) developed by Toyota was employed as the simulation model, and the seat model was derived from the 2012 Toyota Camry full-vehicle model [16], a publicly available vehicle model widely utilized by the global safety and crash research community. The THUMS incorporates detailed anatomical structures, including skeletal components, muscles, internal organs, ligaments, and soft tissues, which enable the comprehensive simulation of biomechanical behaviors across human tissues. As an initiative study to understand an occupant's kinematic and injury responses under different rear-impact speeds, only the 50 th adult male human model was considered since it represents one of the largest population groups. During this study, both the seat and biomechanical models were validated by Postmortem Human Subject (PMHS) experiments from [PERSON] et al. [17,18], and PMHS2 was used as the reference anatomy. The H-point position of the THUMS was adjusted to an 80 mm vertical distance from the head center of gravity to the head restraint upper surface with a 55 mm minimum horizontal gap between the head and head restraint surface, ensuring alignment with the PMHS test configurations. The arms of the THUMS were positioned to naturally hang alongside the thighs through a postural adjustment process. The parameters of the seat belt system are shown in Table 1. A three-point seat belt system without retractors or pretensioners constrained the human body model in the seat. Standard adult seat belts were utilized with 48 mm width and 1.2 mm thickness. The retractor was positioned 72 mm inward from the seatback structural frame. A shoulder guide loop was installed 183 mm lateral from the seat centerline at the headrest's right side, positioned slightly above the backrest upper edge. The guide loop and buckle components were rigidly connected to the seat frame structures. The seat model and human body were positioned and matched in Hyperworks, and the completed finite element model is shown in Figure 1a. The prescribed acceleration pulse [17] in Figure 1b was applied to the vehicle floor. Acceleration and displacement history curves of the head center of gravity and cervical vertebrae were recorded. \begin{table}
\begin{tabular}{c c c c} \hline
**Seat Belt Thickness** **(mm)** & **Seat Belt Width** **(mm)** & **Retractor to Seatback** **(mm)** & **Shoulder Guide Loop to Centerline** **(mm)** \\ \hline
1.2 & 48 & 72 & 183 \\ \hline \end{tabular}
\end{table} 
Table 1: Seat belt system parameters. Figure 1: **(a)** Finite element model assembly for validation; **(b)** sled acceleration pulse. [17]. After the creation of the finite element simulation model, the model was analyzed in LSDYNA, and the corresponding animation result files and time-history data were obtained. As shown in Figure 2, the simulated animation results postprocessed in Hyperview were directly compared with the video footage taken by the high-speed camera at 0, 60, 80, 100, 120, 140, and 160 ms in the PMHS tests [18]. In the PMHS tests, from 0 to 60 ms, the occupant's chest was pushed forward by the seatback, but the occupant's head was delayed due to the inertia effect and remained in the initial position. In the THUMS analysis, during that period, the occupant's chest moved forward while the head remained stationary, illustrating a similar kinematic response with the PMHS tests. From 60 to 120 ms, the PMHS occupant's upper body moved further forward, and the occupant's head began to contact the headrest around 60 ms. After that, the occupant's head remained in contact with the headrest.