## 3 Results
### Injury Response of Occupants in Zero-Gravity Seats with 3-Point Seat Belt  
Table 3 illustrates the head and neck injury metrics of occupants under different impact speeds. In general, the head and neck injury metrics were below their thresholds for both low- and medium-impact speeds. For instance, when the impact speed was 40 km/h, the BrIC, N\({}_{\text{ij}}\), and N\({}_{\text{km}}\) were 0.46, 0.23, and 0.57, respectively, and all of them were smaller than 1. The value of CSDM (0.25) was 21%, which corresponds to the 7% probability of diffuse axonal injury occurrence. The NIC metric was 14.9 m\({}^{2}\)/s\({}^{2}\), which was close to the threshold of 15 m\({}^{2}\)/s\({}^{2}\) specified by the Euro-NCAP. However, when the impact speed increased to 56 km/h, the values of BrIC, N\({}_{\text{ij}}\), and N\({}_{\text{km}}\) increased to 1.16, 0.49, and 3.08, respectively. The values of BrIC and N\({}_{\text{km}}\) surpassed their critical thresholds. The value of CSDM (0.25) increased to 58%, and the probability of diffuse axonal injury was greater than 50%. It is worth mentioning that neck injury mechanisms evolved as the impact speed increased. When the impact speed was 16 km/h, the value of N\({}_{\text{ij}}\), governed by axial force and bending moment at the upper neck, and the value of N\({}_{\text{km}}\), governed by shear force and bending moment at the upper neck, were close. When the impact speed increased to 40 km/h, N\({}_{\text{km}}\) became the dominant injury metric because of the head-neck forward flexion caused by the head rebound between 160 and 180 ms, as shown in Figure 9. When the impact speed became 56 km/h, as illustrated in Figure 10, N\({}_{\text{km}}\) surged to 3.08 due to the collision between the neck and the edge of the headrest.  
Table 4 illustrates the biomechanical evaluation of the cervical ligaments under low-, medium-, and high-impact speeds. With respect to the low- and medium-impact speeds, the headrest was able to provide good support to the head, and the maximum principle strains in five types of cervical ligaments were below the threshold values [37], which were 0.35, 0.34, 1.48, 0.88, and 0.68 for the anterior longitudinal ligament (ALL), posterior longitudinal ligament (PLL), capsular ligament (CL), ligamenta flava (LF), and interspinous ligaments (ISLs), respectively [37]. When the impact speed increased to 56 km/h, all ligament strains were above their thresholds, except for the LF. As illustrated in Figure 10, the large strains were induced by the collision between the head and neck and the headrest at 100 ms. The above comprehensive biomechanical analysis demonstrated that the conventional three-point seat belt system could not effectively protect the occupant under high-speed  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Speed (km/h)** & **BrIC** & **CSDM (0.25) (vol%)** & **N\({}_{\text{ij}}\)** & **N\({}_{\text{km}}\)** & **NIC (m\({}^{2}\)/s\({}^{2}\))** \\ \hline
16 & 0.14 & 0.6 & 0.16 & 0.14 & 6.48 \\
40 & 0.46 & 21 & 0.23 & 0.57 & 14.9 \\
56 & 1.16 & 58 & 0.49 & 3.08 & 72.7 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Summary of head and neck injury metrics for the zero-gravity seat.  
impact. Both the occupant's head and neck experienced severe injuries, evidenced by the high values of the head and neck injury metrics.