## 3 Results
### Occupant Responses in Zero-Gravity Seat with Four-Point Seat Belt at High-Speed Rear Impact  
Figure 11 shows the kinematic responses of the THUMS restrained in the zero-gravity seat by the three-point seat belt system and four-point seat belt system for the 56 km/h high-speed rear impact. In terms of Hyperview, the occupant's kinematic images were extracted at four different moments, and then those images were merged with varying transparencies. Compared with the three-point seat belt system, the four-point seat belt system provided stronger and more balanced restraining effects for both shoulders of the occupant. In terms of Hypergraph, Figure 12 compares the displacement trajectory of the occupant's head relative to the chest when the occupant was restrained via the three-point seat belt system and the four-point seat belt system. When the occupant was restrained by the three-point seat belt system, the maximum relative displacements were 116 mm and 110 mm along the x- and y-direction, respectively. In contrast, when the occupant was restrained by the four-point seat belt system, the maximum relative displacements were 85 mm and 16 mm along the x- and y-direction, respectively, which were 26% and 84% lower than that of the occupant restrained by the three-point seat belt system. The four-point seat belt system had an additional shoulder strap, and accordingly, more balanced forces were applied to constrain the occupant's torso through the double shoulder straps. Therefore, the occupant's head and neck were stabilized along both the x- and y-direction. Meanwhile, the head and neck injury metrics are summarized in Table 5, and the cervical ligament injury metrics are summarized in Table 6. Compared with the three-point seat belt system, the head and neck injury and the cervical ligament injury were significantly reduced. Specifically, the head injury indices BrIC and CSDM (0.25) were reduced by 66% and 70%, respectively, and the neck injury indices N\({}_{\text{ij}}\), N\({}_{\text{km}}\), and NIC were reduced by 22%, 52%, and 39%, respectively. The ALL, PLL, CL, LF, and I \begin{table}
\begin{tabular}{c c c c c c} \hline
**Speed (km/h)** & **ALL** & **PLL** & **CL** & **LF** & **ISL** \\ \hline Thresholds [37] & 0.35 & 0.34 & 1.48 & 0.88 & 0.68 \\
16 & 0.12 & 0.17 & 0.29 & 0.12 & 0.52 \\
40 & 0.25 & 0.29 & 0.51 & 0.28 & 0.53 \\
56 & 0.69 & 1.59 & 2.64 & 0.48 & 1.46 \\ \hline \end{tabular}
\end{table} 
Table 4: Summary of cervical ligament injury in rear impact for the zero-gravity seat. Figure 11: Occupant kinematics in zero-gravity seats with (**a**) three-point seat belt system and (**b**) four-point seat belt system under 56 km/h rear impact. 52%, 57%, 22%, and 46%, respectively. It is worth mentioning that regarding the head injury indices and cervical ligament injury metrics, the CL and LF were below the injury thresholds under the protection of the four-point seat belt system. However, regarding the neck injury metrics N\({}_{\text{km}}\) and NIC and the ligament injury metrics, the ALL, PLL, and ISLs were still beyond the injury thresholds, and other protection strategies still need to be developed to further reduce cervical spine and ligament injuries. The neck injury indices N\({}_{\text{ij}}\) and N\({}_{\text{km}}\) are directly affected by the cervical axial force F\({}_{\text{z}}\) and cervical shear forces F\({}_{\text{x}}\) of the upper neck, respectively. Figure 13a presents the force components that are associated with the corresponding local coordinate system, and the corresponding upper-body structure was prepared in Hypermesh by the variation in the transparency of different components of the human body model. F\({}_{\text{z}}\) with a positive sign represents compression, and F\({}_{\text{z}}\) with a negative sign represents tension, as shown in Figure 13b. When the occupant was restrained by the three-point seat belt system, the head and neck collided with the headrest at 110 ms.