## 3 Results  
Kinematic Responses of Occupants in Zero-Gravity Seat with Three-Point Seat Bells at Different Impact Speeds  
Figure 7 presents the time-history curves of the seatback rotation angle for different rear-impact speeds. In general, the seatback first rotates to a peak value and then rebounds back. As the impact velocity increases, the rotation angle also increases. The maximal rotation angles are 5.1\({}^{\circ}\), 13.1\({}^{\circ}\), and 18.6\({}^{\circ}\) at 16 km/h, 40 km/h, and 56 km/h impact speeds, respectively. The kinematic responses of the reclined occupant restrained in the zero-gravity seat are presented in Figures 8-10 for low-, moderate-, and high-speed rear impacts in terms of Hyperview. The simulation results demonstrated significantly different biomechanical responses for different impact speeds. As shown in Figure 8, when the collision speed was 16 km/h, the seatback slightly rotated backward, and the maximal value was 5.1\({}^{\circ}\). The pelvis moved forward due to the rebound of the seatback foam. However, its movement relative to the seat was limited. During the initial stage of collision, the contact force between the head and the headrest reached the first peak value, 1366 N, at 25 ms due to the pretension of the seat belts. Then, the seatback continued to rotate backward due to the push of the torso. The head slightly detached from the headrest at 33 ms then fell back on the headrest at 58 ms. The contact force between the head and headrest reached a peak value of 1125 N at 88 ms. The head completely detached from the headrest at 131 ms, and the neck started presenting a forward flexion state. As shown in Figure 9, the kinematic responses of the occupant under 40 km/h impact were similar to the responses under the low-speed impact during the 0-100 ms period. However, with respect to the medium-speed impact, the backseat continued to rotate backward until 13.1\({}^{\circ}\), and this resulted in a decrease in the restraining effect of the backseat on the pelvis. At 105 ms, the pelvis showed a strong tendency to move upward along the seatback. Meanwhile, the lower limbs began to significantly flip around the pelvis after 100 ms. In contrast to the low-speed impact case, the kinematics of the lower limbs maintained a similar posture during the whole collision period. With respect to the head contact force, the first peak forces induced by the seat belt pretension were the same for both impact speeds. However, the second contact peak force caused by the relative movement between the head and headrest was higher, reaching 1912 N, and the contact duration time was also longer. In addition, during the later stage of the collision, the neck had a similar forward flexion state as the low-speed impact scenario. Figure 7: Seatback rotation of the zero-gravity seat. As shown in Figure 10, when the collision speed increased to 56 km/h, the maximal seatback rotation angle increased to 18.6\({}^{\circ}\), and the restraining effect of the backseat  
Figure 8: Kinematic response of occupant in 16 km/h rear-end impact. Figure 10: Kinematic response of occupant in 56 k m/h rear-end impact. Figure 9: Kinematic response of occupant in 40 km/h rear-end impact. tinued to decrease. The buttocks completely separated from the seat cushion at 55 ms. Meanwhile, the lower limbs flipped more. In contrast with the low- and medium-speed impact, the kinematic responses of the head and neck were significantly different. At 70 ms, the head center of gravity passed the headrest and continued moving backward until 103 ms, and the neck tension force reached a maximal value of 1582 N. After that, the head continued flipping backward and severely collided with the upper edge of the headrest at 100 ms. And the contact force between head and headrest increased up to 7645 N, which was about four times the maximal contact force in the medium-speed impact.