## 4 Discussion  
The reclined sitting position becomes a general posture during driving, and the current restraint system is not able to protect the occupant. This study proposed a swiveling seat, and conducted 20 simulations to investigate the protection of reclined occupants. The results contribute to promoting the safety of out-of-position occupants in automobile design. In simulation 1, severe submarining accompanied by abdominal injury occurred when a reclined passenger was equipped with a normal three-point seatbelt. The first challenge in developing a novel restraint system for reclined people is solving the submarine problem. The anchor pretensioner, DLT, and belt angle were used in Simulation 5, and submarining was successfully avoided. In this simulation, the pelvis was subjected to a heavy load from the seatbelt, causing a maximum strain of 5.6%, which is higher than the pelvic injury strain of 2.14%. The maximum strain of the pelvis in Simulation 6-8 was decreased to approximately 1% by adopting an extra knee belt and a lap belt. This showed that a large contact area between the restraint system and the occupant could effectively reduce the risk of injury to the pelvis. Some researchers used knee bolsters to increase the contact area and relieve the load applied to the pelvis [26, 27]. The properties and initial position of the knee bolster need to be considered because the large occupant-to-knee bolster distance was not able to reduce the submarining risk, and a hard knee bolster would lead to lower limb injury [18]. Figure 11: The motion trajectories of knees in the sagittal (x–z) plane: (a) Simulations 1–8; (b) Simulations 9–14; and (c) Simulations 15–20. Extra belts eliminate the concentrated load on the pelvis; however, the problems of high lumbar compression forces and bending moments remain unsolved, as shown in Table 3. The GHBMC numerical study simulated by [PERSON] et al. [28] and the PMHS experiments conducted by [PERSON] et al. [29] showed a similar phenomenon in that the lumbar spine of the reclined occupant was loaded in compression and bending during the crash. The rotated seat in this study aims to make the reclined occupant upright during frontal impact to mitigate injury. However, the simulation results of the current study showed that the pelvic angle changed little during the rotational movement of the seat, which can be attributed to the flexibility of the lumbar spine in the biomechanical model. Study conducted by [PERSON] et al. [30] also showed that the pelvis of reclined human body model did not return upright under seat-back repositioning. Therefore, an upright pelvis was not considered necessary to minimize the injury of the reclined occupants because the lumbar force in Simulation 18 (with a high pelvis angle during the impact) was 1870 N, which was lower than the spine injury threshold. In this study, one of the significant factors affecting the low lumbar force in Simulations 17-20 was that the swivel seat allowed passengers a greater forward movement relative to the floor. A large passenger displacement can allow the restraint system to play its full role, and reduce the impact energy that must be absorbed by the occupant and the resultant injury risk. Simulation 17 had the minimum spine force of 1787 N among the 20 simulations, and its knee excursion was 300 mm, which was larger than that of the others. The knee excursion in Simulation 7 was 95 mm, and the axial force in the first lumbar region reached 5359 N, which was the maximum value among the 20 cases. Additionally, the swivel seat in this study lowered the center of gravity of the pelvis during impact, reducing the curvature of the spine and the associated injury risk [31], as shown in Figure 9. In methods wherein the reclined occupant returns to a normal sitting configuration by raising their upper torso, the safety system needs to provide the occupant with an additional upward force, possibly increasing the axial force acting on the spine.