## 1 Introduction
Additionally, the injury assessment focused solely on head and chest injuries, neglecting neck injury metrics such as N\({}_{\bar{\mathrm{y}}}\) or neck ligament strain, which are critical for evaluating occupant protection in highly reclined configurations. [PERSON] et al. [12] investigated the effects of hinge failure on occupant injuries for different reclined seatbacks under various rear-impact speeds using a production seat model and the THUMS (Total Human Model for Safety). The study showed that, under moderate- and high-speed rear impacts, the threshold of hinge failure was reached for a series production seat and the neck injuries were reduced; on the contrary, the head and chest injuries were exacerbated. However, the considered seatback recline angles were 22\({}^{\circ}\) and 45\({}^{\circ}\), which were significantly smaller than the recline angle of zero-gravity seats. Additionally, the research did not address protective strategies for neck injuries, which is highly associated with reclined seating configurations. [PERSON] et al. (2013) investigated occupant injuries for medium- and high-impact speed cases with the consideration of a wide range of seatback recline angles, including 30\({}^{\circ}\), 60\({}^{\circ}\), and 90\({}^{\circ}\). The study revealed that at seatback angles of 60\({}^{\circ}\) and 90\({}^{\circ}\), the head displacement increased and the head detached from the headrest, while the neck experienced stretching, leading to injuries in the anterior longitudinal ligament of the neck. This phenomenon was attributed to two factors. One is that larger seatback angles reduced the headrest's ability to stabilize head posture. Another one is that the conventional three-point seat belts failed to effectively restrain horizontal occupant motion, yielding excessive head displacement. With respect to the widely used three-point seat belt restraint system, the four-point seat belt system seems a better way to mitigate head movement by restraining both shoulders instead of one shoulder. Two types of four-point seat belt systems ([PERSON] et al., 2014), the X4 and V4 belts, have been utilized to reduce chest injuries during frontal crashes. Further, X-type four-point seat belts ([PERSON] et al., 2015) have been utilized to restrain occupants for the comfortability study of highly reclined seats under autonomous driving scenarios. However, the protective performance of seat belts under rear impacts has never been evaluated. A review of the literature indicates that the current research on occupant injuries in rear impacts primarily focuses on reclined seats, and there is a lack of comprehensive studies on zero-gravity seats. For zero-gravity seats, in addition to seatback recline angles, evaluations need to account for seat cushion angle adjustments, which may result in a larger recline angle for occupants and footrest configurations. Further, it is worth mentioning that the head's out-of-position posture and neck injuries remain common issues in highly reclined seats during rear collisions, and currently there is a lack of effective protection strategies for these scenarios. This study aims to investigate the kinematics responses and injury mechanisms of an occupant restrained in a zero-gravity seat under varying rear-impact speeds and evaluate the effectiveness of different restraint systems. Firstly, the THUMS-based finite element model assembled with a production seat and three-point seat belt system were validated by comparing kinematics and head accelerations of THUMS with PMHS (Postmortem Human Subject) test data from standard seating postures. Based on a validated model of the seat and the corresponding seat belt system, a zero-gravity seat model incorporating adjustable seat cushion angles and footrest configurations was created.