## 1 Introduction  
With the continuous advancement of intelligent and connected automotive technologies, driving tasks have been significantly reduced and may no longer be required in the future. Car buyers often look for features that enhance ride comfort, especially given the increasing prevalence of traffic congestion and long commutes in major cities. Research into zero-gravity seats originates from NASA's research in the 1980s on neutral body posture (NBP), a natural floating posture that astronauts experience in a zero-gravity space environment [1,2]. The NBP is characterized by a semi-crouched torso, flexed arms and legs, and bent-forward head and neck. Nissan Motor Company [3] first introduced the NBP design concept into automotive car seat design with a two-piece backrest configuration in 2005 and illustrated that zero-gravity seats can improve support for the spine and the region from the pelvis to the chest, enhancing blood circulation and significantly reducing fatigue during prolonged sitting. As an innovative technological product, automotive zero-gravity seats are increasingly attracting widespread attention from both the consumer and automotive manufacturers, particularly in the highly competitive Chinese electric automotive market. Compared to conventional car seats that only offer seatback recline adjustment, zero-gravity seats additionally provide adjustments for the seat cushion angle, leg support, and armrests [4]. As of today, nearly 20 vehicle models equipped with zero-gravity seats have been introduced to the Chinese market. In 2022, the AITO M7 became the first vehicle manufactured in China to feature zero-gravity seats. The AITO M7's second-row zero-gravity seats feature a range of adjustment modes designed to maximize passenger comfort. With the adjustable backrest, seat cushion, and footrest, the zero-gravity seats provide the passenger with a nearly fully reclined seating posture. Additionally, several mass-produced models, both domestic and international, such as the Kia Carnival, Hyundai Custo, and IM LS7, have also incorporated zero-gravity seats in the front passenger or second-row seating positions [5]. However, this type of seat is currently only designed for use in a parked state due to a lack of safety protection strategies; it is essential to investigate the safety features of such seats to expand their usage scenarios to regular driving scenarios. Currently, research on zero-gravity seats primarily focuses on injury assessment and prevention strategies regarding the lumbar spine and submarining in frontal crashes [6, 7, 8]. However, studies of zero-gravity seats in rear-impact settings are very few and limited, although they are crucial for optimizing automotive seat and restraint system design to mitigate head or neck-related injuries, such as whiplash injuries and for enhancing the occupant's safety by ensuring proper head and neck support, energy absorption, and controlled seatback motion during collisions [9,10]. The most relevant research into rear impacts with zero-gravity seats is the study of rear impacts with reclined seats, since a reclined seating position is an essential feature of zero-gravity seats. [PERSON] et al. [11] employed the Global Human Body Models Consortium (GHBMC) 50 th percentile simplified (M50-OS) male finite element model and a Honda Accord seat model to evaluate occupant injury risks under low-speed (15 mph) and high-speed (35 mph) rear-impact conditions with varying seatback recline angles. The injury metrics included the Head Injury Criterion (HIC), Brain Injury Criterion (BrIC), and maximum chest deflection. The study found that increased seatback recline angles during rear impacts correlated with higher BrIC values, indicating elevated brain injury risks. However, the maximum initial occupant recline angle in this study was only 45\({}^{\circ}\), significantly smaller than the typical angles of zero-gravity seats, which are larger than 50\({}^{\circ}\).