## 1 Introduction  
Automobile manufacturers are increasingly using high-tilt-back seats to improve passenger comfort. The development of highly automated vehicles may introduce occupants with new seating positions and configurations, among which reclined seating is one of the most popular [1; 2; 3]. [PERSON] et al. [4] investigated how 149 participants from China and Sweden wished to sit via the \"Setting the stage\" method. The results showed that comfortable and reclined seats were frequently mentioned, and that participants could accept extra restraints to maintain their current safety level if more sitting postures were available [5]. However, traditional restraint systems, such as three-point seat belts and airgaps mounted on the steering wheels were unsatisfactory in protecting reclined passengers, and even caused serious injuries to passengers, including strangulation across the neck [6; 7]. Therefore, there is a growing need to develop novel safety systems to match reclined sitting postures.  
Several studies have investigated reclined occupant injuries in autonomous driving vehicles based on numerical studies and post-mortem human subjects (PMHS) experiments [8; 9; 10; 11; 12]. The key challenge in protecting the reclined occupants is to balance the problem of submarining with the high lumbar load associated with anti-submarining devices. Frontal impact simulations using the Global Human Body Model Consortium (GHBMC) occupant model showed that a higher pelvic angle resulted in higher risk of submarining [13]. [PERSON] et al. [14] adopted a knee bolster to prevent submarining and observed an increased lumbar flexion load in a human model. The PMHS experiments conducted by [PERSON] et al. [15; 16] also showed that the lumbar spine is subjected to high bending and compression in a reclined posture, owing to the movement of the upper torso.  
Several safety devices have been proposed for the reclining occupant. [PERSON] et al. [17] designed a dual shoulder airbag (DSA) system that could be deployed from both sides of the seatback and seat pan to raise the occupant's pelvis and thigh area to avoid submarining. The study illustrated the effectiveness of the DSA based on head, neck, and chest injuries of the Hybrid III and THOR dummies, whereas the lumbar spine, with a high potential risk of injury, was not evaluated. [PERSON] et al. [18] predicted that submarining was the major challenge for reclined occupants, and adopted a knee bolster to restrain the forward movement of the passenger's knee and reduce submarine risk. A high lumbar force was discovered in the 30\({}^{\circ}\) seatback sled simulation; however, no countermeasures were proposed. Although these methods protect the reclined occupant in situ, high spinal force remains unresolved. Some car manufacturers protect reclined occupants by adjusting the seatback angle to maintain normal posture before a crash [19]. However, in this proactive safety seat, the rotation speed of the seatback was restricted to <10\({}^{\circ}\)/s because of the maximum allowed voltage capacity of the battery, costing 3-5 s to upright the seatback. This means that the car should detect the crash 3-5 s in advance and transmit the signal to the seat to adjust the backrest, which brings a high requirement for predicting the collision risk.  
The current study proposed a swiveling seat in which reclined occupants could move upright using their own inertia during frontal impact. Additionally, related constrained systems for the swiveling seat, such as the belt position and force limit, dynamic locking tongue (DLT), and passenger lap pretension (PLP), were further investigated using a simulation matrix. The crash configuration was derived from the sled test of a full frontal rigid barrier at 50 km/h. Injury indices of the head, thorax, abdomen, pelvis, and lower limbs of the occupants were used to evaluate the effectiveness of the swiveling seat.