# Effect of Seat Condition on Abdominal Injuries to Vehicle Occupants in Frontal Impact Accidents
## 1 Introduction  
Vehicle occupants were killed in 33% (1221) of all traffic accidents (3694) in Japan in 2017 [1] (Figure 1). Therefore, countermeasures to reduce fatal traffic accidents are required. In Japan, when road-users are killed or injured in traffic accidents, police officers and medical doctors determine the main body regions associated with severe injuries that led to death. Results of these investigations reveal serious and fatal injury rates for each main body region, presented in Table 1. The abdominal region is associated with the highest rate of severe injury and fatality of vehicle occupants (Table 1) [1]. The same trend has also been observed in France [2]. Therefore, in this study, we focus on causes of abdominal injuries to vehicle occupants. Figure 1: Distribution of traffic fatalities by crash type in Japan in 2017. Fifty-four percent of vehicles struck in vehicle-to-vehicle accidents in Japan were impacted from the front [3]. A US accident investigation database of occupant injuries in the front passenger seat of impacted vehicles showed that 59% of abdominal injuries were directly caused by seatbelts [4]. Two detailed accident case studies revealed that lap belts caused serious abdominal injuries and residual deformation marks were left behind on seat surfaces, which suggests that the seat moved vertically during the frontal impact [4]. Some studies have attempted to evaluate the relationship between lap belt condition and abdominal injury. [PERSON] et al. performed simulations using a vehicle cabin model with a rigid-seat model to investigate injury risk, considering the abdomen, for the anthropomorphic 50 th percentile male (AM50) and obese male models during frontal impact [5]. In addition, [PERSON] et al. investigated the relationship between lap belt loading and abdominal response using an AM50 dummy and post mortem human subjects (PMHSs), by employing a rigid seat during frontal impact to facilitate the design and development of seat belt tensioning systems [6]. However, only a few studies have focused on the movement of a seat during frontal impact. [PERSON] et al. investigated two accidents in which drivers sustained abdominal injuries owing to car-to-car frontal impacts [7]. These authors indicated the possible cause of abdominal injuries may be due to deformation of the seat during impact. [PERSON] and [PERSON] investigated the relationship between seat height vertical movement and lap belt upward movement, in which an anthropomorphic 50 th percentile male Hybrid III dummy model (hereafter referred to as an AM50 Hybrid III model) was seated in a passenger seat model [8]. Results revealed that when the AM50 Hybrid III model moved 150 mm vertically downward, the lap belt exhibited significant upward movement from its initial position on the dummy's waist. This phenomenon resulted in the abdomen being directly compressed, owing to tightening of the lap belt. In the upper body structure of a Hybrid III dummy, there is a space between the thoracic and waist part [9] because the dummy does not have internal organs. Currently, Toyota Motor Corporation and Toyota Central Research and Development Labs. Inc. have jointly developed a finite element model, the total human model for safety (THUMS) version four, which includes an internal organ finite element model [10]. It is possible that dynamic movement of a seatbelt on a Hybrid III dummy may be different from that of a seatbelt worn by a human, i.e., after a lap belt moves away from the iliac crests of the human pelvis it moves deeply into the abdomen. Thus, the purpose of this study is to clarify the correlation between different downward movement conditions of the seat and those of the lap belt from the iliac crests of a human occupant in the front passenger seat of a car in the event of a frontal impact. We investigate this phenomenon by conducting simulations using an AM50 human model.