# Effect of Seat Condition on Abdominal Injuries to Vehicle Occupants in Frontal Impact Accidents
## 4 Discussion  
In this study, we investigated behavior of the seatbelt and AM50 human model under two seat conditions: vertical movement and lean forward movement. Our results indicate that the lap belt detached from both of the iliac crests during the lean forward movement. Under this condition, the lap belt first detached from the left iliac crest at 75 ms and then from the right iliac crest at 90 ms. In addition, under this condition, tensile force of the lap belt reached a maximum (8.9 kN) at 70 ms before decreasing. After the lap belt detached from both of the iliac crests, tensile force of the lap belt reduced more steeply after 90 ms (Figure 15). It was considered that the relatively large contact force would act on the abdomen as a result of the large tensile force generated by the lap belt. Next, we focused on contact force and deformation to the abdomen of the AM50 human model. In this study, we measured resultant contact force (Fc) applied to the abdomen. Additionally, we measured deformation of the abdomen (D) shown in Figure 16. Deformation of the abdomen is defined as the distance between the abdominal surface corresponding to the center of the lap-belt and  
Figure 14: Time histories of the tensile forces on the (**a**) retractor belt, (**b**) shoulder belt, and (**c**) lap belt. Figure 15: Time history of tensile forces of the seatbelt under the lean forward movement seat condition. the sacrum on the median sagittal plane. The resultant contact force reached a maximum value of 15.0 kN at 70 ms (Figure 17a) and deformation of the abdomen reached a maximum value of 104 mm at 90 ms (Figure 17b). Strain of the deformed abdomen was 68% (=104 mm/154 mm), because the initial distance (D) was 154 mm. These phenomena suggested that a large resultant force was applied to the abdomen, resulting in the lap belt slipping from one iliac crest. After the lap belt slipped from both iliac crests, the abdomen experienced extreme deformation, which indicates possibility for critical abdominal injury. The definition and time history of the vertical distance of lap belt movement are shown in Figure 18. The lap belt slipped upwards by a maximum distance of 4 mm at 50 ms and maintained this upward movement until 80 ms. We consider that although slipping distance was small, the pelvis could be rotated forward along with the lean forward movement of the seat from the initial seat angle of 16\({}^{\circ}\). Time history of the seat angle indicates that it reached 0\({}^{\circ}\), i.e., a horizontal condition, at 76 ms and 4\({}^{\circ}\) at 90 ms (Figure 13b). During these periods, the iliac could be rotated forward, leading to slipping of the lap belt from the iliac crests. Figure 16: Illustration of the resultant contact force (Fc) between lap belt and abdomen, deformation of the abdomen (D), and tensile force of the lap belt (Ft). Figure 17: Time history of (**a**) resultant contact force (Fc) and (**b**) deformation of the abdomen (D) under lean forward movement conditions. Figure 18: Definition and time history of vertical distance (Dz) of lap belt movement: (**a**) definition and (**b**) time history. Results indicated that the lap belt detached more easily from the iliac crests during the lean forward movement. A photograph of the front passenger seat after the full-frontal rigid barrier impact test exhibits residual deformation of the frontal edge of the seat (Figure 2). A similar residual seat deformation was observed in car-to-car real-world frontal impact accidents [7]. Based on this residual seat deformation, a seat would be rotated during frontal impact to a car. Thus, the lean forward movement condition is estimated to be similar to the deformation situation generated by real-world accidents. Findings of the present study suggest that to evaluate an occupant's behavior in detail in future work, a seat model considering rotation should be incorporated into the vehicle cabin model when simulations are performed.