## 3 Finite Element Modeling (FEM) of Human-Seat Interactions
### Boundary Conditions and Initial Setup for Simulation  
Accurately defining boundary conditions is critical for effectively simulating the forces acting on model components in seat-comfort analysis. Two widely utilized approaches, as highlighted by [PERSON] [13], are the gravity-based method and the thigh and buttock model. The gravity-based method ([16; 19; 20; 31; 32; 33; 34]) applies body weight as a gravitational force continuously acting over time at the model's center of gravity, directed downward  
Figure 11: (**a**) Uniaxial tensile testing setup for the fabric adhesive tape; (**b**) Experimental stress–strain curve (blue) compared with the fitted stress–strain curve (red) from the tensile test of the tape. onto the seat cushion surface to simulate static interactions. In contrast, the thigh and buttock model ([35; 36; 37]) distributes body weight across specific anatomical regions, with 55% allocated to the ischial tuberosities, 27% to the femurs, and 18% to the centers of mass of the lower legs. By enabling a more localized and precise simulation of pressure distribution, the thigh and buttock model offers enhanced accuracy and was therefore selected for this study. The body weight distribution in the model was based on the weight of the experimental participant, adjusted to match the half-symmetric design of the model. A total mass of 14 kg, corresponding to an evenly distributed force of 140 N, was assigned to the pelvic and sacral regions to represent muscle and bone mass, as shown in Figure 12a. The weight of the half-upper body, equivalent to 18 kg, was represented as a simplified force of 180 N applied at the ischial tuberosity, effectively replicating the load exerted by the upper body. Figure 12: Cont. The connections between components in the model were established through a connectivity matrix, as shown in Figure 12b. The MSC.Marc software facilitated these connections by using two types of contact interactions: Glue Contact (G), which simulates rigid connections, and Touch Contact (T), which represents flexible surface interactions. These contact definitions ensured accurate modeling of the components' mechanical behavior. To ensure simulation stability, specific constraints were applied to the movement of the model components. Directional constraints limited the seat cushion, pneumatic spring, and human body to motion along the \(z\)-axis, in alignment with gravity. Additionally, contact  
Figure 12: The force distribution on the human body mode (**a**), contact table of the components in the model (**b**), the complete model (**c**), and the desired internal pressure of the pneumatic spring (**d**). constraints were applied to the interface between the flesh and bone layers in the human body model, the layers comprising the pneumatic spring, and the interaction between the seat cushion and the pneumatic spring. An alternative boundary condition, referred to as the collision approach, was implemented, as detailed in previous studies ([37; 38; 39; 40; 41; 42; 43; 44]). In this method, the human body model is positioned 0.1 mm above the seat cushion and allowed to move downward in the z-direction to mimic a natural sitting motion. The initial velocity was set to 0 m/s, with gravity as the sole acting force. This approach prevents mesh penetration during contact, ensuring high accuracy and reliability in the simulation results. The complete finite element model is illustrated in Figure 12c. To analyze the contact pressure distribution between the human body and the car-seat cushion, the internal pressure of the pneumatic spring was adjusted within a range of 0-25 kPa. This variation was modeled as a time-dependent function (Equation (12)), with the target pressure profile shown in Figure 12d. \[p_{s}= \begin{cases}0&\text{kPa {if} 0\leq t<5}\\ 5(t-5)&\text{kPa {if} 5\leq t<10}\\ 25&\text{kPa {if} 10\leq t\leq 15}\end{cases}