## 3 Results
### Main Steps in FEA of Human-Seat Interactions
#### 3.2.3 Material Properties  
Skin Material Properties  
Starting from the external layer of the human body, there is the skin, which has an elasticity behavior. Several papers [18; 23; 24; 37; 41; 42; 44; 46] chose to ignore the skin since it has a negligible effect on the results. Nonetheless, other papers decided to lump the skin with the soft tissue material and model it as a soft tissue [2; 17; 20; 21; 25; 27; 31; 35; 38; 40; 43], which is discussed in the next section. Papers that assigned a material for the skin [1; 16; 30; 32; 47] used a linear isotropic elastic material with an elasticity of 0.15 MPa. Soft Tissue Material Properties  
The second human body layer is the soft tissue, which includes the muscles, fat tissue, tendons structures, and blood vessels. Blood, interstitial lymph, and fluid surround these tissues [46]. Thus, biological tissues are nonlinear, anisotropic, and viscoelastic. Many finite element models have been developed to simulate the soft human tissues, and various approaches have been observed from the papers read for this review paper. Several authors have used a single model to describe the behavior of all the components [1; 2; 17; 18; 19; 21; 23; 24; 25; 26; 27; 28; 29; 32; 37; 39; 40; 42; 44]; however, other papers have used mathematical models for tendons structures, and muscular tissue and a changed one for fat [15; 16; 30; 31; 33; 34; 35; 36; 38; 41; 43]. However, there are a few models that have been used by many papers because of the accuracy of the results that they portray. These models include: (1) [PERSON] [47], which models all tissues by considering a single linear elastic,  
Figure 4: High and buttocks section of a human FE model. Left: Bony structure, Middle: Bony Structure and Human Soft tissue; Right: Full human model with bony structures, soft tissues, and skin [44]. isotropic, and equivalent material simultaneously; (2) [PERSON] and [PERSON] [48], which models all the tissues at once by using a single hyperelastic equivalent material, such as the neo-Hookean formulation; (3) [PERSON] et al. [45], on the other hand, uses a single hyperelastic Mooney-Rivlin formulation that models the tissues simultaneously; and (4) [PERSON] [44], which is the most common method used by many papers [1; 16; 17; 18; 20; 21; 26; 28; 29; 39; 40; 42], which also uses a single hyperelastic equivalent material Mooney-Rivlin formulation to model the soft tissues; however, the skin is not included in this formulation, and hence a linear elastic isotropic material is used for the skin. There are different hyperelastic models that can be used for soft tissue modeling. The neo-Hookean model is a model similar to [PERSON]'s law that is used for materials that undergo large deformation by predicting the stress-strain behavior of these materials. The neo-Hookean model [19] is a very simple hyperelastic model that consists of one material parameter only, which is the initial shear modulus. The stress-strain of the model is linear until it reaches a certain point, where the curve becomes non-linear [49]. The strain energy density function of a non-compressible neo-Hookean model contains a material constant and a Cauchy-Green deformation tensor [49]. This model does not provide accurate prediction at large strains, and the use of this model is good if the results that are being assessed are focused on the pressure distribution rather than the stress values at specific locations [19]. On the other hand, another model is the Mooney-Rivlin model [1; 16; 17; 18; 20; 26; 28; 29; 40; 42; 44], which generalizes the hyperelastic model of the neo-Hookean model. The strain energy density function of the Mooney-Rivlin model is a linear combination of the Cauchy-Green deformation tensor and an incompressible Mooney-Rivlin material. It consists of two empirically determined material constants. In addition, the material constants of the model are related to the linear elastic shear modulus.