## Materials and Methods  
### Establishment of Grasping Motion Simulation  
#### 2.1.1 Construction of Finite Element Model  
The outer surfaces of skin and bones of human hand (right hand) were obtained from the human model database of a computer graphics software Poser 9 (Smith Micro Software Inc., Aliso Viejo, CA, USA). The joint angles of the hand model were adjusted to obtain an initial posture during grasping as shown in Figure 1. The outer surface data in OBJ file format were then checked via a computer-aided design software Rhinoceros (Robert McNeel & Associates, Seattle, WA, USA) and were converted into a polygon model data in STL file format. The model data was then imported into a finite element modeling software HyperMesh (Altair Engineering Inc., Troy, MI, USA). A joint cover consisted of triangular shell elements that were constructed to surround apertures between two adjacent bones. The measure was performed to avoid errors due to excessive deformations of subcutaneous soft tissue (fat layer) pinched by the bones. The bones and subcutaneous soft tissue were meshed using tetrahedron solid elements. Nodes on the boundary between bones and subcutaneous soft tissue were shared. Furthermore, a skin model was constructed on the outer surface of the hand model using triangular shell elements. Human finger skin consists of epidermis and dermis. Thus, the skin model was constructed as a one-layer model because the effect of thin epidermis and dermis was considered to be insignificant. The thickness of the skin model corresponded to 1.75 mm as derived from the total thickness of epidermis and dermis [19]. Figure 2 shows the hand finite element (FE) model. The number of elements and nodes for each component are listed in Table 1. Additionally, in the study, bones, subcutaneous soft tissue, and skin were modeled as uniform isotropic materials. Cartilages and ligaments were not modeled in the hand FE model and their role was substituted by rotational joints connecting all adjacent finger bones. Furthermore, since our interest was in the palmar contact pressure and not in the internal response of hand, we considered that cartilages and ligaments could be omitted. Figure 1: Outer surfaces of skin (**left**) and bones (**right**) at the initial hand posture. The hand and phalanges lengths of the hand FE model were scaled to those of the subject who participated in the previous experiment [6] to appropriately reproduce the hand shape of the subject. Thus, the hand length (L01) based on the AIST Japanese hand dimensions database [20] of the subject-specific FE model, as shown in Figure 3, exhibits a good agreement with the hand length of the subject (L01 = 202.4 mm). Additionally, the phalanges lengths of the subject-specific FE model almost coincided with the phalange lengths of the subject that were measured from the positions of markers attached on the subject's joints in the experiment. A cylinder with a diameter of 60 mm and height of 131 mm was a test object used in the experiment [6] and was selected as a basic grasping object in the study because the typical shape of the gripping part of a product corresponds to a cylinder. Furthermore, the shape was investigated in previous studies [13; 21; 22; 23]. The cylinder FE model exhibits the same dimensions as those of the \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Component**} & \multicolumn{2}{c}{**Number**} & \multirow{2}{*}{**Element Type**} \\ \cline{2-3} \cline{5-4}  & **Elements** & & **Nodes** \\ \hline Skin & 5362 & 2693 & Triangular shell \\ Subcutaneous soft tissue & 30,480 & 7979 & Tetrahedron solid \\ Bones & 13,633 & 4677 & Tetrahedron solid \\ Cylinder & 3198 & 4428 & Hexahedron solid \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Number of elements and nodes as well as type of element used in the hand FE model and cylinder FE model. Figure 3: Definitions of the hand length (L01) based on the AIST Japanese hand dimensions database [20]. Figure 2: Hand FE model and cylinder FE model.