## Materials and Methods
### Validation of the Grasping Motion Simulation
#### 2.2.2 Evaluation Result of the Grasping Motion Simulation  
A comparison of the simulation result and experimental result is shown in Figures 8 and 9. A comparison between the final gripping posture and the hand posture in the experiment indicated a significant similarity. Specifically, the postures of the thumb, index finger, and middle finger in the simulation were almost identical to those in the experiment. Conversely, a difference between the simulation result and experimental result corresponded to the posture of the ring finger. In the experiment, the fingertip of the ring finger was approximately in the same position as the fingertip of the middle finger. However, in the simulation, the fingertip of the ring finger was slightly separated from the fingertip of the middle finger. As shown in the comparison of the contact pressure distribution, the contact pressure at region 2 (which corresponded to the highest pressure in the experiment) was accurately generated in the simulation. Additionally, the contact pressure distribution at the thumb, index finger, middle finger, and fingertip of the ring finger and little finger was also accurately generated in the simulation. Conversely, a difference between the simulation result and experiment result corresponded to the contact pressure distribution at the proximal part of the ring finger and little finger. In the experiment, low contact pressure was produced at the proximal part of the ring finger and little finger. However, in the simulation, contact pressure did not occur at the proximal part of the ring finger and little finger. A quantitative comparison of the contact pressure value obtained from the simulation and experiment is shown in Figure 10. The difference between the simulation result and experiment result was evaluated via Mean Absolute Error (MAE) that is calculated by Equation (9). \[\text{MAE}=\frac{1}{n}\sum_{k=1}^{n}\left|A_{j}-E_{j}\right| \tag{9}\]  
where \(n\) (= 23) denotes the number of regions; \(A_{j}\) [kPa] denotes the contact pressure in the simulation at \(j\); \(E_{i}\) (kPa) denotes the contact pressure in the experiment at \(j\); and \(j\) denotes the region number. Thus, MAE corresponded to 7.4 kPa, thereby indicating the average difference in contact pressure among the 23 regions. Additionally, the simulation result was mostly within the average \(\pm\) standard deviation (SD) of the results of 9 male subjects obtained in our previous experiment [6]. The grasping motion simulation was developed to estimate gripping comfort. Therefore, it is necessary to understand the effect of the difference between the simulation result and experimental result on the gripping comfort estimated via grasping motion simulation. The gripping comfort score was calculated by assigning the contact pressure obtained from the simulation into the regression equation as reported in a previous study [6]. Thus, gripping comfort score was estimated as 0.32. Given that the actual gripping comfort score evaluated by volunteers corresponded to 0.48 in the experiment, the difference between the estimated gripping comfort score and actual gripping comfort score corresponded to 0.16. The gripping comfort evaluation was conducted using the visual analog scale method. The possible range of the gripping comfort score corresponded to \(-1\) (bad) to \(1\) (good) in continuous values. The gripping comfort score corresponds to a value with a range of 2, and thus the difference of 0.16 is sufficiently low when compared to the range of 2 (less than 10%). Hence,  
Figure 9: Comparison of the contact pressure distribution between the FE simulation result and experimental result. Red color denotes high contact pressure while blue–black color denotes low contact pressure. the grasping motion simulation proposed in the study was considered as useful to evaluate gripping comfort during the development process of the gripping part of a product.