## 4 Discussion  
The influence of oversimplified models and the lack of actual circumstances have been underestimated in recent experimental research and simulation analysis of footwear traction [10; 28; 29; 30; 31; 32]. For example, a simplified outside pad model could lead to an overly constrained boundary condition. The lack of actual loading conditions and shoe position lead to the inability to mimic human weight, pressure distribution, and foot angle in normal gait. To the best of our knowledge, the model development and FE analysis setting process in this research could improve the feasibility and applicability of traction performance simulation. One of the most prominent features of this research was the automatic pressure application, which could provide new insights for computational simulation. Instead of the built-in manual pressure generator, the ANSYS ACT extension was used to create an automatic plantar pressure generator. The manual pressure time settings were 4 h at the mid-stance phase, 2.5 h at the push-off phase, and 1 h at the loading response phase. The automatic pressure application time was significantly reduced to 8 min at the mid-stance phase, 3 min at the push-off phase, and 50 s at the loading response phase. The ANSYS ACT extension for the automatic plantar pressure generator is an efficient tool for mimicking physiological loading conditions during the gait motion, and it is particularly advantageous to the footwear industry, especially for customized products. Trend differences have been found between the reaction force calculation during the entire stance phase of the gait in the FE analysis and the coefficients of friction measurements in the experimental test. The trend differences could be caused by the shoe position, sliding direction, or loading condition. The experimental test only considered the condition that the specimen was parallel with the ground and slid in one direction under a load of 4.5 kg. To mimic the human gait motion, the contact angle between the bottom of the shoe and the ground was 7\({}^{\circ}\) at the loading response phase and 7\({}^{\circ}\) at the push-off phase. The footwear was parallel with the ground at the mid-stance phase. The foot angle was 6\({}^{\circ}\) at all three phases. In terms of the ground sliding direction, the horizontal displacement of the ground was from toe to heel at the loading response phase, and was from heel to toe at the push-off and mid-stance phases. Instead of a uniform loading of 4.5 kg, the data of the dynamic plantar pressure were used in the FE analysis. Therefore, the FE results of reaction force had different trends from the experimental test. According to the classical Amontons-Coulomb friction law, the friction force is independent of the apparent contact area [33]. However, this theory could only be correct at the macroscopic scale. Previous research has proven that the real contact area between sliding surfaces plays a role in friction at a microscopic scale, especially for an elastic material during sliding [34; 35; 36]. Some important aspects may be gained from the graph of linear regressions shown in Figure 10. The higher value of the shoe-ground real contact area could achieve a higher traction performance. Some of the results that did not follow this observation perfectly could be caused by the geometry of the tread. When the surfaces of ground and outsole tread push against one another, a shear force is needed to break the adhesive bond of the contacting surfaces [37; 38; 39]. The geometry of the tread may have contributed to the formation and strength of bonds, which could affect the results of the reaction force. The potential limitations of this research should be noted. Firstly, for the surface condition, neither contamination conditions nor the roughness of the surface of the ground were tested. A dry and flat concrete ground was applied in the FE model. Secondly, the loading condition in the FE model was based on a single-subject dynamic plantar pressure measurement, and was applied to the insole.