*4.2. Static Trials*

The results of the static standing trial showed that there was a high linear correlation between the medial arch sensor (5 + 4 + 3/2) with the AI obtained from the static ink (r > 0.7). Meanwhile, five sensor configurations (sensor 1, sensor 3, 3-sensors point (1, 3, 5), 5-sensors point (1–5), and the lateral arch sensor (1+2+ 3/2) showed a moderate correlation with 0.4<r< 0.6.

The ink-type AI has been reported to also be correlated with the navicular height [13,25], the medial arch angle analyzed by X-ray [14,26,27], and the pressure of the hindfoot [28–30] and midfoot [31,32] measured during gait. The results of the current study indicated that the force value measured by the insole-type sensor alone was also associated with the AI measured by the traditional ink one, which may be introduced as another method to distinguish between a flatfoot and normal foot. Although significant differences between normal foot and flatfoot were also found (*p* < 0.05), the current method should be applied with caution for individuals whose AI value falls on the transition area (from normal to flat foot).

The results showed that the force of the center midfoot of the flatfoot was obviously higher than the normal foot, which was consistent with the results of the current study. There were obvious differences between the medial arch of the flatfoot and normal foot. Thus, in the static experiment, it was found that the sensors on the arch could be placed in the position of 3 or 5 for the instruments to judge between flatfoot and common foot in the static design. There was a high linear correlation with the AI from the conventional ink-type measurement, and the comparison between the groups showed that there was a significant difference between the sensing measurements of the position of 3 or 5 when judging between normal foot and flatfoot (*p* < 0.05).
