**2. Materials and Methods**

## *2.1. Subject*

Twenty-one healthy subjects with shoes sized between US9/Euro 42 and US10/Euro 43–44 participated in this study. Ten subjects with flatfoot and 11 with normal foot were classified by ink-type AI. Healthy subjects had no history of affecting postural stability, or gait in the past 12 months, and/or cognitive impairment. This experiment was approved by local research ethics. Subjects who injured their lower extremity six months ago before participation and still suffered from pain, who had surgery on their hip joints, knee joints, and ankle joints of the lower extremity, and who could not walk or stand for the pain in the lower extremity or back were excluded from the study. The subject's characteristics, including their Body Mass Index (BMI) and ratio of foot length and navicular-to-toe length, are shown in Table 1.


**Table 1.** Subject characteristics of 21 young subjects who participated in this study.
