**1. Introduction**

The arch of the foot helps in absorbing and transferring energy when walking or running. Thus, an abnormal arch, such as flatfoot, often leads to an abnormal gait and running pattern. Flatfoot can be divided into flexible flatfoot and rigid flatfoot, with the former, also called functional flatfoot, meaning that the arch is flat when the foot is weight bearing, or that the arch is evident when the toes of the foot point outwards while the subjects stand in non-weight bearing or through the toe-raising of the Jack Test proposed in 1953 [1]. While the latter, also called structural flatfoot, refers to when the arch is not present, whether in weight bearing status or not, and the subtalar joint movement is poor. Ninety-five percent of patients with flatfoot are flexible flatfoot [2]. Patients with rigid flatfoot often suffer from pain or potentially more severe pathological problems, such as tarsal coalition or neuromuscular flatfoot [3], etc. Rigid flatfoot mostly requires surgery rather than being treated by traditional treatments if patients are in pain [2].

Currently, the methods for judging flatfoot have been mainly focused on the medial longitude arch (MLA), and common methods include the traditional arch height, the navicular drop test [4,5], the footprint index classification [6–9], and X-ray [10]. The arch height can be quantified by the difference between the highest point of the soft tissue on the edge of the medial arch and the ground [11]. The navicular drop test (NDT) was first described as a means of representing the height change of the navicular bone under-weight bearing and non-weight bearing [5], with a moderate to good intra-reproducibility of the measurement [12]. The level of navicular drop refers to the height difference of the navicular tubercle in the standing and sitting position, and has been reported to be significantly correlated with the degree of the pronation of the foot of a runner, and has been used for judging flatfoot [4].

The footprint index classification is commonly used in research and the traditional method is mostly conducted with an ink-type instrument, which has advantages over other clinical tests, with its high reproducibility [13,14]. More recently, the plantar pressure system or the plantar scanning system has been used to measure, calculate, and determine the footprint. Although the footprints are measured in different ways, the method of judging flatfoot is the same. There are different calculations in terms of footprint classification, where the common arch index (AI) is obtained while the subjects stand evenly on the ink board one foot at a time [6]. The footprints are divided into three equal areas based on the foot length of the footprints to calculate the AI of the subjects and also quantify the severity of flatfoot [15]. The AI is highly reliable and correlates with X-ray measurements, so has been used for determining flatfoot in research [13]. The judgment for the type of arch relies on X-rays in hospital, which is reliable, but time-consuming and expensive.

Plantar pressure, also known as the pressure between the foot and the support surface, can be calculated by the force of the vertical contact divided by the contact area. Obtaining the plantar pressure information is helpful for assessing clinical problems. For example, the peak plantar pressure of the forefoot and hindfoot as well as the value of the peak forefoot pressure divided by the peak hindfoot pressure of severe diabetic neuropathic patients were significantly higher than that of patients with mild ulceration or with no history of ulceration. This was determined to be the possible reason as to why diabetic patients easily face the risk of foot ulceration. Therefore, plantar pressure can be regarded as a parameter to prevent foot ulceration for diabetic patients [16].

The average pressure has also been studied under 10 anatomical areas, including the medial and lateral heel area, the medial and lateral mid-foot area, the heads of the first, second, and fifth metatarsal area, the great toe, the head of the second metatarsal area, and the lateral metatarsal head area [15]. However, the placement of different insoles could effectively affect these pressure indices [17]. Thus, recent studies have addressed how to simplify the number of sensors in the plantar pressure insole or to place sensors at relatively important positions based on applications to reduce the development cost as well as provide relatively accurate data for launching products in the market to consumers [18]. Reduction of the spatial and temporal image resolution by placing pressure sensors at only seven locations, namely the heel, lateral midfoot, lateral forefoot, great toe, head of the first metatarsal, center midfoot, and center forefoot has been used for measuring and evaluating plantar pressure during activities of daily living for further gait analysis and interpretation of pathological foot anatomy.

A cluster type of sensor placement, being three on the metatarsal and three on the heels with a total of only six points, was developed, with the aim of further comparing the results between those obtained from state-of-the-art equipment with those obtained from sensors on the insole [19]. By placing sensors on the insole, these cost-effective and efficient wearable devices allow the monitoring and analysis of the gait anywhere from large clinics to an individual's home, and can be applied to produce graphics similar to those measured from the force-plate [20,21], where fourteen sensors of the Force Sensitive Resistance (FSR) Model 402 (Interlink Electronics, Camarillo, CA, USA) were placed on the forefoot, midfoot, and hindfoot to measure plantar pressure. In 2016, Liang et al. also developed an insole-based plantar pressure measurement with a fiber-optic sensing system where six fiber-optic sensors were used and embedded in silicone rubber. The results showed that the system could successfully identify the four different foot types and their developed-system could reach a Pearson correlation coefficient of 0.671 compared to the *i-Step P1000* plantar pressure plate [22].

It has been suggested that the traditional visual judgment of flatfoot was too subjective, and that flatfoot should be detected by objective and accurate optical or electronic systems [23]. This study suggested how to design the insole-based plantar pressure instrument for flatfoot. However, it lacked practical clinical verification. However, there are few insole-based plantar pressure measurement systems for flatfoot, therefore, methods of measuring flatfoot with a simplified framework of software, hardware, and design to develop a low-cost wearable insole-based instrument that allows for the measurement of plantar pressure to quickly and successfully screen flatfoot and normal foot during both static and dynamic conditions are of clinical needs.

Thus, the aims of this study were first to perform the correlation between traditional AI and the value of each point of the self-made insole-based sensor to find the effective plantar pressure sensor position and to compare whether there were significant differences between the sensors at specific positions of flatfoot and normal foot from the data of 21 young subjects.
