2.2.1

Static foot posture was assessed using a three-dimensional foot scanner (Dream GP, Osaka, Japan) . Scanning was performed under weightbearing conditions, with 50 % of the body weight applied to the test foot ( Fig. 1 ). As indicators of foot structure related to the pronated foot, we calculated the arch height index, hallux valgus angle, and fifth toe varus angle. The arch height index was determined by dividing the dorsal height at 50 % of the foot length by the truncated foot length. We also assessed foot posture using the Foot Posture Index 6-item (FPI-6), a widely used clinical assessment tool ^, . A higher FPI-6 score indicated a more pronated foot posture, whereas a lower score indicated a more supinated foot posture.

Each measurement procedure. A: 3D foot scanning, B: muscle stiffness assessment using MyotonPRO, C: measurement of ankle anterior displacement, D: measurement of hallux extension range of motion.

2.2.2

The participants were positioned in the prone position with their knee joints fully extended and foot placed in a custom-made device to maintain the ankle in a neutral dorsiflexion/plantarflexion position. In this posture, the stiffnesses of the medial gastrocnemius (MG), lateral gastrocnemius (LG), and soleus muscles were measured using two different devices. These muscles were selected based on previous findings that indicated their association with DROM ^, .

The shear elastic moduli of the MG, LG, and soleus muscles were measured using ultrasonic shear wave elastography (Aplio 500; Toshiba Medical Systems, Tochigi, Japan) with a 5–14 MHz linear probe. The measurement sites were set 30 % proximal to the gastrocnemius muscles and 50 % proximal to the soleus along the longitudinal axis of the lower leg ( Fig. 2 ) ^, . A rectangular ROI was placed at the muscle center, with three circular ROIs inside. Shear elastic modulus (kPa) was calculated for each ( Fig. 2 ). The average of three values was used in the analysis.

Measurement of shear elastic modulus using ultrasonic shear wave elastography. A: Measurement sites, B: ultrasound image with the region of interest placed. (a) indicates the medial gastrocnemius (MG), (b) the lateral gastrocnemius (LG), and (c) the soleus.

Muscle stiffness was also assessed using compressive stiffness (N/m) of the MG, LG, and soleus muscles was measured using a MyotonPRO device (Myoton AS, Tallinn, Estonia). This device is a compact, non-invasive handheld device that applies a brief mechanical impulse (time:15 ms, force: 0.4 N) to the skin surface ( Fig. 1 ) . The measurement sites were the same as those used for ultrasonic shear wave elastography. Measurements were performed three times,

2.2.3

Anterior displacement of the ankle joint was measured using a capacitance-type sensor device (AT measure; Aimedic MMT Co. Ltd., Japan) . The participants were seated at the edge of a table with their knees flexed, and a soft brace equipped with a capacitance-type sensor device was applied ( Fig. 1 ). The ankle joint was placed in slight plantar flexion and the examiner performed an anterior drawer by pulling the foot anteriorly relative to the lower leg. Anterior displacement (mm) was measured five times with the sensor .

2.2.4

The extension range of motion (ROM) of the hallux relative to the plantar surface was measured using a goniometer capable of 1-degree increments. The measurement was performed with the ankle in maximum dorsiflexion while manually stabilizing the first metatarsal head to prevent plantarflexion ( Fig. 1 ). . Measurements were performed three times.

2.2.5

Ankle DROM was measured under three postural conditions ( Fig. 3 ). Under non-weightbearing conditions, measurements were obtained in the prone position with the knee extended and flexed . Using a goniometer, the angle between the plantar surface of the foot and the fibular axis was measured in 1-degree increments. The examiner manually dorsiflexed the ankle to its maximum and measured its angle. In the weightbearing condition, ankle DROM was measured using the weightbearing lunge test . At maximum dorsiflexion without a heel lift, the tibial anterior inclination angle was measured using an inclinometer (Fabrication Enterprises Inc., White Plains, NY, USA) placed 15 cm distal to the tibial tuberosity, and recorded in 0.1-degree increments. All DROM measurements were performed three times.

Measurement of ankle DROM. A: Non-weightbearing with extended knee, B: non-weightbearing with flexed knee, C: weightbearing position.