## 2 Materials and Methods
### Data Acquisition and Statistical Analysis  
Before each measurement, the pressure plates were calibrated according to the manufacturer's instructions. A graphical representation of the plantar pressure images of both feet in the SCFP condition was exported in \"PNG\" format for each participant and then imported into the open-access GeoGebra tool [32]. This tool was used to calculate the following between-feet measurements (BFMs): Inter-Heels Medial (IHMD) and Posterior Distances (IHPD) [cm]; Right Foot Standing Angle (RFSA) [Deg]; Left Foot Standing Angle (LFSA) [Deg], and Between-Feet Angle (BFA) [Deg]. Regarding the calculation of IHMD and IHPD, reference was made to the millimeter scale shown in the image imported into the online tool. These parameters were calculated according to specific points identified on the foot outline (Figure 2). In particular, IHMD and IHPD were calculated as the distances between the most medial points of the left and right rearfoot and between the most posterior points of the left and right heel, respectively. In addition, points of tangency for the forefoot and rearfoot region were identified (Figure 2). The BFA was calculated as the angle between the two tangent lines on the medial region of the foot pressure images. RFSA and LFSA were calculated as the angles between each of the two tangent lines and the longitudinal axis between the feet (dashed gray line in Figure 2). Figure 1: Graphical representation on the sagittal plane of a healthy young adult in upright posture in both SCFP (**a**) and SFP (**b**) with a target placed at 0.7 m from the heels. Representation of anthropometric measurements (height, inter-Acromion and inter-ASIS distances) on the frontal plane (**c**) and of the feet on a transversal plane in both SCFP (**d**) and SFP (**e**). Figure note: self-selected comfortable foot position (SCFP); standardized foot position (SFP); Anterior Superior Iliac Spine (ASIS). The following clinically relevant stabilometric and plantar pressure parameters, defined in our previous study [7], were exported from P-Walk software (G Studio 3.5.26.8): center-of-pressure sway area (CoPsa) [mm\({}^{2}\)], the ellipse containing the trajectory of the CoP recorded during the exam; Length Surface Function (LSF) [mm\({}^{-1}\)], defined as the ratio between distance covered by the CoP during the exam and CoPsa; center-of-pressure speed (CoP-speed) [mm/s]; total foot (Tf), rearfoot (Rf), midfoot (Mf), and forefoot (Ff) loads [%] (load parameters were normalized to body weight); mean and peak pressures (Pmean and Pmax) [KPa] at Rf, Mf, and Ff (mean pressure parameters were calculated as the ratio between weight and area for each region); foot contact area (FCA) [cm\({}^{2}\)], and arch index (AI) [%] that is calculated as the ratio between midfoot contact area and total foot contact area (without the toes). All plantar pressure parameters were calculated for both the left (l) and right (r) sides. In addition, the coefficient of variation (CV) was calculated for all plantar pressure and stabilometric parameters across three trials for each condition of foot placement (SCFP and SFP) in order to assess their intra-subject variability. The Shapiro-Wilk test was used to assess the normality of all AMs, BFMs, CVs, plantar pressure, and stabilometric parameters. According to the type of distribution (parametric or non-parametric), mean with standard deviation (SD) and range (minimum, maximum) or median with 25 th-75 th percentiles were used to report these values. Firstly, a correlation analysis between AMs (height, inter-acromion and inter-ASIS distances) and BFMs (BFA, IHMD, and IHPD) was performed using Pearson's coefficient with a 95% confidence interval. Afterward, the following statistical comparisons were implemented:  
signed rank test to assess the effect of foot positions (SCFP and SFP) on CVs and on all plantar pressure and stabilometric parameters.