## 2 Materials and Methods
### Ultrasonography  
The architecture of the gastrocnemius medialis muscle was assessed using extended field-of-view (EFOV) imaging with a 10 MHz linear probe (38 mm) (product model Z5, Shenzhen, Mindray Bio-Medical Electronics Co., Ltd., Shenzhen, China). This study aimed to examine muscle architecture only at rest (i.e., muscle fascicle optimal length) and not in different joint positions in order to observe growth changes during childhood and adolescence. Athletes lay prone on the physiotherapy bed with their ankle joints off the edge. FL and cross-sectional area of the right leg were measured at the midpoint of gastrocnemius medialis. The EFOV technique allows for the capture of fascicles longer than the probe's length to avoid trigonometric estimations using non-linear aponeuroses. An echo-absorptive marker was placed at the midpoint of the gastrocnemius medialis muscle belly, estimated as the thirty percent of the distance between the tiboifemoral joint to the most prominent point of the medial malleolus. A probe path was drawn on the skin with a permanent pen by real-time imaging to obtain a clear panoramic view parallel to the fascicles' orientation (Figure 1) [33]. Aquasonic gel was used to provide acoustic permeability, while the probe was moving slowly and steadily along the muscle applying constant minimal pressure. For each capture, three different fascicles crossing the  
\begin{table}
\begin{tabular}{l c c c c c} \hline  & **Group 1 (n = 13)** & **Group 2 (n = 14)** & **Group 3 (n = 26)** & **Group 4 (n = 22)** & **Group 5 (n = 14)** \\  & **8–10 y** & **10–12 y** & **12–14 y** & **14–16 y** & **16–18 y** \\ \hline Age (y) & \(9.4\pm 1.0\) & \(11.6\pm 0.3\) & \(13.0\pm 0.7\) & \(15.0\pm 0.6\) & \(17.3\pm 0.9\) \\ Training Experience (y) & \(1.8\pm 0.9\) & \(2.1\pm 1.2\) & \(2.8\pm 1.5\) & \(2.9\pm 1.2\) & \(5.0\pm 1.4\) \\ Height (cm) & \(139.4\pm 9.0\) & \(154.3\pm 6.2\) & \(160.4\pm 5.1\) & \(164.5\pm 7.0\) & \(166.5\pm 6.2\) \\ Sitting Height (cm) & \(72.5\pm 4.5\) & \(79.3\pm 5.9\) & \(84.2\pm 3.3\) & \(87.2\pm 4.8\) & \(90.5\pm 3.2\) \\ Leg Length (cm) & \(70.0\pm 6.4\) & \(78.3\pm 5.2\) & \(80.3\pm 6.0\) & \(82.9\pm 6.9\) & \(84.1\pm 5.5\) \\ Calf Length (cm) & \(34.3\pm 3.0\) & \(36.5\pm 2.8\) & \(37.9\pm 2.0\) & \(39.4\pm 2.0\) & \(37.8\pm 2.9\) \\ Weight (kg) & \(38.1\pm 8.1\) & \(47.6\pm 9.1\) & \(53.6\pm 9.0\) & \(57.2\pm 8.8\) & \(58.4\pm 10.7\) \\ BMI (kg/m\({}^{2}\)) & \(19.4\pm 1.8\) & \(20.0\pm 3.6\) & \(20.9\pm 3.5\) & \(21.2\pm 3.2\) & \(21.1\pm 3.8\) \\ PHY (y) & \(11.3\pm 0.5\) & \(11.5\pm 0.5\) & \(11.7\pm 0.5\) & \(12.2\pm 0.7\) & \(12.9\pm 0.7\) \\ Maturity Offset (y) & \(-2.1\pm 0.9\) & -0.1 \(\pm\) 0.4 & \(1.09\pm 0.61\) & \(2.66\pm 0.51\) & \(4.42\pm 0.83\) \\ \hline \end{tabular}
\end{table}
Table 1: Anthropometric characteristics of the participants (means \(\pm\) standard deviations).  
absorptive marker in the middle of the muscle were measured and the average value was used for further analysis (Figure 2). The EFOV technique was also applied to assess the gastrocnemius medialis cross-sectional area. A probe path was again drawn perpendicular to the calf at the middle of the muscle belly (Figure 1). The probe was moving slowly and steadily on the transverse plane, perpendicular to the skin to obtain a panoramic ultrasound image. The outline of muscle area was measured manually, with image analysis software using a polygon selection tool (Figure 3) (Motic Images Plus 2.0, Motic, Hong Kong, China). Test-retest reliability using ICC (two-way ANOVA random effects model) for muscle FL was (0.971; 95% CI: 0.902-0.991) and CSA was (0.922; 95% CI: 0.752-0.977).