## 1 Introduction  
Growth and mechanical loading trigger molecular pathways and induce muscle morphological adaptations, which are reflected in muscle size increases [1, 2]. Skeletal muscle morphology is macroscopically described using architectural features (such as fascicle length and angle, muscle thickness and anatomical cross-sectional area) linked to muscle function [3, 4]. Muscle growth occurs through the addition of new muscle fibers, the addition of sarcomeres in series or parallel within pre-existing muscle fibers, or through increases in muscle fiber diameter [5, 6]. These growth strategies affect whether muscle fascicles grow longitudinally and/or transversely, which, in turn, modifies the fascicle pennation angle and enhances muscle volume [7]. Simultaneously, the connective tissue, both within (i.e., perimysium and endomysium) and around the muscle-tendon unit (i.e., epimysium), also undergoes growth [8]. Fascicle length (FL) and anatomical cross-sectional area (ACSA) represent muscle geometry and are associated with muscle function and muscle mechanical output [9]. Fascicle length is defined as the straight-line distance between the upper aponeurosis and the deeper aponeurosis, parallel to the lines of collagenous tissue visible on the ultrasound images [4]. Generally, FL reflects the number of sarcomeres in series [4] and is associated with maximal muscle elongation, as well as the muscle length at which the myofilaments optimally overlap [10]. Anatomical cross-sectional area is the largest cross-sectional area along the length of an individual muscle [11; 12]. Muscles with broader fiber diameters have a greater capacity for force and power generation [13; 14; 15]. During growth, the size and length of skeletal muscles are modified to respond to the functional demands of their mechanical environment [16]. Previous cross-sectional studies reported that from childhood to adolescence, gastrocnemius medialis muscle belly and FL increase in proportion to tibia longitudinal growth [1; 17; 18]. Nevertheless, information regarding the typical growth of human muscles at the macroscopic level is derived from a few studies, using either ultrasound imaging or diffusion tensor imaging, the majority of which provided comprehensive measurements of muscle architecture in children [19; 20; 21; 22]. These studies also included small cross-sectional samples (10-30 participants) and provided summary statistics for muscle architecture, making it difficult to compare age-specific measurements. However, understanding changes in muscle architecture due to growth is important because these changes directly affect muscle function and movement efficiency [23]. As muscles grow, changes in anatomical cross-sectional area, fascicle length and angle influence their capacity for force and power production, affecting physical development and sports performance. These insights are essential for optimizing training and injury prevention programs for youth athletes. Furthermore, knowledge of these changes, especially in a large number of participants, helps in understanding developmental biomechanics and clinical and training practices [23]. Muscle size and length grow at different rates which are affected by bone development, growth and hormonal signaling [24; 25]. Nevertheless, the specific changes in muscle architecture (e.g., fascicles, anatomical cross-sectional area) during growth from childhood to adulthood remain largely unknown. Therefore, assessing muscle architecture in the context of age-related changes may provide important information about the properties of muscle tissue. Importantly, children and adolescent athletes are submitted to training loads according to the age-group competitive category that national and international sports federations classify irrespective of their biological age. To date, growth changes during childhood and adolescence in muscle architecture have not been examined in developing athletes.