## 1 Introduction  
Maintaining the integrity of bone tissue is critical to its function and adaptability, particularly during skeletal growth. Behaviors associated with osteoporosis, such as physical inactivity and poor nutrition during childhood, adolescence, and young adulthood, can reduce peak bone mass and increase the risk of osteoporosis later in life [1]. A decrease in bone quantity, particularly bone mass and bone mineral density (BMD), increases the susceptibility of bones to deformity and fragility [2]. Over the last three decades, research has shown that bone quantity alone does not increase fracture rates [3, 4]. Therefore, there is an increased interest in the factors that influence bone quality, including composition, structure, microdamage mechanisms, and remodeling processes. Clinical evaluation of bone fragility and fracture risk typically relies on the measurement of bone mineral density (BMD) using dual-energy X-ray absorptiometry (DXA). However, recent research indicates that DXA-derived BMD correlates poorly with fracture resistance in adults and children [3]. Multisite quantitative ultrasound (mQUS), a non-invasive diagnostic tool that measures the speed of sound (SOS) in the bone, is an alternative method for assessing bone health and predicting fracture strength. This technique has evolved significantly and offers advantages over DXA, such as affordability, portability, and the ability to target specific anatomical sites without ionizing radiation. Research suggests that mQUS can detect aspects of bone quality not captured by DXA, reflecting both bone density and qualitative properties, includingelasticity and trabecular microarchitecture [5; 6; 7]. Therefore, mQUS can potentially assess bone health and integrity [6]. In particular, radius QUS is considered a reliable method in primary healthcare settings to assess fracture risk and to perform osteoporosis pre-screening [8; 9; 10]. The mQUS showed promise in predicting fractures in the radius of 10 to 12-year-olds, indicating its potential as a screening tool for bone fragility. Although it is not consistent with the whole body without head (WBLH) BMD measurements from the DXA [11], it remains a valuable resource. A recent meta-analysis by [PERSON] et al. [12] confirmed these findings and showed an association between measured SOS and the risk of hip fractures, particularly in women. Bone mass is influenced by many factors, with two main body components being the most important determinants: fat mass (FM) and lean body mass (LBM). LBM, which represents the total body weight excluding fat, is crucial for bone health [13; 14]. Skeletal muscle, which makes up a large portion of the LBM, functions as an endocrine organ that secretes muscle factors, such as insulin-like growth factor 1, fibroblast growth factor 2, and brain-derived neurotrophic factor. These substances influence bone metabolism through interactions with osteoblasts and osteoclasts [15; 16]. Although a recent systematic review and meta-analysis demonstrated a positive association between LBM and BMD [17], the impact on bone quality as defined by the SOS parameter remains unanswered. Furthermore, the conclusions regarding the relationship between FM and bone tissue properties are still not clear. [PERSON] et al. [18] observed that high amounts of FM had a negative effect on bone mass, while [PERSON] et al. [19], who stratified the samples by age, found this negative effect only in individuals under 25 years of age. In contrast, the effect was positive in older people. This finding was also confirmed by [PERSON] [17], who pointed out that body fat (%) appears to have a negative effect on bone formation in children and adolescents. In addition, a longitudinal study of adolescents aged 11-19 years found a negative effect of FM on cortical bone strength in boys [20]. Recent evidence suggests that FM may affect bone health through inflammation, oxidative stress, and the shared origin of adipocytes and osteoblasts from mesenchymal stem cell progenitors [21].