## 4 Discussion
### Fat-Free Mass and Bone Tissue  
In the four groups studied (obese and non-obese women and men), a significant positive correlation between SOS in the radius and LBM parameters was observed only in non-obese men, emphasizing the positive effect of lean mass on bone quality. In particular, total LBM, LBM in the trunk, SMM, and BCM were all positively correlated with SOS, supporting the notion that lean mass contributes to bone tissue quality in non-obese men. In contrast, radial SOS was negatively correlated with LBM in obese men, suggesting that SOS tends to decrease with higher LBM; however, the correlation was not significant. However, the differential effect of LBM on bone tissue between obese and non-obese men suggests that lean mass favors bone quality more in the absence of high-fat mass. Furthermore, the correlation of SOS with most LBM-associated parameters in women was insignificantly negative in both the obese and non-obese groups. Our results in men are consistent with those of previous studies that showed a positive association between LBM and bone tissue characteristics. For example, LBM significantly predicted SOS in young Chinese adults [49]. The existence of an interaction between bone and muscle was also evidenced by the bone loss due to SMM decline investigated in the study by [PERSON] et al. [50], who found that the reduction of bone mass in the limbs of older adults was positively associated with loss of skeletal muscle. In addition, the study reported that individuals with a low skeletal muscle mass index (SMI) had a higher prevalence of osteoporosis and lower bone mass at various sites than the control group. Interestingly, [PERSON] et al. [50] focused on the correlation between bone mass and SMM in older adults, while [PERSON] et al. [51] also supported the notion of an association between osteoporosis and SMM loss, particularly in community-dwelling older women. However, [PERSON] et al. [51] found no significant association between osteoporosis and muscle strength, suggesting that the loss of muscle mass rather than muscle strength may be more closely related to bone density. [PERSON] et al. [52] reported that peak BMD and peak LBM are reached in late adolescence to early adulthood, with a significant association between BMD and fracture risk, suggesting that LBM may play a role in bone strength. [PERSON] et al. [53] also found a significant association between muscle strength, which is closely related to LBM, and bone mineralization in young adult women. Finally, [PERSON] et al. [54] showed that handgrip strength, which is an indicator of muscle mass, is strongly correlated with LBM and moderately correlated with BMD. However, it is important to note that although these studies show a positive correlation, the relationship between LBM and bone tissue characteristics may be complex and influenced by various factors such as age, sex, and health status. For example, [PERSON] et al. [16] pointed out that the diabetic subgroup did not show the same robust association as other subgroups, and [PERSON] and [PERSON] [55] suggested that the genetic component of BMD is not significantly reduced after adjustment for lean mass and muscle strength, suggesting that other genetic factors may also play a role in determining BMD. Literature indicates that LBM positively influences bone density and strength through endocrine interactions between muscle and bone, muscle contraction, and physical activity, including specific exercise regimens, which are closely related to mechanical bone stress [56; 57; 58]. Mechanical loading has recently been considered as the main factor influencing bone strength, as the muscles associated with bones produce small levers that require significant pressure to generate torque. The mechanical loading from these forces stresses the bones [59; 60]. Moreover, muscle strength, which is closely related to LBM, has been found to be directly related to BMD, suggesting that the interaction between muscle and bone is mediated by both mechanical and endocrine pathways [57].