Objectives: To explore associations between mechanical load, expressed by structural (appendicular lean soft tissue index, ALSTI; ALST/trunk fat mass, TrFM); and functional lean mass (handgrip strength, HGS; HGS/body mass) with the mechanical capacity of bone, represented by whole-body bone mineral density less head (WBLH BMD) and arms/legs BMD imbalance. Methods: We analysed cross-sectional NHANES data from 5,424 adolescents (10–19 years, both sexes) collected between 2011 and 2020. ALST, TrFM and BMD were determined using whole-body dual-energy X-ray absorptiometry, and HGS was assessed with a dynamometer. All variables were standardised by sex and age group (Z-scores), with Z ≤ -1 indicating low or at-risk values. Results: The odds of low WBLH BMD were higher in participants with low ALSTI (girls: OR= 3.9; boys: OR=4.5, p<0.001) or low HGS (girls: OR=3.1; boys: OR=4.5, p<0.001). Boys with low HGS/body mass had 1.8 times higher odds of low arms/legs BMD (p=0.012), while girls with low HGS/body mass had 4.0 times higher odds (p<0.001). Conclusions: Both structural and functional lean mass deficits are associated with reduced WBLH BMD and arms/legs BMD imbalance in adolescents. These findings reinforce the importance of lean soft tissue in bone health monitoring.