Body composition and physical activity during childhood and adolescence: Relations to biomechanical bone strength

Numerous studies have examined the influences of body composition and physical activity (PA) on bone development, but none have used micro-finite element analysis(mFEA) from HRpQCT scans. Thus, in 150 healthy 8- to 15-yr old boys (n=85) and girls (n=65), we investigated the relations of appendicular skeletal muscle mass(ASM), total body fat mass (TBFM) and PA to distal radius and tibia biomechanical bone strength (failure load), using mFEA. We determined relative ASM and TBFM from whole-body DXA scans. PA strain score (related to mechanical loading) and time spent in vigorous-intensity PA was quantified from a validated questionnaire. Results were similar in analyses stratified by sex; thus, we present analyses from the boys and girls combined. Relative ASM was positively associated with bone strength of the distal radius (r=0.55, P,0.001; Fig. A) and tibia (r=0.61, P,0.001; Fig. B) after adjusting for bone-age, sex, fracture history, relative TBFM and PA. By contrast, after adjustment for the same covariates, substituting relative TBFM for relative ASM, a significant inverse association was observed between relative TBFM and bone strength of the distal radius (r=–0.30, P,0.001), while no correlation was seen at the distal tibia (r=–0.01, P=0.91). In addition, after adjustment for bone-age, sex, fracture history, height and weight, time spent in vigorous-intensity PA was positively associated with bone strength (r=0.28, P,0.01) at the distal tibia only, while PA strain score was significantly associated with greater bone strength of the distal radius(r=0.16, P,0.05) and tibia (r=0.24, P,0.01).These data thus highlight the importance of skeletal muscle mass and PA for optimizing bone strength during growth. Furthermore, fat mass has different ial influences on bone strength at weight-bearing versus non-weight-bearing skeletal sites in children and adolescents. At the distal tibia, fat mass is not associated with bone strength, whereas fat mass is negatively associated with bone strength at the distal radius after adjusting for bone-age, sex, fracture history, skeletal muscle mass and PA. These observations suggest that the strength of the distal radius does not commensurately increase with excess gains in adiposity during growth, which may result in a mismatch between bone strength and the load experienced by the distal forearm during a fall. These findings may, in part, explain why obese children are over-represented in distal forearm fracture cases