Purpose: Bone adapts to changing mechanical loads by altering the structure appropriately. These adaptations should be evident in the bone cross-sectional area (CSA) and section modulus (Z), indices of axial and bending strength, respectively. In this cross-sectional study, we investigated associations between physical activity, CSA, and Z in 467 young children (mean age 5.2 yr). We also examined whether lean tissue mass, which is predominantly muscle, mediates the relationship between physical activity and bone structural measures.
Methods: Physical activity was assessed using accelerometry and questionnaire. Proximal femur measures of the neck, intertrochanteric, and shaft CSA (cm) and Z (cm) were derived from dual-energy x-ray absorptiometry (DXA) scans using the Hip Structure Analysis program. Total body lean mass (kg) was also measured using DXA.
Results: Boys were more physically active than girls. Boys also had greater CSA, Z, and lean mass than girls. At each region, time spent in vigorous activity was positively and consistently associated with CSA and Z in boys and girls (r = 0.19 to 0.32). After adjustment for age, body mass, and height, vigorous activity explained, on average, 6.9% of the variability in CSA and Z. With additional adjustment for lean mass, vigorous activity explained 3.7% of the remaining variability in CSA and Z.
Conclusion: This study demonstrates that everyday amounts of physical activity in healthy, normal children are associated with bone geometry and that differences in lean mass explain some, but not all, of this association. This suggests that, even in young, nonathletic children, bone may adapt to physical activity by structurally remodeling.