Quantifying bone-relevant activity and its relation to bone strength in girls

Physical activity (PA) is critical for maximizing bone development during growth. However, there is no consensus on how well existing PA measurement tools predict bone strength. Purpose: The purposes of this study were to compare four methods of quantifying PA (pedometer, 3-d PA recall (3DPAR), bone-specific PA questionnaire (BPAQ), and past year PA questionnaire (PYPAQ)), in young girls and to evaluate their ability to predict indices of bone strength. Methods: A total of 329 girls aged 8-13 yr completed a pedometer assessment, the 3DPAR, the BPAQ, and a modified PYPAQ. Peripheral quantitative computed tomography was used to assess bone strength index (BSI) at metaphyseal (4% distal femur and tibia) sites and strength-strain index (SSI) at diaphyseal (femur = 20%, tibia = 66%) sites of the nondominant leg. Correlations and hierarchical multiple regression were used to assess relationships among PA measures and indices of bone strength. Results: After adjusting for maturity, correlations between PA measures and indices of bone strength were positive, although low (r = 0.01-0.20). Regression models that included covariates (maturity, body mass, leg length, and ethnicity) and PA variables showed that PYPAQ score was significantly (P < 0.05) associated with BSI and SSI at all sites and explained more variance in BSI and SSI than any other PA measure. Pedometer steps were significantly (P < 0.05) associated with metaphyseal femur and tibia BSI, and 3DPAR score was significantly (P < 0.05) associated with metaphyseal femur BSI. BPAQ score was not significantly (P > 0.05) associated with BSI or SSI at any sites. Conclusions: A modified PYPAQ that accounts for the duration, frequency, and load of PA predicted indices of bone strength better than other PA measures.