K. F. Janz*1, T. L. Burns*2, S. M. Levy*3, J. C. Torner*4, M. C. Willing5, T. J. Beck*6, J. M. Gilmore*3, T. A. Marshall*3. 1Health and Sport Studies, University of Iowa, Iowa City, IA, USA, 2Biostatistics, University of Iowa, Iowa City, IA, USA, 3Preventive and Community Dentistry, University of Iowa, Iowa City, IA, USA, 4Epidemiology, University of Iowa, Iowa City, IA, USA, 5Pediatrics, University of Iowa, Iowa City, IA, USA, 6Radiology, Johns Hopkins, Baltimore, MD, USA.
Intervention studies suggest that a threshold of vigorous-intensity physical activity is needed to impact bone strength in children. However, quantifying this relationship in population-based studies is difficult since children are not cognitively capable of accurately reporting activity intensities. In this observational study, accelerometry was used to investigate associations between activity intensities and femoral neck (FN) bone strength (426 children, mean age 5.2 yr, range 4 to 6 yr). We tested the hypothesis that within the scope of children’s usual activity patterns, vigorous activity is more strongly associated with bone strength than other activity intensities.
Physical activity was measured using 4-d accelerometry readings. This method produced minute-by-minute movement count data. Movement counts were calibrated to oxygen uptake as a multiple of resting metabolic rate (METs). The predictor variables were minutes per day in sedentary and light (< 2.9 METs), moderate (3 to 5.9 METs), and vigorous activity (> 6 METs). FN aBMD, cross-sectional area (CSA), and section modulus (Z) were calculated from DXA outputs (Hologic 2000). Associations were examined using partial correlation coefficients and stepwise linear regression after adjustment for weight and height.
In boys, only vigorous activity entered regression models that included weight and height, the proportion of variance explained after adjustments ranged from r2 = 3.9 to 5.8%. In girls, vigorous activity also entered all models explaining 1.2 to 4.4% of the variance and sedentary and light activity inversely entered the FN aBMD model (r2 = 5.7%). During young children’s "everyday" activity, vigorous activity is more highly associated with FN bone strength than other activity intensities. In girls, sedentary and light activity negatively impacts bone strength. Public health guidelines emphasizing moderate activity may not be appropriate for children’s bone health.