A six-year longitudinal study of the relationship of physical activity to bone mineral accrual in growing children: The University of Saskatchewan bone mineral accrual study

To investigate the influence of physical activity on bone mineral accrual d uring the adolescent years, we analyzed 6 years of data from 53 girls and 6 0 boys. Physical activity, dietary intakes, and anthropometry were measured every 6 months and dual-energy X-ray absorptiometry scans of the total bod y (TB), lumbar spine (LS), and proximal femur (Hologic 2000, array mode) we re collected annually. Distance and velocity curves for height and bone min eral content (BMC) were fitted for each child at several skeletal sites usi ng a cubic spline procedure, from which ages at peak height velocity (PHV) and peak BMC velocity (PBMCV) were identified. A mean age- and gender-speci fic standardized activity (Z) score was calculated for each subject based o n multiple yearly activity assessments collected up until age of PHV. This score was used to identify active (top quartile), average (middle 2 quartil es), or inactive (bottom quartile) groups. Two-way analysis of covariance, with height and weight at PHV controlled for, demonstrated significant phys ical activity and gender main effects (but no interaction) for PBMCV, for B MC accrued for 2 years around peak velocity, and for BMC at 1 year post-PBM CV for the TB and femoral neck and for physical activity but not gender at the LS (all p < 0.05). Controlling for maturational and size differences be tween groups, we noted a 9% and 17% greater TB BMC for active boys and girl s, respectively, over their inactive peers 1 year after the age of PBMCV. W e also estimated that, on average, 26% of adult TB bone mineral was accrued during the 2 years around PBMCV.