Physical activity and bone measures in young children: The Iowa Bone Development Study

Objectives. Physical activity has a beneficial effect on bone development i n circumpubertal children, although its effect on younger children is uncer tain. In this cross-sectional study, we examined associations between physi cal activity and bone measures in 368 preschool children (mean age: 5.2 yea rs, range: 4-6 years). Design. Physical activity was measured using 4-day accelerometry readings, parental report of children's usual physical activity, and parental report of children's hours of daily television viewing. Total body and site-specif ic bone mineral content and area bone mineral density (BMD) were measured b y dual energy radiograph absorptiometry. Results. After adjustment for age and body size, accelerometry measures of physical activity and parental report of usual physical activity were consi stently and positively associated with bone mineral content and BMD in both boys and girls (r = 0.15-0.28). Television viewing was inversely associate d with hip BMD in girls (r = -0.15). The proportion of variance in bone mea sures explained by physical activity in linear regression models ranged fro m r(2) = 1.5% to 9.0%. In all of these models except total body BMD, at lea st 1 and often several of the physical activity variables entered as indepe ndent predictors. Activity variables most likely to enter the regression mo dels were vigorous physical activity (as determined by accelerometry) and p arental ranking of child's usual physical activity. Conclusions. Findings indicate that there are statistically significant and , perhaps important, associations between physical activity and bone measur es during early childhood, well ahead of the onset of peak bone mass. This would suggest that intervention strategies to increase physical activity in young children could contribute to optimal bone development.