Published online September 1, 2008
PEDIATRICS Vol. 122 No. 3 September 2008, pp. e728-e736 (doi:10.1542/peds.2007-2573)
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ARTICLE

Objectively Measured Physical Activity and Bone Strength in 9-Year-Old Boys and Girls

Luís B. Sardinha, PhDa, Fátima Baptista, PhDa and Ulf Ekelund, PhDb,c

a Exercise and Health Laboratory, Faculty of Human Movement, Technical University of Lisbon, Lisbon, Portugal
b Medical Research Council Epidemiology Unit, Institute of Public Health, University of Cambridge, Cambridge, United Kingdom
c School of Health and Medical Science, Örebro University, Örebro, Sweden

OBJECTIVE. The purpose of this work was to analyze the relationship between intensity and duration of physical activity and composite indices of femoral neck strength and bone-mineral content of the femoral neck, lumbar spine, and total body.

METHODS. Physical activity was assessed by accelerometry in 143 girls and 150 boys (mean age: 9.7 years). Measurement of bone-mineral content, femoral neck bone-mineral density, femoral neck width, hip axis length, and total body fat-free mass was performed with dual-energy radiograph absorptiometry. Compressive [(bone-mineral density x femoral neck width/weight)] and bending strength [(bone-mineral density x femoral neck width2)/(hip axis length x weight)] express the forces that the femoral neck has to withstand in weight bearing, whereas impact strength [(bone-mineral density x femoral neck width x hip axis length)/(height x weight)] expresses the energy that the femoral neck has to absorb in an impact from standing height.

RESULTS. Analysis of covariance (fat-free mass and age adjusted) showed differences between boys and girls of ~9% for compressive, 10% for bending, and 9% for impact strength. Stepwise regression analysis using time spent at sedentary, light, moderate, and vigorous physical activity as predictors revealed that vigorous physical activity explained 5% to 9% of femoral neck strength variable variance in both genders, except for bending strength in boys, and ~1% to 3% of total body and femoral neck bone-mineral content variance. Vigorous physical activity was then used to categorize boys and girls into quartiles. Pairwise comparison indicated that boys in the third and fourth quartiles (accumulation of >26 minutes/day) demonstrated higher compressive (11%–12%), bending (10%), and impact (14%) strength than boys in the first quartile. In girls, comparison revealed a difference between the fourth (accumulation of >25 minutes/day) and first quartiles for bending strength (11%). We did not observe any relationship between physical activity and lumbar spine strength.

CONCLUSIONS. Femoral neck strength is higher in boys than girls. Vigorous intensity emerged as the main physical activity predictor of femoral neck strength but did not explain gender differences. Daily vigorous physical activity for at least ~25 minutes seems to improve femoral neck bone health in children.

Key Words: bone strength • children • physical activity

Abbreviations: BMC—bone-mineral content • BMD—bone-mineral density • DEXA—dual-energy radiograph absorptiometry • HAL—hip axis length • FNW—femoral neck minimal width

Accepted Apr 17, 2008.


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