ORIGINAL ARTICLES

Subjective and objective measures of physical activity in relationship to bone mineral content during late childhood: the Iowa Bone Development Study

K F Janz^1,2, H C Medema-Johnson^1,3, E M Letuchy², T L Burns^2,4, J M Eichenberger Gilmore⁵, J C Torner², M Willing⁴, S M Levy^2,5

¹ Department of Health and Sport Studies, The University of Iowa, Iowa City, Iowa, USA
² Department of Epidemiology, The University of Iowa, Iowa City, Iowa, USA
³ Department of Kinesiology, St Ambrose University, Davenport, Iowa, USA
⁴ Department of Pediatrics, The University of Iowa, Iowa City, Iowa, USA
⁵ Department of Preventive and Community Dentistry, The University of Iowa, Iowa City, Iowa, USA

Dr K F Janz, Department of Health and Sport Studies, 130 FH, University of Iowa, Iowa City, IA 52242, USA; kathleen-janz{at}uiowa.edu

Objective: This study compared accelerometry to self-report for the assessment of physical activity (PA) in relation to bone mineral content (BMC). In addition, we compared the ability of these measures to assess PA in boys versus girls.

Methods: Participants in this cross-sectional study included 449 children (mean age 11 years) from the Iowa Bone Development Study. PA was measured via 3–5 days of accelerometry using the Actigraph and 7 day self-report questionnaire using the Physical Activity Questionnaire for Children (PAQ-C). Hip, spine, and whole body BMC were measured via dual energy x ray absorptiometry (DXA).

Results: Partial correlation analysis (controlling for height, weight, and maturity) showed the Actigraph was significantly associated with hip (r = 0.40), spine (r = 0.20), and whole body (r = 0.33) BMC in boys, as was the PAQ-C (r = 0.28 hip, r = 0.19 spine, and r = 0.22 whole body). Among girls, only the Actigraph was significantly associated with hip (r = 0.18) and whole body (r = 0.16) BMC. Both the Actigraph and PAQ-C were significant in hip, spine, and whole body multivariable linear regression models (after controlling for body size and maturity) in boys. Only the Actigraph entered hip BMC regression model in girls.

Conclusions: Our study supports previous work showing associations between everyday PA and BMC in older children. These associations are more likely to be detected with an objective versus subjective measure of PA, particularly in girls.

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