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 yr) from the Iowa Bone Development Study. PA was measured via 3 to 5 d of accelerometry using the Actigraph and 7-d self-report questionnaire using the Physical Activity Questionnaire for Children (PAQ-C). Hip, spine, and whole body BMC were measured via DXA (Hologic 4500). 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 multi-variable linear regression models (after controlling for body size and maturity) in boys. Only the Actigraph entered hip BMC regression models 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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