Two measures of physical activity as predictors of bone mass in a young cohort

Objective: To compare the association of two measures of physical activity with bone mass in healthy children and young adults, as part of a larger study on bone mineral acquisition in youth. Design: Cross-sectional observation study. Setting: General community, outpatient study. Participants: Subjects included 103 non-Hispanic white female (n = 54) and male (n = 49) healthy volunteers aged 9 to 25 years. Main Outcome Measures: Self-reported physical activity was measured by a 3-day activity diary of all activities and a questionnaire designed to capture recreational activities throughout the year. Activity was expressed as hours per week of weight-bearing and non-weight-bearing activity. Bone mass at the hip, spine, and whole body was measured by dual x-ray absorptiometry. Results: The activity measures were not well correlated with each other. In males, weight-bearing and non-weight-bearing activity reported in 3-day diaries was positively associated with bone mass at the hip, spine, and whole body (p < 0.05). Among females, only weight-bearing activity measured by the yearly questionnaires was significantly positively associated with bone mass (p < 0.05). In males and females, weight-bearing activity was more highly correlated with bone mineral than was non-weight-bearing activity. In addition, the associations between activity and bone mass varied by skeletal site. Conclusions: The association between physical activity and bone mass varied both in direction and in significance depending on the physical activity instrument used. Gender differences were observed in the associations between specific activity instruments, type of activity (weight-bearing and non-weight-bearing), and bone mass at different skeletal sites. Variability associated with the two physical activity measures may contribute to discrepant findings in this study and in the literature.