Correlates of objectively and subjectively measured physical activity in middle school youth

Background: Children are not meeting the recommended 60 minutes of moderate to vigorous physical activity (MVPA) per day, increasing their risk of obesity and other health complications. Numerous correlates have been identified in the literature; however, the majority of studies have relied on self-reported (subjectively measured) physical activity as the dependent variable. Therefore, these studies may not identify true physical activity correlates since subjective measures rely heavily on one's attitudes and can share variance with subjective outcomes unrelated to actual physical activity behaviors. Therefore, the current investigation was designed to investigate the relationship between hypothesized correlates of physical activity and objectively and subjectively measured MVPA. Methods: Middle school children (N=285, 113 male) who participated provided a minimum of four monitored days of physical activity (via accelerometer) and completed a survey which assessed physical activity, athletic competence, perceived appearance, enjoyment of physical activity, and self-efficacy for physical activity. Height and weight were measured and BMI calculated. Results: Hierarchical regression models controlling for sex, race, and BMI Z-score indicated that athletic competence, enjoyment of physical activity, and self-efficacy for physical activity were significant predictors of self-reported physical activity. However, in a separate model, sex, BMI Z-score, and enjoyment of physical activity were significant predictors of minutes of MVPA measure via accelerometry. Conclusions: Measuring physical activity via self-report versus accelerometry produces drastically different results in a sample of middle school youth. Future studies should use caution when identifying outcome measures if the intent is to identify modifiable correlates of physical activity in youth.