Purpose: The primary purpose of this study was to investigate changes in exercise intentions and behaviors among children across time. Then, we investigated how determinants in the Theory of Planned Behavior (TPB) predicted exercise intentions and behaviors, and explored if demographic predictors contributed to predicting behaviors. Design and methods: A three-wave, 12-month longitudinal study was conducted. A proportional stratified random sampling method was adopted, and 1997 children from 11 elementary schools in Taipei City were recruited. Numbers of participants were 1074, 1064, and 995 at times 1, 2, and 3, respectively. Results: Children's exercise intentions and behaviors significantly changed (both p <.05) during a 6-month interval. Attitudes, subjective norms, and perceived behavioral control (PBC) at time 1 could respectively explain 51.0% and 17.1% of the variance in time 1 and 2 intentions (F(3, 1068) = 372.20, F(3, 1059) = 73.92, both p <.001). PBC was the strongest predictor of the intention to exercise. Intentions were the immediate determinant of exercise behaviors. PBC not only indirectly affected exercise behaviors through intentions but also directly affected exercise behaviors. Gender and sports club participation directly affected children's exercise behaviors. Conclusions: The findings support the TPB model being suitable for use in longitudinal studies; its core constructs significantly predicted children's exercise intentions and behaviors. Practice implications: This study highlights that clinical practitioners and school nurses working with children can help youth engage in regular exercise by enhancing their intentions and perceived behavioral control, and cultivating positive attitudes and subjective norms when planning exercise intervention programs.
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