Growth Trajectories of Young Children's Objectively Determined Physical Activity, Sedentary Behavior, and Body Mass Index.

PURPOSE: This study examined trajectories in children's physical activity (PA), sedentary behavior, and body mass index (BMI) for both genders, and relationships among these trajectories, from childhood through early adolescence.
METHODS: A total of 261 seconds and third-grade children (135 girls; Meanage = 7.81; 73% White) from two U.S. elementary schools participated in this study. Their objective PA, sedentary behavior, and BMI were measured yearly from 2012 to 2015. The outcome variables were accelerometer-determined daily moderate-to-vigorous PA (MVPA), sedentary behavior, and BMI-calculated as height divided by weight squared. A series of latent growth curve models (LGCM) were employed to analyze the trajectories in the outcome variables and relationships over time through AMOS version 23 in 2017.
RESULTS: The models generally fitted the data well. In detail, children's MVPA increased slightly by the end of the first year and then declined during follow-ups (p < 0.05), with boys having more MVPA time. Sedentary hours increased at 2-year follow-ups (p < 0.05), but decreased slightly at year 3 follow-up in both genders. BMI increased gradually for boys (p < 0.05), particularly in year 3. Trajectories in MVPA were negatively related to BMI trajectories (p < 0.05), and trajectories in MVPA and sedentary behavior did not interact to affect BMI trajectories.
CONCLUSIONS: Maintaining or increasing MVPA and limiting sedentary behavior should be components of efforts to prevent excess weight gain during the transition from childhood to early adolescence. Children's MVPA and sedentary behavior are independent determinants of BMI changes.