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Background: External contexts, including the social and physical contexts, are independent predictors of momentary physical activity and sedentary behaviors. However, no studies to date have examined how external contexts are related to overall momentary movement behavior compositions using compositional data analysis. Therefore, this study aimed to determine differences in momentary movement behavior compositions between different social and physical contexts in adolescents. Methods: Overall, 119 adolescents (mean age 14.7 y, SD = 1.44) provided details about their momentary physical and social contexts over 4 days using ecological momentary assessment. Sedentary behaviors, light-intensity physical activity, and moderate to vigorous physical activity were assessed using ActiGraph GT3X+ accelerometers. Compositional multivariate multilevel models were estimated to determine if movement behavior compositions differed between contexts. Results: Participants engaged in significantly less sedentary behaviors when outdoors compared with indoors and replaced it with moderate to vigorous physical activity. Participants also engaged in significantly less sedentary behaviors when with friends or friends and family and replaced it with light-intensity physical activity. Conclusion: These results highlight the potential of targeting external contexts to increase physical activity and to reduce sedentary behavior in adolescents’ daily lives. These factors could be targeted in mobile health and just-in-time adaptive interventions to improve young people’s movement behavior compositions.

The study investigated associations between selected physical activity correlates among 299 adolescents (90 boys, age 12–14 years) from 3 English schools. Physical activity was assessed by self-report and accelerometry. Correlates represented biological, predisposing, and demographic factors as described in the Youth Physical Activity Promotion Model. Boys engaged in more self-reported (p < .01) and accelerometer assessed physical activity than girls (p = .02). Positive associations between sex (male), BMI, Perceived PE Ability, Perceived PE Worth, number of enrolled students, and physical activity outcomes were evident (p < .05). School-based physical activity promotion should emphasize sex-specific enhancement of students’ perceived PE competence and enjoyment.