BACKGROUND: Despite the numerous benefits of engaging in physical activity (PA), many adolescents fall short of meeting daily recommended levels of PA, and adolescent girls are more likely to be inactive or insufficiently active compared to boys. Social-ecological models suggest that, in part, behavior can be explained by individual-level factors (e.g., motivation) and factors that are external to the individual (e.g., social environment) and that these influences across multiple levels may interact to influence behavior. Despite the fact that one's motivation, social environment, and PA levels can change on a day-to-day basis, few studies have evaluated the extent to which relations between these constructs are connected at the day level. The purpose of this study was to examine the association between adolescent girls’ daily intentions to engage in PA and their subsequent PA that day, with a focus of investigating whether social support moderates these associations. METHODS: This study included adolescent girls n=66; ages 12-18 years; 13.64% self-reported meeting PA guidelines at baseline) who received beginning and end-of-day questionnaires for 28 days. Beginning-of-day questionnaires assessed intentions to engage in PA that day. End-of-day questionnaires assessed their perceptions of social support for PA from family, friends, and teachers and asked girls to report their PA for that day. RESULTS: Participants provided valid beginning and end-of-day data on 75% of possible days (n = 1,336 person-days). Results from a linear multilevel model indicated a significant interaction between daily intentions and social support in predicting PA, such that on days when adolescent girls reported more social support to engage in PA than usual, the association between intentions and subsequent behavior was weaker, compared to days when girls reported less social support than usual (β = 0.01, p = 0.04). DISCUSSION: These findings suggest that when adolescent girls are experiencing particularly high levels of social support (compared to their typical levels of social support), girls’ own motivation may not be as important in determining their PA behavior. Efforts to increase PA in adolescent girls should consider developing intervention programs that are adaptive to the needs of these girls in the context of their daily lives.

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