Background and significance: Physical activity participation tends to decline during adolescence, with a more rapid decline observed among adolescent girls compared to boys. The drop out has been attributed to competing priorities including work, socializing, an increased school load and other leisure activities. This research was grounded in self-determination theory and assessed a large cohort of adolescent girls to determine the longitudinal nature of the relationship between participation in, and motivation for, physical activity.

Method: 732 adolescents girls (12 years and 16 years) completed three waves of a comprehensive sport and physical activity survey over three years. Exercise motivation (behavioural regulation in exercise questionnaire), physical activity (24 hour recall MET) and activity drop-out were assessed in each wave of the survey. 301 participants were included in the final analyses after completing all three waves of the survey. Structural equation modelling was used to assess the relationships between variables over time. Chi square analyses were used to compare drop in organised vs. informal activities.

Results: Girls with higher levels of autonomous motivation reported greater amounts of physical activity. Motivation was positively related to physical activity in subsequent waves but previous physical activity was the strongest predictor of future physical activity. Both previous physical activity and exercise motivation predicted subsequent exercise motivation (path coefficients ranged from 0.21- 0.35; ps< 0.01). Drop-out was varied according to motivational orientation in that autonomously motivated girls tended to drop out of organised sport and activities but adopted informal activities instead.

Conclusions: This research confirms that past physical activity predicts future participation, however, motivation also influences ongoing participation in physical activity. The activity profile of adolescent girls changes over time according to their motivational orientation. Health professionals may consider a girl's motivation for exercise when recommending activities, as autonomously motivated girls may thrive in informal exercise settings.