Background/Purpose The decline of physical activity in adolescents has been a long-time concern. Although, a few determinants have been identified in descriptive research, the extent to which adolescent physical activity behaviour change over time, in association with specific motivational determinants, has remained unknown. The study aimed to examine Finnish adolescents' physical activity and motivational determinants change trajectory over a three-year period (Grade 7 to Grade 9). Based on the expectancy-value model of achievement choice, students' and their parents' expectancy beliefs and task values (attainment, utility, and interest) in physical education were hypothesized to be predictive of their initial physical activity levels and change trajectory.
Method Students (N = 812) aged 11-15 years completed measures of self-reported physical activity five times. Furthermore, students' and their parents' (n = 225) expectancy beliefs, task values, and self-esteem were assessed in the beginning of Grade 7.
Analysis/Results Latent growth models revealed a declining trend in physical activity with gender differences in terms of motivational determinants (cov = -.545, SE = .005, r = -.189, 90%, CI [-.248, -.131]). The girls' physical activity level was predicted by parents' physical education expectancy beliefs and the change trajectory was determined by their personal self-esteem. The boys' physical activity level was predicted by their personal self-esteem and task values, as well as their parents' values towards physical education.
Conclusions The results indicate the complexity of students' physical activity and reiterate the importance of student's own and parents' beliefs and values. The determinants for change, however, remained unclear, which warrants further study.