Aim: To examine: (1) the developmental trajectory of physical activity as well the influence of body mass index (BMI) and friend support on initial physical activity and change of physical activity; and (2) the stability of physical activity in adolescent girls.
Methods: Participants were 193 urban adolescent girls and their best friends. Physical activity was measured using the 3-day physical activity recall. Best friend social support was assessed using a questionnaire. BMI was calculated based on height and body mass of the participants. Data were collected on four occasions over a 1.75-year period.
Results: The stability coefficients for moderate, vigorous and total physical activity were statistically significant and ranged from 0.25 to 0.62. There was a curvilinear decrease in physical activity and a curvilinear increase in BMI and friend social support across four measurement occasions. Latent growth modelling (LGM) revealed that initially higher level of friend social support was associated with higher physical activity, and higher level of BMI was associated with lower levels of physical activity. With the physical activity change factor, there were statistically significant and positive direct effect from friend social support change factor, and statistically significant and negative direct effect from BMI change factor.
Conclusion: Change in friend social support was positively and change in BMI was inversely associated with a change in adolescent girls' physical activity.