Abstract Background: Physical activity provides extensive health benefits during adolescence; however, many adolescents do not achieve the recommended level of regular physical activity (RPA). The age-related declin in regular physical activity throughout adolescence has been documented in some studies in western countries but by our search, there has been no study in eastern countries. Objectives: The aim of this study was to determine the decline in RPA among Iranian girl students aged 10 to 16 and the related factors. Methods: This cross-sectional study included 1164 students who were selected via multi-stage random sampling in Hamadan, Iran, in 2015. They filled out a self-administered questionnaire. A number of 105 students out of the above-mentioned sample were selected randomly for using pedometers. The questionnaire included demographic variables, physical activity questionnaires (PAQs), questions on height and weight measurements. Join Point regression, t-test, ANOVA, and logistic regression were used to obtain PA decline age, associations, and odds ratio. Results: The results showed that about 83% of the students did not engage in 60 minutes moderate to vigorous physical activity (MVPA) per day. In addition, 46% of the students watched TV more than 2 hours/day. There was a significant association (P < 0.01) between RPA, parents’ support and demographic variables (age and mothers’ education). There was a steady decline in RPA from ages 10 to 16. Annual percent change (APC) was -5.06. Interest in RPA and access to facilities showed higher odds ratio for engaging in the recommended level of RPA. Conclusions: This report showed RPA declined by age during adolescence in girl students. Motivating students toward RPA and providing facilities to do RPA can prevent this problem.
