Physical activity, mental health and academic achievement in adolescents : A longitudinal study exploring the role of physical activity on dimensions of mental health and academic achievement in Norwegian adolescents at lower secondary school

Introduction: Good mental health during adolescence is seen as important for positive social and educational outcomes, as well as a healthy transition to adulthood. Similarly, achieving academic success at school is associated with later educational success, higher income and better physical and mental health. Mental health and academic achievement during adolescence thus serve as important determinants of future health and wellbeing. Consequently, it is important to explore factors that may promote mental health and academic achievement during adolescence. Physical activity (PA) has been identified as a factor that may promote both mental health and academic achievement among young people. Nevertheless, research in this field to date has been replete with methodological issues related to measurement of PA, mental health and academic achievement as well as primarily reliant on cross-sectional study designs. Purpose: The overall purpose of this thesis was to explore the cross-sectional and longitudinal relationships between objectively measured PA and mental health (both in terms of mental health problems and mental well-being) and objectively measured PA and academic achievement among Norwegian adolescents. Methods: This thesis is based on a three-year prospective cohort study of 599 lower secondary school students (aged 12 to 16 years) recruited from 11 schools in the east and west side of Norway. Mental health was assessed by the Strengths and Difficulties Questionnaire, the Warwick-Edinburgh Mental Well-Being Scale and the Self-Perception Profile for adolescents. Academic achievement was measured by grade point average from midterm grades and PA was measured objectively using accelerometers over a period of 7 days. Data were collected annually from 2016 to 2018. Main results: Paper I found small cross-sectional associations between PA and mental wellbeing, as well as with domain-specific self-esteem. There was no association between PA and mental health problems or academic achievement. Paper II found no longitudinal relationship between change in volume or intensity of PA and academic achievement, neither through direct nor indirect associations. Paper III found a longitudinal relationship between increase in sedentary time and higher levels of mental wellbeing among boys, as well as a small relationship between increase in PA volume and mental wellbeing in girls. No relationships were present for change in sedentary time, volume or intensity of PA and