The aim of this thesis is to examine the association between physical activity and mental health among Norwegian adolescents.

The thesis includes four studies among adolescents in grade 10, aged 15-16 years. Studies I, II and III are based on the Norwegian Youth Health Surveys and study IV is based on data retrieved from Ungdata. In 2000-2003 (Youth Health Survey), the adolescents completed a paper and pencil self-administered questionnaire at school during lesson time. In both 2009 (Youth Health Survey) and 2017 (Ungdata), the adolescents completed an anonymous web-based questionnaire.

The Rasch analysis from study I showed that overall, the HSCL-10 showed good reliability and the items worked well. One item “Sleeping difficulties”, in the HSCL-10 instrument clearly misfit and some items worked differently for boys and girls. Study II showed that the association between physical activity, screen-based sedentary behaviour and psychological distress was weak. In study III, a significant association between physical activity and incident use of hypnotics was shown at short-term follow up. No significant associations were found for later use of antidepressants. Study IV showed that participating in a sports club was significantly associated with lower odds of depressive symptoms.

Mental health and physical activity are both complex phenomena. When investigating the association between adolescent`s physical activity and mental health, it is important to look at physical activities in different contexts, not only volume and frequency.

Mental health is considered to be an important public health issue, particularly during adolescence. The World Health Organization has reported that half of all mental health disorders in adults start by age 14, but most of them are undetected and untreated. Furthermore, there are concerns about how to measure mental health, and a need for valid instruments has been identified.

This thesis considers the relationship between physical activity and mental health in Norwegian adolescents. It consists of four studies, all based on data collected from surveys among adolescents aged 15-16 years old. The results indicate a weak association between physical activity and mental health, and that participating in a sports club is associated with fewer symptoms of depression. Furthermore, the psychometric analysis of the Hopkins Symptoms Checklist-10 instrument using Rasch Measurement Theory is a step towards more valid assessment of psychological distress among adolescents.

This thesis contributes to the field of public health sciences by underlining the need for broader approaches in studies of the association between physical activity and mental health using proper measures.

Den här avhandlingen belyser sambandet mellan fysisk aktivitet och psykisk hälsa bland ungdomar. Den läggs fram i en tid med ökande psykisk ohälsa och stigande oro för ökande fysisk inaktivitet och stillasittande. Avhandlingen består av fyra artiklar som baseras på data insamlade bland 15-16-åringar i Norge. Resultaten visar att sambandet är svagt mellan hur ofta ungdomar är fysiskt aktiva och hur väl ungdomar mår psykiskt. När kontexten för den fysiska aktiviteten beaktas framträder ett tydligare mönster: de som är aktiva i en idrottsförening rapporterar signifikant mindre psykiska besvär än de som inte regelbundet är fysiskt aktiva. Detta samband kvarstår även efter kontroll för föräldrarnas socioekonomi och efter kontroll för andra hälsorelaterade vanor. I avhandlingen redovisas också psykometriska analyser baserade på Rasch Measurement Theory av en skala för psykologisk distress och en fördepressiva besvär. Avhandlingen pekar på vikten av bredare analysansatser när fysisk aktivitet och psykisk hälsa studeras och på vikten av reliabla ochvalida mått på psykisk hälsa.