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Résumé (anglais)

Background Physical and sedentary activities have been identified as potentially modifiable risk factors for many diseases, including mental illness, and may be effective targets for public health policy and intervention. However, the relative contribution of physical activity versus sedentary behaviour to mental health is less clear. This study investigated the cross-sectional association between physical activity, sedentary activity and symptoms of depression and anxiety at age 14-15 in the National Longitudinal Survey of Children and Youth (NLSCY). Methods Respondents aged 14-15 years between 1996 and 2009 who reported on symptoms of depression in the NLSCY were included (n=9702). Multinomial logistic regression was used to assess the relationship between physical and sedentary activity and symptoms of depression and anxiety. Joint models including both physical and sedentary activity were also explored. Models were adjusted for sex, ethnicity, immigration status, family income, parental education, recent major stressful life events and chronic health conditions. Results The odds of having moderate and severe symptoms of depression and anxiety compared with no symptoms was 1.43 (1.11 to 1.84) and 1.88 (1.45 to 2.45) times higher, respectively, in physically inactive youth relative to physically active youth. The odds of having moderate and severe symptoms of depression and anxiety compared with no symptoms was 1.38 (1.13 to 1.69) and 1.31 (1.02 to 1.69) times higher, respectively, in sedentary youth relative to non-sedentary youth. In joint models including both physical and sedentary activity, sedentary activity was not consistently associated with symptoms of depression and anxiety. Conclusions Both physical inactivity and sedentary activity appear to be significantly related to symptoms of depression and anxiety. The importance of distinguishing these two behaviours has relevance for research as well as policies targeting physical activity and mental health in youth.