Introduction : Seulement 7% des Canadiens de 17 ans et moins pratiquent 60 minutes d’activité physique modérée à vigoureuse quotidiennement. La majorité dépasse le temps d’écran quotidien recommandé de deux heures. Plusieurs études transversales indiquent que les deux comportements évoluent indépendamment et ne seraient que faiblement corrélés. Objectifs et hypothèses : Les objectifs de ce mémoire est d’identifier des trajectoires d’activité physique et des trajectoires de temps devant l’écran durant l’adolescence, par sexe, et de décrire la distribution des individus selon l’appartenance simultanée aux deux trajectoires. Nos hypothèses stipulent que des trajectoires différentes seront observées pour les deux sexes et que les deux comportements ne s’influenceront pas. Méthodes : Les données proviennent d’une étude prospective de 1294 adolescents de la région de Montréal, recrutés en 1999 à l’âge de 12-13 ans. Des questionnaires ont été remplis en classe à chaque 3 mois, de la première à la cinquième année du secondaire. Des modélisations de trajectoires de groupe ont identifié des trajectoires d’activité physique et de temps devant l’écran. Une modélisation de trajectoires jointes a rapporté des probabilités d’appartenance aux trajectoires des deux variables. Résultats : Cinq groupes ont été identifiés pour les trajectoires d’activité physique pour les deux sexes. Quatre groupes ont été identifiés pour les garçons et 5 pour les filles concernant le temps passé devant l’écran. 57% des garçons et 46% des filles ont fait des activités physiques pendant 6-7 jours par semaine, durant toute l’adolescence. Toutes les trajectoires de temps devant l’écran accumulent plus de deux heures d’écran quotidiennement. Les probabilités conditionnelles suggèrent une relation entre l’activité physique et le temps devant l’écran. Conclusion : Le développement de l’activité physique et du temps devant l’écran est hétérogène durant l’adolescence. Leur coévolution doit être prise en compte par les professionnels en santé publique. / Introduction: Only 7% of Canadians age ≤ 17 years engage in the recommended 60 minutes or more of moderate-to-vigorous physical activity (PA) daily. Further, most youth surpass the recommended screen time maximum of 2-hours daily. Many cross-sectional studies suggest that PA and screen time are only weakly correlated and that they evolve independently. Objectives and hypotheses: The first objective of this MSc thesis was to model PA and screen time trajectories during adolescence, in boys and girls. The second objective was to describe the distribution of participants according to concurrent membership in the two sets of trajectories. Our hypotheses were that trajectories differ by sex and that PA trajectories are independent of screen time trajectories. Methods: Data were drawn from an ongoing longitudinal study of 1294 adolescents age 12-13 years recruited in 1999-2000 in 10 Montreal-area high schools. Self-report questionnaires were completed during class time, every 3 months from grade 7 to 11. Group-based trajectory modeling identified PA and screen time trajectories. Joint trajectory models provided membership probabilities in both PA and screen time trajectories. Results: Five groups of PA trajectories were identified in both sexes. Four and five screen time trajectory groups were identified in boys and girls, respectively. Half (57%) of boys and 46% of girls engaged in PA 6-7 days weekly during the entire 5-year follow-up. All screen time trajectories were above the recommended 2-hours daily. Conditional probabilities suggested weak associations between PA and screen time. Conclusion: Patterns of PA and screen time are heterogeneous during adolescence. Their co- evolution may need to be considered by public health practitioners.

En raison des progrès technologiques, les comportements sédentaires (souvent devant un écran) sont devenus les activités de loisir préférées pour beaucoup d’enfants, au lieu de s’engager dans le jeu actif, des sports ou d’autres activités requérant de l’effort physique. L’utilisation de transport actif est aussi en déclin, la majorité des enfants se rendant à l’école en voiture ou dans d’autres moyens de transport passifs. Pourtant, l’inactivité et la sédentarité constituent des facteurs de risque pour de multiples problèmes de santé physique et mentale. Tel que décrit dans le premier chapitre, les bienfaits de l’activité physique sur le bien-être sont bien documentés et les risques associés aux comportements sédentaires sont de plus en plus étudiés. Néanmoins, la plupart des études ne regardent ces deux aspects que séparément et ne s’intéressent qu’à l’impact immédiat sur une dimension spécifique du bien-être. Le but de cette thèse doctorale était donc d’étudier le rôle que l’activité physique et les comportements sédentaires, à la fin de la maternelle, jouent sur le bien-être physique, cognitif et psychosocial au début de l’adolescence (deux périodes de transition importantes pour les enfants). Le premier article (Chapitre II) a examiné les associations prospectives entre la participation aux activités physiques pendant les loisirs, à l’âge de 6 ans, et le rendement scolaire à l’âge de 12 ans. Les résultats montrent que les enfants qui participent davantage aux sports et à d’autres activités physiques structurées et non structurées présentent de meilleurs résultats scolaires et un engagement en classe plus élevé six ans plus tard que les enfants qui s’adonnent moins à ces activités. Le deuxième article (Chapitre III) a examiné les liens prospectifs entre un mode de vie qui tient compte, simultanément, de la participation à des activités physiques pendant les loisirs, de l’utilisation de transport actif et du temps consacré aux comportements sédentaires (ordinateur, télévision et jeux vidéo), à l’âge de 6 ans, et des indicateurs reliés au rendement scolaire et au bien-être physique et psychosocial, à l’âge de 12 ans. Les résultats montrent que les enfants qui mènent un mode de vie plus actif/moins sédentaire avant de commencer la scolarité présentent, ultérieurement, des habitudes de vie plus saines (plus d’activité physique et moins de temps d’écran) et moins de troubles émotifs et de victimisation, comparativement aux enfants menant un mode de vie moins actif/plus sédentaire. Dans les deux articles, les associations spécifiques pour chaque genre ont également été explorées. Les résultats des analyses fondées sur le genre montrent que même si tant les filles que les garçons bénéficient d’être plus actifs/moins sédentaires, chaque sous-groupe éprouve les bienfaits différemment. Une discussion des principaux résultats et des implications est présentée dans le dernier chapitre. En somme, cette thèse souligne l’importance de promouvoir un mode de vie actif dès un âge précoce et de développer des communautés qui offrent aux enfants de multiples occasions de se maintenir actifs. Ceci permettra de prévenir les nombreux risques associés à l’inactivité et la sédentarité et de contribuer au bien-être des jeunes, à court et à long terme. / Due to advances in technology, sedentary behaviors (which typically involve screens) have become the preferred leisure activity for many children, instead of engaging in active play, sports, or other activities requiring physical effort. Due to urban sprawl, the use of active transportation is also in decline and most children travel to school by car or other passive means of transportation. However, physical inactivity and sedentariness are both risk factors for multiple physical and mental health problems. As described in the first chapter, the protective benefits of physical activity on well-being are well documented and the risks associated with sedentary behaviors are increasingly being studied. Nevertheless, most studies look at these two aspects separately and focus solely on the immediate impact on a specific sphere of well-being. The aim of this doctoral thesis was therefore to study, during two important transition periods in development, the links between physical activity and sedentary behavior in kindergarten and physical, cognitive, and psychosocial well-being while children transition out of elementary school. The first article (Chapter II) examined the prospective associations between participation in leisure-time physical activity at age 6 and academic performance at age 12. The results suggest that children who participate more in sports and other structured and unstructured physical activities perform better in school and present a higher classroom engagement six years later, than children who are less involved in these activities. The second article (Chapter III) examined the prospective links between a lifestyle that takes into account, at the same time, participation in leisure-time physical activity, use of active transportation, and time spent on sedentary behaviors (computer, television, and video games) at age 6, and several indicators related to academic performance and physical and psychosocial well-being at age 12. The results reveal that children who lead a more active/less sedentary lifestyle before starting school present healthier lifestyles (more physical activity and less screen time), fewer emotional disorders and less victimization later in life, compared to children leading a less active/more sedentary lifestyle. In both articles, gender-specific associations were also explored. The results of the gender-based analyses suggest that even if both girls and boys benefit from being more active/less sedentary, each gender experiences these benefits differently. A discussion of the main findings and implications is presented in the last chapter. In summary, this thesis highlights the importance of promoting an active lifestyle from an early age and the need of developing communities that provide children with multiple opportunities to stay physically active. This will help prevent the many risks associated with inactivity and sedentariness among youth and thus contribute to their well-being, both in the short and long term.

Background. Emerging evidence suggests that youth with attention deficit/hyperactivity disorder (ADHD) may engage in sub-optimal health behaviors including high levels of screen time, low physical activity participation, and consumption of poor diets. These are independent risk factors for adverse health outcomes, and health-related behavior patterns established in childhood can track into adulthood. Thus, identifying and addressing dietary and physical activity habits in sub-populations of youth have important implications for health over the lifespan. The specific aims of this dissertation were to: (1) compare screen time between youth with and without ADHD and to assess its relationship to ADHD symptomatology; (2) compare participation in physical activity (PA) between adolescents with and without ADHD and to assess the relationship of PA participation to ADHD symptomatology; and (3) evaluate the association of diet quality and dietary patterns to ADHD symptomatology among youth ages 8-15 years. Methods. The aforementioned outcomes of interest were analyzed using data from the continuous National Health and Nutrition Examination Survey (NHANES) 2001-2004. These waves of NHANES included a structured DSM-IV-based interview administered to parents that identified youth with ADHD and also yielded symptom counts for hyperactivity/impulsivity and inattention. Screen time and physical activity data were obtained from questionnaires that queried the amount of time spent watching television, playing videos, or using the computer outside of school time, and also surveyed the types, frequency, and duration of PA in which youth participated. Diet quality and dietary patterns, which included consumption of sugar-sweetened beverages (SSBs), total calorie intake, and eating frequency, were obtained by a 24-hour dietary recall using the Automated Multiple Pass Method of interviewing. Linear and logistic regression models adjusted for sociodemographic factors and anxiety/depression were employed to address the specific aims. Results. The findings suggest that youth with ADHD are at the same, if not higher, risk for engaging in suboptimal health behaviors. Overall, youth participating in NHANES engaged in excessive amounts of screen time, failed to acquire sufficient physical activity, and consumed diets of poor quality. However, our findings suggest that ADHD symptomatology places youth at higher risk for sedentary behavior and poor diet quality. Relative to screen time, youth with ADHD showed a trend toward increased screen time, as did youth who took medication. ADHD symptoms were also associated with over two hours of daily TV viewing and overall increased screen time, and this was particularly true for children ages 8-11 years. Relative to physical activity, the outcomes did not differ between youth with and without ADHD, but the majority of youth did not meet the recommended guidelines of 60 minutes or more of moderate-to-vigorous PA each day. Diet quality was poor across the population of youth who participated in NHANES, and hyperactive/impulsive symptoms were associated with an even greater decrease in diet quality in both children and adolescents. In males, the presence of hyperactive/impulsive symptoms was associated with a decrease in diet quality, whereas in females, inattentive symptoms accounted for a decrease in diet quality. No differences in the other dietary patterns (i.e., SSB consumption, total energy intake, and eating frequency) were observed. Conclusions. The diagnosis of ADHD and/or its symptoms are associated with less-than-recommended levels of screen time and poor diet quality, though youth in general were found to be engaging in suboptimal sedentary, physical activity, and dietary behaviors. The mechanisms for why youth with ADHD may have increased vulnerability to poorer health behaviors are not yet well understood. The findings from this dissertation support the need for ongoing efforts to address lifestyle factors among the nation’s youth generally, but may also stimulate new hypotheses about the needs of youth with ADHD from both public health and clinical perspectives, and encourage research on the implications of ADHD symptomatology on health-related behaviors and lifestyle factors.