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Study1 investigated PA prevalence in a Spanish sample, and longitudinally investigated changes (drops/ increments) in different intensities of PA (moderate-vigorous vs. light) throughout early adolescence. In line with worldwide trends, Study1 showed that PA prevalence in a sample of Spanish adolescents is low. It was also found that there were no big drops in PA levels; in fact, there were slight increments for some groups. Furthermore, Study1 focused in a less researched question: if there are changes in the amount of PA performed during adolescence, are these changes different across different types of intensities? When considering PA as a global measure, changes overtime were not always observed, but they became clear when we discriminated between two levels of intensity: moderate-to-vigorous physical activity (MVPA) vs. light physical activity (LPA). Specifically, there were significant increments in LPA for boys, and in MVPA for girls. Finally, investigating potential changes overtime in PA intensities across different weight-statuses, increments in LPA in overweight/obese boys and increments in MVPA and global PA in overweight/obese girls were observed. This information is valuable for future interventions. In particular for understanding which kind of intensities need to be targeted more heavily and/or which ones may be easier to invite adolescents to practice. 

 

Study2 is a cross-sectional analysis that investigated whether high levels of BD may be a barrier to engage in MVPA. In addition, Study2 aimed to add to the existing literature on the impact of TV and magazines on BD, by investigating the statistical association of computer time exposure and BD. Study2 showed within a large sample of Spanish adolescents that BD  can work as a barrier and not as a motivator to PA engagement.  Importantly, it was found that the use of computers during leisure time was negatively associated with girls’ body image. Findings of the present study along with previous research findings have implications for the development of programs aimed at preventing the broad spectrum of weight related disorders with a focus on improving body satisfaction and physical activity simultaneously, as well as the critical consumption of messages delivered via new technologies. 
 

Conclusions
 

The present thesis contributes to the analysis of issues that are relevant for public health interventions as these issues have important physical and psychological consequences. Moreover, it considers a sample of Spanish adolescents, which contributes to the understanding of this phenomena in the less studied non-Anglo-Saxon population. 

The findings of this thesis are an addition to the literature and highlight that only a very small percentage of young adolescents meet the PA recommendation and that interventions to increase participation are needed. In line with this, the results of this thesis suggest that interventions targeting specific intensities of PA for different groups based on their gender and weight status, may be beneficial. Likewise, from this thesis it was observed that BD can work as a barrier to PA adherence, and that therefore, weight-related interventions should improve body image and PA simultaneously, while critical consumption of mass-media interventions should include both an internet technology and a social network component.