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• Time spent by parents watching television was positively associated with screen-based sedentary behavior in boys and girls. • Coparticipation with parents and siblings in physical activity was negatively related with screen-based sedentary behavior in boys and girls, respectively. • Coparticipation with friends in playing videogames (in boys) and in surfing the Internet (in girls) was positively associated with screen-based sedentary behavior and negatively related with educational-based sedentary behavior. • Coparticipation with friends in physical activity was negatively related with accelerometer-based sedentary behavior and screen-based sedentary behavior both in both boys and girls. The aim of the present study was to analyze the associations of youths' sedentary behavior (SB) with parents' and siblings' SB and physical activity (PA), as well as the associations of youths' coparticipation with parents, siblings, and friends in PA and SB with youths' SB. The sample consisted of 1543 youths (12.02 ± 2.51 years; 788 boys) enrolled in the baseline cohort of the UP&DOWN study. SB was assessed by accelerometry and questionnaire. Participants reported the time spent by their parents and siblings watching television, playing videogames, surfing the Internet, sitting/resting, and doing PA. Further, participants reported coparticipation with parents, siblings, and friends in these activities. Linear mixed models, including school and city as random effects, were performed. Parents' television time was positively associated with youths' screen-based SB. Coparticipation with friends in playing videogames (in boys) and in surfing the Internet (in girls) showed a positive association with screen-based SB and a negative association with educational-based SB. Moreover, coparticipation with siblings and friends in PA was inversely associated with accelerometer-based SB in boys and girls. Our results emphasize the important role of social modeling in the development of sedentary lifestyles in youths. Interventions aimed at reducing health risk behaviors in youths could be more effective if they are oriented from a social perspective that involves their families and networks of their closest friends. [ABSTRACT FROM AUTHOR]

Objectives: To develop a questionnaire able to assess time spent by youth in a wide range of leisure-time sedentary behaviors (SB) and evaluate its test-retest reliability and criterion validity.Design: Cross-sectional observational.Methods: The reliability sample included 194 youth, aged 10-18 years, who completed the questionnaire twice, separated by one-week interval. The validity study comprised 1207 participants aged 8-18 years. Participants wore an accelerometer for 7 consecutive days. The questionnaire was designed to assess the amount of time spent in twelve different SB during weekdays and weekends, separately. In order to avoid usual phenomenon of time over reporting, values were adjusted to real available leisure-time (LT) for each participant. Reliability was assessed by using Intraclass Correlation Coefficients (ICC) and weighted (quadratic) kappa (k), and validity was assessed by using Pearson correlation and Bland-Altman plots.Results: The reliability of questionnaire showed a moderate-to-substantial agreement for the most (91%) of items (k=0.43-0.74; ICC=0.41-0.79) with three items (4%) reaching an almost perfect agreement (ICC=0.82-0.83). Only 'sitting and talking' evidenced fair-to-moderate reliability (k=0.27-0.39; ICC=0.34-0.46). The relationship between average sedentary time assessed by the questionnaire and accelerometry was moderate (r=0.36; p<0.001). Systematic biases were not found between questionnaire and accelerometer sedentary time for average day (r=0.05; p=0.11) but Bland-Altman plots suggest moderate discrepancies between both methods of SB measurement (mean=19.86; limits of agreement=-280.04 to 319.76).Conclusions: The questionnaire showed moderate to good test-retest reliability and a moderate level of validity for assessing SB in youth, similar or slightly better to previously published in this population. [ABSTRACT FROM AUTHOR]