***TASK*** the task is to summarize an input biomedical literature in six sentences 
    ***INPUT*** the input is a biomedical literature 
    ***OUTPUT*** the output is the summary of an input biomedical literature in six sentences 
    ***DOCUMENTATION*** 
    ***EXAMPLES*** 
    Input: Background This study aim ed to evaluate the associations of selected demographic , individual , social , and environmental factors with moderate-to-vigorous physical activity ( MVPA ) in a sample of children and adolescents . Methods  MVPA  was assessed among youth ( n = 294 ) 10 - 17-years-old using the ActiGraph accelerometer . Youth completed measures of demographic and individual variables related to physical activity ( PA ) , perceived social support by parents and peers , and perceived neighborhood characteristics . Parents completed the long-form of the International Physical Activity Question naire . The Physical Activity and Media Inventory was used to measure the home environment and Geographical Information Systems software was used to measure the physical neighborhood environment . Bivariate correlations and hierarchical multiple regression were conducted stratified by gender . Results Boys participated in significantly more MVPA than girls . In hierarchical analyses , peer support , home PA equipment , and temperature were significantly associated with MVPA among boys whereas distance to the school they attended was associated with MVPA among girls . The final models accounted for 25 % and 15 % of the variance in MVPA among boys and girls , respectively . Conclusions Important differences exist among the individual , social , and environmental factors related to MVPA between boys and girls . Boys ' levels of activity appear to be influenced by factors closely linked to unstructured and social types of activities whereas girls ' activities relate to internal and external barriers as well as their proximity to their schools . The prospect i ve contribution of these important individual , social , and environmental factors to changes in MVPA among children and adolescents remains to be determined PURPOSE The objectives of this study were to describe longitudinal trends in adolescent physical activity in a sample of U.S. adolescents and to assess the effect of multiple individual , parental , and environmental factors on initial level and rates of change in adolescent physical activity . METHODS Study subjects were 12,812 boys and girls 10 to 18 years of age who were participating in the Growing Up Today Study and their mothers . We used accelerated longitudinal analysis to describe trajectories of physical activity from 1997 - 1999 , and r and om effects linear mixed models to determine which factors were independently associated with baseline physical activity and changes in physical activity over time . RESULTS Mean hours of physical activity ranged from 7.3 - 11.6 hours per week in boys and from 8.0 - 11.2 hours per week in girls . Physical activity was best modeled as a quadratic function of age , increasing until early adolescence and declining after age 13 in boys and girls . Multivariable modeling demonstrated that variables associated with physical activity level at baseline in boys and girls were age , body mass index , psychosocial variables , personal attitudes about body shape , perceived peer attitudes about body shape/fitness , parental attitudes about physical activity , parental physical activity , and environmental barriers to physical activity . Age was the only factor that predicted change in physical activity over time . CONCLUSIONS Interventions to increase physical activity in adolescents should begin before adolescence . Interventions may be more effective if they are multimodal and focus on modifiable individual , parental , and environmental factors OBJECTIVE To test whether self-efficacy for overcoming barriers to physical activity has direct , indirect ( i.e. , mediated ) , or moderating relations with naturally occurring change in perceived social support and declines in physical activity during high school . METHODS Latent growth modeling was used with measures completed in the 8th , 9th , and 12th grade s by a cohort of 195 Black and White girls . RESULTS Self-efficacy was stable and moderated the relation between changes in physical activity and perceived social support . Girls who maintained a perception of strong social support had less of a decline in physical activity if they also had high self-efficacy . However , girls having high self-efficacy had a greater decline in physical activity if they perceived declines in social support . CONCLUSIONS R and omized controlled trials of physical activity interventions based on social cognitive theory should consider that the influence of girls ' perceptions of social support on their physical activity may differ according to their efficacy beliefs about barriers to physical activity Background Few studies have examined the mediators of behavior change in successful  school-based physical activity interventions . The aim of this study was to explore potential mediators of physical activity in the Fit-4-Fun program for primary school children . Design Group r and omized controlled trial . Methods Four primary schools were recruited in April , 2011 and r and omized by school into intervention or control conditions . Participants included 213 children ( mean age = 10.7 years ± 0.6 ; 52.2 % female ) with the treatment group ( n = 118 ) completing the 8-week multi-component Fit-4-Fun program .  Participants were assessed at baseline , 3- and 6-months . Physical activity  was measured using Yamax SW700 pedometers ( mean steps/day ) and question naires were used to assess constructs from Social Cognitive Theory and Competence Motivation Theory . Hypothesized mediators measured included social support from peers , parents and teachers ; physical activity self-efficacy ( barrier and task ) ; enjoyment ; and perceived school physical environment . Mediation was assessed using Preacher and Hayes ’ multiple mediation regression SPSS macro . Action theory ( A ) , conceptual theory ( B ) and the significance of the product of coefficients ( AB ) are reported . Results The intervention had a significant effect on physical activity ( p<0.001 ) . The action theory test results revealed significant treatment effects at 3-months for perceived school environment ( A=0.28 , p<0.001 ) ; and at 6-month follow-up for perceived school environment ( A=0.058 , p<0.001 ) , teacher social support ( A=0.54 , p<0.05 ) and enjoyment ( A=-0.23 , p<0.05 ) . The conceptual theory test revealed a significant relationship between changes in teacher social support and changes in physical activity at 6-month follow-up ( B=828 , P<0.05 ) . Teacher social support was shown to have a significant mediating effect on physical activity ( AB = 445 , CI = 77 - 1068 steps , proportion= 13 % ) , and perceived school environment approached significance ( AB = 434 , CI= -415 to 1507 steps , proportion= 13 % ) . Conclusions The Fit-4-Fun program successfully targeted social support for physical activity provided by classroom teachers which contributed to improved physical activity in children . These results demonstrate that classroom teachers play a key role in influencing physical activity behavior outcomes in children . Trial Registration No : Aims : Habits of leisure time sports activity , similar to other health behaviours , are established during late childhood and early adolescence . While regular leisure time sports activity is a natural part of children 's lifestyle , it starts to decrease during adolescence , particularly among girls . Among the factors influencing children 's sports activity , the role of parents , peers , and other members of children 's social networks seem to be crucial . The purpose of this study was to determine the role of social influences in early adolescents ' leisure time sports activity and to examine gender differences . Methods : Data were collected from middle school students using r and omly selected classes from four schools in distinct school districts in Szeged , Hungary , using a self-administered question naire ( n = 548 ) in which a response rate of 91 % was achieved . Respondents were 10—15 years of age ( mean = 12.2 years , SD = 1.2 years ) with 54.9 % of the sample male and 45.1 % female . Results : Results show significant others as an important influence on sport-related behaviours and suggest that during early adolescence girls ' sports participation is particularly influenced by the social influence of peers ( e.g. friends , classmates , boy/girlfriend ) . Conclusions : Our findings suggest that health promotion programmes should build on possible social influences , i.e. the role of peers , parents and significant others in general , in fostering adolescents ' physical activity 
    Output:
    
Longitudinal research supported the cross-sectional analyses .