The purpose of this study was to investigate the influences of intrinsic motivation, social affiliation orientations and reciprocal social support for physical activity (PA) on underserved youths' afterschool period moderate-to-vigorous (MVPA) changing trajectories across the 16-week Connect through PLAY intervention, a social-motivational climate intervention. A total of 113 youth (61.06% African American, 56.64% girls) provided full data. Youths' intrinsic motivation, social affiliation orientations and social support were measured by youth responses to a set of surveys at baseline and post-intervention. Youths' afterschool period MVPA was measured using data from 7-day ActiGraph accelerometer wear at baseline, midpoint and post-intervention. Hierarchical linear modelling analysis found that youth daily afterschool period (3pm-6pm) MVPA increased, on average, 37.94 min across the 16-week intervention. Increases in intrinsic motivation, social affiliation orientations and social support were positive predictors of youth afterschool MVPA changing trajectory. The findings clarify the contributions that a social-motivational climate intervention can have on youth afterschool period MVPA through increasing youth intrinsic motivation, social affiliation and reciprocal social support.
Keywords: Afterschool programs; moderate-to-vigorous physical activity; social affiliation orientations; social-motivational climate; youth.