Project Summary Half of U.S. adults are insufficiently physically active despite the known health benefits. Theory-based physical activity interventions can facilitate short-term increases in physical activity; yet, little progress has been made in achieving long-term maintenance of these improvements. Epidemiological evidence has shown a positive link between social support from existing social ties and regularly engaging in physical activity; however, experimental evidence focused on these relationships remains equivocal perhaps because most interventions do not involve one’s existing social ties in the behavior change process or fail to use effective strategies for harnessing the social environment. Recent calls within healthcare and the scientific community have been made to identify effective strategies for mobilizing existing social relationships for lasting physical activity increases. We propose to test the effect of digitally delivered social support training for promoting sustained moderate-to-vigorous physical activity (MVPA) increases, using a cluster randomized controlled trial. Self-selected teams of 3-8 insufficiently active adults (about 60 teams comprised of a total of N=300 participants) will be randomized to receive either a 3-month theory-based technology-delivered physical activity intervention (mobile-compatible web app with behavior change content, Fitbit physical activity tracker, tailored goals, and feedback; Team) or the same intervention plus digitally delivered social support training (Team+Training). Teams randomized to receive social support training will be provided with a series of online interactive training modules that highlight methods of providing social support in an autonomous way (non- judgmental, empathetic, refraining from control). They will also be provided with an online crowdsourcing communication tool where they can communicate with intervention staff and their peers about challenges and solutions regarding the exchange of social support for physical activity, as well as team skills-building feedback to reinforce the knowledge and skills acquired via the modules. The specific aims are: Aim 1: To test the effect of Team+Training versus Team among self-selected teams of insufficiently active adults (N=300; 40% minority) on changes in MVPA measured by accelerometry at 3 months. Aim 2: To compare the effect of Team+Training versus Team on changes in key psychosocial constructs (social support; autonomy support; motivation; accountability) at 3- and 12-months. Aim 3: To assess MVPA change measured by accelerometry at 12 months in Team+Training versus Team. Aim 4: To explore whether social support mediates increased and sustained MVPA within the overall sample. The proposed research will advance public health efforts to improve metabolic and cardiovascular health by providing evidence on a novel and promising strategy to leverage existing social relationships for sustained physical activity behavio...