BACKGROUND: Social networks can influence physical activity, but little is known about how best to engineer online and in-person social networks to increase activity. PURPOSE: The purpose of this study was to conduct a randomized trial based on the Social Networks for Activity Promotion model to assess the incremental contributions of different procedures for building social networks on objectively measured outcomes. METHODS: Physically inactive adults (n = 308, age 50.3 (SD 8.3) years, 38.3% male, 83.4% overweight/obese) were randomized to one of three groups. The Promotion group evaluated the effects of weekly emailed tips emphasizing social network interactions for walking (eg, encouragement, informational support); the Activity group evaluated the incremental effect of adding an evidence-based online fitness walking intervention to the weekly tips; and the Social Networks group evaluated the additional incremental effect of providing access to an online networking site for walking as well as prompting walking/activity across diverse settings. The primary outcome was mean change in accelerometer-measured moderate-to-vigorous physical activity (MVPA), assessed at 3 and 9 months from baseline. RESULTS: Participants increased their MVPA by 21.0 min/week, 95% CI 5.9 to 36.1, p = 0.005, at 3 months, and this change was sustained at 9 months, with no between-group differences. CONCLUSIONS: Although the structure of procedures for targeting social networks varied across intervention groups, the functional effect of these procedures on physical activity was similar. Future research should evaluate if more powerful reinforcers improve the effects of social network interventions. TRIAL REGISTRATION NUMBER: The trial was registered with the ClinicalTrials.gov (NCT01142804).

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