Testing Responses of Young Adults to Intervention Messages for Promoting Physical Activity Trial

The goal of this clinical trial is to test a precision (person-specific and context-sensitive) messaging algorithm for increasing physical activity and slowing weight gain in insufficiently-active young adults. The main questions it aims to answer are: * Does physical activity increase more when text messages are sent based on a precision rule for selecting and timing messages compared to when the same message content is selected and sent at random or not at all? * Do biological or social characteristics of young adults make them more likely to respond positively to the precision messaging intervention than either of the other two interventions? Participants will be provided with education about health-enhancing physical activity and given an activity tracker to wear for 12 months. They will then be randomly assigned to one of three groups. Participants in one group (Precision AIM) will receive up to 4 messages/day selected and timed based on a person-specific algorithm that forecasts possible message effects periodically throughout the day. Messages will be drawn from one of three content libraries: move more, sit less, or inspirational quotes. Participants in a second group (Random AIM) will receive 4 messages/day selected at random from the same three content libraries and delivered at random times within their availability window. Participants in the third group (No AIM) will receive not motivational messages but will randomly assigned to the Random AIM group will receive up to 4 messages/day drawn at random from three content libraries at randomly-selected times. Step counts and weight will be assessed at baseline, and at 3 months, 6 months, 12 months, and 18 months. Researchers will compare Precision AIM, Random AIM and No AIM groups to see if physical activity increased more and weight gain was slower in Precision AIM than Random AIM or No AIM after 3, 6, and 12 months of intervention, and 6 months after the intervention is complete (18 months).