Our goal is to develop personalized decision rules for selecting the frequency, timing, and content of messages to promote physical activity. The objective of this project is to develop personalized dynamical models of physical activity (PA) under different weather and temporal conditions as well as in response to different types of intervention messages. This approach relies on having extensive observations within the person under varying conditions to develop a dynamical model of how different conditions interact with each other to predict how behavior changes in response to text messages. Complementary sub-models will be estimated for each participant to describe their behavioral responses under different weather conditions. Healthy but insufficiently active young adults (n=80) will wear an activity monitor and receive a variety of randomly-timed and randomly-selected notifications via smartphone. GPS coordinates at the time of messages delivery and receipt are recorded and used to look up weather indices at that location at that time. The work is exploratory/descriptive as we will be developing models to describe participants' responses to messages under different weather conditions. This work is needed to develop the decision rules for a subsequent behavioral intervention that will be developed and tested in a future project.

Inclusion Criteria:

• Participants capable of reading, speaking and understanding English and of giving informed consent.
• Participants between the ages of 18-29 years.
• Participants must be free of visual impairment that would interfere with the use of a smartphone.
• Participants must own and carry an iPhone (running iOS version 10 or higher) or Android (running operating system 7 or higher) smartphone during waking hours.

Exclusion Criteria:

• Participants engaging in 90 or more minutes of moderate- or greater intensity PA each week.
• Participants engaged in organized programs with mandated physical activity (e.g., varsity sports, ROTC).
• Participants with contraindications to normal physical activity on the Physical Activity Readiness Questionnaire.
• Participants who require an assistive device for mobility or have any other condition that may limit or prevent participation in moderate-intensity physical activity.
• Participants who are pregnant or planning to become pregnant within the next 6 months.
• Participants with a prior diagnosis of cancer, cardiovascular disease, diabetes or metabolic syndrome.

6 months

The COVID-19 pandemic was declared in March 2020 during enrollment and data collection. Some participants completed data collection before the pandemic; others completed data collection during the pandemic. The pandemic adversely impacted physical activity worldwide and may have impacted results from this study.