Characteristics of Tailored Text Messages that Maximize Physical Activity amongst Cardiac Rehabilitation Enrollees: Secondary Analysis of a Micro randomized Trial (Preprint)

Background: Emerging data suggest text message-based mHealth interventions may enhance physical activity levels in patients with cardiovascular disease enrolled in cardiac rehabilitation. Optimal characteristics of text messages that lead to maximal patient engagement and drive meaningful behavioral change are not well understood. Objective: To understand the impact of text message-level and participant-level characteristics that are hypothesized to lead to increased physical activity after delivery. Methods: The VALENTINE (Virtual Application-supported ENvironment To INcrease Exercise) study was a randomized controlled trial to evaluate a mobile health intervention delivered to low and moderate-risk adults enrolled in cardiac rehabilitation. Embedded within this study was a micro-randomized trial focused on the effect of text messages on physical activity levels amongst intervention participants. Participants in the intervention group received text messages through a smartwatch (Apple Watch or Fitbit Versa) that were tailored to the time of day, day of the week (weekday versus weekend), weather, and time since enrollment in cardiac rehabilitation. Text messages also differed in content type (walking vs. anti-sedentary) and level of personalization (inclusion of participant name or not). Delivery was randomized at four user-selected time points daily, with participants having a 25% probability of receiving a notification at any time point. The primary outcome was step count 60 minutes after a decision point. The current analysis focuses on the text message and participant-level factors that moderate the intervention’s effect on the primary outcome. Given potential measurement differences determined a priori, analyses were stratified by device type and phase of cardiac rehabilitation and adjusted for age, sex, and baseline activity status using a generalization of regression analysis. Results: Over 70,552 randomizations occurred in 108 participants (mean age 59.5 SD 10.7 years; 32.4% female; 17.6% non-White; 63.0% Apple Watch users) over six months. Overall, no text message characteristics, (including personalization with participant name) or participant characteristics (including baseline physical activity) consistently impacted text message responsiveness for either device type. Although not consistently significant between device types and across phases of the trial, there was a trend towards increased responsiveness to text messages that promoted walking (as compared to anti-sedentary messages) and when delivered to younger (age < 65 years) and male participants. Conclusions: In this randomized clinical trial, we found that tailored text messages improve physical activity levels amongst cardiac rehabilitation enrollees in the initiation phase, but this effect was not explained by text message or participant-level moderators. Additional work is needed to explore the impact of tailoring based on an extended set of personal and environmental factors to optimize delivery and efficacy of text message-based interventions. Clinical Trial: Unique identifier: NCT04587882