Motivating Activity Through Text Communications - Helping Adults Increase Movement

The goal of this clinical trial is to compare the effect of different types of text messages on physical activity levels of middle-aged and older adults and to determine whether these messages are effective in adults age 40 and older who are engaging in less than 90 minutes per week of moderate or higher intensity physical activity. The main questions it aims to answer are:

* Which types of motivational messages show the greatest effects on changes in daily step counts in a 3-month precision text messaging intervention? * What are the effects of personalized text messages on automatic affective evaluations, habit strength (experienced automaticity), exercise identity, working memory capacity, and processing speed?

Participants will:

* Wear a Fitbit activity tracker continuously for 3 months and are encouraged to continue wearing it through 6 months. * Wear an activPAL activity monitor on their thigh for 7-day assessment periods at the beginning of the study, at 3 months, and at 6 months. * Complete 3 cognitive assessments ("brain games") per day on their smartphone during the 7-day assessment periods at baseline, 3 months, and 6 months. * Be asked to set goals to increase their daily steps over the first 3 months of the study. * Receive up to 4 motivational text messages per day for 3 months to encourage physical activity. * Keep study-related apps (Fitbit app, AIM app, MetricWire app) open in the background on their smartphone. * Complete questionnaires at the beginning of the study, at 3 months, and at 6 months