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

Inclusion Criteria:

• Be 40 years of age or older
• Own a smartphone running an Android or iOS operating system
• Have proficiency with the English language
• Be capable of providing informed consent
• Be able to perform moderate physical activity without restrictions
• Be willing to complete the entire 6-month protocol
• Be willing to establish a 12-hour window - beginning no later than 9:45 am and ending no later than 9:45 pm - for receiving text messages.

Exclusion Criteria:

• Residing outside of the continental United States
• Self-reporting 90 minutes or more of moderate-to-vigorous intensity physical activity per week
• contraindications to normal physical activity as identified on the Physical Activity Readiness Questionnaire (PAR-Q)
• Mobility restrictions that interfere with unassisted physical activity
• Being pregnant or planning to become pregnant during the study
• Planning to undergo surgery during the course of the study protocol
• Having a prior diagnosis of Alzheimer's disease or a related dementia
• Evidence of mild cognitive impairment as indicated by a mini-Montreal Cognitive Assessment (MoCA) test score of 12 or higher
• Concurrent participation in another study involving weight loss or physical activity
• Failure to respond to at least 85% of Ecological Momentary Assessment (EMA) prompts during the 7-day qualification phase.

De-identified individual participant data (IPD) underlying the results reported in primary and secondary publications will be shared. This includes baseline behavioral metrics, coarsened demographics, post-intervention outcomes, and follow-up data. To protect participant privacy, all data will be stripped of direct identifiers, ages will be binned into 5-to-10-year bands, geographic markers restricted to broad regions, and timeline dates converted to "days since randomization." Raw, free-text qualitative journal entries will be suppressed entirely due to re-identification risks.

Data and supporting documentation will be made available concurrently with the primary peer-reviewed publication of the trial results and will remain accessible for 5 years post-publication. This immediate availability complies with the NIH Data Management and Sharing Policy.

Access is restricted to qualified external researchers affiliated with an accredited academic or non-profit research institution. Applicants must submit a formal, written research proposal detailing a sound scientific hypothesis and an analytical plan, alongside documentation of local IRB approval/waiver. Approved requestors and their sponsoring institutions must sign a legally binding Data Use Agreement (DUA) that strictly prohibits any re-identification or external data-linking attempts.