Behavior modification programs hold promise for increasing levels of physical activity (PA) for individuals who are insufficiently active. However, existing interventions, which typically prescribe uniform PA goals across participants, are limited by their insensitivity to changing individual needs and circumstances over time. An alternative approach is to continuously adjust goal difficulty to match fluctuations in individual performance, or adaptive goal-setting (AGS), which evidence suggests may more effective for increasing PA than non-adaptive approaches. Still, no prior studies have examined the psychological mechanisms targeted by AGS, which limits the ability to further refine and disseminate this technique. In this exploratory study, several candidate mechanisms of AGS (expectancy beliefs about goals, perceived value of goals, affective appraisal of goals, implicit attitudes towards exercise) will be examined. Adult participants interested in increasing their level of physical activity (N = 36) will be randomized to receive 6 weeks of either adaptive goal-setting (AGS) or non-adaptive, static goal-setting (SGS) as part of a remote, low-intensity PA intervention. The primary aim of the study will be to evaluate the hypothesis that AGS, as compared to SGS, results in greater increases over time to four hypothesized psychological mechanisms. The secondary aim will be to evaluate whether post-intervention increases to any among these three mechanisms mediate the relationship between intervention type (AGS vs. SGS) and increases to PA over the course of the intervention.

Interested participants will schedule and complete a phone screen with a researcher to determine eligibility, and if deemed eligible, will complete an initial hour-long baseline assessment with a member of the study team. The baseline assessment will be conducted over video call using the secure, easy-to-use and HIPAA-compliant Zoom service hosted by Drexel University. During the baseline assessment, the assessor will electronically obtain the participant's informed consent to participate in the study, guide the participant on using a mobile phone application to track their daily steps (Google Fit), verbally provide participants with psychoeducational content on safely increasing their physical activity, and administer a psychological task (Implicit Association Test) as well as questionnaires (Demographics; Physical Activity Enjoyment Scale; Treatment Self-Regulation Questionnaire; Self-Efficacy for Exercise Scale). Immediately following the assessment, participants will be randomized to receive one of two possible remote physical activity interventions: adaptive goal-setting (AGS) or static goal-setting (SGS). Randomization will follow a stratified procedure based on age (cutoff point = 45 years), and permuted block randomization will be used within strata to assign individuals to an intervention condition (AGS or SGS). Both interventions will be preceded by a 10-day baseline "run-in" period during which participants will be asked to record their activity but will not receive any daily step goals. In addition, in both interventions, a brief (10-minute) mid-study (3-week) check-in call will be conducted with each participant by a member of the study team to encourage continued compliance with step goals and problem-solve any barriers to increasing physical activity. Participants will also complete a brief survey once each week over the course of the intervention period. At the conclusion of the intervention period, participants will complete several post-intervention questionnaires/tasks and will then be compensated for their participation.

Participants in the adaptive goal-setting condition will begin receiving a new step goal each day over text message (SMS) following the baseline run-in period (starting Week 2). These messages will continue to be delivered each day for the entire duration of the 6-week intervention period (through Week 8), and will contain information about the participant's daily-assigned step goal as well as their number of steps on the previous day in reference to their previous day's goal. Goals will be calculated using a 60th percentile method (consistent with prior research on adaptive goal-setting), such that each day's goal will be set to the 60th percentile value of the last 9 days of available step data for that participant. Goals will be calculated by an algorithm stored on a remote database that automatically extracts user data from the Google Fit cloud server (Google Fitness Store). The database will store daily steps data for each participant, use these data to calculate new daily step goals and transmit them over SMS using a commercial cloud communications platform. The maximum steps goal permitted by the algorithm will be 20,000 steps/day, to ensure that goals remain within an attainable range for most individuals. Participants in the static goal-setting condition will receive the same exact step goal each day (10,000 steps), consistent with prior research, and will receive daily text message updates providing a summary of their previous day's steps and encouraging them to reach their 10,000-step goal for that day.

Inclusion Criteria:

1. Aged 18-75 years old
2. Reporting a desire to increase physical activity in order to lose weight, prevent weight gain or improve physical health
3. Possessing a smartphone that can connect to a WiFi (wireless fidelity) network
4. Daily access to a WiFi connection
5. Ability to understand and provide informed consent
6. Proficiency in speaking, reading, and writing English
7. Currently report engaging in insufficient levels of physical activity (<150 minutes/week of moderate-to-vigorous physical activity)

Exclusion Criteria;

1. Present involvement in another physical activity promotion or a behavioral weight loss program
2. Pregnant or planning to become pregnant within the next 6 months (exclude for either)
3. Current BMI <18.5 kg/m^2
4. Diagnosis of a medical condition that could limit the ability to increase physical activity
5. Acute suicide risk (exclude if present in the past 2 weeks or if any attempts were made in past 2 years)
6. Current substance use disorder
7. Current diagnosis of psychosis
8. Current diagnosis of bipolar disorder
9. Currently meet criteria for loss-of-control eating
10. Presence of compensatory behaviors (vomiting, laxatives, diuretics, exercise)

Participants will not be informed of their assigned condition, though may infer it through the types of texts that they receive.

The principal investigator and outcomes assessor will each be blinded to a participant's assignment while the participant is enrolled in the study, and will be unblinded at the conclusion of the study prior to performing statistical analyses.