The smartwatches we planned to use were discontinued.

Participants (N = 300) will be randomly assigned to: 1) an equipment-only comparison group that receives a smartphone and an activity monitor wristband but does not receive smartphone intervention prompts, or 2) the intervention group who will receive an activity monitor wristband that works in conjunction with a smartphone to reduce sedentary time via smartphone prompts during prolonged sedentary bouts. All participants will be provided with a Fitbit® wristband activity monitor and an Android smartphone on the same day of the week (i.e., Mondays). Participants will complete a 1-week baseline accelerometer and EMA period prior to the start of the intervention. Participants will be followed for 26 weeks after the intervention begins with follow-up visits at 6, 13, and 26 weeks. The primary outcome will be accelerometer-measured time spent in sedentary activity. Sedentary breaks, total active time, and time spent in light, moderate, and vigorous activity will be additionally evaluated as outcomes. Contextual variables and potential treatment mechanisms will be assessed via traditional questionnaire measurement and daily random EMAs at baseline and all follow-up visits. Brief smartphone-based daily diary assessments will be completed daily for the duration of the study.

Inclusion Criteria:

• A score of ≥ 4 on the REALM-SF85 indicating > 6th grade English literacy level, must be at least 18 years of age
• A BMI ≥ 25 (overweight/obese), not meeting current physical activity guidelines as measured by the 7-Day Physical Activity Recall
• Do not have a contraindication for physical activity based on the Physical Activity Readiness Questionnaire (PAR-Q)
• Able to attend all study visits, and possess a valid home address and a functioning telephone number.

Exclusion Criteria:

• Participants who have physical disabilities that restrict lower limb function and/or reading skills below a 7th grade reading level.