Self-efficacy to regulate physical activity (SERPA) will be measured at W1 and W2 using a 13-item version of the SERPA scale. This scale will assess participants' confidence in their ability to overcome barriers to sustaining weekly PA. Previous measurement studies of the SERPA scale indicate a two-factor structure reflecting different types of barriers to PA. Items are rated on a five-point self-efficacy scale (0 = no confidence to 4 = complete confidence).

Total scores range from 0 to 52, with higher scores indicating better outcomes.

Self-Efficacy for Whole Person Health (SE-WPH) will be measured at W1 and W2 using a newly developed 9-item self-reported scale. The SE-WPH assesses confidence in engaging in and sustaining behaviors that support whole person health across physical, psychological, and social domains, aligned with the Whole Person Health Index (WPHI). Items reflect key areas including overall health and quality of life, social connections, health behaviors (diet, physical activity, sleep, stress), and health self-management. Items are rated on a 5-point scale (0 = no confidence to 4 = complete confidence). Psychometric evaluation (reliability and validity) will be conducted prior to hypothesis testing. Total scores range from 0 to 36, with higher scores indicating greater self-efficacy.

Self-Efficacy to Engage (SE-E) will be measured at W2 among participants in the CM group upon access to the intervention, using a previously developed 7-item self-reported scale. The SE-E assesses confidence in engaging with the online intervention and is conceptually aligned with subjective engagement. An example item is: "How confident are you in your current ability to pay close attention when engaging with this online intervention?" Items are rated on a 5-point scale (0 = no confidence to 4 = complete confidence). Although previously developed, the scale was only used in a feasibility trial; thus, reliability and validity will be evaluated prior to hypothesis testing. Total scores range from 0 to 28, with higher scores indicating greater engagement self-efficacy.

Physical Activity Outcome Expectations (PAOE) will be assessed at W1, W2, and W3 using a modified 13-item self-reported scale capturing beliefs about expected outcomes of physical activity (PA). The scale was adapted from the Multidimensional Outcome Expectations for Exercise scale, grounded in social cognitive theory and encompassing physical, social, and self-evaluative domains. Modifications were informed by responses from 30 adults with obesity in a 2025 feasibility trial, using inductive analyses and independent item review to enhance relevance to obesity-specific PA contexts. An example item is: "Physical activity will improve my stamina for daily activities." Items are rated on a 5-point scale (1 = strongly disagree to 5 = strongly agree). Psychometric evaluation will be conducted prior to hypothesis testing. Total scores range from 13 to 65, with higher scores indicating more positive outcome expectations.

Subjective engagement (SE) will be assessed among participants assigned to the CM group after intervention access is provided, using a newly developed subjective engagement scale. This 7-item measure is designed to assess participants' subjective engagement with the web-based intervention, consistent with conceptualizations of engagement in the online behavioral intervention literature. An example item is: "I paid close attention when engaging with this online intervention." Responses will be recorded using a five-point Likert scale (i.e., 0 = not at all to 4 = completely). Because the engagement scale is newly developed, evidence regarding the validity and reliability of scores derived from this measure will be evaluated using data collected at W1.

Total scores range from 0 to 28, with higher scores indicating better outcomes.

Support from Obesity Care Centers (SOCC) will be assessed at W1 using a 7-item self-reported scale informed by the 2025 CM feasibility trial. The SOCC assesses experiences with and perceptions of physical activity (PA)-related support received outside the CM intervention. It includes a screening item on receipt of PA support, followed by conditional items assessing type of support and satisfaction. Participants reporting support describe it in open-ended form and rate satisfaction on a 5-point scale (1 = very dissatisfied to 5 = very satisfied). Additional items assess referrals to PA resources and perceived preferences, needs, and barriers. Data will be used descriptively to contextualize PA support and inform intervention refinement. Total scores range from 7 to 35, with higher scores indicating greater perceived support.

Self-reported PA will be measured at W1, W2, and W3 with the short form of International Physical Activity Questionnaire (IPAQ). This questionnaire assesses the frequency and duration of walking, moderate-, and vigorous-intensity PA over the past 7 days. Average minutes per day of moderate-to-vigorous physical activity (MVPA) will be calculated based on the IPAQ guidelines.