Usability will be assessed via the post-questionnaire based on System Usability Scale (Brooke, 1996) as part of study feasibility. The ten items are rated on a six point Likert-scale coded from 0 to 5 with some items having to be reversed. A score is built as the sum of the items (after reversing the pertinent ones). Thus, scores range from 0 to 50 with higher scores indicating better usability.

Engagement will be assessed via the post-questionnaire based on Digital Behaviour Change Interventions Engagement Scale (Perski et al., 2020) as part of study feasibility. The eight items are rated on a seven point Likert-scale from "not at all" (1) to "extremely" (7). After reversing the pertinent items, an overall engagement score can be built as the mean of the items. Thus, scores range from 1 to 7 with higher scores indicating greater engagement.

Satisfaction for each intervention component will be assessed via the post-questionnaire based on two items from the Questionnaire to Measure Patient Satisfaction (ZUF-8; Attkisson & Zwick, 1982) for each intervention component as part of study feasibility. Items are rated on a Likert-scale coded from 0 to 3. Scores are built as the sum of the two items. Thus, satisfaction scores range from 0 to 6 for each intervention component with higher scores indicating higher satisfaction.

Needs assessment for personal contact will be assessed via the post-questionnaire roughly based on the satisfaction, usability and engagement scales mentioned above and as part of study feasibility. Items are rated on a 5-point Likert scale from "not agreeing at all" (1) to "agreeing completely" (5). After reversing the pertinent items, an overall score can be built as the mean of the items. Thus, scores range from 1 to 5 with higher scores indicating a higher need for personal contact.

Motivation for physical activity will be measured by Situational Motivation Scale (SIMS) at the pre- and post-questionnaires as well as in the shortened form during the weekly questionnaires (4 items; short form not validated). The SIMS includes 16 items rated on a 7-point Likert-scale ranging from "not at all in agreement " (1) to "completely in agreement" (7). Four items form each of the following subscales: intrinsic motivation, identified regulation, external regulation, and amotivation. A score for each subscale is built as the mean of the four items (or, in the case of the weekly questionnaire, the rating of the single item directly). Thus, scores range from 1 to 7 with higher scores indicating a higher motivation.

Further indicators of motivation will be the motivational factors of the transtheoretical model (TTM) based on validated items from Lee et al. (2001) and Paxton et al. (2008). Stage of change will be assessed using one question with five answer alternatives at every questionnaire. The answer directly indicates the stage the person is at (precontemplation, contemplation, preparation, action, or maintenance). The question will be included once for physical activity motivation and once for sedentary behavior motivation (the latter also based on Han et al., 2017).

The 5-factor model of processes of change from the transtheoretical model for physical activity will be assessed with 22 items in the pre- and post-, and 5 items in the weekly questionnaires based on Paxton et al. (2008). All items are rated on a 5-point Likert-scale from 1 to 5 and a score for each of the five factors is built as the average of the corresponding items. Thus, scores range from 1 to 5. Higher scores indicate a more frequent use of each process of change.

Decisional balance (as a factor of the transtheoretical model for physical activity) will be assessed with 13 items in the pre- and post-, and 4 items in the weekly questionnaires (based on Paxton et al., 2008). Items are rated on a 5-point Likert-scale from 1 to 5. Two scores are built, one for pros and one for cons, as the mean of the corresponding items. Thus, scores range from 1 to 5 with higher scores indicating more importance of the pros or cons. Decisional balance for sedentary behavior will be measured as well with 16 items in the pre- and post-, and with four items in the weekly questionnaires (based on Han et al., 2017; Paxton et al., 2008). Scoring is exactly the same as for decisional balance for physical activity.

Self-efficacy (as a variable of the transtheoretical model) will be measured once for physical activity and once for sedentary behavior using 7 items in the pre- and post-, and 1 item in the weekly questionnaires (based on Paxton et al., 2008; Han et al., 2017). Items are rated on a 5-poin Likert-scale ranging from 1 to 5. A score is built by averaging the items. Thus, scores range from 1 to 5 with higher scores indicating higher self-efficacy.

Temptation (as a factor of the transtheoretical model for physical activity) will be assessed with seven items during the pre- and post-questionnaires and- with two items during the weekly questionnaires (based on Paxton et al.; 2008). The items are rated on a continuous scale from 0% to 100%. A score for affect and one for competing demands is built as the average of the corresponding items. Thus, scores range from 0% to 100% with higher scores indicating higher temptation.

self reported social support pertaining physical activity will be measured based on items by Rackow et al., 2017 (not validated, but based on previous literature). Single items are used in each questionnaire to measure the following subscales: quantity of practical social support, quality of practical social support, quantity of emotional social support, and quality of emotional social support. Items are rated on 6-point Likert scales ranging from 0 to 5. The single ratings for quality already represent a score for quantity of practical and emotional support, accordingly. Thus, scores range from 0 to 5 with higher scores indicating a higher quantity of social support. In order to take quality into account, scores ca be built by multiplying the quantity with the quality item once for practical support and ince for social support. These scores would range from 0 to 25 with higher scores indicating more and better support.

habit will be measured via self-report based on validated Self-Report Habit Index (Gardner et al., 2012). The four items are rated on a 5-point Likert-scale from 0 to 4. A score is built by averaging the items. Thus, scores range from 0 to 4 with higher values representing a higher automaticity (a stronger habit). The scale will be applied once for physical activity, once for sedentary behavior, and once for a reduction of sedentary behavior.

positive and negative affect will be assessed with a validated scale by Krohne et al.(1996), the short and German version of the Positive and Negative Affect Schedule. Ten items are rated on a 5-point Likert-scale from 0 to 4, five items measure positive and five negative affect. Scores are built as the mean of the corresponding items for positive and negative affect. Thus, scores range from 0 to 4 with higher values indicating higher levels of positive/negative affect.

Self reported health-related quality of life assessed with the validated instrument MacNew Heart Disease Health-Related Quality of Life Questionnaire (MacNew; Höfer et al., 2004). Originally, the scale includes 27 items rated on a 7-point Likert-scale from 1 to 7. However, we excluded one item as recommended in the literature due to very low response rate in previous research. A global quality of life score can be built by averaging the remaining 26 items. Furthermore, scores for a physical, an emotional and a social subscale can be built by averaging the corresponding items. (We recommend following the factorial structure proposed by Bermudez et al. (2022) given that the original structure has shown some issues.) Thus. all scores range from 1 to 7 with higher scores indicating higher quality of life.

Self reported physical activity will be assessed with an adapted version of the Godin-Shepherd Leisure-Time Physical Activity Questionnaire (Godin, 2011). Participants indicate on how many of the last seven days and for how many minutes per day they engaged in each type of physical activity (light, moderate, vigorous). Scores for each type of activity are built by multiplying the amount of days with the amount of minutes. Another score will be built for moderate-to-vigorous physical activity in minutes per week by adding the score of vigorous und moderate activity. Thus, higher activity scores indicate more minutes per week engaged in each type of activity. Device based assessment of physical activity will be done with a triaxial accelerometer. As with the self-report outcomes will me light, moderate, vigorous and moderate-to-vigorous physical activity per week in minutes (with higher scores indicating more minutes engaged in each activity per week).

Self reported physical activity will be assessed with the Measure of Older Adults' Sedentary Time (MOST; Gardiner et al. 2011). Participants indicate on how many of the last seven days and for how many minutes per day they engaged in seven different sedentary activities. A score is built by multiplying the amount of days with the amount of minutes for each sedentary activity and summing them up. Thus, scores represent amount of sedentary time per week in minutes (with higher scores indicating more sedentary time). Device based assessment of sedentary behavior will be done with a triaxial accelerometer. Outcome will be sedentary time in minutes per week (thus, higher scores indicate more sedentary time).