BACKGROUND: The physiological and psychological benefits of regular physical activity (PA) and reduced sedentary behavior are well documented. Nevertheless, the majority of adolescents in western countries, e.g., Switzerland, do not meet the recommendations for moderate to vigorous PA of at least one hour per day. Planning interventions, defined as the creation of plans when, where and how to engage in a behavior change, have been proposed as an effective way of translating intentions into action. AIM: The project aims at closing the empirical gap of the question whether or not planning interventions of PA in adolescents are successful and which planning intervention is the most effective one.

In detail, it is aimed for to examine changes in (daily) moderate to vigorous physical activity in adolescents' friendship dyads following (a) an individual planning intervention, (b) a collaborative planning intervention or (c) one of the two no-planning control conditions. Changes in (daily) physical activity will be examined over a period of 6-months. By investigating the intervention effect systematically by means of daily diaries, long-term follow-ups and objective measurements this project will provide a novel knowledge about individual self-regulating mechanism and social exchange processes as the mediating mechanism of the effectiveness of individual and collaborative planning for health behavior change in adolescents. DESIGN: Single-blind four-arm parallel-group cluster-randomized controlled trial with longitudinal design. The assessment of the main and secondary outcomes is conducted at the baseline, at 1 week after the first intervention session, and at 1-, and 6-month follow-ups. The intervention consists of one planning session and one booster session after 1-month. PARTICIPANTS: The sample will consist of 400 friendship dyads between 14 and 18 years of age. A cluster-randomization will assign participants to one of the two planning groups (collaborative or individual) or one of the two control groups. Both partners of the friendship dyad will complete all questionnaires. OUTCOMES: Physical activity constitutes the main outcome, whereas self-regulatory strategies like planning, self-efficacy, as well as social exchange processes like social support are secondary outcomes.

The physiological and psychological benefits of regular physical activity (PA) and reduced sedentary behavior are well documented. Nevertheless, the majority of adolescents in western countries, e.g., Switzerland, do not meet the recommendations for moderate to vigorous PA of at least one hour per day. However, changing PA seems to be a major challenge even for individuals with good intentions to do so. This phenomenon is the so called intention-behavior gap. Planning interventions have been proposed as an effective way of translating intentions into action. Planning includes action planning (i.e., individual creation of plans when, where and how to engage in a behavior change) and coping planning (i.e., what an individual will do in case barriers arise). So far, good evidence has been found for individual planning increasing PA among adults. Such research with adolescents, however, is in its early stages. Moreover, recent research suggests that an individual's behavior change is also dependent on members of the social network. Consequently, the idea of conjoint planning was introduced recently. Conjoint planning comprises two planning formats, namely collaborative and dyadic planning. Dyadic planning implies creating plans together with a partner, but executing the behavior individually. In contrast, collaborative planning is defined as conjoint planning of at least two individuals when, where and how to perform a behavior together. An example would be "If we go shopping, we will take the bicycles to get there". It is assumed that conjoint planning stimulates social exchange processes between the planning partners. Social exchange processes are interactions between individuals that influence an individual's behavior, emotions, and cognitions and have been shown to play a significant role for health behavior change. A central component of social exchange processes is social support. Regarding PA in adolescents, the influence of peers and friends become increasingly important due to increased autonomy in adolescence. However, past research on health behavior change in adolescents has not yet examined effects of conjoint planning to enhance PA in adolescents. Although a few studies showed promising effects of conjoint planning in adults and adolescents, some important questions still remain open, which will be addressed in this project focusing on individual and collaborative planning. Our research questions and hypotheses are the following:

Research question 1: Is planning an effective strategy to promote PA in adolescents when using daily based measures of PA and long-term follow-ups?

Hypothesis 1.1. Adolescents, who take part in one of the two planning interventions, will show higher levels of (self-reported/objectively measured) PA compared to the control groups when comparing PA on a daily basis, one week (directly after the intervention), one month (after the booster session) and six months after the intervention.

Hypothesis 1.2. Adolescents in the individual and collaborative planning intervention will report higher PA one month and six months after the intervention compared to the control groups.

Research question 2: Do collaborative and individual planning differ regarding their effectiveness to promote PA in adolescents?

Hypothesis 2.1. Adolescents, who take part in the collaborative planning intervention, will show higher levels of (self-reported/objectively measured) PA compared to the individual planning intervention when comparing PA on a daily basis, one week (directly after the intervention), one month (after the booster session) and six months after the intervention.

Hypothesis 2.2. Adolescents in the collaborative planning intervention will report higher PA one month and six months after the intervention compared to the individual planning intervention and the control groups.

Research question 3: Does the planning intervention increase individual self-regulatory processes over time?

Hypothesis 3.1. Adolescents in the individual and collaborative planning intervention will report higher daily self-efficacy one week (directly after the intervention), one month (after the booster session) and six months after the intervention compared to the control groups.

Hypothesis 3.2. Adolescents in the individual and collaborative planning intervention will report higher self-efficacy one month and six months after the intervention compared to the control groups.

Hypothesis 3.3. Adolescents in the individual and collaborative planning intervention will report higher daily (self-reported) planning one week (directly after the intervention), one month (after the booster session) and six months after the intervention compared to the control groups.

Hypothesis 3.4. Adolescents in the individual and collaborative planning intervention will report higher (self-reported) planning one month and six months after the intervention compared to the control groups.

Research question 4: Does the collaborative planning intervention increase social exchange processes over time?

Hypothesis 4.1. Adolescents in the collaborative planning intervention will report higher daily levels of social support one week (directly after the intervention), one month (after the booster session) and six months after the intervention compared to the individual planning condition and the control groups.

Hypothesis 4.2. Adolescents in the collaborative planning intervention will report higher levels of social support one month and six months after the intervention compared to the individual planning condition and the control groups.

Research question 5: What are the underlying mechanisms of the planning interventions in adolescents regarding PA?

Hypothesis 5.1. The effect of the individual planning intervention on daily (self-reported/objectively measured) PA, one week (directly after the intervention), one month (after the booster session) and six months later is mediated by changes in daily individual self-regulatory strategies like self-efficacy, and self-reported planning.

Hypothesis 5.2. The effect of the individual planning intervention on (self-reported) PA, one month and six months later is mediated by changes in individual self-regulatory strategies like self-efficacy, and self-reported planning.

Hypothesis 5.3. The effect of the collaborative planning intervention on daily (self-reported/objectively measured) PA, one week (directly after the intervention), one month (after the booster session) and six months later is mediated by changes in daily levels of social support, as well as by changes in daily individual self-regulatory strategies like self-efficacy, and self-reported planning.

Hypothesis 5.4. The effect of the collaborative planning intervention on (self-reported) PA, one month and six months later is mediated by changes in levels of social support and changes in individual self-regulatory strategies like self-efficacy, and self-reported planning.

Design: This study is designed as a single-blind four-arm parallel-group cluster randomized controlled trial with longitudinal design. The design will comprise a baseline assessment, the intervention and two follow-ups one and six months after baseline. The sample will consist of 400 friendship dyads between 14 and 18 years of age. Recruitment will take place at schools. Therefore, a cluster randomization based upon the schools of all dyads to the four intervention arms is used. In addition, a blocked randomization is used to ensure an equal allocation ration of all a school types (level A; secondary school; vocational school) to the study conditions. All participants being interested in the study, fill in a pre-screening online questionnaire (runin) with the inclusion and exclusion criteria. Afterwards, the friendship dyad will be contacted to make an appointment for the first baseline measurement (T1). Subsequently, adolescents fill in the end-of day diaries and wear the accelerometers for eight days. After this week, the dyads will be invited for the intervention session (T2). All participants will receive an education and general motivational treatment to enhance daily physical activity and are then blocked randomly assigned to one of four intervention conditions: (a) a collaborative planning intervention, in which both adolescents form action and coping plans together to increase daily physical activity together, or (b) an individual planning intervention, in which both adolescents form such action and coping plans individually, or (c) a no-planning control condition, in which both adolescents perform a distraction task together, or (d) a no-planning control condition, in which both adolescents perform a distraction task alone, separately from each other. After the intervention session all adolescents fill in the end-of day diaries and wear the accelerometers for the following seven days. Adolescents of the two planning conditions will receive weekly messages to rethink and adapt their plans with the request to send them to the experimenters. One month (T3) after the intervention all participants are invited for the booster session. Adolescents of the two planning conditions will work on their plans again to adapt or improve them. After this session all participants fill in the end-of day diaries and wear the accelerometers for the following seven days. On a monthly basis all participants of the planning conditions will receive messages to rethink and adapt their plans with the request to send them to the experimenters. Six month after the intervention the participants will be invited to the six-month follow-up (T4). Again, all participants subsequently fill in the end-of day diaries and wear the accelerometers for the following seven days. During all sessions and the daily diaries both partners of the friendship dyad will complete all questionnaires. Self-reports of PA and sedentary behavior, as well as objectively measured PA as the main constructs will be assessed at all time points. Self-regulation constructs (e.g., self-efficacy) will be assessed at time point 1, 3, and 4, whereas social support will be assessed at time T2-T4. In addition, the body weight, height, and body fat tissue will be measured objectively at T1, T3, and T4. The Ethics Committee of the Faculty of Arts and Social Sciences of the University of Zurich, ref: 2017.10.3 approved the study.

Results of this project aims at closing the empirical gap of the question whether planning interventions of PA in adolescents are successful and which planning intervention is the most effective one. By investigating this systematically by means of daily diaries, long-term follow-ups and objective measurements this study will provide novel knowledge about social exchange processes as the mediating mechanism of the effectiveness of collaborative planning for health behavior change in adolescents. Consequently, effective theory-based and evidence-based interventions to promote PA in adolescents can be developed.

Inclusion Criteria:

• Healthy adolescents (age 14-18)
• Same sex of the adolescents of each friendship dyad
• Access to internet (smartphone or at home)
• Adolescents less active than 1 hour of moderate to vigorous physically active per day
• Adolescents with the intention to be more physically active
• Parental consent at the baseline for adolescents under the age of 16
• Adolescents consent at the baseline

Exclusion Criteria:

• Restrictions on being physically active as assessed by a risk-check form modelled after the Physical Activity Readiness Questionnaire
• Romantic relationship with participating friend
• A BMI below 17
• Insufficient comprehension of the German language
• Pregnancy
• Participating in other intervention programmes targeting physical activity

In line with the recommendations of the Swiss National Science Foundation the data underlying publications will be shared (anonymised) to make the published results reproducible.

The study protocol will be published before the end of the study. In line with the recommendations of the Swiss National Science Foundation the data underlying publications will be shared to make the published results reproducible after submission of the papers.

Upon reasonable request.

Friendship dyads are assigned to the 4 conditions without being informed that 4 conditions exist.