Regular physical activity reduces the risk of a number of grave chronic diseases, including many forms of cancer. Guidelines indicate that increasing moderate physical activity by 30 to 60 minutes on at least 5 days of the week should already be associated with such significant health benefits. However, recent epidemiologic information indicates that only about 20% of the German adult population fully meet this important health behaviour goal.

Despite this relatively moderate recommended increase in physical activity and an apparently high acceptance of goals to increase moderate daily physical activity, many people fall short of goal achievement, as the implementation and maintenance of behaviour change often fails. Therefore, aside from enhancing motivational factors, interventions promoting health-behaviour change (e.g., increase in physical activity), are now commonly extended to address volitional strategies, such as action planning. Action planning aids individuals in translating their goals into behaviour. By forming action plans, individuals link situational cues ('when' and 'where' specifications of critical situations) to behavioural responses ('what to do' or 'how to act') in an 'if-then'-format. Available findings suggest that action planning aside from its direct effects on behaviour change, including increase in physical activity, also fosters other self-regulation strategies that are effective in the implementation and maintenance of changes in health-related behaviour. Research on action planning so far mainly focuses on its benefits for the individual's self-regulation of behaviour. To date there is little evidence on the role of the close social environment in planning health-behaviour change. However, married and cohabiting individuals often try to and succeed in coregulating their partners' health behaviour, including partners' levels of physical activity. Therefore, in the proposed study, an extension of individual action planning to the level of the dyad, i.e., dyadic planning, will be examined, along with associated mechanisms fit to aid the process of increasing physical activity. Dyadic planning refers to creating together with a partner if-then plans on when, where and how the individual target person will implement a new behaviour (e.g., be more physically active). We hypothesise that dyadic planning leads to higher increases in daily physical activity when compared to individual planning or no planning.

In the proposed randomised controlled trial, we will compare changes in daily moderate physical activity in N = 346 adults (primary outcome) and their partners who receive a general motivational treatment to enhance daily physical activity and are then randomly assigned to one of three intervention conditions: (a) a dyadic planning intervention, in which target persons form action plans to increase daily physical activity together with their partners, or (b) an individual planning intervention, in which target persons form such action plans individually and partners receive a distraction task, or (c) a no-planning control condition, in which target persons do not receive instructions for action planning, but perform a distraction task together with their partners. The study will entail an experimental longitudinal design with a baseline assessment (T0), an intervention session one week following baseline (T1) and follow-up assessments at 1 week (T2), 6 (T3; primary endpoint), 19 (T4), 26 (T6), and 52 (T7) weeks following the main intervention session. A booster intervention (T5) will take place 20 weeks after the main intervention session, i.e. 1 week following T4. Objective measurement of daily physical activity will include 4 one-week (T0, T2, T3, T7) accelerometer assessments. Additionally, further objective measurements of cardiorespiratory fitness, BMI, and waist-to-hip ratios along with self-report assessments from target persons and partners will be collected. Proposed mechanisms of the total intervention effect along with covariates will be assessed mostly via self-report.

Results of this study will contribute to the still limited understanding of the interplay of individual and dyadic regulation in the increase of daily physical activity and its maintenance, both in persons targeted by the intervention and in their partners. Findings will guide further development of this economical dyadic planning intervention that can be straightforwardly implemented in clinical settings to support the initiation and maintenance of preventive behaviour in couples. Furthermore, this theoretically-derived dyadic planning intervention can easily be extended to other domains of health-behaviour change.