Background Physical activity has many health benefits, especially in people with a chronic disease. Behaviour change interventions appear more effective than just advice when increasing physical activity. However, most studies that compare different interventions overlook interaction effects between methods and their parameters for effectiveness.

Objectives Present study examined the effectiveness of a motivational and an action planning intervention for increasing physical activity in patients with a chronic disease. Their interaction as well as various potential moderators and mediators were studied in order to identify which intervention is effective in which patients.

Methods In a healthcare centre specialised in the treatment of people with a chronic disease, participants whose physiotherapist would advise them to be more physically active were randomly assigned to one of four interventions in a factorial design: 1) a control condition with only advice, 2) a motivational intervention with information about health benefits and selecting personally relevant motives and social support sources regarding physical activity, 3) an action planning intervention in which participants specified which physical activities they would do, when, where and for how long and 4) a combination of these interventions. Prior to the intervention, participants' leisure time physical activity in the past week was measured with items from the SQUASH. Also behavioural intentions, expected social support and planning behaviour with regard to leisure time physical activity were measured. Directly after the intervention, positive outcome expectations and self-efficacy were measured. After a week, physical activity measures were administered again.

Results 208 Participants (71% female; mean age 62 years; 40% with a rheumatic disease) completed the study. Figure 1 displays the average change in physical activity for each intervention. Multiple regression analyses showed that both the motivational intervention (β=0.22) and action planning (β=0.31) significantly predicted a positive physical activity change and that these effects appeared additive as opposed to catalytic. The effects were not moderated by intention, expected social support or planning behaviour, nor mediated by outcome expectations or self-efficacy. The motivational intervention's effectiveness was weakly correlated with the number of personally relevant exercise motives identified (r=0.22) and the action planning's effectiveness was weakly related to the number of plans made (r=0.19) and to one's outcome expectations after the intervention (r=0.21).

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Conclusions These data suggest that using a motivational or action planning intervention seems beneficial for increasing physical activity in patients with a chronic disease, independent of one's intentions, expected social support and planning behaviour. The data also suggest that action planning is preferable over a motivational intervention and that the added value of a motivational intervention is minimal if action planning is used.

This study was limited by only measuring physical activity after a week and by using self-report. Future studies should investigate longer term effects, other potential moderators and mediators and interaction effects between other promising behavioural change methods.

Disclosure of Interest None declared