Behavioural intentions as well as action planning can facilitate the adoption and maintenance of physical activity under certain conditions. The present study examined levels of plan‐specific self‐efficacy and habit strength as possible conditions that may modify this relationship. Design:
As a secondary analysis of a larger randomized trial to improve physical activity, n = 225 recipients of a planning intervention were followed up at five measurement points over one year. Methods:
Two‐level models were fit. Within‐person levels, that is, fluctuations of intention and action planning around person means, were modelled to predict self‐reported moderate‐to‐vigorous physical activity. Moreover, between‐person, that is, average person, levels of self‐efficacy and habit strength were specified as putative moderators of this relationship. Results:
The within‐person intention–activity relationship was moderated by between‐person levels of habit strength, yielding a compensatory effect: higher‐than‐usual intention predicted physical activity only when average activity habit levels were low. The within‐person planning–activity relationship was moderated by between‐person levels of self‐efficacy, yielding a synergistic effect: higher‐than‐usual planning combined with high average self‐efficacy resulted in highest physical activity levels. Conclusion:
Higher‐than‐usual intention may only be required in the presence of low activity habits. Moreover, high self‐efficacy seems to be required to translate higher‐than‐usual action planning into augmented physical activity because self‐efficacious individuals may invest more efforts to enact their plans.