Motivational factors for initiating and maintaining physical activity among adults aged over fifty targeted by a tailored intervention.

To provide insight into the motivational working mechanisms (i.e. mediators) of an effective physical activity (PA) intervention for adults aged over fifty. The mediation model ( = 822) was investigated in an RCT for the total intervention population, participants who were not norm-active at baseline (targeting PA initiation) and norm-active participants (targeting PA maintenance) separately. Potential mediators (attitude, self-efficacy, intention, action planning and coping planning) of the effect on PA (6-months) were assessed at baseline, 3 and/or 6 months. The intervention resulted in a decrease in intention (= -.209; =.017), and an increase in action planning (=.214; =.018) and PA (=.220; =.002). Intention and action planning did not mediate the effect on PA. Self-efficacy, although not significantly influenced by the intervention, was found to be the only motivational variable that predicted change in PA (=.164; =.007). These results were confirmed among participants initiating PA. Among norm-active participants no significant intervention effects were identified. The motivational factors cannot explain the intervention effect on PA. Most likely, the effect can be explained by an interaction between the motivational factors together. Differences between participants initiating versus maintaining PA, highlight the importance of performing mediation analyses per subgroup.