The present study investigated whether psychosocial determinants mediate the effect of a telephone coaching intervention on physical activity levels. Two hundred eighty-eight adults were randomly assigned to a six-month telephone coaching intervention (n = 12 calls) or a control group receiving a single written recommendation. Seven psychosocial determinants as defined in the MoVo model as well as objective and self-reported physical activity levels were measured after 6 and 12 months. Participants also reported which taught intervention strategies (behavior change techniques) they perceived as most useful. Structural equation modeling was used to determine the mediating role of psychosocial determinants. Up to 227 participants with complete data on psychosocial determinants and physical activity were included in the mediation analyses. Compared to the control group, a greater increase in self-reported and objectively assessed physical activity levels was observed the coaching intervention group. The mediation analyses showed that the intervention had a positive effect on self-efficacy, outcome expectations and intention strength after 6 months and on action planning and barrier management after 6 and 12 months. Increases in objectively assessed physical activity after 6 months were mediated by increased barrier management. None of the other psychosocial determinants worked as mediating factors on self-reported or objectively assessed physical activity. The participants perceived 'action planning' and 'problem solving' as the most useful strategies to increase their physical activity levels. Further understanding of working mechanisms of remote physical activity promotion is needed.

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