Intervention-engagement has received little attention in sports medicine as well as research and promotion of physical exercise. The construct is important, however, in the understanding of why interventions work. This study aimed at shedding more light on the interplay of engagement and the subsequent effectiveness of physical exercise interventions. A three-stage model differentiating among nonintenders, intenders, and actors informed the intervention design in this study. In an internet-based randomized controlled trial (RCT) with two measurement points, n = 326 participants received a stage-matched, stage-mismatched, or control treatment. Assessed variables were goal setting, planning, behavior, and intervention-engagement. It was found that regarding goal setting, nonintenders in the stage-matched intervention and those who engaged highly in the stage-matched intervention improved significantly over time. Regarding planning, intenders in the matched condition as well as all actors increased their levels over time. Regarding behavior, nonintenders and intenders having engaged highly in the intervention improved more than those having engaged little. In order to help nonintenders progress on their way toward goal behavior, it is necessary that they engage highly in a stage-matched intervention. Implications for exercise promotion are that interventions should also aim at increasing participants' intervention-engagement.

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