Physical activity motives have a direct effect on mental health.

Physical activity (PA) motives are associated with both moderate-to-vigorous intensity PA (MVPA) and mental health. Studies examining whether PA motives relate directly to mental health or indirectly through MVPA are lacking. This study examined the direct effect of five PA motives (i.e., enjoyment, competence, fitness, social, appearance) on mental health and their indirect effects through MVPA in adolescents. A total of 424 participants (57.1% females) ages 14-15 years from the longitudinal MATCH study were included. Mediation analyses, based on the counterfactual framework, assessed the natural direct effect of PA motives on mental health, and the natural indirect effects through MVPA. Separate models were conducted for each PA motive. Natural direct effects were observed for enjoyment ( β ̂ [95%CI] = 2.12 [0.34, 3.90]), competence ( β ̂ [95%CI] = 1.58[0.28, 2.88]), fitness ( β ̂ [95%CI] = 1.42[0.04, 2.80]), and social ( β ̂ [95%CI] = 2.32[1.03, 3.60]) motives. No natural direct effects were observed for appearance motives. A natural indirect effect through MVPA was observed for fitness motives, and no other natural indirect effects were found. Interventions and public health strategies in adolescents need to acknowledge the importance of enjoyment, competence social and fitness motives in PA to promote mental health, and integrate specific recommendations on the importance of the reasons why adolescents participate in PA.