A sport-specific ACT group intervention for promoting athletes' mental well-being: a randomized controlled trial. - Ronkainen, Hannaleena; Lundgren, Tobias; Kenttä, Göran; Ihalainen, Johanna K.; Kalaja, Sami; Valtonen, Maarit; Lappalainen, Raimo (Journal of Applied Sport Psychology, 2026)