Concept and study protocol of the process evaluation of a pragmatic randomized controlled trial to promote physical activity in outpatients with heterogeneous mental disorders—the... oleh David Victor Fiedler, Stephanie Rosenstiel, Johanna-Marie Zeibig, Britta Seiffer, Jana Welkerling, Anna Katharina Frei, Thomas Studnitz, Julia Baur, Florian Helmhold, Andreas Ray, Eva Herzog, Keisuke Takano, Tristan Nakagawa, Saskia Kropp, Sebastian Franke, Stefan Peters, Anna Lena Flagmeier, Lena Zwanzleitner, Leonie Sundmacher, Ander Ramos-Murguialday, Martin Hautzinger, Thomas Ehring, Gorden Sudeck, Sebastian Wolf