Background To achieve real-world impacts, behavior change interventions need to be scaled up and broadly implemented. Implementation is challenging however, and the factors influencing successful implementation are not fully understood. This study describes the nationwide implementation of a complex theory-based program targeting physical activity and sedentary behavior in vocational schools (Lets’s Move It; LMI). The implementation primarily involved a systematic and theory-based training and user manual for school staff. We explore how the perceived acceptability of this training (in line with the Theoretical Framework of Acceptability) relates to (un) successful implementation. The study evaluates (1) the experienced acceptability of the training and anticipated acceptability of later delivering the program; (2) reach and implementation, including adaptations and barriers; (3) whether acceptability ratings predict teachers’ intentions for implementation.

Methods Upper secondary school staff from vocational and high schools (n = 194) enrolled in a two-part training, covering implementation of the LMI program and training in motivational interaction styles. One hundred fifty-one participants attended both parts of the training. Participants reported their perceived acceptability of the training and their implementation efforts in online questionnaires at baseline, after training sessions and at long-term follow-up. Qualitative data (open-ended questions) were analysed with content analysis to collate responses. Quantitative data analyses involved correlations and logistic regression.

Results Participants rated the training as highly acceptable on all dimensions (average ratings exceeded 4.0 on a 5-point scale). The implementation reached at least 6100 students and 341 school classes. Most teachers intended to continue program implementation. Acceptability ratings explained 51.7% of teachers’ intentions to implement the student program (훘2 = 30.08; df = 8; p < .001), with affective attitude, perceived effectiveness and self-efficacy the most influential. Teachers commonly reported condensing program content, and reported deficits of time and collegial support as common barriers to implementation.

Conclusion High acceptability and reach of the training indicate strong potential for implementation success. Multiple facets of acceptability seem important to successful implementation. Future research should explore ways to improve acceptability, thereby promoting successful implementation in real-world settings.