Strengths and limitations
This study has several strengths. First, this RCT included a long follow-up period up to 24 months post study entry (i.e., 12 months following the intervention period) to elucidate causal mechanisms of the PrevOP-PAP intervention in the long term. The intervention program PrevOP-PAP was based on theoretically-derived health behavior change techniques to allow for testing underlying processes of behavior change [However, some limitations must be acknowledged. Despite the advantages of mediation analyses to understand the causal mechanisms of this complex intervention over time, this modelling approach also comes with drawbacks. Given the complexity of OAK symptoms, it seems likely that further non-hypothesized factors may have explained changes in OAK symptoms over time which, however, were not captured using this theory-guided approach. Future research may apply data-driven approaches such as Bayesian Networks to further elucidate intervention mechanisms of the PrevOP-PAP intervention [Second, intervention delivery and data analyses in this RCT were unmasked due to ethical and practical reasons. Participants were informed that they would be randomly allocated to either the PrevOP-PAP intervention condition or the PrevOP-PAP active control condition and study personnel were aware of delivering the intervention treatment. Masking of statistical analysis is desirable for future RCT evaluation.Third, attrition rates were elevated across two years of the study period (29%) with the majority of participants dropping out during the intervention period within the first year. This may be explained by a high participant burden due to intensive intervention delivery and repeated measurements in crossed psychological PrevOP-PAP and medical PrevOP-MMT trials. However, it must be noted that the sample size of completers in PrevOP-PAP was sufficient to detect the proposed mediation effect with a power of 0.80 as the overall sample size estimation was higher for the PrevOP-MMT. Moreover, we aimed to reduce potential selection effects due to attrition by conducting intent-to-treat analyses with full maximum likelihood estimation and considering potential dropout mechanisms as covariates in our sensitivity analyses [Fourth, despite numerous advantages of the objective assessment of PA, this approach may also have drawbacks. Whereas participants may have increased their engagement in joint-friendly MVPA that are particularly recommended in the context of OAK such as swimming or riding a bike [