Thank you for this detailed and well-written protocol. I have only a few comments:

1) The design of the study is not sufficient to answer the question of whether ACT works. The reason is, as you might know, the lack of an appropriate control group. We may find that the functioning of those in the active treatment group is indeed improved compared to treatment-as-usual group after the intervention period. But we cannot know that the observed effect is not due to 'non-specific' factors of the active treatment rather than the theory-based factors. Particularly, we cannot tell if the effect is due to differing levels of therapist contact between the two groups, or differing expectations for the two treatments. I wouldn't ask that you redesign the study to include blinding of participants and an active control group (matched for expectancy and therapist contact, etc., but with no component based on ACT-theory) which would help answer whether the treatment works, but a simple wording change would be good. Could you remove the word 'controlled' from the title and 'RCT' from the manuscript? That way any results from the study will not misinterpreted as controlling for the non-specific effects of the treatment.

2) The development of the PACT protocols, and the training and supervising of the physiotherapists, was carried out by a number of the authors. This is a competing interest and needs to be included in the 'Conflict of Interest' section.

3) The Design section on page 5 says the trial is 'single-blind'. 'Single-blind' would usually refer to the participants being blind to which intervention they are receiving. I think you are referring to the blinding of the statistician doing the analysis rather than the participants or the researchers so it would be better to remove the phrase.

4) The section on Blinding says that no hypothesis has been proposed to the participants as to the superiority of PACT over usual care, but there may be subtle transmission of the idea that it is effective in the Patient Information Sheet, and from the physios delivering the treatment.