Table 5 Description of identified themes, subthemes and examples of illustrative quotes

1. Perceived added value of model

1a. Stratified approach appreciated but only needs some flexibility

“This time the physiotherapist really listened to me and gave me exercises tailored to my situation. While usually, a physiotherapist gives you a squat exercise, while you cannot do this exercise. But now, the physiotherapist did some subtle changes to the exercises, so I could really perform them and keep performing them.” (P13-OS)

“Usually, we provided one standard treatment program to everybody. I suppose that this was too heavy for some and too little for others. So I think it is really smart to distinguish subgroups and to differentiate the treatment to each subgroup.” (PT17)

“This differentiation [of people with low versus high muscle strength] was something we always did.” (PT6)

“When you see a patient for the first time, you see quite easily in which subgroup he or she will be allocated, which was confirmed by the stratification algorithm.” (PT10)

“Perhaps, a patient who only just reaches the cut-off of 12 repetitions [at 30-s chair stand test] should possibly be allocated to the low muscle strength subgroup.” (PT5)

1b. Exercises: effective and easy-to-perform

“Most of the exercises were very easy to perform, so I could do it at home as well.”(P7-LMS)

“I see a clear difference with the exercises from my previous physiotherapist. Those were a lot lighter and not really to strengthen my muscles but to make my muscles more flexible. But the physiotherapist from the OCTOPuS-trial really focused at strengthening of the upper leg muscles.” (P11-OS)

“What helped me [in persisting to perform home exercises] is that when I did not exercise for a day, my knee became more painful, so you think: “that’s your own fault, you should have done your exercises.”(P6-LMS)

1c. Patient education: important for coping and self-management skills

"It was a shock when I heard that I have OA, because I was young and believed that I should be very cautious. But now it was explained that this is not the case, and that I can still do almost anything.” (P2-HMS)

1d. Booster sessions: contrary beliefs regarding their necessity

“It varies whether a booster sessions is helpful. Some of my patients had almost no symptoms anymore at the end the treatment, so I hesitated to suggest a booster session. But some others still had significant symptoms at the end, who I wanted to come back after a while.” (PT9)

“The physiotherapist had no control at all whether I did my home exercises or not. While when you visit the physiotherapist every week, you are inclined to keep performing the exercises every week.” (P11-OS)

2. Difficulties in realizing the potential of combined treatment in ‘obesity subgroup’

2a. Obesity is a difficult topic to address

“The name of the subgroup was tricky, because then you directly address obesity. Patients mostly do not appreciate this.” (PT4)

“There is quite a threshold for me to discuss obesity, but as the treatment protocol recommends to address this, I just did this and I felt less burdened.” (PT1)

2b. More is needed to reach sustainable weight loss

“When you lose weight, you benefit from two sides: you are feeling fitter and can perform your exercises more easily, so that is really positive.” (P14-OS)

“It makes the treatment more professional, for example in your communication to the patient, as you can say: “I have discussed this with your dietician…” It gives you more confidence in your treatment, and I assume that it gives the patient more confidence in the treatment as well. Because there are two professionals working together and knowing more than one.” (PT1)

“For me this is a very good combination. As the knee complaints can be reduced by the physiotherapists as well as by losing weight.” (D2)

“I think that when you really strive for long-lasting weight loss, than the 3 h [maximal time than can be reimbursed] is not sufficient. While especially when you combine your treatment with a physiotherapy treatment for at least 1 year, than you can expect some real change.” (D4)

“Because I already started with a fitness-coach, I did not see the need for consulting a dietician as well.” (P11-OS)

2c. Poor collaboration between physiotherapist and dietician

“I have to confess that the collaboration did not go very easy so far.” (PT10)

“It is really nice that she [the dietician] is located inside our practice, so we can easily and shortly consult each other on our shared patients.” (PT3)

“This [collaboration with physiotherapist] was a difficult component, which was mostly due to the location where I see those patients. As I am only located at the physiotherapy practice once every 2 weeks, I do not see the physiotherapists on a regular base. Mostly, we only discussed those patients that did not go very well, but it was difficult to initiate a more structural consultation.” (D1)

3. Mixed feelings on minimal supervision of ‘high muscle strength subgroup’

“Sometimes, a patient had to train at home for 6 weeks without any supervision, so as a physiotherapist you lack control in preventing joint overloading. I can only try to advice the patient where they should pay attention to and how to recognize overloading, but you cannot check this.”(PT1)

”I would have preferred some more contact, as this would have motivated me to continue.” (P1-HMS)

“They were mostly relatively young male people doing sports. So yes, those people do not have to train under supervision.” (PT5)

“I think this is a better way, because now the responsibility is fully yours and you know what you should do. I like that” (P3-HMS)

4. Barriers for knee OA treatment in general

4a. Lack of motivation

“Personally, I find it very hard to keep being motivated, and to make time to do your exercises.” (P11-OS)

4b. Comorbidity

“One patient had balance problems, therefore I could not let her perform the exercises on her own. That’s why I could not strictly follow the treatment protocol.” (PT1)

4c. Costs

“Because of the start of a new year, I have to pay my own risk first. For me this is too much money in comparison to the small benefits [of the diet intervention].” (P15-OS)

“I did not exceed the recommended 3 h in my patients, but I think that more is necessary. That is widely known. However, these extra hours will not be reimbursed and people are not willing to pay for this.” (D3)

4d. Personal factors

“Because something happened in his family, he wanted the treatment to be on hold for a while.” (D5)

4e. COVID-19 lock-down

“Only just after overcoming my barriers and visiting the gym 2 times a week, the corona-crisis started and the gym closed.” (P15-OS)