Table 2. Themes and subthemes with illustrative quotes.

Theme	Subtheme	Illustrative quotes
No awareness of/experience with HIIT and ambivalence about exercise capabilities	N.A	I understand very little of it at the moment, until I actually get shown what I am supposed to be doing (12 Baseline). I have seen some of it on the Biggest Loser and stuff like that. But they were using ropes and things like, um, weights and things so I don’t know (06 Baseline). I’m not fit you know…. I don’t know if I can do HIIT, it might be too high to achieve (06 Baseline). I am nervous, uh, unsure about my heart rate going high, um, my, I have been told I probably can’t get my heart rate up high because of my size (10 Baseline). Oh, I’m feeling positive about starting the program. Um, how the body handles it as we go on, that is another question (12 Baseline). That’s what I am scared about with the home-based that, without that motivational part of it (09 post self-directed HIIT)
Multiple medical and social barriers to commencing and continuing HIIT	Multimorbidities and multifarious specialist appointments	Yeah just joints, knees, feet and work (13 Baseline). Well, the only thing that I can see that would stop me is pain…pain in my knee, everything else will be fine (17 Baseline). At the moment, I don’t want to push myself too much, ……. because of the knee and the ankle, and now my hips hurting and my arms are hurting, I am starting to find it very difficult to do what I need to do to get work (13 post self-directed HIIT). Um, I have, and my wife, we have many appointments. ……. I have appointments for my NASH and my knees and diabetes and my wife has health conditions too (08 Baseline)
	High burden of carer responsibilities	Well, my husband has dementia, I am his carer so I will need to bring him with me (10 Baseline). The only thing that got in the way was getting my husband ready to get here. That was quite stressful. But he then got into a routine, and he really enjoyed coming over…… I was worried that I would send him a bit more confused (10 Post-HIIT). …….If my husband gets sick, and I need to be with him and get him places, and you know, to appointments, or be with him in hospital or whatever and that is very unpredictable (16 Baseline)
Exercise specialist support was a highly valued enabler	HIIT education and logistics support	I guess being told what to do and how to do it. (04 Baseline). I understand very little of it at the moment, until I actually get shown what I am supposed to be doing (12 Baseline). I won’t be as confident, because I won’t be familiar with the machine. And you girls won’t be there, and I won’t have that support. But, um I think I am confident enough within myself to do it, but I can imagine people who aren’t confident, or who don’t go out a lot, or who have never been to a gym before (10 post self-directed HIIT). We put the monitor on and then we go in [to the gym], the only problem was the fact that the monitor, um the watch lost contact on the treadmill, I couldn’t figure out why… it would just lose it and go away (14 post self-directed HIIT). You know, whether I will be able to create the situation where I am getting the, you know, the heart rate up, and doing the four intervals. Whether I can actually do that and sustain that, or whether, I will find that it felt like too much and I couldn’t finish it. That worries me (16 post self-directed HIIT). The positivity of the controlled sessions means that I have the power to do it, I have the ability to do it, and that’s what has kept me doing it (09 Post-HIIT). Well, I would come here, everything would be set, I would have the ah, um, hate, the heart rate monitor, and all that all set-up, all made for myself. The machine was generally in my place ……we would do the HIIT, I would recover, and then I would go home (08 Post-HIIT). Very difficult to organize myself to do it. Life gets in the way (13 post self-directed HIIT)
	Feeling safe	The positivity of the controlled sessions means that I have the power to do it, I have the ability to do it, and that’s what has kept me doing it (09 post self-directed HIIT). It has been really great because there’s different people available, um physical stuff, like with my foot. It was a bit of a pain, but we always just kept looking for a way around that (16 Post-HIIT). I think the arm ergometer, ah, the equipment to get my heart rate up without my knees being limiting. I think I will be good on that and that will help (08 Baseline)
	Social connection	Um well, the people that were taking me on the day, they were always really encouraging, and it was ah, it was a pleasure to interact with them, they are all very nice people, so it was never a drag to have to come in, it was always something to look forward to (17 Post-HIIT). The team, if it wasn’t for that team, I would have gone home long ago (11 Post-HIIT). I have enjoyed the social part of it (09 Post-HIIT). I mean, I always find that I make it difficult because, like if I had, if I was doing it with someone else, I would honor the commitment better (16 post self-directed HIIT). The motivational part of it I think is, you know, what worries me is that when this part of it stops, just going on the home-based that, I might think well I’m a bit tired tonight, I will do it tomorrow night. And before you know it, the week is gone, and then another weeks gone and That’s what I am scared about with the home-based that, without that motivational part of it, the support, having professional support, not just support, but having somebody who knows what they are doing, who knows how, how the program is done, how the program is run (09 Post-HIIT)
	A point of commitment and accountability	I think committing to something, I feel um, I need to do it now (07 Baseline). I guess being told what to do and how to do it, and just committing (04 Baseline). oh, well I think coming in to a regular session, you know, really commit and um, with the right equipment for my body size (10 Baseline). Um yeah, I figured that if I could make the commitment, if you guys could make the commitment for my benefit, then it was in my interest to make the commitment and I was cool with that. (09 Post-HIIT). Um while I am confident to do it and I have geared up to do it, I just find that ……… I may go home and say oh I’ll do a thing tomorrow night, whereas coming here, I know I had to do it and I wanted to do it. (09 Post-HIIT). I would come here [supervised setting] to do anything, but I still probably wouldn’t go to a gym (09 Post-HIIT). Um, telling other people, like even if I am not doing it with someone, telling other people….that I am going to do it, and having someone say, “did you do that?”, helps (16 post self-directed). I have a treadmill at home, I have some hills out on the street I can walk, so there’s the gym….so, I possibly could [do HIIT at home], But I need to be pushed too, I will need to be pushed to do it (07, Post-HIIT). There’s been some weeks where I haven’t done it [due to] health ….. there are some weeks where I have, but it wasn’t my favorite part of the thing, you know, it’s very very hard doing it by yourself (09 post self-directed HIIT). Um, the fact that, well in my case, when I get home, psychologically I am home and I tend to unwind, and you hop on the treadmill and you think, 10–15 minutes will do me, instead of a 40 minute session (09 post self-directed HIIT). ..hard to fit in, unless I did it first thing you know, a priority, and sometimes I’d commit to it and just get it done, you know, but um other times I think I would do it later and then never did. Its um not like I don’t really have the time, but things just come up, like family issues, and I need to pick up my grandson or just friends call by and um, I just didn’t prioritize it (07 post self-directed HIIT)
HIIT was enjoyable and provided holistic benefits		It was good, I enjoyed it, a lot (17 Post-HIIT). Absolutely marvelous. I have never done anything like this before and it has been a real pleasant experience (08 Post-HIIT). Well, the most notable thing is the energy that I have. I get a lot more done…. More energy and more confident that I can do it (10 Post-HIIT). I have got more confident, I found I could remember more, It improved my memory… [more confident in] everything throughout my whole life, I felt a lot more confident (11 Post-HIIT). My knee, I was um, actually surprised it felt better after the exercise (13 Post-HIIT). And I feel more flexible in the legs, being on the bike really helped with my knees (17 Post-HIIT). I am not having trouble getting out of bed in the morning physically, I can swing my legs, my sciatica, ah, is going away, and in conjunction with the leg exercises I am doing, I feel like I am turning the clock back (08 Post-HIIT). Um, overall, physically I feel much healthier and much stronger. …… And this has enabled me to get back to the same level of strength and wellbeing I felt…… Its ah, you know I have recovered my base physical strength (08 Post-HIIT). Something I have noticed greatly has changed is my breathing, the indigestion has sort of gone, um, because of this program…. (07 Post-HIIT). I don’t think, oh God I gotta walk up a flight of stairs. Ah well I just know that I can walk up that flight of stairs or walk to the shops or do the full round of shopping without fearing that I’ve got to get back to the car because my back is hurting or whatever. So I do feel stronger muscularly (10 Post-HIIT). Um, before it was difficult for me to put shoes on, I had to sit down to put shoes on. Now I can stand and put shoes on. And just basic, um, not losing my breath so much (17 Post-HIIT). Yeah I have to do this, I gotta do this. And it actually helped me out with certain things in my life. So it gave me a lot more stamina riding motorbikes and stuff like that, which is what I like to do (13 post self-directed HIIT). Oh I loved it. Yeah, it was really, um, well it really helped me because I don’t usually do exercise and I am not good at being consistent about it. So, it was helpful to have to come. And I guess, I wouldn’t of, if I didn’t enjoy it, I wouldn’t have made myself keep coming. (16 Post-HIIT). I was very nervous in the beginning, but as time went on, I enjoyed it more and more, just as time went on, I slipped more into the way things were going and it was great I enjoyed it (11 Post-HIIT). Ah I just generally feel like I have more energy ….. I just notice that I walk with a bit more of a spring in my step and you know, I haven’t been sick even, you know, [when] other people being sick around me and I haven’t got sick (16 Post-HIIT)

HIIT, high-intensity interval training.