As a teenager, Rob Ghahremani loved working out in his local gym, but he couldn’t use the resistance area because of the steps. The 27-year-old broke his spinal cord in a car accident when he was 10 months old and uses a wheelchair. “Accessible is a funny word, isn’t it?” he muses. “That gym was supposedly accessible, but you couldn’t get to the resistance area unless someone lifted you down.”

At least Ghahremani could get into the gym. Lauretta Johnnie, 51, once got caught in an entry turnstile and had to ask the receptionist to open the service entrance. “The receptionist kept saying: ‘Oh gosh, I’m so sorry!” recalls Johnnie, who is plus-size. “I don’t know who she was embarrassed for: me or her.”

When you think of fitness, who do you see? People like Ghahremani and Johnnie, being carried down steps or ushered through service entrances by blushing staff? Probably not. After all, when you can’t physically get into a space, the message is clear: you don’t belong here.

But Ghahremani and Johnnie do belong. Both are personal trainers involved in pioneering work to redefine an industry that has prioritised the young, slim and able-bodied. For an industry worth so much money – just under £5bn, according to a report out this year – the fitness world has been remarkably late to embrace consumers who don’t fit this mould. But it is finally evolving – albeit slowly – thanks to the inclusive-fitness champions proving that exercise is for everyone.

It is a message they are spreading through social media, which has made non-stereotypical gym users more visible. Plus-size Instagram fitness stars such as MyNameIsJessamyn and Nolatrees have thousands of followers. Sport England’s 2015 This Girl Can campaign was a viral hit, widely lauded for depicting a diverse group of women grunting and sweating while exercising, to break down the embarrassment many women feel in the gym. It was hugely successful: one study estimates that 2.8 million British women became more active in the year following its launch.

“Fitness operators are realising that they need to be more inclusive and are starting to understand the barriers that stop people from exercising,” says Helen Fricker of the market research firm Mintel. She estimates that a quarter of British women would swim more at leisure centres if there were female-only sessions, while 10% of all adults would prefer the option to wear less-revealing clothing while exercising. The most common reasons for this, research suggests, are body confidence and religious motivation.

Some gyms already offer female-only sessions and allow swimmers to wear modest swimwear, such as burkinis. But more work is needed. A report released last month by the government found that 3.5 million disabled people are at greater risk of poor health due to inactivity, and that disabled people are twice as likely to be inactive as people without disabilities. “Gyms need to be doing more to promote an inclusive and non-judgmental space,” says Fricker.

Why are gyms such frightening places for so many people? The main reason seems to be that fitness culture has been synonymous with weight loss since at least the 1960s. To be fit is to be slim, the logic goes, meaning that fat people are automatically viewed, in the often cult-like environs of a gym, as a work in progress, rather than people who simply enjoy exercising for its own sake. It wasn’t always like this. Modern gym culture was invented 130 years ago by Swedish doctor Gustaf Zander. Recognising that industrialisation had led to a sedentary class of workers, Zander developed exercise machines that would be the forebears of today’s gym apparatus. In Victorian times, exercise – or physical therapy, as it was often called – was a way to improve overall health.

But with the explosion of mass gym culture in the 60s and 70s, things changed. Exercise was no longer seen primarily as a means to get fitter, but as a way to lose weight, with Debbie Drake and Jane Fonda beamed into living rooms. As one doctor wrote in 1971, in an article in Vogue entitled “Fitness Forever”: “It’s less and less acceptable to be either obese or out of shape … fitness has to do with vanity, an entirely normal thing. Anyone who says he doesn’t want to look neat and trim, ie sexy, is a damn liar.” As a result of this shift in cultural messaging, gyms became exclusionary places for the best part of half a century. It is only in recent years that a new wave of body-positive fitness pioneers has shown that fat people can be fit, too.

“The paradigm of health is changing,” says Louise Green, author of Big Fit Girl: Embrace the Body You Have. “We’ve been working in this weight-loss culture for so long, and it is a multibillion-dollar industry that fails people.” Green cites Weight Watchers’ recent rebrand to WW as evidence that the diet industry is in its death throes. But there is still a way to go. “A size-16 person is virtually invisible in the fitness industry unless they’re the ‘before’ picture in a weight-loss ad.”

“People see you as a project to be solved,” Johnnie says. “They think there’s a slim black girl inside me.” For fat people in gyms, a veil of observation settles on them like a fine sheen of sweat. Johnnie remembers one particular incident that left her feeling uncomfortable. “I got off the treadmill and this personal trainer comes towards me, gives me a thumbs-up and says: ‘Well done, I’ve been watching you.’ I wanted him to get lost.”

If overweight people are hypervisible in gyms, older people tend to disappear. But social media has allowed 57-year-old Claire La Terriere to meet the fitness industry on her own terms. A long-time fitness enthusiast, La Terriere joined Instagram at the encouragement of her daughter. After building up 13,000 followers, La Terriere began running fitness retreats for older clients in her Norfolk home. Overwhelmingly, her clients are “terrified of fitness … They don’t realise you can be 58 and work as hard as I do, without injuring yourself or keeling over and dying.”

Like La Terriere, 36-year-old Khadija Safari had to create a space for herself in the fitness world. After converting to Islam in 2009, Safari struggled to find a women-only gym to pursue her love of Thai boxing, so she set up her own. She now runs a chain of gyms in south-east England that cater for Muslim women. “It allows them to come in and remove their hijab while training,” Safari says.

Muslim women have higher levels of inactivity than their peers, with barriers to entry including a lack of single-sex spaces in which they can exercise without wearing hijab and few female sporting role models. Although evidence is limited, a report from the Women’s Sport and Fitness Foundation found that only 12.5% of Asian women – the majority of whom are Muslim – do enough exercise, compared with 18.8% of white women.

“A lot of the women who train with me did PE at school,” Safari says, “but when they reached their teenage years they dropped out of fitness because they didn’t feel comfortable training with men.” She is working to re-engage Muslim women with sport, by training them to become Thai boxing instructors and set up gyms of their own. “If I choose to wear hijab and I want a service to suit my needs, it’s my responsibility to go out there and make it happen.”

Safari says her classes play a social role – Muslim and non-Muslim women come to her classes and train together. “People criticise women-only gyms by saying Muslim women are segregating themselves,” Safari says. “It’s actually the opposite. It’s about integrating into society, with women learning about each other in a neutral environment.”

Like Safari, Johnnie doesn’t only train people like her. “I had a client on Sunday who was a slim man,” she says. “I asked him: why did you come to me? Because normally my clients are overweight. And he said: ‘I need a trainer.’” Johnnie was stunned. “I told myself, Lauretta, this guy just wants you because you’re a good trainer. This is when inclusivity is working.”

But fitness can do more than integrate marginalised communities. “It’s wild how the things you think you won’t survive help other people,” says Jessamyn Stanley (AKA MyNameIsJessamyn). A 31-year-old, plus-sized yoga teacher, she preaches a message of self-love and body-positivity to her 370,000 Instagram followers. (“You are powerful, you are strong,” she intones in one video.) Sometimes, after her classes, young girls will confide how much her message means to them.

“That really hits me in the gut,” Stanley says. “Because I was a mess at that time in my life. I was not happy to be alive, very much in a dark space. Now I’ve got girls coming to me at the age that I was, and they’re saying: ‘How do I get through this shit?’”

Were it not for fitness, Adam Foster, a former soldier, says he would not be alive. After he was injured in an IED blast in Afghanistan in 2009, Foster had chronic back and neck pain. The pain was just about bearable, but the 12-day migraines were not. Doctors were no use, variously diagnosing him with PTSD or complex regional pain syndrome, or attributing the pain to scar tissue. In desperation, he even spent £1,000 on a pain-management course, but it was a disappointment. He turned to fitness to survive. Without it: “I’d have killed myself, most definitely.”

Now a personal trainer, Foster says the majority of his clients also have chronic pain syndromes, such as fibromyalgia. “When you’ve been in pain for a really long time,” Foster says, “you become sensitive to pain and depression. Becoming physically fit allowed me to feel human again.” With his fibromyalgia patients, he starts slowly. “We do gentle exercises where we get used to the joints being used again, so flexion work with weights, for example.”

Ghahremani is the only disabled trainer at the national chain Pure Gym. Recently, he had a client with no hands who was an amputee below the knees. Other trainers might have struggled to devise a suitable training regimen, but Ghahremani wasn’t fazed. He devised a regimen that included mat-based floor work on her glutes, and resistance bands on her upper body. “Sometimes, as a personal trainer, you’ve got to get creative,” he says.

If there is one thing that unites this group of fitness fanatics, it is that they have had to plough their own furrow in an industry that seemed to ignore their needs. It is easy to look at moves to make fitness more inclusive and think: we are getting there. But Stanley cautions that changing hearts and minds “will take generations”. We are only in the infancy of making the fitness world more inclusive. Who will lift the heavy weights when we are done with the warm up? The younger generation, of course.

“Children now are so much more woke than I am,” Stanley says. When thoughtful, inquisitive kids come up to her to have their picture taken after her yoga events, she allows herself a moment of hopeful optimism. “I think: what’s that kid’s life going to be like when they’re my age? How evolved are they going to be?”