Lisa Andrews was looking for a quick fitness fix. The 34-year-old had “a bit of weight to lose” a year after having her first baby and, being both time-poor and on a budget, she decided to do it with the help of an online 12-week training programme she’d seen advertised on Facebook. “There were hundreds of transformations on there,” Lisa tells me. “I was so excited to start. The programme had several different levels so you could begin at whatever level you thought worked for you. Stupidly, I picked intermediate. It was really challenging, with daily sets of high-intensity exercises, and I would frequently feel exhausted and totally out of breath by the end of it – but I was on a high. As I got fitter, I began to really love the training. I looked forward to it, talked about it all the time, got friends to sign up. I became quite evangelical. Sometimes I’d even do two sessions a day. I’d skip other activities to work out – because if I had to miss a session, I’d feel depressed and worried it would derail my progress.”

But when “niggling pains” in her feet and ankles developed into something more severe, Lisa was unable to go to work. An X-ray confirmed that she had stress fractures in two places in her foot. Bound up in a big boot-like aircast, she struggled to walk for weeks and was told to avoid any weight-bearing training for months, until the bones have fully healed. “I had become obsessed,” she says now. “I was completely into it and the ‘community’ of people online doing the same thing. I’d be on Instagram all the time, looking at other people’s transformations. I do feel silly. I should know better – but it is psychologically intoxicating.”

Using Instagram, blogs and YouTube to get fit is fast becoming de rigueur. And despite getting collectively fatter and more sedentary, the British spend record amounts of money exercising. Figures from the 2017 UK State of the Fitness Industry report show that the sector is worth more than £4.7bn annually – up more than 6% on the year before. A quick search for the #fitspo hashtag on Instagram brings up almost 47 million images – people in workout gear lifting weights, close-ups of ultra-defined abs, bulbous biceps, “transformation” pictures (taken before and after fat loss) – each one advocating a programme more punishing than the last.

These days, hardcore fitness sells. Even Nike, which made its name with that inclusive Just Do It tagline, has taken to lambasting joggers in its latest ad campaign: “If You Like It Slow, Jog On”, or “You Win Some Or You Win Some”, proclaim its new billboards. Gyms run “go hard” promotions, with discounted packages for those taking up unlimited classes for short periods of time, such as 10 classes in 10 days – the kind of training that many dub “binge workouts”.

The new stars create a false sense of what healthy looks like. They’re also paid to shift products

But nowhere is full-on training more powerfully advocated than on social media, where inspirational quotes such as “Pain is Weakness Leaving The Body” and “Sweat Is Your Fat Crying” are liked and shared millions of times. In the age of “wellth”, a well-honed tricep is more desirable than the latest pair of designer shoes. The so-called world of “fitspo” began as a niche way for gym nerds to share tips and document how their bodies changed, before spreading into a whole lifestyle movement. Instagram’s short videos lend themselves to fitness content; people started following routines in the gym.

Fitness movements have been around a long time – think back to Jane Fonda, The Green Goddess and Mr Motivator – but working out has become a lot more complex since the aerobics days, says Rick Miller, a clinical and sports dietician. “Increasingly, there seems to be this feeling of, ‘Why would I go for a gentle 5km jog or a moderate aerobic session when I can do a punishing high-intensity set?’” he tells me over lunch. High-intensity training (mixing all-out bursts of activity with short rests) gets mixed reviews from health professionals: some swear by the fast results, while many believe that unsupervised exercise of this kind can cause health problems.

“Many young people I see are completely obsessed with Instagram fitness stars,” Miller says, “and they follow workouts from so-called trainers they don’t know, which may not be right for their body or their levels of fitness. Fitness athletes are stars online, but their followers often try to train at the standard of a professional athlete, without the core level of fitness. Following these kinds of workouts can very often lead to injury and burnout. Were I to recommend some of the things that fitness bloggers recommend – levels of exercise, nutritional advice – I would get struck off.”

The National Careers Service advises that training to become a fitness instructor can be done on the job at a gym, as an apprentice, or via a college course. Becoming a personal trainer (PT) is more advanced. PTs are usually self-employed, and they need insurance, first-aid training, an awareness of anatomy and physiology, and a qualification, which takes anything from six weeks to three months to achieve. Increasingly, trainers tell me, gyms are looking for another asset in their PTs: they want them to be photogenic, with a big social media following.

Zanna Van Dijk: ‘When I look back at my old posts, I cringe.’ Photograph: Getty Images

Some Insta-fitness personalities have personal training qualifications, but many do not. Often, there is no way of telling who is trained and who isn’t, without asking them. Anyone with more than 100,000 followers, however, regardless of their qualifications, is deemed an “influencer”, courted by brands eager to reach their followers. That’s a fact that angers many offline personal trainers, who feel that the unqualified yet famous ones devalue their profession. “Online programmes want people to feel as if they have their own – affordable – personal trainer,” one tells me. “As some of them are totally unskilled and the programmes are really ‘one size fits all’, the reality couldn’t be further from the truth. It makes reputable personal trainers seem outrageously expensive.”

It is a sentiment echoed by one health and beauty magazine editor, who asks to remain anonymous because her views don’t tally with that of her employer. “These days, a strong Instagram following, good gene pool and even better spray tan can make you a fitness star, regardless of what qualifications you have. Not only do many of these ‘fitness stars’ know little about what constitutes safe exercise (the truth is that no amount of likes come in handy when you need to solve a gym-induced injury), they also create a false sense of what fit and healthy looks like – and it doesn’t always look 21 and great in a bikini. Add to that the fact that these social media stars get paid to shift fitness gadgets, gimmicks and protein shakes, and you’ve a whole load of dangerously misguided followers.”

No one would deny that people becoming more active is anything other than a good thing. Millennials claim to enjoy working out as much as going out; gyms have become stylish, social spaces where people spend their Friday nights and Saturday mornings, often doing back-to-back classes. Spinning, boxing and hybrid cardio-barre workouts at city-centre-based studios often have waiting lists for evening or weekend sessions, when people would traditionally be kicking back with a drink (fewer people aged between 16 and 24 drink than ever before, according to the Office of National Statistics). Gyms are designed with sleek interiors and high-impact feature walls – all the better to post to Instagram.

And while the rest of the fashion sector struggles, activewear – now not so much a genre of clothing as a way of life, led by leggings and crop tops – has become big business. Morgan Stanley forecasts the workout clothing sector to be worth $83bn a year globally over the next three years. Gymwear is no longer old jogging bottoms or baggy T-shirts; it’s cut-outs and mesh – clothes you can wear all day, seven days a week.

It’s a warm Monday lunchtime and I am sitting next to a bread oven in a sourdough bakery in Battersea, south London. Where else to meet a 24-year-old qualified personal trainer and full-time fitness blogger? This is one of Zanna Van Dijk’s favourite hangouts: when she’s not working out (or “socialing” herself doing so), Van Dijk and her boyfriend run an Instagram account dedicated to where to find the best brunch. There is much deliberation about the type of alternative milk to be served with her americano. Later this afternoon, she tells me, she is getting the symbol for Earth tattooed on her wrist because, “I’m a vegetarian for the planet.”

It used to be that running a marathon was hardcore. Now that’s not enough: you have to do a multi-day ultra-marathon

Van Dijk is tall, about 6ft, and lean. She has long, blond hair, immaculate makeup and more than 180,000 followers on Instagram. She studied speech therapy at Sheffield University, but after graduating went into fitness blogging full time. “For me, fitness started as a way to lose my ‘Fresher’s 15’ [a reference to the weight first-year university students can gain]. I documented it, picked up 35,000 followers and didn’t know what to do with them. So I took a year off, moved to London, started to work as a PT, made an income and forged partnerships with brands. I did a six-week intensive course and got it sponsored, as long as I blogged about it. As my online profile grew, I reduced my personal training work – now I train people one morning a week. Otherwise, I’m editing videos or blogposts – I do three of these each a week. I’ve written a book, I’ve brought out jumpers [which say ‘Coffee and carbs’ on them], and I’m an Adidas ambassador.”

Van Dijk admits things were quite different when she was starting out. “When I look back at my old posts, I cringe. I think: ‘Gosh, you knew nothing! You had completely the wrong end of the stick!’ I used to try and be super-lean, and now I really don’t care if I am lean or not – I want to be fit.” She breaks off to vlog, before we look through her Instagram demographics together.

“My following is 81% women, 19% men,” she says. “The biggest audience is 25- to 34-year-olds, more older men, more younger women, mostly London, mostly UK.” More older men? Isn’t that a bit creepy? Van Dijk doesn’t respond. Does she feel a responsibility to her followers? “You want to be 100% honest and share everything, but the other day, I did a video where I showed my body. It was all about self-confidence and self-love, which is what I am all about, but somebody commented: ‘I just think this video is drawing attention to different people’s bodies and their appearances.’ That wasn’t its intention, that’s how it’s being perceived.”

Van Dijk argues that her followers shouldn’t compare themselves with her. “It’s really hard. I train four days a week or maybe five,” she says. “A lot of young girls will look at me and think: ‘I want to look like she looks and I want to do what she does,’ and that’s when I have to be so careful.”

In that sense, she feels she has to protect people from themselves. “If you’re someone who has a negative mindset or is in a vulnerable place, you can easily access material that you could use badly. If you’re someone with an eating disorder or an obsession with exercise, Instagram is not a good place to be.”

Celebrity trainer James Duigan (right): ‘There’s no guarantee you’re doing things right online’

How much responsibility do online trainers really bear for people copying the workouts they recommend? Jean-Claude Vacassin, the founder of boutique London gym W10, is not a fan of fitness via social media or, as he terms it “excer-train-ment”. “What people see on social media is marketing,” he tells me on the phone. “Extreme fitness sells, it’s exciting. It used to be that running a marathon was hardcore. Now, that’s not enough: you have to do a multi-day ultra-marathon. A lot of these online training regimes are aimed at millennials who want to buy on the first click and transform their body on the second – and they push themselves too hard. No one wants to spend eight weeks moving more and eating less these days because, sadly, people don’t believe basic exercise, done well, is going to get them anywhere. There’s this idea that it’s boring.” He cites the case of a builder who got a deal with a supplement company because he works out a lot and has hundreds of thousands of Instagram followers. “But does that mean he knows what he’s doing? No! He’s a builder, not a personal trainer.”

Vacassin adds: “In our gym, we have gym standards. People undergo an assessment before they get a programme. Hiit [high intensity] training and complicated exercises under fatigue should not be in 90% of people’s fitness regimes because they don’t have the physical capability. These online accounts trick people into thinking this is easy. No one posts a bad workout. No one posts the workout they missed. No one posts the depression they have when they get injured or the relationships it costs them. All you see is the good stuff.”

Deep squats, lunges, deadlifts and high-intensity cardio are the mainstay of online workouts, and keep Cameron Tudor, owner of West London Physiotherapy, in business. “We’ve seen an increase in the numbers of clients coming to us having injured themselves doing online workouts,” he says. “People get hurt largely because the message is: ‘This is what I do and there’s no reason it won’t work for you.’ Extrapolated across the population, that’s not going to be good. While it’s a great thing that people are being encouraged to be active, if you’ve never lifted a barbell and then start lifting 10kg, you’ll put your tissues at risk.” Part of the problem, Tudor says, is in the age differences. “The trainers are usually in their early 20s, but a lot of the people using the programmes are mid-to-late 30s and 40s. That matters, because your tissues are far more resilient when you’re under 30.”

Many online workouts feature models and they look so compelling. But regular people won’t achieve the same results

All exercise carries some risk of injury, but the lack of supervision means that online programmes can carry more risk. Cara, 28, from Birmingham, was doing an online squats challenge when she damaged her sciatic nerve. “I am cross about what happened to me,” she says, “but I’m not sure what anyone can do about it. It was my decision to do the programme. I just didn’t know it wasn’t the right thing for me.”

Natalie Burley, 37, from Chichester, swapped daily sessions on her exercise bike for an online programme to regain some fitness after her second child. In her fifth week, she began experiencing knee pain. “A physio told me I’d inflamed the ligaments on the outside of my knee and I had to rest for six weeks. Now I have to wear a knee support.”

Fitness stars themselves aren’t immune from both physical or emotional injuries as a result of their jobs. Van Dijk tells me she broke her hand doing box jumps last year. Fitness Instagrammer Queen City Sweat (almost 50,000 followers) wrote a post in June admitting she had become “addicted” to exercise in 2016, blaming the pressures of social media. “It becomes so easy to start comparing yourself to others on here, which led me to develop a mindset of ‘How skinny can I get?’ rather than ‘How healthy can I be?’” she wrote.

According to a 2008 Journal of Health Psychology study, women reported an increased negative mood, depression and anxiety after only 30 minutes of viewing fitness magazines that promote an “athletic ideal”. Social media means you don’t have to buy a magazine to see these images; they’re in your newsfeed. The BMJ has identified exercise addiction as a growing problem, affecting up to 10% of the exercising population. Meanwhile, research from Flinders University in Australia found that online “fitspo” images mostly depict the thin or athletic ideal for women or the muscular ideal for men which, says clinical psychologist Dr Lisa Orban, can lead to psychological problems, too. “Images seen on Instagram can represent one uniform, idealised standard of attractiveness – one not achievable to most young people.”

I ask celebrity personal trainer James Duigan if he has benefited from Instagram’s fitness culture. “Massively,” he says from his gym in west London. “Social media helped my business Bodyism, and I admit that. But I think there’s a difference between that and photos of people advertising products and selling exercise and nutrition programmes, which can be physically and emotionally harmful.”

Duigan made his name training the likes of Elle Macpherson and Rosie Huntington-Whiteley – both enormously successful models, neither with particularly achievable physiques – but he is unequivocal about his issues with online fitness programmes. “Too many of these videos feature complex moves and people get hurt,” he says. “From a physiological perspective, there’s no guarantee you’re doing things right online.”

Duigan tells me the story of an 18-year-old client who has just joined his gym, after becoming obsessed with an online workout “advocated by very thin models and reality TV stars”. He sighs: “She developed an eating disorder and was under medical supervision for 18 months. It makes me angry. Many online workouts feature models and they look so compelling, playing into our deepest insecurities. But regular people won’t achieve the same results. ”

Lisa Andrews has now made a full recovery, but is determined not to succumb to online training a second time. “I have deleted social media from my phone so I can’t fall back into that vortex. And I’ve joined a gym where they’ve made a programme especially for me. It’s early days and I know it will take time, but I’m having fun again.”

Some names have been changed.

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