It’s lunchtime on a Wednesday and I am in a dark room, shaking intensely. My forearms are propped up on a big black machine called a “Sweatlana”. Like five other quaking women beside me, I am furiously plank-crunching, attempting to move the machine’s carriage forwards and backwards using only the force of my abs.

“Come on team!” bellows a high-energy instructor over the booming music, urging us all to get “comfortable with discomfort”. Our trembles, she says, are a sign of reaching “second-stage muscle failure” which is not, as it sounds, fatal, but, apparently, a state to aspire to if we want to get stronger.

Welcome to Solidcore, a 50-minute workout which combines two zeitgeisty forms of exercise: pilates and strength training. Classes take place in dark, blue-lit studios which look like petite nightclubs. Celebrity fans include Malia Obama and Sydney Sweeney and the cult-like nature of classes has been parodied in an SNL sketch.

Legions of self-identifying “Solidcore girlies” proudly post TikTok videos about how sore they are after class. In September, the LVMH-backed private equity firm L Catterton bought a majority stake in the company, valuing it at between $600m and $700m, with plans to grow the chain from 130 US locations to 250 globally by 2028, including plans to open more than 25 new US locations this year.

Like Barry’s Bootcamp and SoulCycle – the latter of which pioneered the concept of boutique exercise classes as a nightclub-cum-church almost 20 years ago – Solidcore is cult-like. Giddy enthusiasts say their bodies have never looked better, and wax lyrical about leaving classes floating on a cloud of endorphins and achievement.

Coaches are charismatic leaders: the most popular have waitlisted classes and are celebrated for their toughness. Studios offer a sense of belonging. Instructors know your name and, as Valerie Weiss, a film-maker and producer who has done 153 classes, explains, there are rituals. Clients are photographed with boards reading “200 shakes superstar” or “50 classes stronger” when they reach milestones. “I liked being celebrated for the hard work that it was,” she says. “The teacher who did my 100th class always remembered it, talked about it in the class to the other students. You kind of feel like you matter, and that it matters whether you show up or not.”

Encouraging clients to do more classes is, obviously, very good for business. Solidcore’s prices vary by region but they can be body-quaking: in Brooklyn a peak single class is $43. A pack of five costs $213.18; a month of unlimited is $432.63. When I attended, I was advised that it would take about 40 classes until I fully mastered the movements. The fact that those 40 classes would cost, say, $1,510, if I paid for eight a month, went unmentioned.

For some fans, those prices are not just worth paying but part of the appeal, adding to a sense that they are making the best choice possible for their bodies. Many also said they felt they were extremely unlikely to skip a class knowing they had paid top dollar for it.

Physically, as well as financially, the struggle is part of Solidcore’s appeal. Jessica Beaugris, a cosmetic dermatologist and influencer who has done about 650 classes, says that just hearing someone does pilates tells you something about them. “It’s just a reminder that it’s somebody who has grown and has done phenomenal things because it is a very difficult workout.”

She is proud of the discipline she feels she has shown by attending so many classes. It “reminds me that I have control over my life, that I am the one who makes decisions”.

One fan, Montse Lewin, 26, describes Solidcore as “military training for the pilates girlies” and likens the intense shaking, in some classes, to “an out-of-body experience”. Nicole Lavery, a wellness coach who has done more than 1,100 classes, says that when she makes friends through Solidcore “we always joke that it’s trauma bonding”.

Sami Jo Negron, a 29-year-old Tampa-based radio DJ on 93.3 FLZ, had a typical initiation. She did not like her first class but was so sore afterwards she thought, “I want to feel that way again – believe it or not. I do realize that sounds crazy but I felt like it was doing something for me that I had not achieved in other workouts.” A year later, she says, “I’m definitely one of the obsessed people.”

I, too, have felt this ache. I did two Solidcore classes. After my second, I had to push myself up in bed using my hands – an uncannily similar sensation to the one I experienced while recovering from an emergency C-section.

***

Solidcore was launched in 2013 as a single studio in Washington DC by Anne Mahlum, a fitness enthusiast who did a hardcore pilates class in Los Angeles, then tried SLT – another hybrid, hardcore pilates class that uses the strapline “sore not sorry” – and loved it. Word of mouth was wild from the start, she recently told Fortune, because after class clients tended “to go tell everyone they knew how sore they were”. She opened five new studios in the first year. In 2014, Michelle Obama was spotted working out there at precisely the moment the world’s media was obsessed with her biceps.

Solidcore has had its controversies. Initially, Mahlum used Megaformer machines designed by the fitness inventor Sebastien Lagree (the “Lagree method” is another hybrid, hardcore take on pilates). Mahlum later created her own machines and in 2015, Lagree sued Mahlum, who counter-sued and won. Meanwhile, in the great “girl boss” takedown of 2020 (in which a host of female business leaders, including Yael Aflalo of Reformation, Steph Korey of the luggage company Away and Audrey Gelman of the Wing, resigned after accusations of inequity and poor staff treatment were reported in the media), BuzzFeed reported on allegations by a former coach, Emily Collinson, that Solidcore was flouting Covid regulations. Current and former staffers told the site that Mahlum was fostering a toxic work environment, including alleging bullying and harassment. Mahlum tells the Guardian that the Buzzfeed piece was “a take-down article filled with only anonymous sources of people airing their dislikes and grievances” and says that “nothing in the article was ever substantiated”. As for Covid, she says: “I absolutely stood up for Solidcore during this time as the restrictions, especially in DC, that were being placed on us directly contradicted more lax restrictions placed on other in-person businesses, like gyms and restaurants.”

She adds: “Until society accepts that top priorities for women leaders is not to be nice or liked, these kinds of articles will continue to happen.”

Mahlum remained CEO for nine months, then moved to another executive role within Solidcore. By 2023, when Mahlum left, the company had raised its second chunk of private equity money, enabling it to grow while others contracted during the tail end of pandemic lockdowns. She sold her shares for over $88m (she received another $10m in the deal with L Catterton).

Pilates, as it was originally conceived by the German physical trainer Joseph Pilates more than 100 years ago, is a low-impact exercise that focuses on core strength, breath and mind-body connection, using controlled, fluid movements. It is excellent for posture and associated with the prevention and treatment of injuries.

Unlike mat pilates, which can be attempted for free by anyone with access to the internet, it is pilates’s reformer iteration that dominates social media now. Reformer classes tend to be expensive, because they generally require large, expensive machines which take up a lot of studio floor space. As such, pilates has been associated with cash- and time-rich older white women for decades – whether Upper East Siders or celebrities like Madonna, who literally sang its praises in her guns-of-steel era (“I do yoga and pilates / And the room is full of hotties,” she rapped in 2003’s American Life).

Recently, however, younger women have started to dabble, taking inspiration from the likes of Ariana Grande, Bella Hadid, Kendall Jenner and Hailey Bieber, who are continually photographed outside the exclusive Forma Pilates in LA. In 2022, one Forma Pilates fan, the model Lori Harvey, went viral for crediting her superhero abs to the exercise, during an Essence interview at the Met ball.

When she saw that clip, Negron admits, “I was like, ‘Say less! I want Lori Harvey’s abs!’” Negron says she has also been influenced by “the ‘pilates princess’ movement on TikTok. I feel like it went from like a peer pressure to try it to having to live the pilates lifestyle, get the cute workout set, drink matcha lattes.”

Not only does the reformer feel exclusive, it is also great content, says Rina Raphael, author of 2022’s The Gospel of Wellness: Gyms, Gurus, Goop and the False Promise of Self-Care: “They all look really cool on the reformer. Like, they’re almost harnessing a horse.”

In the pandemic, Raphael points out, yoga was very democratised through free YouTube classes such as Yoga with Adriene. For some, yoga now feels associated with the “anxiety-ridden pandemic era, when we really needed to calm ourselves down”. Pilates, by contrast, feels right for now. “There’s a sense that it’s challenging – there is some sort of psychological appeal to it as something you have to master – it’s about strength.”

See every little muscle in your stomach

The full pilates lifestyle – complete with Stanley cups and grip socks embroidered with slogans (“Hot girls do pilates!”) – undeniably dovetails with the Ozempic-fuelled return to the skinny body ideal. Tiktok offers countless “pilates transformation” videos, in which soft, medium-sized users are replaced with their smaller, harder selves. There are so many videos of tiny waists in tiny workout sets, sometimes with rib bones and spine visible in the stretch of “snatched” skin between the lycra. One devotee describes it on TikTok as an exercise that “makes you look dried from the inside out – you can see every little muscle in your stomach”.

Though the mega-influence of Harvey, and the rise of Black and brown teachers seeking to make the exercise more inclusive, has broadened pilates’s customer base, the aesthetic still veers towards skinny white women. “As a curvier girl myself I’ve felt that intimidation,” says Negron of some pilates classes “where I was definitely underestimated for my size”. Solidcore isn’t like that for her she says: “It feels a lot more inclusive.”

The focus on skinniness is a misrepresentation of the benefits of traditional pilates. After going viral, Harvey clarified that her abs were not the result of pilates alone, but also a very low-calorie diet and going to the gym five or six times a week, sometimes working out twice a day.

Meanwhile, the phrase “long, lean muscles”, omnipresent in pilates content, isn’t accurate. According to Chris Gagliardi, a scientific education content manager at the American Council On Exercise, pilates may “restore” the length of muscles that have become temporarily shortened by sitting at a desk all day, but this would not be noticeable visually, beyond potential improvements in posture and symmetry. It can, however, help build what many describe as “lean” muscle, he says: defined muscles that aren’t bulky, because it focuses on “muscular endurance rather than muscular hypertrophy”.

But, he points out, “dramatic changes such as those seen in the types of before and after pictures that individuals post online are usually achieved by making multiple lifestyle changes at the same time.” Changes in muscle definition will usually only be visible if a person also loses weight or if they are very thin already.

Hardcore and hybrid iterations of pilates, like Solidcore, SLT and another competitor called, unambiguously, Tremble, may incorporate elements of cardio, or, like Solidcore, may be designed to build muscle in a way that feels more similar to strength training, and which may (in combination with a calorie-controlled diet) be associated with weight loss. These hybrid classes are sufficiently different from actual pilates that Solidcore has recently decided to stop using the word pilates at all in its branding.

Rather much of Solidcore’s marketing focuses on the phrase “second-stage muscle failure”. This is Solidcorese, not industry standard, but, explains Gagliardi, it does make sense. The idea is to push through the feel-the-burn stage of muscle effort to get to a level of “breakdown in the way our muscles are working together. Our body is having to recruit accessory muscles to get the job done” – hence the shaking.

That may be fine for many people, “especially when done in a supervised setting”, he says. Gagliardi does have concerns, though, about the risk of injury. Because you’re aiming for failure, and for your body to shake somewhat, it can be hard to maintain good form and control of movement. But aiming for shakes, a central part of the Solidcore brand, is not something Gagliardi would recommend his own clients do, in part because, he says, “the evidence doesn’t show that training that way is going to get you a better workout.” (Solidcore says that its coaches are trained to prevent overexertion and will move clients on to the next exercise if they reach a stage where they are unable to complete repetitions or maintain proper form.)

Add to this that some users attack their Solidcore schedule with too much intensity: Solidcore recommends classes should be taken one or twice a week for newcomers and twice to four times for regulars. But all of the power-users I spoke to were going at least four times a week. Some went more. One, Lewin, who admitted she has long struggled with body image issues, went every day during her first month, and says: “I’m not going to lie, I was in a lot of pain, using heat patches or taking a bunch of baths every day. I was in the best shape I’ve ever been – I had the best abs I’ve ever had in my life, I was very snatched – but it was hurting me.”

One user told me that the current beauty ideal, as she sees it, is: “BBL, slim arms, not bodybuilder abs but some sort of abs. For those of us who can’t afford BBL surgery, or liposuction,” she said, a class like Solidcore felt like the best way.

Raphael isn’t sure why this surprises some of us: “It’s like how people are shocked that skinny is back in – they’re like: ‘but we have all these Dove commercials!’ But people are still people, and there is still a large group who judge their progress according to how they look, who want to view fitness through a lens of achievement. That is just part of living in a society that is very focused on attractiveness.”

There is something encouraging, nevertheless, in women strength training, a movement partly fuelled by rising awareness of its proven benefits, particularly for older women. “A decade ago, the perception was that weightlifting was for men,” says Mikala Jamison, a health and culture writer and author of the Substack newsletter Body Type. “Now, you see women talking about wanting to become ‘muscle mommies’, wanting to eat more protein.”

Building muscle at a class like Solidcore – with an instructor leading you, so you don’t have to devise your own programme or pay for an even more expensive, albeit more targeted, personal trainer at the gym – was a huge part of the appeal for many women I spoke to, who hated the male-skewing weight rooms in gyms. “You’re like, ‘Oh, I look like this needly little whatever,’” said Michelle Triolo, a relator from Annapolis. “There’s always a bunch of dudes. I feel uncomfortable going to specific machines. It’s intimidating, because you feel like you’re getting checked out the entire time,” says Lewin.

Many fans described Solidcore as an investment in self-preservation in a hostile world. Weiss, the film-maker, for example, thought an increased emphasis on strength was a symptom of a time in which “women’s rights have been challenged so much. I think people are like: ‘I can’t necessarily rely on my government to keep me safe. So I need to know how to fight.’”

It’s important to realize, though, that boutique exercise classes are not the only way – perhaps not even the best way – to get such results. Jamison, who is a former spinning instructor, says she is always wary of boutique classes that tell customers their class is “the best possible exercise, the hardest possible workout”.

She says the right exercise is in fact “very individual” and that “superlatives have no place in this conversation”. The key, says Gagliardi, is “consistency in your training and finding something you actually enjoy doing”.

Covid lockdowns demonstrated that many of us could get a lot benefits at home, for free. And yet, says the trend forecaster Eryn Murray of WGSN, it is the Solidcore model that feels ripe for expansion now.

Some of this is because women want to be strong and want to gather in groups and receive in-person advice. Some of it is a triumph of branding, and is capitalism doing what capitalism does, pushing one of the most expensive forms of exercise possible – the kind of pilates that requires the use of an enormous machine – because few were getting rich from YouTube’s free yoga videos. Some of it, depressingly, is about the decisive return of the very skinny, expensively toned body ideal which is unobtainable for most, the same time-consuming, expensive, distracting ideal that – however much we push against it – the zeitgeist always seems to want to snap back to.