The naked female body. It's rather a shock to encounter it in the middle of your afternoon. One moment you're walking off the street into the gym, head full of thoughts of work and life, then bam! There it is, an exclamation point of flesh, as subtle as a brick to the head. There's no getting around it. Literally, in my case. In the smart communal changing rooms at one of London's chicest boutique fitness outfits, I step purposefully (eyes forward, humming to affect insouciance) past a woman wrapping her hair in a towel. She is quite, quite naked, and though I really am trying not to, I can't help but notice her full (luxuriant, you might even say) Seventies bush.

It's not that I'm unused to nudity (my own or other people's), it's just that women's bodies, up close and personal, seem charmingly at odds with the environment in which we currently come into contact with them: the ramped-up, high-spec, fully fitted and kitted communal changing rooms of the fanciest fitness studios. Here, the air is purified, the lighting is flattering, and the amenities are hotel-standard: Bumble & Bumble shampoos and Ila body wash, GHD straighteners, tongs, spare elastics, tampons, deodorant and, at all-female members' club Grace Belgravia, hairdressers summoned from nearby Neville to blow-dry your post-yoga mop into perfect bouncy waves.

You would have thought that with all these treats and added extras the most obvious luxury would be privacy; indeed, at Grace Belgravia there are specially designed shower cubicles with in-built changing areas for those leery of flashing the flesh. But, for most, it's about getting in the altogether, all together. Is this some subliminal way of body shaming us into exercise? (We might appear to be keeping our eyes on our lockers but there is some serious side-eye going on in these rooms, a high-volume game of "Are my thighs bigger than hers?"). Or is it just that we're more comfortable with nudity these days, a little more European than English in our attitudes, and a little too busy to care? The more mundane reasoning finds that where space is at a premium, communal changing simply makes more sense per square foot.

© Mario Testino

I tend to go to class dressed for class and then dart home afterwards fora shower rather than face the queues. If I absolutely have to get changed, then it's done as unobtrusively and quickly as possible, preferably in a shadowy, unvisited corner. My idea of hell is a locker in the main thoroughfare of a communal changing room. Really it's the embarrassment of getting my kit off in public - all that red-faced wrangling with straps, the strategically placed towels, the knickers put on inside out: it's a setting rich with potential humiliation. (I might be OK with public nudity, but really it is other people's public nudity.) A cursory glance around these spaces reveals I'm certainly in the minority in my thinking. But if I was in the best shape of my life, would I still be so very Victorian about getting it all off? What would it take for me to join in with the rudey nudies? At this stage I'm thinking a personality transplant.

For most, though, the communal experience is a heartening one. (Unless you have models in your midst, in which case it's advisable to clear out as quickly as possible or risk a serious blow to your self-esteem.) Rhian Stephenson, CEO and top trainer at spinning outlet Psycle (and owner of one of the best bodies in town), says, "It's about intimacy between women and a sense of community. We don't talk about weight here, we talk about energy and happiness, so subsequently I don't think people feel as body shy."

There's an audience in a changing room, and often it's an appreciative one. I've been complimented on a particular pair of Lucas Hugh leggings several times while emptying my locker after a class. Tamara Hill-Norton, founder of Sweaty Betty, says she is frequently approached in the changing rooms of her local health club by women praising a certain piece of kit or pointing out flaws in her designs, criticism that she says is invaluable. She sees the trend in wider terms of style. "In fitness studios, women used to cover up in baggy things, now they're wearing tiny crop tops to rock their six-pack or toned arms. Women are definitely more confident about showing off their bodies in the studio, so why wouldn't that translate to the changing room?"

There should be a German word to describe the exquisite horror of making accidental eye contact with an acquaintance as they take their nude stroll from the shower

"The Instagram generation aren't body shy," says Elissa El Hadj, founder of Form Studios. "Their mantra is, 'If you've got it, flaunt it.' These women empower themselves by training hard and following a healthy lifestyle." She continues, "Cellulite-free limbs and pert buttocks should be proudly on show in the changing room for other women to admire and hopefully derive inspiration from." (A pert bottom also makes a particularly good target at which to aim your thought daggers.)

"Why not be proud of what you've achieved?" asks model Tallulah Harlech, a regular at Barry's Bootcamp. "I feel good about my body, so mostly I'm pretty naked in the changing room. Women dress for women more than for men, and I think we work out not only to look good in clothes but for our bodies to be looked at and admired by other women, too."

It's a case of picking your moment."I was at a class with a friend," Harlech continues, "and there was a moment in the changing room when we were both butt-naked and about to have a full-on catch-up, and it was like, 'Hang on, maybe I'll just put my bra and knickers on before we continue?"' Nudity between strangers is fine, but with friends it's awkward. There should be a German word to describe the exquisite horror of making accidental eye contact with an acquaintance as they take their nude stroll from the shower. Tamara Hill-Norton recalls a design trip to Morocco and a fateful hammam where "they got us to take everything off. I mean, I know my team really well, but they were my work colleagues."

© Patrick Demarchelier

That's why the design of these spaces is so important; not just in the obvious ways of enough bench space and clean showers, but as environments that quiet the mind and quell anxieties. At the Bulgari Hotel, home to Lee Mullins's Workshop Gymnasium, the changing rooms also feed the spa and the air is filled with calming plinky-plonky music that might usually accompany a 90-minute aromatic massage. It's all warm wood and stone floors and Espa body cream and miles of mirror space — including flattering floor-length mirrors angled just so to make your legs look two sizes leaner and a foot longer. In Grace Belgravia's changing room, there is a similarly hushed, relaxing atmosphere, conjured by pink leather banquettes and marble rain showers. The steam room area smells of dried eucalyptus and suntans, like holidays, and you can order a kale salad while still in your dressing gown. It's the kind of spot where the Queen's cousin is in your yoga class and the pavement outside is nose-to-bumper Bentleys. The lockers, as the manager explains, "have hanger space for ballgowns". Of course they do. As founder Kate Percival says, "The changing rooms are designed to be as luxurious and as well-equipped as one's bathroom at home." Or, in this case, much, much better.

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Small things make all the difference. At Psycle, alongside the Bumble & Bumble hair products, the changing area — a human soup during class changeover — is scented by Ila, "something Zen but energising", says Rhian Stephenson. While in the changing rooms at James Duigan's smart new Bodyism gym on Westbourne Grove, towels are embroidered with empowering words such as "Love" and "Compassion". "My main priorities were making people feel at home and providing absolute comfort," says Duigan. "The way that it smells, the purified air, the crystals hidden under the floorboards - even the lighting is designed to make people feel better. Not only is it flattering - no downlighting - we've also put vitamin D lights in there so you get your vitamins and your full light spectrum, to help combat depression in winter."

There is all manner of dubious, fascinating behaviour on show - from a woman plucking her bikini line while blocking a row of lockers to another blow-drying her hair in sunglasses

Stories of communal-changing-room mishaps range from the comedic (standing up from a lower locker too quickly and brushing one's cheek against a stray bosom) to the frankly weird (coffee granules used as a shower exfoliant). The issue with changing rooms, with being at one with your fellow woman, is that hiding in a crowd leads people to think they're invisible. There is all manner of dubious, fascinating behaviour on show - from a woman plucking her bikini line while blocking a row of lockers to another blow-drying her hair in sunglasses, to beauty regimes of such elaboration and precision (the delicate placing of two tissues on the weighing scales before stepping on to them, the body creaming as art form) - it's akin to wisdom imparted by geishas.

It's the proximity to the physical in these spaces that makes the etiquette so necessary. No queue-barging for the showers. No staring. No commenting on a stranger's intimate tattoos. No stealing your neighbour's towel. "No defecating in the only available loo before class," says Harlech, firmly. "And could you please not sit there eating that sandwich?" Bench space is sacred. If you're on the phone, get out the way. "Think about your underwear," says Tamara Hill-Norton. "If you're going to be semi-naked, grey knickers are probably not the way to go." But by all means approach your trainer to tell her how life-changing her classes are. And feel free to compliment someone on their excellent leggings.

If you think it bad here, imagine what it's like in America — birthplace of fitness trends. "I was at Soulcycle with a friend in New York," recounts Stephenson, "we were late for a class so we ran downstairs, whipped off our clothes, looked back and there were 15 men watching us. The changing rooms were unisex." Truly the stuff of my nightmares.

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