Peta Bee investigates the rise of the craze that's seen everyone from Jennifer Aniston to Gisele Bundchen ditching cargo pants for sculpted sweats

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Rarely am I someone who would get excited about a shopping bag. But this is different. It carries more than the beautiful, sought-after garments inside it. Emblazoned with aphorisms such as ‘Friends are more important than money’ and ‘sweat once a day’, it gives me instant cachet, I discover, and elevates me to the status of someone perceived to have her life well and truly sorted. By purchasing something from  Lululemon Athletica , the Canadian brand behind the bag, you buy straight into a lifestyle choice that tells people you are fit, spiritual and healthy in equal measure. By implication, you are suddenly right up there in the bendy stakes with Avril Lavigne and Scarlett Johansson, Megan Fox and Reese Witherspoon, all of whom wear its subtly logoed yoga apparel inside and out of the gym.

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Since the opening of the UK’s first Lululemon  store in Covent Garden in the Summer (several more are planned this year), I know women so desperate to get their hands on its best-selling Wunder-Under leggings and signature Groove Pants, cut with flat seams from buttock-boosting, trademarked fabrics, that they willingly take a day off work to get there. Such is the Lulu effect. As a brand it has inspired cult-like devotion since it was first founded in Vancouver in 1998, before spreading to the rest of Canada, then California and eventually hitting Manhattan in 2007. It currently has close to 250 stores in Canada, the US, Australia, New Zealand and now Europe, with reported quarterly sales of around £318 million and millions of customers who call themselves Luluheads and gush in Lululemon blogs and on dedicated Facebook pages about the clothing range’s ability to flatter the female form.

Not that the brand is alone in making fitness clothing aspirational. Others like Sweaty Betty , Lucas Hugh , Hey Jo  and Stella McCartney  have jumped on the bandwagon and now design the kind of upmarket sweat gear that was unheard of a little over a decade ago. It’s no coincidence that the growth in the market has mirrored a simultaneous rise in the number of personal trainers, boutique gyms and niche workout studios available not just to wealthy celebrities, but to all of us. What we want when we buy high-end workout gear is to give the impression that we can afford the luxury lifestyle that allows us to swan from one class to another or as if we are nipping to meet a personal trainer.

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Whereas casual once meant donning cargo pants, skinny jeans and ballet flats, it has been supplanted by sculpted sweat pants with wide waistbands and stylish lounge-wear teamed with a pair of Nike  Flyknits. It’s the new stealth way of dressing for everyone from Jennifer Aniston and Kate Hudson to Gisele Bundchen and Teri Hatcher, all of whom adopt the look for everything from shopping to lunch dates. Teresa Rione, Nike’s  European communications director, says that the company is selling more high-end women’s fitness clothing than ever before. “That’s where the biggest growth area is in the sportswear market,” Rione says. “But women aren’t just wearing our clothes to the gym - they wear it for coffee and lunch dates, even to work.”

In short, luxury workout gear has become the wardrobe staple of the effortlessly chic. Even for New Yorkers, who would once have balked at the idea of wearing fitness clothing anywhere other than the confines of a yoga studio, it has become the default uniform to carry them from workout to brunch and then school pick up. In some ways, clothes are only part of the luxury fitness brand story. Many of the companies selling them now offer a shopping experience like no other. Customers at NikeTown , Sweaty Betty , Lululemon  and the Reebok  flagship store in central London are encouraged to attend a burgeoning range of free events that includes running and circuits, self-defence and goal-setting courses as well as yoga sessions.

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In Manhattan, twice weekly Lululemon yoga classes attract 400 barefooted yogis, most of whom are dressed head to toe in Lulu tanks and leggings, to parks where they follow instruction from one of New York’s most sought after yoga teachers.  “We also held a trial yoga practice at the Royal Opera House the other week and had 700 applicants for the 300 places,” says Amanda Casgar, a Lululemon executive who oversaw the launch of the first London shop. “There is such enthusiasm for it.” Step inside a store and you’ll find the sales assistants are called ‘educators’ or ‘leaders’ and work closely with local ambassadors who, in London’s case, include leading yoga instructors and representatives from top gyms like Barry’s Bootcamp  and the Third Space.

There is, of course, no irony lost in the fact that once lured inside you are likely to be tempted by a pair of expensive trainers, butt-hoisting pants or a lime green sports bra. But it’s difficult not to get drawn in. Step inside any luxury fitness clothing emporium and you are made to feel that you are joining an elite club or gate-crashing a members-only gym. Which is precisely the goal. They are, after all, trying to sell us something we didn’t know we couldn’t do without.