NEW YORK, United States — What is this, a luxury boutique? Full of leggings?

Jennifer Bandier is selling workout couture, and her store, Bandier, is quickly becoming something of a celebrity destination. Her fans include Kim Kardashian, Olivia Culpo, and Liv Tyler. Jordin Sparks hosted an in-store fitness session in August. The company, owned by Bandier and her husband, is rapidly expanding.

While there are some high-end online activewear shops, such as Carbon 38 and Net-a-Sporter, few have dared to venture into the world of boutiques. Bandier (ban-DEER) is filling that old store concept with items that aren't traditionally considered luxury. Instead of designer handbags or dresses, it sells pricey workout gear. Most are emerging labels that many customers haven't heard of, like Splits 59, Good Hyouman, and Prismsport. There's a small sampling of ubiquitous brands, a careful selection of gear from Nike and Reebok. The wares mostly run from $35 sports bras to $400 jumpsuits.

"If you're a woman, chances are you're wearing leggings," Bandier said. "Or you want wear comfortable clothing. People get home from work, they want to wear a comfy T-shirt."

High fashion has long shunned activewear as unworthy of the couture treatment. But lately there's been an influx of extravagant workout wear, with upstart companies creating specialized labels and stretching for new levels of status. Thanks in part to the rise of boutique fitness, designers are selling performance leggings that can cost $300, $400, or more.

That's what drives Bandier's business. The Broadway location, in New York City's bustling Flatiron District, is ringed by such trendy workout spots as Flywheel, Tonehouse, and SoulCycle (two studios). The various concepts are on a tear; SoulCycle filed for an initial public offering of stock in July. Many have a noisy nightclub vibe, using music to get people hyped and keep them coming back for more. Within these communities, what you wear often matters.

"We do tremendous outreach with the studios," said Bandier, sitting on a black couch in the middle of the store, surrounded by racks of athletic wear, from capri leggings to cami bra tops. "We try to have them do private sessions with people here." A Fifth Avenue location that's in the works will have a dedicated studio within the store for fitness classes, panel discussions, and more, she said.

It was hard to hear the 45-year-old founder over the noise as pop and hip-hop hits blared from the speaker system, making the shop feel more like a fierce spin studio than a fancy fashion boutique. "I love dancing, so I'm always moving," Bandier said, as she swayed in her seat, black Saint Laurent pumps tilting back and forth. "I like to blast music."

A former music executive who managed R&B icons TLC, Bandier started her retailer as a summer experiment in 2014, opening a seasonal boutique way out east in beachy Southampton. Now she aims to open three new stores, and estimates she'll haul in $15 million in sales over the next 12 months. She's currently seeking outside funding to bankroll her ambitious expansion plans, after launching an e-commerce arm in April.

Michael Tesler, a strategist at consulting firm Retail Concepts, said the activewear industry is so stuffed with different kinds of stores it's difficult to stand out. All players are looking up at Lululemon, and most, including Target, Athleta, and Lucy, are trying to defeat the leader with lower prices. Bandier is the opposite.

"There's a lot of room to go above Lululemon and beat them on the quality piece," Tesler said. But botched operations, bad merchandising, and overexpansion are all threats. "There's so many wrong exists you can take off the highway," he said.

There's also the danger of being hitched to the boutique fitness craze. If classes like SoulCycle and Pure Barre lose their popularity, Bandier will be looking for a new customer for lines that are pretty far out there. If that ever happens, she hopes the appeal of her clothes will reach a broader audience.

For now, Bandier is feeding off the popularity of what the fashion industry has dubbed "athleisure," workout wear that can be worn casually on the street. She knows her good fortune can't last forever, as fashion trends continue their perpetual cycling.

"Listen, I'm all about ready-to-wear — I'm wearing it right now," she said, pointing to her Stella McCartney blazer and Alice & Olivia blouse. "I'm realistic. But at the moment, for us, it seems like the market's growing."

It is. The U.S. women's activewear market represents more than $18 billion in annual sales, up 21 percent from a year ago, according to research firm NPD Group.

With that in mind, Bandier is thirsting for growth. Her new stores will open on Fifth Avenue and in luxury malls in Manhasset, N.Y., and Dallas. These high-end malls are packed with some of the swankiest brands around, such as Chanel, Prada, and Dior. Some of the merchandise at Bandier can cost much more than the usual legging, such as $350 bottoms from Lucas Hugh, but that's nothing compared with a $4,000 fur boot from Tom Ford.

If it wants to fit in, Bandier needs to provide an opulent aura consistent with its surroundings. The merchandise already has that vibe under Chief Merchant Jayne Harkness, a veteran executive who spent time at Barney's, Coach, Calvin Klein, and Isaac Mizrahi. Flanked by luxury labels, the new stores will be sleeker than the current shops, with lots of white space, fancy mannequins, and glossy displays. Seating areas outfitted with headphones will provide hangout spots.

Exclusivity is key. Many pieces at Bandier are launched early at its stores or are outright exclusive designs created through partnerships with the labels. The store maintains a steady churn of these limited items to prevent shoppers from going elsewhere to buy the brands it sells. For instance, Reebok re-cut a style of its FuryLite shoes with a floral print. Bandier plans to come out with its own private activewear label.

Flipping through a rack of printed leggings from Zara Terez, Bandier pointed out a couple of pairs covered in emojis and patterns of kale. She finds new designers by voraciously searching fashion blogs, visiting stores, and ripping out hundreds of pages from magazines. At this point, designers tend to come to her as much as the other way around. She's not worried about finding more.

What would worry her? "If people stopped working out."

By Kim Bhasin; editor: Peter Jeffrey.