Mainstream retailers have caught on. Fast-fashion chain Forever 21 Inc. began selling activewear in February, following Gap, which introduced its GapBodyFit line last fall. Nordstrom Inc. and Target Corp., which have their own private-label brands, have been expanding their women's sportswear offerings. Even lingerie company Victoria's Secret sells yoga pants and "bralettes."
"It's an underserved, underrepresented market that's growing," said Howard Tubin, a retail analyst at RBC Capital Markets. "There is certainly competition, but there's room for retailers to pick off a little bit."
Brands are eager to quickly roll out new merchandise because the core activewear shopper is the ultimate consumer: female, relatively young, fashion-forward, and willing to spend money.
"We are hungry to win this girl over," said Adrienne Lofton Shaw, a marketing director at Under Armour.
Retailers say they're not only filling a void in the sports apparel market, which has predominantly catered to men, but also correcting a long-standing flaw in athletic clothing for women.
"The assortment that has historically been available to them has generally been reverse-engineered from a male garment, made smaller, and made pink," said Scott Key, senior vice president at Athleta, which plans to open 50 stores by 2013.
"Women are demanding true performance items, but that are attuned to their specific needs."
One of those needs is clothing that is attractive. Women such as Claire Parker, 29, say they work out to feel good but want to look good while doing it, too.
Before going for a run, Parker stopped by a Lululemon store to buy two running tops. Although the yoga-inspired brand's merchandise is "ridiculously priced" - she spent $125 on the shirts - she said the clothing was worth it because it "hides all the imperfections" and "decreases jiggle."
"Let's face it," she said. "You never know who you're going to meet at the gym."
Retail analysts say activewear brands have successfully broken the barrier between workout attire and street clothes, which has further boosted sales. "There are people who don't do anything athletic who are wearing it; it's become more of a fashion statement," said Pamela Quintiliano, a research analyst at Oppenheimer & Co.
Despite toting a $1,000 Marc Jacobs handbag while shopping at the Grove in Los Angeles recently, Stacy Lee, 19, was dressed in a sweatshirt, flip-flops, and Lululemon yoga pants, which she called "my favorite pants in the world."
"I probably wear these every other day," she said. "I don't feel like if you wear workout clothes that you have to look grubby."
Retail analysts credit Lululemon, based in Vancouver, British Columbia, with fueling the activewear trend and making it cool to wear workout clothes to the mall, out for coffee, or to run errands.
Founded in 1998, the company has had phenomenal growth recently as the overall retail industry has seen modest performance. Lululemon, which has 142 stores, plans to open 25 new locations this year. Its stock is at all-time highs, with shares up 75 percent for the year. In its most recent quarter, Lululemon reported that its revenue rose 35 percent year-over-year to $186.8 million and sales at stores open at least a year rose 16 percent. The company's $33.4 million profit was up 70 percent from $19.6 million in the year-earlier quarter.
Shoppers appreciate that the company does more than just sell products, noting the weekly yoga classes and running clubs that the retailer offers. Other brands, including Athleta and Nordstrom's Zella activewear line, have also hosted in-store fitness events.
"Companies like Lululemon, they don't sell clothes - they sell a lifestyle," said Lee, a student at the University of California-Berkeley. "So I think people bought into that and felt like they needed the clothes to be part of the lifestyle. I have."
As competition has heated up, high-performance brands have bristled at suggestions that anyone can make a quality athletic product, arguing that there's a difference between authenticity and jumping on the bandwagon.
"When I look at the market, there's a lot of 'me too,' " Lofton Shaw of Under Armour said. "Everyone's chasing trends."
For their part, general retailers are touting affordability and accessibility.
At Target, which has been adding products to its exclusive C9 by Champion activewear line, the discounter wants to offer athletic merchandise without "the intimidation factor of walking into a specialty boutique or a sports store," spokesman Joshua Thomas said.
The most expensive activewear item at Forever 21 is a long-sleeved sports top with a partial zipper for $22.80. The Los Angeles-based chain also carries sports bras for $8.80 and workout pants for $12.80.
Victoria's Secret, meanwhile, is upping the glam factor. Its Bling Bootcut Yoga Pant features rhinestones and animal prints; the product's online shopping Web page recently said: "Look hot when you work out or hang out. Be a yoga superstar."
For shoppers such as Lynn Brown, 41, who said she dreads going to the gym, not looking "like a frump" is of utmost concern.