There's much activity in women's activewear

Athletic clothing has historically been confined to the gym, where women would huff and puff in unglamorous getups - think oversize men's T-shirts and baggy sweatpants.

But now that more women are pursuing active lifestyles - juggling Pilates, spinning classes, and hikes with busy professional and personal lives - what was once a limited market for fitness apparel has taken off.

High-end brand Lululemon Athletica Inc. has been adding to its yoga and running lines, experimenting with cycling products, and opening stores in new markets (the Philadelphia area is home to four: on Walnut Street and in Haddonfield, Wayne, and King of Prussia). Gap Inc.'s Athleta, until recently a catalog- and online-only company for women, will open seven stores this year. Performance athletic company Under Armour Inc. is introducing a new yoga collection this summer and redesigning its sports bras and underwear for 2012.

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.