The rate of growth in the U.S. retail athletic apparel market is surging, even as Americans' rate of participating in most sports is in decline.
The result is a phenomenon the apparel industry calls "athleisure"—a bright spot in a sluggish business thanks to Americans who are increasingly donning sneakers in the boardroom and yoga pants at brunch.
Analysts at Barclays estimate the U.S. athletic apparel market will increase by nearly 50% to more than $100 billion at retail by 2020, driven in large part by consumers snapping up stretchy tees and leggings that will never see the fluorescent lights of a gym.
Demand for yoga gear, for example, is outpacing growth of the sport itself. Yoga participation grew 4.5% in 2013, according to the Sports & Fitness Industry Association.
Meanwhile, sales of yoga apparel were up 45%, according to Matt Powell, an analyst for SportsOneSource, a sporting-goods industry tracker.
"Everyone is wearing yoga pants, even people who aren't doing it," said Karen Score, the owner of Yoga Mandali, an independent yoga store in Saratoga Springs, N.Y.
Ms. Score, who also runs an adjoining yoga studio, is drawing up brochures for fall classes with the tag line: "Do you wear yoga pants? Why not try yoga?"
Lauren Wheeler-Woodburn estimates that she owns at least 25 pairs of yoga pants.
As a graduate student at the University of Southern California and social-media strategist, she says she wears them mostly every day, for class or to work, or just sitting at home lounging.
"I sound like the yoga pants version of a crazy cat lady," said Ms. Wheeler-Woodburn, who prefers Lululemon but dons other brands too.
The 25-year-old isn't a diligent yogi, though she practices at home sometimes. For her, the clothing isn't an athletic utility but a wardrobe staple. Yoga pants, she said, are easy to clean, don't need to be ironed and, at $90 a pop at Lululemon, are cheaper and more versatile than even her favorite jeans, for which she pays upward of $200 a pair at Nordstrom.
The trend isn't limited to yoga. Organizers of a trade show for traditional outdoor and camping retailers earlier this month debuted a new exhibit devoted to so-called urban wear for "millennials" who wear their boots and flannels with no intention of actually hiking.
For men, retailers are rolling out new versions of jogger pants—sweatpant-like trousers with elastic cuffs at the ankles. Mr. Powell, of SportsOneSource, said they have been a main topic of discussion at apparel trade shows throughout Las Vegas this week.
Tracksmith, an online apparel boutique, made its debut in July offering preppy, $90 men's running shorts.
Not long afterward, a parody site called Running Team JVA mocked Tracksmith's marketing. "Running is free. But it shouldn't be," the site reads.
Matt Taylor, a co-founder of Tracksmith, which sold out its first shipment of inventory, said he thought the site was "pretty clever."
Betabrand, a San Francisco-based apparel startup that crowdsources ideas for items and crowdfunds the production, said the debut of its chino-styled dress-pant yoga pants earlier this year was so successful the company has put ideas for other projects on hold to focus on the athletic apparel business, particularly for women.
The Sports & Fitness Industry Association tracks participation rates across six categories. Over the past five years, participation in individual, racket and team sports fell, and was flat for outdoor, water and fitness sports.
Meanwhile, the size of the U.S. market for workout clothes grew by 5% a year on average, from roughly $54 billion to $68 billion, according to analysts at Barclays.
Athletic apparel manufacturers and retailers are reaping the benefits. Under Armour Inc., UA +0.63% once known largely for its compression gear worn by football players, expects its annual revenue to surge 29% this year to $2.91 billion, fueled by its expansion into women's wear, youth apparel and footwear.
Consumers "have and will continue to want versatility, to look great everywhere, in the gym, on the street, in class," said Henry Stafford, president of Under Armour North America.
Dick's Sporting Goods Inc., DKS +1.88% reeling from downturns in both the golf and hunting categories, said this week it is looking to grow its other businesses, and has already begun allocating more store space to sports apparel, particularly for women and children in time for back-to-school shopping. Bon-Ton Stores Inc. BONT -1.20% in May said it would "outsize the active athleisure category across all zones." Kohl's Corp. KSS +1.97% will begin selling flashy sweatpants by Juicy Couture in the fall as a means "to capture more of the athleisure market."
In March, Andy Mooney, chief executive of surf and snowboard apparel maker Quiksilver Inc., ZQK -2.20% said the company was de-emphasizing the core adventure-sports community and "going back to what we did when we first started, which is, we were both functional and fashionable at the same time."
The changes are even being noted in high-end markets. Online luxury apparel retailer Net-a-Porter in July unveiled a channel devoted exclusively to high-fashion athletic looks, named Net-a-Sporter.
Luxury brand Christian Dior CDI.FR +0.60% unveiled some sneaker-inspired pumps for its fall ready-to-wear line. A glazed rubber pointy-toed upper is paired with a rubber sole inspired by running shoes. The heels are available for preorder in Dior stores starting at $1,450.
The athletic apparel market isn't without its challenges. Lululemon Athletica Inc., LULU +0.03% which has been wrestling for a year-and-a-half with supply chain and quality issues, cut its outlook for the year as it works to clear excess inventory.
Alexandra Medina, of Costa Mesa, Calif., likes to wear yoga pants around town, which is standard fare in her community. She said she likes to work out, but can't always find the time in days that are consumed with running her flooring company and chasing her 19-month-old daughter.
"When you put on your workout apparel," she said, "you think, 'Huh, maybe I should think about working out today.' "