Under Armour Inc (UA.N), the No. 2 U.S. sportswear maker, on Thursday reported a better-than-expected 31 percent jump in revenue, allaying concerns that apparel sales were slowing, sending its stock up as much as 21 percent.

The company's shares were up 19.3 percent at $81.84 in heavy trading after it also forecast full-year 2016 revenue above Wall Street estimates.

Analysts and investors have been spooked by results from department stores and sporting goods retailers, such as Macy's Inc (M.N) and Dick's Sporting Goods Inc (DKS.N), who had warned of poor sales during the holiday quarter.

Dick's Sporting Goods, Under Armour's biggest customer, cut its same-store sales expectations for the holidays in November, saying it was seeing lower demand for apparel and footwear.

But Under Armour's sales of apparel rose 22.2 percent to $864.8 million in the fourth quarter, driven by demand for training, running, golf and basketball clothing. Apparel accounts for nearly three quarters of the company's total revenue.

"The growth last quarter affirms that the momentum continues for them. Even apparel continues to be very strong, which people had thought would slow by now, but it hasn't," FBR Capital Markets analyst Susan Anderson said.

Under Armour has been quick to cash in on the new trend of "athleisure," a mash-up of athletic and casual clothing growing popular even in formal settings like offices, which has helped it maintain a revenue growth momentum of more than 20 percent for 23 straight quarters.

Sales have also been boosted by the company's sponsorship deals with popular sports personalities such as basketball star Stephen Curry and golfer Jordan Spieth, and English Premier League club Tottenham Hotspur.

Overall, sales rose 31 percent to $1.17 billion, beating analysts' average estimate of $1.12 billion.

"If you look at the numbers versus Nike Inc (NKE.N), they're still very, very tiny, so I think we can continue to see this growth over the next 3-5 years," Anderson said.

Another bright spot was footwear sales, which nearly doubled to $166.9 million, driven by new products in running and demand for NBA-star Curry's signature basketball shoe line.

The company forecast full-year 2016 sales of $4.95 billion, topping analysts' average expectations of $4.91 billion.

Net income rose 20.4 percent to $105.6 million, or 48 cents per share, in the quarter ended Dec. 31, beating the average estimate by 2 cents.