Fastest-Growing Companies rank: No. 51
CEO: Kevin Plank
The business: Athletic apparel and gear, including shoes, equipment, and accessories
While Nike (NKE) still sets the bar for legendary endorsements — the Swoosh claims Tiger Woods, LeBron James, and, of course, Michael Jordan — Under Armour has spent the past three years stockpiling an impressive roster of its own, signing MLB stars Buster Posey and Bryce Harper, NFL quarterbacks Cam Newton and Tom Brady, and two of the most godlike Olympians, Michael Phelps and Lindsey Vonn. The company is shelling out big bucks for these stars: Total contractual obligations for 2013 were nearly $58 million, more than double the $26 million committed in 2009. Under Armour (UA) isn’t just turning heads with bigtime endorsement deals — it’s also demonstrating some outside-the-box product innovation. The company’s new Speedform shoe, which has no insole and minimal stitching, is produced in a Chinese bra factory. It’s fitting that the new shoe falls under the company’s fastest-growing product division; Under Armour’s footwear sales totaled $239 million in 2012, an increase of more than 31% over 2011.
This story is from the July 1, 2013 issue of Fortune.