A poster hangs over the desk of Kevin Plank, CEO of Under Armour, the red-hot athletic-apparel brand that has joined Nike, Adidas and New Balance as a major player on the market. Under Armour pitchman Eric Ogbogu, a former NFL lineman, is flexing his impressive pecs; underneath him, the tagline reads protect this house.
That slogan has been at the center of the company's marketing campaign, and Under Armour has an equally muscular business: over the past five years, the company's comfy, moisture-wicking shirts and shorts have helped it grow at a blistering 65% annual rate. Under Armour, which had $640...