Kevin Plank's improbable hit -- a sweat-wicking undershirt -- kicked off the fastest-growing category of sportswear. Now Nike and other megabrands are in hot pursuit of Under Armour. Welcome to the disrupter's dilemma.
Like many upstarts, Under Armour made a splash with a breakthrough product, then promptly found itself facing Goliaths. Here's how an un-likely giant killer is protecting its house from megabrands like Nike.