Although we don't believe in timing the market or panicking over market movements, we do like to keep an eye on big changes -- just in case they're material to our investing thesis.

What: Shares of performance athletic apparel maker Under Armour (NYSE: UA) dashed up as much as 11% today, fueled by a solid fourth-quarter earnings report.

So what: Fourth-quarter revenue was up 36% from last year while diluted earnings per share jumped 47% from the prior year. It's hard to argue with that. Even after excluding a $0.04 EPS bump from a tax benefit, the company's bottom line still easily topped the $0.37 that analysts were looking for. And as if that wasn't enough, the company also raised its full-year outlook for 2011. It now sees revenue growth of 25% to 27% for the year.

Now what: If there's anything to potentially quibble with, it's Under Armour's stock price, which, after doubling over the past year, now trades at 38 times expected 2011 earnings per share. Of course a high earnings multiple has been a pretty consistent presence throughout Under Armour's history as a public company and -- recession aside -- its blazing growth has thus far kept the stock chugging along. The company is now hoping that the introduction of its Charged Cotton T-Shirt -- its first product in the large market for cotton athletic apparel -- will help keep the growth engine's furnace well-stoked.

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Under Armour is a Motley Fool Rule Breakers pick. Under Armour is a Motley Fool Hidden Gems choice. The Fool owns shares of Under Armour. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors.

Fool contributor Matt Koppenheffer does not own shares of any of the companies mentioned. You can check out what Matt is keeping an eye on by visiting his CAPS portfolio, or you can follow Matt on Twitter @KoppTheFool or on his RSS feed. The Fool's disclosure policy prefers dividends over a sharp stick in the eye.