What: Under Armour (NYSE:UAA) continues to be the fastest runner in the athletic apparel and footwear business, turning in another quarter of huge growth. The company reported fourth-quarter financial results before market open on Jan. 28, and Mr. Market is happy. As of this writing, shares are trading up 18% at 11 a.m. ET.

So what: In the fourth quarter, Under Armour grew revenue by 31%, finishing the year up 28% from 2014. Operating income and net income increased 21% apiece, a slower rate than sales growth as the company continues to invest aggressively in expanding its international presence, product development, and its connected fitness initiative. 

The fastest-growth category continues to be footwear, which saw sales nearly double in the quarter, and the fastest-growing segment is international sales, with a 70% increase in revenue overseas. But even the core apparel business continues to do very well, with revenue growth of 22% in the quarter. 

For the full year, sales increased 28%, led by 57% growth in footwear and 69% international sales growth. Earnings per share and operating income increased 11% and 15% for the full year, coming in with slower growth than net revenue because of the investments in growth mentioned above, as well as acquisitions in the first quarter, and the impact of foreign exchange due to a strong dollar, as international sales become a bigger part of the sales mix.

Now what: Shares of Under Armour are still down from their all-time high reached in late 2015, and were actually down for the past year before today's big jump. However, investing in Under Armour has always come with some risk, because the stock has traded at such a high premium valuation because of its high rate of growth.

But as this quarter demonstrated, the company's performance usually warrants that premium valuation because of its execution on management's strategy for aggressive growth. Simply put, Under Armour's combination of growing brand recognition, its association with a number of popular world-class athletes in multiple sports, and its focus on high-performance products, continues to produce stellar results quarter after quarter. 

Looking for more in-depth coverage of Under Armour's earnings? Stay tuned here for our full earnings review later today.