Shares of Under Armour (UA -4.30%) started the day almost 5% higher after the apparel company reported better-than-expected earnings for the first quarter of the year, though they were sold off throughout the day, leaving the stock slightly below breakeven as of 2 p.m. EDT.

Under Armour's top-line performance was particularly impressive. For the three months ended March 31, it notched sales of $472 million. That equated to a 23% increase over the same time period last year, during which it sold $384 million worth of product. The improvement, according to the company's earnings release, was largely attributable to the "introduction of new Baselayer product and strong sales of Fleece."

On the bottom line, Under Armour earned $8 million, or $0.07 per share. While this was 47% less than the prior-year period, the decline was the result of the "planned timing of marketing expenditures." This was reflected in analyst estimates, which called for earnings of $0.03 per share.

In a prepared statement, the company's founder and chief executive officer, Kevin Plank, remarked:

In the first quarter, we drove growth in excess of 20% for the 12th consecutive quarter in total revenues and the 14th consecutive quarter in apparel revenues. This growth is the direct result of our enhanced design and innovation, including new and improved HeatGear Sonic Baselayer and the attention-grabbing UA Alter Ego line, featuring iconic superheroes such as Batman and Superman. Our Youth product is stronger than ever and we continue to see traction with our expanded Women's lines in Studio and ArmourBra. Momentum is also evident in Footwear with solid sell through of our latest product in the running platform, Spine Venom.

Earlier this year, The Motley Fool picked Under Armour as one of America's best-run companies -- click here for the article announcing its selection. With sales growth like this, it's easy to see why.