New York (CNN Business) Under Armour stock fell sharply Tuesday as competition grows over sneakers and athletic clothing in the United States.

Despite a strong US economy and upbeat consumers , Under Armour's sales in North America declined 3% during its most recent quarter compared with a year ago, the company reported Tuesday.

Under Armour's business with outside retailers and its direct sales through its own website and stores both fell last quarter in its home market. Under Armour also lowered its forecast for the region, signaling that the brand expects struggles to persist.

Under Armour's UA Heading into Tuesday,stock had gained more than 50% this year. But investors sold off the stock after Under Armour missed Wall Street's expectations and cut its forecast for North American sales. Shares sank around 15% Tuesday morning.

Under Armour's international business is strong. International sales grew 12% last quarter, buoyed by growth in Asia and Europe. However, investors focused on weakness in North America, Under Armour's most important area. North America accounts for nearly three-quarters of the company's revenue.

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