Under Armour tumbles over lowest sales growth in six years

US sportswear brand Under Armour tumbled 16.2% to $31.77 in pre-market trading on Tuesday after a disappointing third quarter earnings report in which the company saw its weakest sales growth in the last six years.

The second-largest sportswear group in the country had said in July that its third quarter was likely to see a slowdown in sales, mainly due to the bankruptcy of retailer Sports Authority.

Net sales in North America actually grew 15.6% in the quarter to September 30, but that was well below the 20% that the company has been used to breaking.

"While we expect to continue to significantly outpace the apparel industry, the growth rate going forward will be less than expected from our Investor Day in 2015," the company said during an earnings call on Tuesday morning.

Overall, profit for the quarter was $128.2m, or 29 cents per share, up from $100.5m, or 23 cents from the same quarter last year.

Investors appear to have been put off by Under Armour's plans to invest heavily in order to further expand its market share, as CEO Kevin Plank told analysts on Tuesday.