By Gayathree Ganesan and Siddharth Cavale

(Reuters) - Under Armour Inc's <UA.N> <UAA.N> revenue increased at breakneck speed for more than six years, averaging a quarterly growth rate of 20 percent, as shoppers couldn't get enough of their Stephen Curry basketball gear and Bandit running shoes.

Then, the company shocked Wall Street on Tuesday with a big drop in holiday-quarter sales growth and issued a glum forecast for the year, as it grapples with excess inventory amid a glut in the broader athleisure market.

Now, analysts say, a roughly 15 percent growth rate could be the new normal for Under Armour.

"This is the first time since Under Armour has been a public company where sales guidance clearly missed the consensus and a sign it is no longer able to find enough growth in new channels to offset weakness in its core business," Morgan Stanley analyst Jay Sole wrote in a note dated Jan 31.

Under Armour ended the year with inventories that were 17 percent higher the year before and analysts said the company's margins would be hit through 2017 as it discounts to get rid of old stock.

Inventory problems aside, Under Armour is contending with a host of other issues.

While the biggest concern is that the company lacks a true fashion line, Under Armour is also struggling with intensifying competition from Nike Inc <NKE.N> and Adidas AG <ADGn.DE>, which have stepped up discounts.

The company's Class A shares have lost more than half their market value over the past 12 quarters, including a 23.4 percent decline on Tuesday.

The Class A stock was flat at $21.47 on Wednesday.

Under Armour's problems have been compounded in an industry already reeling from the bankruptcies of big footwear retail chains The Sports Authority and Sport Chalet, which have disrupted distribution channels.

To be sure, the Under Armour brand is still a powerful one.

It was the No. 2 sportswear brand in the United States for two years, before being recently displaced by Adidas, according to market research firm NPD.

Under Armour has also successfully expanded into new categories including sportwear for kids and outdoor products, and signed deals with Major League Baseball, high profile athletes, and professional teams.

NOT TOO COOL FOR SCHOOL

Under Armour focuses on selling shoes and apparel that are solely meant for improving sports performance rather than as a fashion accessory, while Adidas and Nike have much broader ranges of fashion offerings.

But in the United States, three-quarters of all sports shoes are not used for sports, according to market research firm NPD.

For example, Under Armour's Curry shoes are made for on-court performances, unlike Nike's Air Jordans that have become the stuff of fashion lore.

Under Armour Chief Executive Kevin Plank himself admitted on the company's earnings call on Tuesday that it needed to become more fashionable at a time when consumers were spoilt for choice.

"Under Armour is great on the field but is yet to be too-cool-for-school," Susquehanna Financial analyst Sam Poser said.

The company has said it will invest aggressively in its premium Under Armour Sportswear business, under which it sells button down shirts, bomber jackets and parkas at high-end stores such as Barneys.

It is also looking to expand its top brands to more retailers, much like its new deal with department store operator Kohl's Corp <KSS.N> to sell its products in more than 1,100 stores.

Analysts, however, don't expect its fledgling fashion business to take off anytime soon.

"They are going to be moving gradually towards more fashion products ... and I expect we'll see the ramp up a little bit and maybe see some less dependence on the basics," NPD analyst Matt Powell said.

"But I don't expect a dramatic shift to their high-fashion overnight."

(Reporting by Gayathree Ganesan and Siddharth Cavale in Bengaluru; Editing by Sayantani Ghosh)