Nike's stock falls behind rivals amid Rio Olympics

San Francisco: Nike has top billing among sports brands at the Rio Olympics, but a week into the Games, its stock is losing to Adidas and Under Armour UA.N.

The quadrennial global sports event exposes apparel brands worn by top athletes to hundreds of millions of viewers, but growing competition and higher costs may be reducing the benefits of major endorsement deals.

Since the Summer Olympics started last week, shares of official games sponsor Nike NKE.N had gained 1.0 percent as of Friday. But rival stocks have dashed ahead, with Under Armour up almost 3 percent and Adidas ADSGn.DE nearly 6 percent, after a strong quarterly report.

Neither Adidas nor Under Armour are official Olympics sponsors. But Under Armour sponsors about 250 athletes, including U.S. swimming legend Michael Phelps. Adidas supplies uniforms to teams including Britain and Germany, and it is releasing special-edition sneakers lined in bronze, silver and gold.

Shares of Nike, which also sponsors the U.S. team, had delivered strong Summer Olympics performances until the last two Games.

The stock surged by a range of 4 percent to 19 percent during the six summer Games from 1984 to 2004. But it fell almost 3 percent during the 2008 Olympics in Beijing and the same amount during the 2012 Games in London, according to Bespoke Investment Group.

Under Armour listed its shares in 2005.

"You have increased competition. Under Armour is a public company out there now competing for more sponsorships. It's more expensive for the companies involved," said Bespoke co-founder Paul Hickey.