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.
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 have risen just 1% as of Friday.
