BEIJING—Two athletes in Sunday’s marathon will have an extra spring in their step—quite literally.

Angola’s Joao N’Tyamba and Ecuador’s Franklin Tenorio have both opted to race in the controversial Spira Stinger shoe that has two stainless steel springs bedded in its sole.

US track and field authorities have banned the distinctive yellow footwear, saying it gives athletes an unfair advantage, while the international athletics federation has yet to rule on the issue, opening the way for the Spira’s debut at an Olympics.

The shoe’s creator says other contesters could still protest about its use and criticized the continuing confusion.

“My shoe certainly provides no more of an advantage than the Speedo swim suit and no one is saying Michael Phelps should be barred,” Andy Krafsur, CEO of Spira Footwear, told Reuters in a telephone interview from his home base in El Paso, Texas.

Phelps and countless other swimmers broke a slew of world records in Beijing swimming in the space-age Speedo garb.

Wearing the right shoes is vital in a marathon.

They have to be tough enough to get the runner round the 26.22-mile (42.195-km) course, gentle enough to protect the hard pressed feet and light enough to keep tiredness at bay.

Other racers are also opting for unusual footwear, including the fancied US athlete Ryan Hall, who will be using innovative Asics shoes which have rice husks meshed into the sole to absorb water and improve traction.