The latest designs have produced fast times in international races. But they have also spurred yet another debate about the advance of technology and the gray area where innovation meets extremely vague rules about what is considered unfair performance enhancement for the feet.

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The shoes came in the colors of a tropical drink, lime and orange and pink, as if the logo ought to be an umbrella instead of a Nike swoosh. You half expected the insoles to smell of rum and coconut.

If the color scheme suggested frivolity, race results did not. The shoes cushioned the feet of all three medalists in the men’s marathon at the Rio Olympics last summer. Later, in the fall, they were worn by the winners of major marathons in Berlin, Chicago and New York.

The latest shoe designs have produced fast times and impressive results in international races. But they have also spurred yet another debate about the advance of technology and the gray area where innovation meets extremely vague rules about what is considered unfair performance enhancement for the feet.

Where to draw the line of permissible assistance?

Many sports have struggled with the answer. Swimming allowed record-setting, full-body suits, then banned them after the 2008 Beijing Olympics because they gave an unfair advantage in buoyancy and speed. And track and field wrestled with the issue of prosthetic blades worn by the South African sprinter Oscar Pistorius.

The latest issue is shoes. Track’s governing body, the International Association of Athletics Federations (IAFF), said in an email that it had received a number of inquiries about elite runners wearing new designs made by various companies. Its technical committee will meet within two weeks to “see if we need to change or review approvals.”

Bret Schoolmeester, Nike’s senior director of global running footwear, said, “We’re very confident we’re doing things within the rules and above board.”

On Tuesday, Nike unveiled a new shoe, a customized version of the one worn by the marathon winners in Rio and other recent high-profile races, as part of the company’s bold — some say gimmicky — attempt to break two hours in the marathon in early May.

Adidas, whose shoes have been worn by the last four men to set the world marathon record, also recently unveiled a shoe for its own, less publicized attempt to lower the current record from 2 hours 2 minutes 57 seconds to 1:59:59 or faster.

George Hirsch, the chairman of New York Road Runners, which organizes the New York City Marathon and more than 50 other races, said everything from elite races to age-group competitions could be affected by the latest shoe technology. It would be impossible to check the shoes of hundreds or thousands of runners before each race, he said.

“This is a game changer, in the sense that if the shoe companies get patents and these shoes go onto the market, and they’re in wide use, it does make you wonder if it’ll be a level playing field if people can use these advantages,” Hirsch said.

All shoes are considered to enhance performance. Otherwise, everyone would run barefoot. But at what point is the line of inequitable advantage crossed? No one seems to know precisely.

The Nike shoe used by medalists in the Olympics, which will retail in June for $250, is called the Zoom Vaporfly. The shoe to be used for the Breaking2 project, as Nike calls its effort to crack the two-hour mark in marathoning, is a customized version called the Zoom Vaporfly Elite, which the company refers to as a “concept car” model.

Three East African marathon runners sponsored by Nike, including the 2016 Olympic champion Eliud Kipchoge of Kenya, will attempt to break two hours on a Formula One racetrack outside Monza, Italy. And Nike has said the attempt will not meet all of the requirements necessary for a certifiable record.

Some critics have accused Nike of staging a publicity stunt, or a marketing campaign, instead of a credible sporting event.

The runners will wear shoes that have been individually tuned, as if they were violins. The question is whether the shoe model used in the Olympics, and in big-city marathons, along with the new version, conforms to the footwear standards of the IAAF, which are imprecise.

The shoes weigh about 6.5 ounces and feature a thick but lightweight midsole that is said to return 13 percent more energy than more conventional foam midsoles. Kipchoge said he likes the cushioning and reduced pain in his legs in recovering from long runs.

Embedded in the length of the midsole is a thin, stiff, carbon-fiber plate that is scooped like a spoon. Imagined another way, it is somewhat curved like a blade. The plate is designed to reduce the amount of oxygen needed to run at a fast pace. It stores and releases energy with each stride and is meant to act as a kind of slingshot, or catapult, to propel runners forward.

Nike says that the carbon-fiber plate saves 4 percent of the energy needed to run at a given speed when compared with another of its popular racing shoes.

If accurate, said Tucker, the South African sports scientist, that is “the equivalent of running downhill at a fairly steep gradient” of 1 to 1.5 percent.

“That’s a massive difference,” he added.

The IAAF finds itself inundated on many fronts, like corruption, doping and the permissible levels of testosterone in female athletes. And it has long appeared ill- equipped to define what should be allowed on the legs and feet of runners.

The IAAF’s Rule 143 now says that shoes “must not be constructed so as to give an athlete any unfair additional assistance, including by the incorporation of any technology which will give the wearer any unfair advantage.”

What constitutes an unfair advantage? It is not explained.

The rule does say that “all types of competition shoes must be approved by the IAAF.” But Nike said that it was unaware of any formal approval process and that shoe companies do not routinely submit their shoes for inspection.

Nike officials said they were working closely with the IAAF on course design and drug testing for the Breaking2 project and would be “sharing” the shoes with the governing body. They also noted that carbon-fiber soles have been used before in the running-shoe industry.

“We’re giving our athletes a benefit within the rules as they’re written,” said Schoolmeester, the Nike executive, adding, “We’re not using any sort of illegal springs or anything like that.”

Tucker, an exercise physiologist at the School of Medicine of the University of the Free State in Bloemfontein, South Africa, said he thought the Nike shoe “probably should be illegal” because it purports to act as a spring. If it were barred, he said, it should be done in conjunction with a rewriting of the IAAF’s vague rules.

Nike’s fastest marathon runner to date, Kenenisa Bekele of Ethiopia, belongs to a competing project to break two hours, organized by Yannis Pitsiladis, a sports scientist in England. At the Berlin Marathon in September, Bekele wore the Zoom Vaporfly and ran the second-fastest marathon ever in 2:03:03. He plans to wear the same model to make a world-record attempt at the London Marathon in April.

This year, Pitsiladis had a CT scan performed on the shoes that Bekele wore in Berlin. That’s when he first noticed what appeared to be a carbon-fiber plate in the midsole.

Because the plate appears to be a springlike device, Pitsiladis said, he expected the shoe to be barred. But as long as it is not, Bekele plans to continue to use it. He said through Pitsiladis that he liked the cushioning and the fact that his calf muscles did not get sore on long runs.

“Will he be allowed to use it” in London, asked Pitsiladis, a professor of exercise science at the University of Brighton in England. “Or, after the race, will someone tell me the world record is no longer valid because you used a banned shoe?”

Jos Hermens, a former Dutch long-distance runner whose management company represents Kipchoge, the Olympic champion, Bekele and other top marathon runners, said he would be “surprised and disappointed” if the latest model were barred.

The sport should continue to welcome technological advances, he said, just as it did when tracks upgraded from cinder to synthetic rubber, pole vault poles evolved from bamboo to fiberglass and shoes began to incorporate air bladders and gels for cushioning.

“We’re not living in medieval times,” Hermens said. “There are going to be new techniques and materials. It’s time to show something to go forward instead of tipping backward.”