Anyone who saw Eliud Kipchoge of Kenya break the two-hour marathon barrier in October very likely saw something else, too: The thick-soled Nike running shoes on his feet, and, in a blaze of pink, on the feet of the pacers surrounding him.
These kinds of shoes from Nike — which feature carbon plates and springy midsole foam — have become an explosive issue among runners, as professional and amateur racers alike debate whether the shoes save so much energy that they amount to an unfair advantage.
A new analysis by The New York Times, an update of the one conducted last summer, suggests that the advantage these shoes bestow is real — and larger than previously estimated. At the moment, they appear to be among only a handful of popular shoes that matter at all for race performance, and the gap between them and the next-fastest popular shoe has only widened.
We found that a runner wearing the most popular versions of these shoes available to the public — the Zoom Vaporfly 4 per cent or ZoomX Vaporfly Next% — ran 4 to 5 per cent faster than a runner wearing an average shoe, and 2 to 3 per cent faster than runners in the next-fastest popular shoe. (There was no meaningful difference between the Vaporfly and Next% shoes when we measured their effects separately. We have combined them in our estimates.)
This difference is not explained by faster runners choosing to wear the shoes, by runners choosing to wear them in easier races or by runners switching to the shoes after running more training miles. In a race between two marathoners of the same ability, a runner wearing these shoes would have a significant advantage over a competitor not wearing them.
The shoes, which retail for $250, confer an advantage on all kinds of runners: Men and women, fast runners and slower ones, hobbyists and frequent racers. Many other brands, including Brooks, Saucony, New Balance, Hoka One One and Asics, have introduced similar shoes to the market or plan to. These shoes may provide the same advantage or an even larger one, but most do not yet appear in sufficient numbers in our data to measure their effectiveness.
These findings are based on more than twice the data that produced our previous estimates. Our data now includes results from more than a million marathons and half marathons combined since 2014.
More important, nearly all of these new results take place in a world in which runners had begun to wear these shoes in large numbers. In this period, an overwhelming share of runners — particularly faster ones — reported switching to these shoes, presumably looking for any edge they could get on race day.
Consider that in the final months of 2019, about 41 per cent of marathons under three hours were reported to have been run in these shoes (for races in which we have data). What makes these shoes different is, among other things, a carbon-fibre plate in the midsole, which stores and releases energy with each stride and is meant to act as a kind of slingshot, or catapult, to propel runners. The shoes also feature midsole foam that researchers say contributes to increased running economy.
Whether the shoes violate rules from track’s governing body, World Athletics, depends on how one interprets this sentence from its rulebook: “Shoes must not be constructed so as to give athletes any unfair assistance or advantage.” It does not specify what such an advantage might be.
“We need evidence to say that something is wrong with a shoe,” a spokesman for the governing body, then called the IAAF, told The Times last year. “We’ve never had anyone to bring some evidence to convince us.”
In an announcement earlier this year, the group said, “It is clear that some forms of technology would provide an athlete with assistance that runs contrary to the values of the sport.” It has since appointed a technical committee to study the shoe question, and to make a report with recommendations. (The report was originally intended to be released to the public by the end of the year; it will now reportedly be released in 2020.)
When we asked Nike last year about whether its shoes might violate IAAF rules, a spokesman said the shoe “meets all IAAF product requirements and does not require any special inspection or approval.”
On Thursday, the company said in a statement, “We respect the IAAF and the spirit of their rules, and we do not create any running shoes that return more energy than the runner expends.”