A variant of the shoe that powered the first ever sub-two-hour marathon could be yours later this year. The Air Zoom Alphafly NEXT%, a version of the prototype Eliud Kipchoge wore when he ran a marathon in 1:59:40 in Vienna last October, will be released in summer 2020, according to Nike.
With a thick foam sole and added cushioning under the forefoot, the shoe looks like it has been pulled out of a sci-fi film. Running purists say the technology could harm the integrity of athletics, while Nike calls it a sign of "game-changing progress."
Long distance times have tumbled since Nike's Vaporfly range was introduced in 2016, including Brigid Kosgei's 2:14:04 marathon in Chicago last year, knocking over a minute off Paula Radcliffe's previous record that had stood for 16 years.
In a review of a range of leading running shoes, World Athletics recently approved Nike's Vaporfly range, but banned any shoes with soles thicker than 40mm or with more than one plate to enhance their spring.
From April 30, it will also ban from competition any shoes that have not been on public sale for four months.
The new rules mean the prototype Eliud Kipchoge wore in Vienna are now non-compliant, but the latest Alphafly shoes -- which are believed to have a sole that is 39.5mm thick -- are passable.
"For runners, records like the four-minute mile and two-hour marathon are barometers of progress," said Tony Bignell, VP of Footwear Innovation at Nike.
"These are barriers that have tested human potential. When someone like Eliud breaks them, our collective belief about what's possible changes."
According to a 2017 study in the journal Sports Medicine, Nike's Vaporfly shoes provide a 4% boost in running economy -- the amount of work a runner must do at a given speed -- compared with other top racing models.
Nike says the carbon fiber plate offers a sensation of propulsion, while the cushioning minimizes energy loss.
Leading sports scientist Ross Tucker has argued that the latest advancement in shoe technology is damaging for running.
"Is Kipchoge an outlier of immense athletic potential? Or is he a simply a very good runner who is benefiting from the immense improvements that his shoes provide? Perhaps both," Tucker told CNN last year.
"But the point is we don't know with absolute certainty. Running, especially marathon running, is supposed to be the purest thing humans put themselves through. It's just about feet, legs, lungs, heart and brain. These shoes create the same problems that doping throws up."
Kipchoge likened running a sub-two hour marathon to Neil Armstrong's historic moon landing in 1969 and the Kenyan sees nothing wrong with the advancement in shoe design.
"They are fair," he told the Telegraph newspaper. "I trained hard. Technology is growing and we can't deny it -- we must go with technology."
World Athletics president Sebastian Coe said it was not his organization's place to "regulate the entire sports shoe market," but added it should "preserve the integrity of elite competition."
The price of Nike's new shoe nor its exact release date has yet to be confirmed.