'It feels like running on trampolines' - Kipchoge & Kosgei's marathon trainers
It was a landmark weekend for Kenyan marathon runners.
Both Kipchoge and Kosgei wore bespoke versions of trainers designed to improve running economy by 4%.
It has been reported a group of athletes have complained to the IAAF about the trainers, and the sport's governing body said this week it had set up a "a working group to consider the issues".
So what's the story?
The science behind the shoe
The idea of modifying equipment to improve performance is obviously nothing new, with the concept of 'marginal gains' bringing that into sharper focus.
In this particular case, the Nike Vaporfly has a carbon-fibre plate fitted in its chunky foam sole which supposedly helps propel runners forward.
Dr Thomas Allen - senior lecturer in sports engineering at Manchester Metropolitan University - told BBC Sport: "You can't radically change a shoe, but reducing the mass of it might give you a 1% gain.
"Adding more cushioning to the midsole might give you another 1% gain. And if you introduce a stiff plate to the midsole, does that give you another 1% gain?
"This technology isn't new - a university in Canada is considered to be the first to have come up with, and test, the idea of putting a stiff plate in the midsole of a running shoe.
"Nike appear to have gone a step further, as looking at one of their recent patents they seem to have put three stiff plates in the forefoot of the midsole.
"The idea behind those is running efficiency improves."
Sports scientist Professor Ross Tucker says the improved performance is down to a reduction in energy loss.
"Think of a rubber ball bouncing rather than a squash ball," he wrote on Twitter.
"You could previously run a speed of X, using a certain amount of energy. If you can use less energy, you can increase your speed before you hit that energy ceiling.
"Companies have been trying for decades to do this, but failed for various technical reasons.
"This new foam, plus innovation, overcame the barrier. Other companies are now trying to engineer the same."
'It feels like running on trampolines'
Kosgei's historic run in Chicago means the men's and women's marathon and half-marathon world records are now held by athletes who wore the same footwear.
And the top-10 finishers in the men's race in Chicago all wore Vaporflys. One - American Jake Riley - has said the trainers make it feel like he is "running on trampolines".
Dr Allen says it is difficult to determine whether the shoes give athletes an advantage over runners of a similar ability who are not wearing them.
"Manufacturers work on a project five to 10 years ahead of time and typically claim all sorts of things about their products," Dr Allen said.
"There are also question marks around how the shoes degrade, which happens from the moment they are worn.
"Lab tests are not the perfect environment - they are controlled and simplified so they don't mimic the conditions out on the road."
Trying to find the right balance
In a 2017 study published in Sports Medicine, every one of 18 runners tested at the University of Colorado had better running economy in the Vaporflys - the forerunner to the new model - than wearing two other types of popular racing shoes. The average improvement was about 4%.
A 2018 follow-up by the New York Times delivered a similar conclusion. With a more extensive sample - 500,000 marathon and half-marathon runners - it recorded a six-minute improvement for a three-hour marathon runner, and about an eight-minute improvement for a four-hour finisher.
Current rules state "shoes must not be constructed so as to give athletes any unfair assistance or advantage - and any type of shoe used must be reasonably available to all in the spirit of the universality of athletics".
In its statement on Tuesday, the IAAF said: "It is clear that some forms of technology would provide an athlete with assistance that runs contrary to the values of the sport.
"The challenge for the IAAF is to find the right balance in the technical rules between encouraging the development and use of new technologies in athletics and the preservation of the fundamental characteristics of the sport: accessibility, universality and fairness."