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Two new laboratory studies of the shoes suggest that those claims are credible.The new studies, which were not funded by Nike, find that wearing the shoes makes runners more efficient than wearing other shoes, although probably not for the reasons that many runners might expect. Together, the experiments help to explain what can make one runner swifter than another and also raise interesting questions about whether it is cheating if part of the answer is their shoes.Eliud Kipchoge and fellow runners train in Tiergarten in Berlin in the week leading up to the marathon.Since its introduction about a year ago as an aspect of Nike’s ambitious project to breach the two-hour barrier for the men’s marathon, the neon-salmon hued Vaporfly 4% has become a common sight at major road races, despite its hefty $250 price.They were on the feet of Eliud Kipchoge and Abraham Kiptum when the men set world records this year in, respectively, the marathon and half-marathon.

The shoes, which contain a curved, carbon-fiber plate embedded in a thick layer of a unique, lightweight foam, gained their name and reputation after research, funded by Nike, showed that they improved experienced male runners’ metabolic efficiency by about 4 percent. In other words, the men used less energy to run at a given pace, which should allow them to run faster with less effort.Those results were met with calls from some people to ban the shoes, since the carbon-fiber plate was thought to be acting like a spring, creating extra propulsion and an unfair advantage compared to other shoes.But that original research had not examined just how the shoes improved efficiency or whether the plate was the key element. They also had not included women runners or compared the 4% models to more-specialized racing shoes, such as spiked, speedy racing flats.The two new studies set out to fill those gaps.The men and women had benefited equally from the 4% shoe and their efficiency gains had been unrelated, it seems, to that shoe’s featherweight.

The first, which was published in October in Sports Medicine, involved both male and female runners, all of them experienced competitors. It was paid for by grants at Grand Valley State University. The researchers asked these runners to wear the 4% shoe, a minimalist, spiked track-racing flat (also made by Nike) and a different marathon running shoe (made by Adidas). The runners then strode on a treadmill for five minutes at a time, at speeds ranging from about 5.5 minutes per mile to about 7 minutes per mile, a pace that represented easy running for this group.Throughout, the volunteers wore masks that tracked their oxygen consumption, which is one measure of efficiency.They turned out to be most efficient in the 4% shoe, even when compared to the skinny track spikes and even after the researchers used lead pellets to add weight to the 4% shoe so that its mass equaled that of the other, slightly heavier marathon shoe. This year, the mega-event — which brought in $7.8 million in GMV during its first fete — marked its 10th anniversary.