The Race To Build The World’s Fastest Shoe
The big brands are ramping up their research, but how much of a boost can a shoe offer? And which one is right for you?
We get to the heart and sole of an increasingly controversial issue in our sport
NIKE DIDN’T TELL RODGER KRAM AND WOUTER Hoogkamer much about the prototype shoe, at first. ‘We knew it had a new foam, called Pebax and they showed us the carbon [fibre] plate,’ recalls Kram. The exercise physiologist and longtime director of the University of Colorado’s Locomotion Lab, and Hoogkamer, then a postdoctoral researcher there, were writing a journal paper, published in March 2017, detailing how an elite runner might break the two-hour marathon barrier. One factor they detailed was shoe design, and the two had been given what would become the Nike ZoomX Vaporfly 4% to test.
Explaining how shoe design might help break two hours was tricky, says Kram, because while they had already completed testing on the new shoe, they couldn’t include their non-peer-reviewed data in a peer-reviewed journal. So they focused on the weight of the foam midsole, which, based on previous studies, they estimated could improve an athlete’s running economy by around one per cent.
‘The world record at that time [2:02:57, set by Dennis Kimetto in 2014] was in the Adidas Boost shoe,’ says Hoogkamer, now a professor at the University of Massachusetts. But, he adds, ‘We didn’t say you can make the foam better; we said, “Boost is pretty heavy, so maybe you can make it lighter.”’
As for the Vaporfly testing itself, Kram and Hoogkamer approached it with one simple question: was it faster?
In May of the following year, Nike provided a teaser of an answer with their Breaking2 event, where Eliud Kipchoge came within 25 seconds of the two-hour mark on the Formula One Autodromo Nationale track in Monza, Italy. Even though it missed the mark, it was a striking improvement over Kimetto’s record and the running community debated what was responsible for the two per cent time drop – the deadflat track, the pacers, the car-mounted clock that also functioned as a draft vehicle, or the prototype shoe Kipchoge wore.
Hoogkamer and Kram’s next study, published that autumn with the innocuous title A Comparison of the Energetic Cost of Running in Marathon Racing Shoes, offered a more detailed assessment. The •
prototype shoe, compared with two top existing models – Nike’s Zoom Streak 6 and the Adidas Adizero Adios Boost 2 that Kimetto used in 2014 – reduced the metabolic cost of running by four per cent. That energy saving, Hoogkamer and Kram calculated, ‘should translate to 3.4 per cent improvement in running velocity at marathon world-record pace (20.59km/h).’ What’s more, because they normalised shoe weight across all three test samples, the benefit they found wasn’t from lighter weight.
The study was instantly controversial. It was funded by Nike, for starters (Kram is also a paid consultant) and two Nike employees, Geng Luo and Emily Farina, were listed as co-authors. Subsequent studies not associated with Nike have validated the results, although sample sizes have all been small and the resulting effect varied.
Immediately, people questioned the role of the carbon fibre plate. ‘Any device inserted into the shoe, and which purports to add to energy return…should be banned,’ wrote exercise physiologist Ross Tucker on his Science of Sport blog. Tucker maintains that the plate is essentially a spring, pointing out that this is how Nike refers to a carbon plate in a previous patent. It was recently reported that World Athletics will not ban the shoe but there will be a ban on any new shoe technology until after this year’s Tokyo Olympics.
Lost in the clamour over the benefit of the carbon plate was the role of the foam itself. In both their initial study and an early 2018 follow-up, focussing on the biomechanics involved, Hoogkamer and Kram pointed to one major contributor to the improvements in running economy: ‘For now, the elastic properties of the Nike Prototype shoes provide the best explanation for the metabolic energy savings.’ In plain English: the new foam was noticeably springier, which made runners faster.
This foam, which Nike branded ZoomX, helped usher in a new era in shoe design and materials, especially midsole foam. Although the race for a breakthrough foam started in earnest in 2013 with Adidas’s first Boost-equipped shoe, Nike’s bold performance claims around the Vaporfly 4% and NEXT%, right down to the names, have changed the conversation. ‘When we started doing our outsoles, our focus was reinventing cushioning,’ says Olivier Bernhard, cofounder of On Running. ‘Everyone you talk to now, it’s about durability and resilience, rebound.’
As shoemakers continue to try to improve performance, and research accumulates showing a modest but undeniable benefit to next-generation materials, the foam wars have only become more heated.
BREAKING THE MOULD
Recreational runner Andy Jacques-Mayne is well aware of the shift in conversation. Jacques-Mayne is a former pro cyclist, a sport that prizes attention to technical detail and the marginal-gains ethos of seeking even the smallest improvements in efficiency. On paper, he’s a classic Vaporfly buyer, noting that ‘from a performance standpoint, buying shoes that supposedly offer free speed sounds great’, although that’s far from his only consideration. But for Jacques-Mayne, like a lot of runners, foam technology is an afterthought.
Going back almost 50 years, the primary technology in running shoes, midsole foam, has hardly changed. From Bill Bowerman’s original Nike Cortez to most of today’s shoes, the material of choice is EVA, or ethylene vinyl acetate. Only recently has that started to change.
EVA makes an ideal midsole: in foam form, it’s exceptionally light and provides great cushioning. It’s also cheap and easy to make into a foam. And with decades of experience, shoe companies and the foam manufacturers who turn the raw EVA into sheets of midsole foam or compressed midsole blanks have learned little tweaks to the formula and manufacturing that offer a seemingly endless amount of variation to how a shoe feels.
But there are drawbacks: EVA’s performance is highly dependent on temperature; in hot weather it tends to feel like a marshmallow, while in cold weather the midsole hardens and loses its cushioning. It has a relatively high compression set, which means it packs out and permanently loses its cushioning quickly. And while it offers good cushioning, it isn’t great at energy return, where a sole is compressed and then rebounds, returning some of the force back to the runner.
While foam suppliers can tweak EVA’s performance, they can’t fully overcome those basic limitations because they’re baked into its chemical composition. EVA is what’s called a random copolymer, says Polymer Solutions founder Dr Jim Rancourt, a materials scientist with 40 years of experience in plastics, and a longtime runner. Ethylene is a hard plastic. Vinyl acetate is basically glue. Combine them, and you can get a solid-but-spongy product tuned by the ratio of one to the other. But that’s about all you can control.
‘The analogy I use is Lego blocks,’ says Rancourt. Say ethylene [the hard component] is blue blocks and vinyl acetate [the soft component] is yellow. A shoemaker can ask a foam supplier to adjust the amount of each in the blend, but unlike other materials, EVA’s blue and yellow blocks will connect in a random pattern. That basic ratio is a crude way to control properties such as softness. Another method is to vary the density of the foam by putting less or more into a mould, but less dense foams generally lose their rebound characteristics more quickly. •