The Race to Build the World’s Fastest Running Shoe – Runner’s World

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 [fiber] plate,” recalls Kram. The exercise physiologist and longtime director of the University of Colorado Boulder’s Locomotion Lab, and Hoogkamer, then a post-doctoral researcher there, were writing a journal paper, published in March 2017, detailing how an elite runner might break the mythical two-hour marathon barrier. One factor they detailed was shoe design, and the two had been given what would become the 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 percent.

“The world record at that time [2:02:57 by Kenyan Dennis Kimetto in 2014] was in the Adidas Boost shoe,” says Hoogkamer, now a professor at the University of Massachusetts Amherst. However, 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 Breaking2, where Eliud Kipchoge of Kenya came within 25 seconds of the two-hour mark on the Formula One Autodromo Nationale track in Monza, Italy. Even unsuccessful, it was a striking improvement over Kimetto’s record, and running fans debated what was responsible for the two percent time drop—–the dead-flat track, the pacers, the massive car-mounted clock that also functioned as a draft vehicle, or the prototype shoe Kipchoge wore.

Hoogkamer and Kram’s next study, published that fall with the innocuous title “A Comparison of the Energetic Cost of Running in Marathon Racing Shoes,” offered a more definitive 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 percent. That savings, Hoogkamer and Kram calculated, “should translate to ~3.4 percent improvement in running velocity at marathon world record pace (20.59 km/h).” What’s more, because they normalized shoe weight across all three test samples, the benefit they found wasn’t from lighter weight.

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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 coauthors. 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 fiber 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.

Lost in the clamor 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 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 nowadays, 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 gotten more heated.

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 prototypical 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.

For 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. Crucially, it’s also cheap, and it’s 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 in stride.

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 temps 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 Jim Rancourt, Ph.D., a materials scientist with 40 years of experience in plastics, and a longtime runner. Ethylene is a hard plastic used in products like milk jugs. Vinyl acetate is basically Elmer’s glue. Combine them, and you can get a solid-but-spongy product tuned by the ratio of each monomer. But that’s about all you can control.

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“The analogy I use is Lego blocks,” Rancourt says. 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 like softness. Another method is to vary the density of the foam by putting less or more into a mold, but less dense foams generally lose their rebound characteristics more quickly.

The final option is to blend EVA with other materials, or just use something different altogether. Shoemakers have for years tried to find alternatives, all the way back to Nike’s original Air Tailwind and its air cavity in 1978. In the 1980s, says Spencer White, vice president of human performance for Saucony, that company experimented with a material from DuPont called Hytrel. “It was just as bouncy and lively as the foams we’re working with now, but we never figured out how to make it inexpensive enough to make a whole midsole out of it,” he says.

By 2007, the German chemical company BASF had found a way to puff, or expand, thermoplastic polyurethane (TPU) into little spongy pellets that sprang back quickly when compressed. They called the product Infinergy but struggled to identify an application for it—that is, until they figured out how to fuse the pellets into a single layer of foam, and signed an exclusive footwear license to Adidas to use it as Boost, which debuted in 2013.

But perhaps nothing has been as revolutionary as a plastic developed by the multinational chemical company Arkema, called Pebax.

Like a lot of plastics, Pebax has been around for decades. In solid form, it adds flexibility to rigid ski boot shells, and it’s used in heart catheters. But the foam form wasn’t patented until 2004, and Nike and Reebok were the first to adapt it for running shoes.

Pebax is a brand-name play on the generic name: polyether block amide (PEBA). Like TPU, it’s what’s called a block copolymer, a definition that’s essential to its advantages. To go back to Rancourt’s Lego analogy, Pebax is also made of blocks. But instead of being randomly arranged, Arkema can actually join chains of blocks of a single color, and vary the length of those chains, alternating hard and soft sections precisely to control the amount of cushioning and rebound.

So as a product, Pebax isn’t one specific thing, says Kevin Hanrahan, chief marketing officer for Arkema’s technical polymers unit. “It’s a range from very rigid to very soft, and you can get the energy-return properties of a polyether without compromising softness.” That’s also technically the case for TPU or any other block copolymer, says Rancourt. But there are certain advantages to some over others. Weight, for instance: Arkema says Pebax is as much as 20 percent lighter than TPU-based foams like Boost. TPU, in turn, has the advantage of long life, since it has a lower compression set than Pebax or EVA. And both TPU and Pebax offer consistent performance in a wide temperature range. But maybe the signature advantage among these foams is Pebax’s rebound, or energy return.

“Energy return” is a bit of a misnomer; it’s more like “less energy lost.” Any foam midsole, when compressed under load, will store a certain amount of energy on a footstrike that rebounds back when the runner pushes off. Shoes with midsoles based on EVA foams have a range of energy return depending on how they’re formulated, but generally, they top out around 65 to 70 percent. TPUs like Boost are better, at 70 to 76 percent. But the two best shoes Runner’s World has ever tested in terms of energy return are the Vaporfly 4% and 4% Flyknit, which offer as much as 82 percent return in deflection and rebound testing. (Kram and Hoogkamer measure it even higher, at 87 percent.)

Why does energy return matter? Iain Hunter, Ph.D., a professor of exercise science at Brigham Young University, found a clue that he wrote about in a June 2019 study in the Journal of Sports Sciences, where he attempted to replicate Hoogkamer and Kram’s 2018 metabolic cost comparison study. Both studies found runners had a longer stride in the Vaporfly than other shoes, which makes sense from an increased efficiency perspective: The farther you travel per stride, the faster you’ll go at a given cadence.But Hunter also measured a few parameters that the CU study didn’t, like vertical oscillation—essentially how high the runner is bouncing with each stride. There, Hunter found that the Vaporfly led runners to bounce higher, which normally would correlate with lower running economy, “unless it’s not the body that’s generating the energy to make that happen,” Hunter says. His theory: The Vaporfly sole’s bounciness returns more of the energy to the runner than other foams, which produces a longer stride for a given amount of ground time.

Hoogkamer also thinks that because Pebax is a softer-feeling material than the TPU-based Boost, there’s more compression to begin with, leading to an overall higher rebound. “The actual joules, or energy units, stored in the compression phase are a lot higher because it’s such a soft shoe,” he says.

The CU study found slightly higher peak forces on impact for the Vaporfly (the opposite of the BYU study), but because the shoe is so soft, Hoogkamer theorizes that it actually feels less harsh to the runner, which enables the runner to take longer strides. That may matter over the course of a season as much as during a single race. At the July 2019 Footwear Biomechanics Conference, Nike research scientist Brett Kirby presented data showing that runners wearing the Vaporfly sustained less muscle damage than a control group wearing other shoes. Reduced muscle soreness suggests they could sustain higher training loads.

Whatever the mechanism, it’s undeniably effective. In addition to the CU and BYU studies, Kyle Barnes, Ph.D., a professor of exercise science at Grand Valley State, found similar running economy changes in a comparison with the Adizero Adios Boost 3. And in a messier but far larger sample size, the New York Times’s Upshot examined roughly half a million marathon times from public data like Strava and concluded that, on average, runners wearing the Vaporfly ran three to four percent faster than similar runners in most other shoes, and one percent faster than the next fastest shoe, Nike’s Zoom Streak. (The authors noted that “the analysis suggests that, in a race between two marathoners of the same ability, a runner wearing Vaporflys would have a real advantage.”)

As a result of all the data, and Nike’s aggressive marketing, shoe companies are racing to develop faster foams and smarter ways to use them. White says Saucony has tested dozens of foams over the years, and almost used Boost before Adidas snapped up the exclusive. On Running has its CloudTec system and new foam formulations like Helion. Reebok uses Pebax in its Floatride midsole. Under Armour uses a TPU foam called Infuse in its Hovr line. Skechers’s Hyper Burst foam is a CO₂-infused EVA, and the DNA AMP midsole in the Brooks Levitate is another polyurethane foam from BASF called Elastopan. Pebax isn’t even the only PEBA out there; a similar product, called Vestamid, is available from the German chemical company Evonik. And Adidas, which has been a little quiet on the foam front besides tweaked versions of its midsole tech like Boost HD, is rumored to be working on a new shoe that was supposedly tested at the Berlin Marathon, and which may be its answer to Pebax.

Most runners within range of the hallowed three-hour marathon mark, if offered a simple equipment choice to knock six minutes off their time, would probably take it in an instant. But two of the downsides of Pebax foam are its cost and its high compression set. The Vaporfly 4% and Next% shoes are $250—a lot of cash for a shoe that may only last a handful of races. That’s significantly more than most runners are willing to pay. “The bulk of the market has crept up a little but [the average price] is still at $120 to $130,” says Saucony’s White. “In the big races you’ll see a lot of high-end shoes at the front, but it falls off pretty quickly after that.”

Beyond the cost issue, runners become loyal to shoes for many reasons, like a shape that properly fits their feet and the way the design supports their stride and biomechanics. A 2015 meta–review of studies of running shoes and injuries, by University of Calgary exercise scientist Benno Nigg, Ph.D. suggested that runners “intuitively select a comfortable product using their own comfort filter that allows them to remain in the preferred movement path.” Foam, especially the new supersoft and bouncy stuff, seems to fit right into that ideal. But it’s an open question as to how much runners actually care about it, whether it’s called ZoomX or Boost or whatever. “I suspect most runners have no idea what we’re talking about,” says Saucony’s White. “The majority just want a good shoe that feels great.”

There’s also the still-evolving discussion of what provides the performance benefit, including the exact foam formulation, which are often variations on the base material. Nike’s version of Pebax provides excellent energy return, for instance, but Reebok’s Pebax-based Floatride foam, used in several of its top shoes, returns values in the RW tests that you’d associate with a quality EVA shoe, but below Boost or ZoomX.

And we don’t yet know how much of the improved running economy lies in other aspects of shoe design, like air pods or carbon fiber plates, and whether that’s because those elements act as propulsive springs, or if they merely stabilize soft foams to maximize energy return. In another presentation at the Footwear Biomechanics Conference, Emily Farina of Nike’s Sports Research Lab showed data suggesting that, depending on its shape, the carbon plate in the Vaporfly might be responsible for more of the running economy improvement than researchers had thought, although the exact mechanism wasn’t explained.

If Nike, which has a division devoted to exploring questions like this, knows any more, it isn’t saying. The company declined multiple requests to make Farina and other Sports Research Lab staff available for interviews, but e-mailed back a one-line statement: “The magic of the Nike ZoomX Vaporfly Next% and the Nike Zoom Vaporfly 4% is in the full system and how all the elements work together, not in any single ingredient.”

That sounds like bland corporate jargon, but there may be more to it. Among Nike’s various patents around the Vaporfly is a pending application from 2018 titled “Stacked Cushioning Arrangement for Sole Structure” which looks similar to Kipchoge’s shoes from the INEOS 1:59 Challenge. The patent details a shoe design with several distinct layers of foam, as many as three carbon fiber plates, and two pairs of fluid-filled forefoot cushioning chambers sandwiched between the plates. So it seems pretty clear that the plate (or plates) matter.

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But foam absolutely matters as well, even if most runners don’t pay that much attention to it. (It doesn’t help that shoe companies insist on hype-y names like ZoomX or Boost or Helion and liberal use of marketing word salad to describe technologies they didn’t invent.) Perhaps the biggest reason foam matters is the trickle down. Pebax is expensive now, but costs may come down as foam midsole suppliers learn more effective production techniques. And while a “halo product” like the Vaporfly is good marketing now, Nike may ultimately choose to spread its R&D costs across more models, at lower prices.

EVA, for all its drawbacks, isn’t going away: It’s still light, cheap, and easy to tweak. On Running’s new Helion foam is an EVA-based product, says Bernhard, who adds that in the beginning, the company used a mix of EVA and polyurethane foams but in testing, they found that EVA worked best with its sole structure.

On Running’s experience suggests that future breakthroughs may come as much from structural elements like sole designs or plates as from new foams, although Rancourt says other block copolymers are out there that might provide similar performance characteristics to Pebax.

For all its promise, Pebax hasn’t taken over running yet. Absent broader availability of models like the Vaporfly, or a breakthrough that provides significant price drops, it may not. Nike itself seems to acknowledge this with shoes like the race-oriented Zoom Fly 3, a near-clone of the Vaporfly down to the carbon plate, but made with its React foam. That shoe costs $90 less.

Despite its competitors’ efforts—and calls from non-Nike athletes for equipment regulations—Nike held its lead in the foam wars through 2019 with Vaporflys winning the Berlin, New York, and Chicago marathons, while Kipchoge broke the two-hour barrier with his latest prototype. But ultimately, runners and not shoe companies will decide the fate of shoe technology like the Vaporfly, and those considerations come down not just to performance, but fit and even philosophical approach to the sport.

Mindful of the importance of on-bike form to his cycling career, Jacques-Mayne says he pays close attention to his biomechanics and stride (midfoot, with a touch of heel). He’s run in Hoka One One, Adidas, and Nike, among other brands, but trains in New Balance’s 880, for its neutral support and “just right” amount of padding.

The Vaporfly wasn’t available when he got into running, so it wasn’t among the shoes he tried when searching for a fit. But even now, he adds, he’s more interested in gains from improved training and stride mechanics. “I was a novice runner with a huge motor, so growing the mechanics and muscles to unlock speed was way more important,” he says. And he’s a little wary of improvement that isn’t earned. While the idea of knocking four percent off his PR sounds great, he says, he hasn’t felt the need to try Vaporflys.