When the public first noticed Nike’s new Alphafly, it didn’t look like much: In August 2018 the French Instagram account @RUN’IX posted a photo of Eliud Kipchoge flanked by smiling runners. There, atop the red Kenyan soil, Kipchoge is wearing a nondescript black upper attached to a chunky slab of what looked like the Vaporfly’s ZoomX foam. This was the first spark of speculation on how Nike planned to follow up on its world-beating Vaporfly, though Kipchoge had already been testing prototypes since January.
In the two years prior, Nike’s new Vaporfly, unique for its thick, bouncy Pebax-based foam midsole and carbon-fiber plate, had upended the less-is-more maxim governing racing flats by winning—sometimes sweeping—nearly every major marathon it entered. And in the month after those first mysterious photos, Kipchoge, wearing Vaporflys, would claim his first marathon world record in 2:01:39 at the 2018 Berlin Marathon.
More photos of the new prototype, midsole seemingly (bewilderingly) thicker than the Vaporfly’s, surfaced every few months until, in October 2019, in Vienna, Kipchoge wore a polished version of the still-unnamed shoe to clock the first sub-two-hour marathon (albeit not an official record) at the Ineos 1:59 Challenge. The following June, the Air Zoom Alphafly NEXT% was released—and, last year, was updated as the Alphafly 2. Meanwhile, Kipchoge has used Alphaflys to lower his official world record to 2:01:09 and claim his second Olympic gold medal in the marathon.
The Alphafly, with its air chambers, is an obvious evolution from the Vaporfly. And it retains the mechanically familiar block of responsive foam stabilized by a heel-to-toe carbon-fiber plate. That’s no longer a mystery. But how, exactly, does that system translate physiologically to the fastest marathon ever run? Biomechanists are still figuring that out.
The premise of the Alphafly is simple, says Carrie Dimoff, a Nike director of innovation who leads the Alphafly development team: “If we can store more of your energy and return more of that energy, you can run more efficiently.”
All of running is energy maintenance, from our shoes to our stride. “We are essentially giant springs as we run,” says Geoffrey Burns, PhD, a sports physiologist at the U.S. Olympic and Paralympic Training Center. In the air, between footfalls, we’re gathering potential energy both from the forward motion of our previous step and as gravity pulls us down. When our foot strikes the ground, our bodies begin accepting and storing that now-realized potential energy, mostly in our muscles and tendons. Then, in dynamic concert, these muscles and tendons recycle that energy, adding a bit of effort to maintain our kinetic momentum, and push us forward off the ground and back into the air. Researchers quantify how much energy we each require to run—our “running economy”—by measuring the amount of oxygen our muscles demand during an effort that we can sustain for hours.
In 2017, researchers from the University of Colorado and Nike published a study in Sports Medicine showing that runners averaged a 4 percent improvement in running economy wearing a Vaporfly prototype compared to running in Nike’s racing flat, the Zoom Streak 6, or the 2016 Boston Marathon–winning Adidas Adizero Adios Boost 2, also a flat. Months later, Nike tied its shoes to this improved running economy when it launched the Zoom Vaporfly 4%.
The highly cushioned Vaporfly was a paradigm shift, sending shoe developers, footwear researchers, and runners on a quest for shoes promising biomechanical efficiency. “We’d been saying forever that minimal was better,” Dimoff says. “We needed these specific study numbers to explain why people should trust, suddenly, this very tall shoe.”
The idea that a shoe with those attributes could be fast had been a revelation at Nike, too, and in June 2016, almost a year prior to Kipchoge’s first sub-two marathon attempt wearing Vaporflys, a development team was already working to see how far they could push this cushioned propulsion in a new model. “We weren’t looking to invent a companion shoe to the Vaporfly,” Dimoff says. Instead, they were asking how they could make the system more energy-efficient.
“We started thinking about the Nike Zoom Air technology,” Dimoff says, referencing Nike’s trademark air-filled-rubber cushioning, which debuted in the Air Tailwind running shoe at the 1978 Honolulu Marathon. As a component, they believed these air units could return more energy to the runner than Nike’s springiest foam. “We thought we could make the system more efficient by swapping out some foam for air,” she said.
To find out, Nike researchers started conducting mechanical tests on individual components—air units, foam, plates—to answer some initial questions: How much energy was stored? What shape is the deformation curve? Which returns the most energy?
They designed and built the air units first, and then began cobbling prototypes around them. Systems of components came next, with partial builds going through further mechanical testing. By late 2016, wear-testers were already putting the first of hundreds of prototypes through paces on treadmills and across Nike’s campus. Throughout the process, Dimoff says, they were looking for “the right balance between what the science told us was performing the best and what the runners told us was the most comfortable; the ones they had the most confidence in.”
Finally, in January 2018, Dimoff and team took the first of at least five rounds of prototypes for Kipchoge to test at his training camp in Kaptagat, Kenya. “We think we have something even better,” the design team had told him. Kipchoge was game to try it. “He just put them straight on and ran,” Dimoff says. “There was no hesitation. He just saw innovation.”
The heart of the Alphafly—and its Vaporfly cousins—is not the much-discussed carbon-fiber plate, but the ZoomX foam. ZoomX is Nike’s custom formulation of Pebax, a proprietary polyether block amide (PEBA) thermoplastic elastomer (flexible plastic). A PEBA’s chemical structure can be precisely ordered to exhibit a staggering range of properties: hard enough to form the top layer on skis, soft enough to be a waterproof jacket layer, or, as with ZoomX, puffed into a foam. Nike has been deploying it as the hard outsole on the bottom of its cleats since the 1990s and, in 2000, as a plate, in the Shox R4 running shoe.
This capacity for precision tuning is also what allows PEBA foams to be made softer or firmer, bouncier or more rigid. Nike refused to discuss the development of its Pebax foam, but did say it manufactures ZoomX to “produce different benefits” for different shoe models. “In the Alphafly and Vaporfly, we’re manufacturing [ZoomX] to produce the most energy return,” says Nike Running footwear senior product manager Elliott Heath. “Whereas, in the Invincible [trainer], we manufacture the material a little differently to produce cushion for everyday runs.”
Critically, for the Alphafly and Vaporfly, PEBA foam could be optimized to produce properties previously thought contradictory: tall but lightweight, plush yet springy. In a shoe midsole, all foam acts as a spring: It absorbs and stores potential energy when compressed by the foot (known as compliance) and returns a portion of that energy as the foot rolls onto its toes and pushes off, when the foam rebounds to its original shape (known as resilience).
Midsole foams—EVA, TPU, and PEBA—vary in their capacity for compliance and resilience. And none give back as much energy as they get. At its best, the standard EVA foam used in most shoes returns around 65 percent of the energy put into it, Burns says. The Adidas Boost 2’s fairly unique TPU foam, measured in the original Vaporfly 4% study, returns a laudable 76 percent. And in that same study, the Vaporfly’s ZoomX foam returns an unprecedented 87 percent.
“Everybody at the time made a huge deal about this carbon-fiber plate, but that foam is the magic, that’s the game changer,” Burns says about the study.
But the amount of energy that midsole foams return is only half the equation, Burns says. There’s also how much energy is stored. Consider the thin rubber soles on minimalist shoes, he says. They return nearly all the energy put into them. But because they can hardly be compressed, they store virtually no energy and return little.
The energy returned by the three foams in the Vaporfly 4% study—65, 76, and 87 percent—was a percentage of the total energy those foams were able to store to begin with. And when the researchers measured this storage capacity, they found the Vaporfly prototype’s ZoomX, when compressed, stored nearly twice the energy of the other shoes. When this energy storage capacity was compounded by the foam’s superior energy return, overall, the Vaporflys returned more than twice the amount of energy per step. The authors credit “its substantially greater compliance rather than the greater percent resilience.”
“Everybody at the time made a huge deal about this carbon-fiber plate, but that foam is the magic, that’s the game changer.”
The foam’s weight, or lack thereof, is what makes all this possible. “For a very resilient spring, the longer it is—the stack height of a shoe—the more energy you can store,” Burns says. But increasing the stack comes with a weight penalty. Studies have quantified this “cost of cushioning.” Every 3.5 ounces (100g) of added weight per shoe reduces running economy by roughly 1 percent. The Adidas Boost 2’s TPU improved on traditional EVA, but that racing flat weighs roughly the same as the Alphafly 2, which stacks its lightweight ZoomX foam twice as high.
“So,” Burns says, “a super shoe is not just a high-energy-return foam, it’s not just a compliant foam, it’s not just a lightweight foam. A super shoe needs all three of those things.”
And what about that carbon-fiber plate? When the Vaporfly debuted, the running world seized on this and armchair biomechanists labeled it a spring that propelled the runner forward (akin to the carbon-fiber blades that South African sprinter Oscar Pistorius raced on at the 2012 Olympic Games).
Cries of technological doping rang out—at least until Nike’s competition produced their own super shoes. A 2022 study in which Wouter Hoogkamer, who led the original Vaporfly study and now conducts research at University of Massachusetts Integrative Locomotion Lab, took a table saw to the forefoot of a pair of Vaporflys, making six lateral cuts through the bottom of the shoe and the plate, and then compared subjects’ performance in them versus wearing an intact Vaporfly. While the study tracked only 13 participants, the sliced plates worsened subjects’ running economy by only 0.5 percent. And, lining up with previous studies on carbon-fiber plates, the intact Vaporflys produced longer ground-contact time and a noticeable forward shift in ground reaction forces, suggesting that the plate, instead of acting as a spring, is guiding forces into the forefoot, acting like a stiff lever, and increasing the stance’s propelling phase.
This fits with Dimoff’s description of the plate’s purpose. “It stiffens the shoe to reduce joint work and allow your stride to be more efficient,” she says. Highly pliant cushioning inherently lacks not only stability, but also coordinated forces. The plate brings the center of pressure to where we need it under the foot, says Iain Hunter, PhD, an exercise science professor specializing in running biomechanics at Brigham Young University. In 2019, Hunter published a study in the Journal of Sports Sciences validating the original Vaporfly study. During our foot strike, “the force under the foot drifts forward, builds, and peaks around the middle of the time we’re on the ground,” he says. The foam compresses, storing energy, and then the plate, because of its rigidity, positions the foam to “give that energy back close to the time the person needs it.”
The plate’s sculpted shape, with a spoon-like dip in the forefoot, makes this choreography possible. “We want your foot to roll efficiently,” Dimoff says. “A flat plate would be a slappy experience. That would actually add work.”
The Alphafly uses the same plate, tweaked to distribute force across the dual Zoom Air chambers. The energy return of the units tests upwards of 90 percent, Dimoff says. The puck-like pods include tensile nylon fibers, tethering the bottom of each chamber to its top to create a flat floor and ceiling that loads and returns forces vertically.
In October 2020, Dimoff’s team began Alphafly 2 prototyping with a new goal: Improve the shoe while making its advantages accessible to more runners, those training to break three, four, and five hours in the marathon. The Alphafly 2’s base was slightly widened, the air chambers now sit on a sliver of foam, the heel drop was bumped from 4mm to 8mm, and padding was added to wrap the heel and top of the foot in a closer, softer hug. This overall tune-up increases lateral stability and smooths the transition into the forefoot.
The shoe’s foam, plate, and air units remain the same, running just as efficient as the generation before it. Not that anyone but Nike knows exactly how or why any Alphafly always tests out on top. When I began reporting this article, I wanted to identify the technological linchpin responsible for the Alphafly’s ability to improve running economy. PEBA foams and carbon-fiber plates are no longer unique to Nike, after all. But I learned, and was reminded, in every conversation and each study I read, that the mechanics of this shoe (and other super shoes) are a product of an intricate and interdependent architecture. Dimoff spelled this out for me: “There are lots of really high-performing materials and components to the shoe,” she says. “But what makes them special is the way they come together in a system.”
The Alphafly’s eminence is, broadly, easy to explain: It’s simply a better system and a better spring. Even so, when the world’s top running biomechanists and kinesiologists have taken the shoe, or the Vaporfly, into a lab to define why it’s superior to the competition that’s emerged, the results are inconclusive. They’ve admitted as much: “The better running economy…cannot be currently explained from a biomechanical standpoint,” a group of researchers from the University of Lausanne Institute of Sport Sciences and the Volodalen Swiss Sport Lab wrote in a 2022 Footwear Science paper.
There are theories, all with evidence, on how these shoes improve running economy. In both the original Vaporfly study and Hunter’s follow-up, subjects wearing Vaporflys showed slightly longer stride lengths compared to the same control shoe, the Nike Zoom Streak. Hunter’s study also found those wearing Vaporflys ran with greater vertical oscillation, more bounce in their step, while Hoogkamer’s study, in an adjacent data point, found they ran with greater ground reaction force. Hoogkamer’s study showed that heel-strikers saw slightly greater improvement in running economy than midfoot-strikers. Are they utilizing more of the foam? Studies adding carbon-fiber plates to traditional midsole foams have shown the same benefits for heel strikers. Maybe these runners are simply leveraging the full potential of the plate.
A 2022 Footwear Science study by Dustin Joubert, PhD, assistant professor of kinesiology at St. Edward’s University in Austin, Texas, presented results from the only head-to-head research I could find that measured the Alphafly against other super shoes (the Hoka Rocket X, Saucony Endorphin Pro, Asics Metaspeed Sky, New Balance RC Elite, Brooks Hyperion Elite 2, and the Vaporfly Next% 2). On average, the 12 subjects showed the greatest improvement in running economy in the Alphafly (3 percent), with statistically similar results for the Vaporfly and Metaspeed Sky (at 2.7 and 2.5 percent, respectively). Joubert doesn’t know exactly why.
He found correlations hinting at how the shoes are working and who is benefitting, though with only 12 participants, none of the explanations “popped out” as much as he would have liked. Similar to Vaporfly studies, Joubert’s study found that the Alphaflys produced the longest stride length, and that the shoe’s lowest responders, those who saw the smallest gains in running economy, ran with slightly lower vertical oscillation than the other runners.
In a follow-up study, published this year in the International Journal of Sports Physiology and Performance, Joubert, with Burns, measured the running economy gains in subjects wearing Vaporflys while running 8:00- and 9:30-minute/mile pace, in contrast to the 6:00- to 7:00-minute/mile paces typically employed in these studies. The results showed that the Vaporflys, at slower speeds, still improved running economy relative to a traditional shoe, but less so, with subjects averaging a 1.4 percent improvement and 0.9 percent improvement, respectively.
He just put them straight on and ran. There was no hesitation. He just saw innovation.
To Joubert, the studies converge. In Alphaflys and Vaporflys, larger ground reaction force—stronger, faster, bouncier running—seems to generate greater improvements in running economy. “You can only get back what you put in,” Joubert says. He thinks a runner’s weight, too, could factor, especially in the bigger Alphafly. “If you’re really lightweight and barely getting off the ground, how are you benefitting from these 40 millimeters of stacked foam?”
The hypothesis gaining the most traction comes from Nike researchers’ 2019 Footwear Science abstract showing that a group of Portland Marathon finishers wearing the Vaporfly 4% exhibited lower blood markers for postrace muscle damage. Anecdotal evidence for this is strong, as well: On big runs in the Alphafly or Vaporfly, your legs seem to last longer, they’re not so taxed at the end, and they’re less sore the next day. “Exercise physiology is coming to accept that muscle cramping late in the race has a lot more to do with muscle damage than hydration or electrolyte imbalances,” Joubert said. “From personal experience, the last marathon I ran, in the Alphafly, I was at mile 22 thinking, ‘I’m not supposed to feel this good. When are things going to fall apart?’”
But, as Joubert and others told me, none of this is evidence that the Alphafly is the superior shoe, or that any super shoe will be super for everyone. Across studies, at all speeds, individual responses to these shoes range from 3 percent detriment in running economy to something like a 6 percent gain.
“We [each] have a unique interaction with the ground,” Burns says. And this extends to shoes, too. “The way you move through the foot strike and load that foam, and move over that plate, is going to differ for every single person.” He sees the leading hypotheses proposing why the shoes are beneficial—the foam is returning more energy to the runner, the plate is guiding contact forces—and why some see improved running economy and others don’t, and he says they’re plausible. “But nothing is clear yet.”
It’s already happening again. This time the first hints of the next Nike super shoe popped up at the December 2022 California International Marathon—six months since the Alphafly 2’s release—where a racer was seen wearing a prototype with a more angular chunk of white ZoomX under another plain black upper. At this January’s Houston Half Marathon, Conner Mantz was spotted in similar shoes en route to his 6th place finish. Both pairs appeared to have white tape concealing the air units. In March, wearing a polished version, painted with a swoosh, air units visible, Tsehay Gemechu took 2nd place and Suguru Osako placed 9th among the women and men, respectively, at the Tokyo Marathon.
Nike remains tight-lipped; a spokesperson declined to call it an Alphafly, referring to it as the company’s “latest marathon innovation.” But the familiar air units and similar foam contours are compelling evidence for a system update. Compared to the Alphafly 2, the prototypes appear to add foam midfoot. This could improve the shoe’s transition from heel to toe-off, says Runner’s World testing director Jeff Dengate. “There’s also less rubber used in the back half of the shoe. Nike probably didn’t see much wear there, so the little patch there may save weight.”
Nike did share one detail, and perhaps it’s the only one that matters: The fastest marathoner of all time is currently training in these shoes.
John Tyler Allen has been researching, testing, and reviewing running gear for Runner’s World since 2018. His broader writing features in-depth reporting on eccentric characters and compelling narratives. His work has appeared in The Atlantic, New York magazine’s Bedford + Bowery, World Literature Today, and other publications. Once a collegiate sprinter, he’s still discovering the nuances of running for more than 60 seconds. Connect: johntylerallen.com