'It stretches the limits of performance': the race to make the world's fastest running shoe

Natasha Cockram never really cared about shoes. When the Welsh runner entered her first marathon in 2017, she wore a pair of two-year-old Nike racing flats that cost her £15 at an outlet store. And she was a talented athlete: a former junior cross country and middle distance champion, she had won an athletics scholarship to the University of Tulsa in Oklahoma. She studied psychology and raced hard.

“What I’ve always loved about running is that it was so accessible,” Cockram, who is 27, says when we first speak in early September. “All you needed was a pair of trainers. It didn’t matter what they were – anyone could just do it.”

But “just doing it” didn’t seem enough for the brand that built an empire on that phrase. In the year that Cockram began her marathon journey, Nike revealed a radical new shoe during Breaking2, its unsuccessful attempt to smash the two-hour barrier in the men’s marathon with the Kenyan athlete Eliud Kipchoge. The neon Vaporfly shoes, which had thick foam soles embedded with carbon fibre plates, would shake up distance running with their outlandish looks and claim to save a runner 4% in energy expenditure – equivalent to several minutes in a marathon.

The shoes soon inspired accusations of technological doping, not only challenging the purity of a great Olympic event but causing the biggest ethical schism in sports equipment since Speedo’s shark-inspired suits rocked swimming in 2008. The slippery material, versions of which other brands swiftly produced, enabled swimmers, including Michael Phelps, to glide more quickly through the water, triggering a wave of new world records, before being banned by Fina, the sport’s governing body, in 2009.

Runners said Nike’s Vaporflys offered a similar advantage; they felt as if they contained springs, and experts lined up to cry foul. Ross Tucker, a leading South African sport scientist, called them “the shoe that broke running”. Nevertheless, they rapidly sold out, contributing to a near tripling of Nike’s share price and triggering an industry arms race that is still playing out among its rivals.

In 2018, Cockram, who grew up on a farm in Cwmbran, near Cardiff, ran two more marathons in her trusty pale-blue discount shoes. But they were getting old, and the runner was growing curious. The day before the Houston Marathon in Texas, the following January, she found a pair of Nike’s new Vaporfly 4% shoes. They had bright orange uppers above thick white soles that looked like little marshmallow lifeboats.

In Houston, Cockram pulled her personal best down by 90 seconds to 2hr 34min. Nine months later, she came fifth at the 2019 Dublin Marathon, breaking the 31-year-old Welsh women’s marathon record with a time of 2hr 30min 50sec. That Cockram had run so fast, only two years after taking up the distance, gave her hope that she could qualify for the Tokyo Olympics.

Eighty seconds short of the elite athletes’ qualifying time, Cockram began targeting the 2020 London Marathon. By then, her flashy new shoes, which are notoriously quick to wear out, needed replacing. They were also outmoded: while its competitors scrambled to catch up, Nike had released even more outlandish models. The latest – the Alphafly Next% – was ready in time for Kipchoge’s second, successful assault on the two-hour barrier, in October 2019. The use of the prototype shoes contributed to the sense among critics that the event, in which Kipchoge ran behind 40 pacemakers (who were themselves precision guided by laser projections on the road), was as much a laboratory experiment as an athletic feat.

Meanwhile, Cockram had gone from plucky newcomer to Olympic hopeful, and the London Marathon was six weeks away. The stakes suddenly felt very high. “Everything was about the new shoes and how I’d be disadvantaged if I didn’t have a pair,” she recalls. Without a sponsor or agent, the athlete, who works in human resources for Norfolk police, went shopping again. But the £260 Alphaflys were sold out everywhere. She turned to Twitter in late June 2019. “As one of the minority of unsponsored athletes in this year’s London elite field, can anyone recommend where I’d be able to buy some Nike 4% or next percent. Or any racing shoe in size 4. Not fussy – just a pair of racing shoes.” She added a shoulder-shrug emoji.

Days later, a box arrived at the home Cockram shares with her partner, a weightlifter who works in a poultry plant. Nike had seen the tweet and sent a pair of Alphafly Next%. The runner put them on straightaway, noticing how weirdly springy they felt around the house, even compared with her 4% shoes. Her next training session – a 10-mile run – was the best she’d done. “I think I dropped a 4min 55sec mile, and I’ve not run a sub-five minute mile since my track days,” she says.

The Alphafly Next% were even crazier to look at. As well as the carbon plate and even thicker foam under the heel, Nike had added an air pocket to the forefoot for yet more cushioning. They now looked less like lifeboats, more like hydroplaning speedboats. “They looked almost like… I don’t know how to say it, the closest you can get to cheating with shoes,” Cockram says. She stashed them away to keep them fresh for the race of her life. There were five weeks to go.

***

Shalaya Kipp remembers a box of Nike prototypes arriving at the Locomotion Laboratory, at the University of Colorado in Boulder. The lab, which is packed with exercise machines and monitoring equipment, studies the biomechanics of walking, running and cycling, and occasionally does work for brands, including Nike. It was May 2016, and Kipp was a research student at the lab, studying physiology. The 30-year-old from Utah is a former ski racer who had fallen in love with the simplicity of running, and had competed in the steeplechase at the 2012 Olympics. “It was top secret at the time – we had to sign nondisclosure agreements,” says Kipp.

Nike had predicted that the plain white prototypes would save a runner 3% in energy expenditure, which would result in a higher speed. “Three per cent is huge,” says Kipp. “When Adidas released their Boost foam, they were bragging about 1%.” The Adidas foam, made up of tightly packed pellets, was light while also cushioning a runner’s legs, reducing fatigue and flagging. Launched in 2013, it soon featured across the brand’s running range. In 2014, Kenyan runner Dennis Kimetto broke the men’s marathon world record in Berlin while wearing Adidas Boost, with a time of 2hr 2min 57sec. It was the fourth time in a row that an Adidas athlete had broken the record.

The Vaporfly prototype was Nike’s fightback. To crush Adidas, the brand would apply all its resources to breaking the mythical two-hour marathon barrier. The potential 3% energy saving would be enough to close the gap on Kimetto’s record; achieving the four-minute mile of its day would be an athletics coup and a marketing masterstroke.

Kipp was sceptical but tried on a pair of the prototypes in the lab. They were unassuming – “ugly” even, she remembers – and all in a men’s size 10. “I was walking around like a clown but could tell this was something special,” she says. “They just felt so springy.”

Only 10 years earlier, distance running had been locked in a very different arms race. As part of a “barefoot” running craze, brands had focused on featherlight shoes with barely-there soles. Nike had looked into minimalist shoes in its early research for what became Breaking2. But runners complained that they were too unforgiving; fatigue trumped any weight advantage.

Nike’s own scientists, led by Matthew Nurse, a biomechanics researcher at the brand’s Oregon HQ, had begun to look for a solution in much thicker foam. But it needed to be lighter. The breakthrough lay in Pebax, a plastic that has been used for years in dozens of applications, including catheter pipes. Produced in raw granules by Arkema, a French company, its chemical structure is a chain of alternating soft and rigid blocks, the ratio of which can be tweaked precisely. Together, the blocks offer toughness and flexibility at a very low weight, as well as a strong energy return, or bounce. “The nature of the chain is not a big secret,” says François Tanguy, a scientist and European manager for Arkema. “How we make it is a very well-kept secret.”

By turning Pebax granules into a foam, Nike got what it needed: a Boost-killer that it would market as ZoomX; an unusually soft, light sole that would return rather than absorb energy, while also reducing fatigue in the brutal last miles of a marathon. A carbon plate – a feature that Reebok and Adidas had experimented with in the 1990s – added structure and support, reducing energy-wasting flex in the toes.

To test Nike’s 3% hypothesis, Kipp and the team searched for 18 men with size-10 feet who could run a 30-minute 10km. “It was like finding fast Cinderellas,” Kipp recalls. In five-minute treadmill sets of ratcheting speeds, the men wore three different shoes: Kimetto’s record-breaking Adidas shoe; the best Nikes then available; and the prototype. Breathing apparatus supplied the data to calculate energy expenditure.

“Some athletes loved the shoes and some hated them – but it didn’t matter, because even the guys who hated them were still using less energy,” Kipp says. It was almost unheard of in a test like this for nobody to do worse, she says. “After about the first six guys, we knew the advantage was going to be higher than 3%.”

The average energy saving ended up being 4%, which brought the two-hour barrier within reach. Nike was so confident, it named the shoe the Nike Vaporfly 4% when it was released in the summer of 2017. By then, it was already giving pairs to its sponsored runners. Three Nike athletes wore the shoes at the 2016 Rio Olympic Marathon, where they looked to the casual observer like the rest of the brand’s Olympic range – then within the rules. Kipchoge won with a time of 2hr 8min 44sec, in a Nike clean sweep of the medals, ahead of Feyisa Lilesa of Ethiopia and the American Galen Rupp.

That was a warm-up for May 2017, when Nike publicly revealed the 4% shoe at Breaking2. Kipchoge followed 30 pacemakers around an Italian Formula One track, missing out on the historic barrier by only 25 seconds. Nike had failed, but the genie was out. “I was really excited about the research because here was innovation at its best,” says Kipp, now a PhD student in kinesiology and exercise science at the University of British Columbia in Vancouver. “But I don’t think we really thought about the ripple effects this shoe was going to have.”

***

Nike’s assault on the marathon was swift and unprecedented. In 2018, Kipchoge wore Vaporflys when he took 78 seconds off Kimetto’s world marathon record in Berlin – the biggest leap in men’s times for half a century. Last year, Vaporflys bounced on to 36 out of 42 podium places at the seven major marathon championships, including 12 out of the 14 top steps for men and women. At the Chicago Marathon, a young Kenyan named Brigid Kosgei wore the shoes when she took more than a minute off Paula Radcliffe’s 16-year-old women’s world record, finishing in 2hr 14min 4sec.

The new shoes have always been hard to find, but neon swooshes also swept through amateur races and even parkruns on the feet of those who could secure a pair. Analysis of marathon times on Strava, the popular fitness app, seemed to back up word-of-mouth reports that the shoes gave an even greater advantage to slower runners, thanks to their cushioning powers. Ebay buzzed with Vaporflys and Alphaflys, many of them used, despite their running lives of about 200 miles (that foam is built to be fast, not last). New pairs have fetched more than £1,000.

Nike’s triumph climaxed in Vienna, where the British chemicals company Ineos was sponsoring Kipchoge’s 1:59 Challenge, wearing the AlphaFly Next%. Nothing was left to chance, and Kipchoge beat the two-hour barrier by 20 seconds. “It was the biggest marketing event for a shoe in the history of sport,” says Yannis Pitsiladis, a fast-talking professor of sport and exercise science at the University of Brighton, and perhaps the most prominent critic of marathon super shoes. Pitsiladis first began to worry about Vaporflys when he got hold of a pair in Rio and put them through a hospital scanner to reveal the plate inside. “I’d guessed there was something strange about it,” he says.

Pitsiladis, whose father left Greece as a child, has a big interest in shoes. In 2014, he had launched his own attempt to take an athlete through the two-hour barrier. The Sub2 Project was a collaborative experiment that Pitsiladis hoped would lead to advances in nutrition, biomechanics, training and strategy; everything except doping.

“I launched Sub2 with a view to cleaning up sport,” says Pitsiladis, who sits on the medical and scientific commission of the International Olympic Committee. “But I opened a Pandora’s box. Not only does the drug problem remain as bad as it was, but now we also have technological doping.”

Like Nike, Pitsiladis initially thought minimalist shoes could be the best route through the two-hour barrier. But Rio signalled Nike’s move towards more, not less. The discovery also came as earlier cooperation between the scientist and the brand was falling apart. (Pitsiladis accuses Nike of taking his Sub2 idea and running with it, which Nike denies. The company, which declined to make lead researcher Nurse or anyone available for an interview, also rejected accusations of “technological doping”, adding: “The Nike Air Zoom Alphafly Next% is compliant with World Athletics regulations.”)

But disguised, hi-tech prototypes went against Pitsiladis’ principles. He insists he is not anti-tech, but says sport is meaningless if performances can’t be compared from one year to the next, or even within the same race. “It’s accepted that Formula One is about showcasing engineering,” he says. “Nobody will say that if you put the best driver in the worst car he’ll win the championship. But athletics is about the individual.”

Pitsiladis felt powerless as he watched Nike break the two-hour barrier. Two weeks earlier, he had tried to make a point by staging a marathon down a long hill in Spain. “We only missed it because most athletes couldn’t get visas,” he says. “It was a meaningless event to show that you can run downhill, put giant fans behind you – or do what Ineos did. But it’s not a true test of human performance.”

Pitsiladis rejects the suggestion of sour grapes in his view of Nike’s innovations. He’s certainly not alone in having concerns. Liz McColgan, the Scottish former 10,000 metres world champion, has said: “All athletes should be on the start line as equals, and presently they are not.” Unsurprisingly, Kipchoge defends his shoes. “We need to accept change,” he told a press conference this year. Even Radcliffe – also a Nike athlete, it should be noted – was sanguine. “It’s just innovation,” she told ITV’s Good Morning Britain after losing her world record. “Same as my shoes were better [than] my predecessors’.”

Nobody suggests, meanwhile, that any shoe can make a champion; Kipchoge would be insanely fast in wooden clogs. A study published earlier this month by Nike and Exeter University of the Breaking2 athletes revealed that they succeeded thanks, in large part, to their extraordinary ability to manage oxygen intake efficiently while operating just below the exertion level that results in terminal fatigue – or “hitting the wall”, in endurance speak.

But under mounting pressure last year, World Athletics (formerly the IAAF), agreed to investigate all new long-distance shoe technology. In January this year, it announced new rules. Any competition shoe had to have a maximum sole height of 40mm and contain a single plate. To ban secret prototypes, shoes would also need to have been available for sale for four months. (The rules don’t dictate how “available” a shoe should be.)

The rules were good for Nike, stopping short of outlawing any of its new range (it denied rumours, based on a patent filing, that Kipchoge had broken the two-hour barrier on prototypes containing three carbon plates and a sole height of more than 40mm). Pitsiladis was dismayed. He believes that Tokyo, now delayed until next July, will go down as “the Games that killed off distance running”. He’s also concerned about the effect of Nike’s innovations on the track, where records at shorter distances are also being smashed using other lines of shoes based on the same technology. “Someone’s got to do something,” he says.

But Nike are not the sole innovator in super-shoes. The brand’s rivals, including Adidas, are locked in a frantic race to catch up.

***

One morning in early September, Sam Handy was standing at the socially distanced finish line of a half marathon in Prague. As a rare opportunity to run after months of cancellations, the elite-only race attracted some big names. Handy had eyes on one runner: Adidas-sponsored Peres Jepchirchir, a Kenyan former women’s world record holder at the distance.

“I was a bag of nerves, biting my fingernails,” says Handy, speaking a couple of weeks after the event from Adidas HQ near Nuremberg. As head of Adidas’ running studio, Handy, who is British, has been a key player in the race to beat Nike. “Losing the world record we’d held for so many years was a salutary lesson,” he says. “Nothing switches you on more than not winning.”

In May this year, Adidas launched its first major counterattack: the Adidas Adizero Pro, featuring a new foam and a carbon plate. It joined a bulging shelf of super-shoes from New Balance, Reebok, Saucony, On, Brooks, Hoka and Asics. All work according to the same principle as the Vaporfly 4%, with a squidgy sole and carbon plate. (Nike wasn’t the only brand to welcome the new regulations, which Handy also heralds as “a great leveller”. He, like all the brands, rejects accusations of technological doping, insisting the new tech is just evolution.)

Then, in June, Adidas revealed its answer to the Alphafly. The Adizero Adios Pro (£170) has thicker foam and – rather than a carbon plate – five “energy rods”, laid out like a second skeleton under the forefoot. These fall within World Athletics’ definition of a “plate” and “allow the runner to toe off more naturally,” Handy says.

Adidas had not revealed what advantage the Adios shoe might offer, but Prague was its first big test. Jepchirchir destroyed the field, breaking the world record for a women’s-only half marathon, with a time of 65min and 34sec. At the finish line, Adidas marketing director Caio Amato wrote the time on another Adios shoe with a marker pen and handed it to Jepchirchir. She held it up for the cameras like a trophy, her fingernails painted in matching pink and white.

“What Peres did was remarkable,” says Handy, still buzzing. “We feel we’re in the game in a big way… It’s all back on now.”

Pitsiladis is sceptical about the levelling effect of these rival shoes. “You’d be making a big assumption that all companies can equally finance the developments required,” he tells me. Even if startlines become equal again, he argues that expensive, barely obtainable shoes threaten the democracy of running – the very reason that gifted athletes, often from poorer countries, have been able to propel themselves to glory.

Handy says Adidas plans to release greater numbers of the Adios shoes, which have also sold out at record speed. “But this is not a shoe built to democratise running,” he adds. “It’s about allowing a pro athlete to stretch the limits of performance.”

***

The victims of the arms race seem to have been those athletes stuck in the wrong shoes. Wilson Kipsang and Kimmetto, the last two world record holders before Kipchoge, have faded in Nike’s shadow. Other hamstrung athletes have looked for ways to fudge – or even tear up – their sponsorship deals. Nike shoes taped up in black to disguise their logos have become a common feature of elite races. Smaller brands have compromised, allowing their runners to wear Nike shoes: better to have fast vests than slow everything.

After acquiring her own pair of Alphafly Next% for the London Marathon, Natasha Cockram soon had more to worry about than shoes. Days after her best-ever training session, her right ankle exploded in pain. A scan revealed a cyst on a ligament. She couldn’t run for three weeks before the event. “I was convinced it was over,” she says.

Two days before the race, with a supply of pre-approved painkillers, Cockram checked into a secret hotel in Windsor. All athletes were required to quarantine before the race, which was for elite runners only and would involve 19 laps of St James’ Park. She had to wear a “bump”, or proximity collar, which beeped and flashed blue if she drifted within two metres of anyone else. “You wanted to talk, but it wasn’t worth the risk,” Cockram says.

On the eve of the race, the shoe brand reps arrived like foam-bearing Santas. An official checked the shoes against the new list of approved models. It was a nervous affair for some. “The Hoka athletes panicked because their shoes weren’t on the list,” Cockram says. But it turned out to be an admin error.

After a 3.30am “shakedown” run in the rain-soaked hotel grounds, Cockram’s ankle felt OK. She boarded the athletes’ bus to the park, where each runner changed in a private isolation tent.

Shoes on the start line were diverse by recent standards, but cold, wet conditions made for a slow race. New shoes from Asics and Adidas prevented a Nike sweep of the podiums (the rival brands finished second in the women’s and men’s race respectively). And, in a sign, perhaps, of the beginning-of-the-end of Nike’s supremacy, the seemingly invincible Kipchoge would finish only eighth. He had not lost a marathon for seven years. He was human, after all (he blamed cramp and a blocked ear).

Cockram was running her own race against the Tokyo qualifying time for women, of 2hr 29 mins 30sec. Three miles in, her ankle was holding up. She stayed on pace until fatigue struck with eight miles to go. But when the strongest two British women in the race pulled up with injuries, a consolation prize was within her grasp. Cockram nudged ahead of another Alphafly-wearing Brit, crossing the line in 13th place as the new British champion. “I was so tired it took a while to sink in,” she tells me in her lunch break the next day.

Cockram was four minutes off Olympic pace, yet relieved even to finish. The shoes felt good. She’ll now get more physio and look for her next chance to qualify.

When we spoke in September, I asked her if she used to resent running in her old discount shoes while others swooshed ahead in unobtainable Nikes. If anything, she told me, she resented having them more: as grateful as she is to own a pair, she misses the days when shoes didn’t matter.

“Because if I go out and run another PB, I’ll always have that thought in the back of my mind,” she says. “Was it the shoes? I didn’t resent not having them, because I could go into a race knowing that it was all me.”