For 17 months at the end of the 2000s, swimming went through a golden age. Between 2008 and 2009, more than 130 world records, across all distances and strokes, were smashed. American swimmer Michael Phelps was in his prime, and after the 2008 Beijing Olympics there were only four events – the men's 400- and 1,500-metre freestyle and women's 100-metre breaststroke and butterfly – that survived the world record onslaught.

Many of the records set in this period still remain today, with swimmers unable to match the performances of the previous generation. But the brief period of success was mired in controversy.

Swimming's record-breaking months were largely down to Speedo. The company's LZR Racer swimsuit line, which hit the market in February 2008, covered almost all of a swimmer's exposed skin and compressed the body into a more streamlined shape. The polyurethane-based material also trapped air, giving extra buoyancy.


The swimsuits sparked an arms race between sport manufacturers: Speedo’s rivals rushed to create their own similar suits, each wanting to give athletes an extra edge. Swimming's governing body, FINA, was caught out by the rapid technological progress. “The regulations that FINA had in place at the time didn't anticipate some of the technological advances that particularly Speedo and other brands were driving,” says Mike Caine, a professor of sport technology and innovation at Loughborough University.

By the time it came to swimming's world championships in 2009, FINA had stepped up to questions raised around the ethics of using high-tech swimsuits to obliterate the previous best efforts of humans. The governing body tested almost 400 models of swimsuits and approved 202 for use in the event. At the start of 2010, it introduced new regulations on the coverage and materials of suits, effectively banning the performance-enhancing technologies.

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Over the last three years, a similar situation has crescendoed in competitive running. The new culprit? Nike's Vaporfly trainers.

The Vaporfly shoes include a carbon fibre plate and a wedge of soft, energy-returning foam that help runners move at least four per cent more efficiently. The claim was first made by Nike but has since been backed-up by academic studies. The shoes, first introduced in 2016 and currently in their third iteration (with a fourth at a prototype stage), have been worn to break multiple world records and are seen on the feet of the leading runners in races ranging from five kilometres to road-based ultramarathons. Only now, three years after the Vaporfly trainers first emerged, are running shoe rivals releasing their own versions of footwear with carbon fibre plates installed.


But the success of Nike's trainers has divided the running community. Critics of the shoe say they enhance human performance beyond iterative advancements and amount to “technological doping”, while fans can’t get enough of them. Now, ahead of the Tokyo Olympics this summer, running's governing body World Athletics has intervened, publishing new rules on what is allowed from running shoes.

The rules state that, from April 30, any shoe must have been available for purchase for four months before it can be used in competitions, and prototypes cannot be used during races. More significantly, World Athletics ruled that Nike's Vaporfly shoes and records broken using them are legal, but that an upcoming pair of AlphaFly trainers being trialled won't be allowed in the Olympics due to the thickness of their soles. The new rules state that soles can't be thicker than 40 millimetres and can't contain more than one piece of carbon fibre (or other rigid material) in the form of a plate.

At the point of writing, Nike has not fully responded to the World Athletics changes but has consistently said its shoes do not break current rules. “We respect the IAAF and the spirit of their rules, and we do not create any running shoes that return more energy than the runner expends,” a spokesperson said before the change.

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Despite the pass for the Vaporfly shoes, the controversial trainers have transformed the sport of running. The dominance of the trainers means that running will never be the same again and elite running has become a messy affair.


Kenya's Brigid Kosgei breaks Paula Radcliffe's 16-year-old marathon world record at the 2019 Chicago Marathon Getty Images / Quinn Harris / Contributor

The success of the Nike ZoomX Vaporfly Next% – the third and most recent iteration of the Vaporfly – has been phenomenal. Since the release of the trainer, which comes in distinctive fluorescent green and pink, records have collapsed. The £240 shoes have rewritten running's record books and simultaneously have become one of the most dominant and hyped pair of trainers ever.

In 2018, Kenyan Eliud Kipchoge slashed 78 seconds off the men's marathon world record, finishing in a time of 2:01:39, and lowering the record by the biggest margin in more than 50 years. Kipchoge also made history at the end of 2019 by becoming the first human to run under two hours for a full 42.2 kilometre marathon. He wore a prototype of the shoe, dubbed the AlphaFly, which is currently being trialled by other elite runners. The AlphaFly is believed to have a sole height of 51mm – higher than the new rules from World Athletics allow - although this has not been confirmed by Nike.

Perhaps more impressively, Kipchoge’s fellow Kenyan Brigid Kosgei smashed the women's world record by 81 seconds last year, beating Paula Radcliffe's 16-year-old best that no other runner has since come close to. Elsewhere, analysis of world rankings has shown that, during 2019, twice as many men and women ran faster than 2:10 and 2:27 for a marathon than before the shoe's debut in 2016. For elite athletes, a Vaporflys could make a reduction of one to two minutes across an entire marathon. It’s potentially the difference between coming first and coming fifth.

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The shoes haven't just been involved in the destruction of long-distance records. Versions of the trainers designed for use on asphalt running tracks were also worn by Dutch athlete Sifan Hassan as she became the world champion over the 1,500m and 10,000m distances. And last year in Japan, a country with a hyper-competitive running scene, around 84 per cent of athletes in an annual relay race wore the Nike shoes. Runners wearing trainers from homegrown brand Asics plummeted from 51 to just seven.

The fast-growing ubiquity of the shoe at the highest levels of the sport has led to suggestions of unfairness. “The danger with technology is that the moment it starts to influence performance more than the normal difference between two humans, then the human integrity of the result is gone,” says Ross Tucker, a sports scientist and co-host of the Science of Sport podcast. “Every single course record has basically gone. The world record is gone, the top five in the world have all been running that same shoe.” Tucker believes Nike's Vaporfly has created a large performance advantage that “splits the sport into haves and have-nots”.

Nike's own claims that the first versions of the shoe provided four per cent efficiency gains has been borne out by studies and analysis. One study, which was conducted independently of Nike but funded by the brand, found that all 18 runners involved in a test used less energy when running in the Vaporfly than they did in other Nike shoes or those from rivals, including adidas. None of the other shoes tested had the same blend of Nike foam or a carbon plate.

Analysis from The New York Times looked at race data from 500,000 marathon and half marathon times set since 2014. The results were comprehensive: people in Vaporflys ran three to four per cent faster than people of similar abilities who were wearing other shoes. “In a race between two marathoners of the same ability, a runner wearing Vaporflys would have a real advantage over a competitor not wearing them,” the newspaper reported.

The Vaporfly doesn't just provide a benefit for elite runners – who would likely be winning races even without the shoes. Wouter Hoogkamer, an academic at the University of Massachusetts, has studied the impact of Nike's technology on amateur runners. (Hoogkamer was also behind the Nike-funded independent study looking at the initial claims of the Vaporfly). His research found that runners who move at speeds slower than nine minutes per mile could see higher percentage increases in pace than those who are already performing at the top of their sport (running a shade above four minutes per mile).

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One of the issues around regulating the shoes is that we don’t know the exact reason of why the Vaporfly makes runners more efficient. The combination of the carbon fibre plate and “ZoomX” foam (also known as Pebax) provides the overall energy gains, but neither one of them can be singled out as the advantage-adding factor. It's believed that the spongy foam absorbs energy with each of the runner's strides and the carbon fibre plate works in tandem to reduce the strain that's put on the runner’s calves.

To reveal its true secret, a control shoe would have to be created for testing. Evaluating runners wearing a Vaporfly without a carbon fibre plate, or one with a plastic plate, would reveal which element of the trainer really saves runners energy.

The incredible science behind Eliud Kipchoge's 1:59 marathon Sport The incredible science behind Eliud Kipchoge's 1:59 marathon

The running shoe industry has been scrambling to catch up with Nike's innovation. Meanwhile, a handful of elite runners who are contractually obliged to other brands through sponsorships have found themselves nevertheless wearing Nike's shoe. “I lost my regular shoes in flight and am waiting for them,” Derera Hurisa, the male winner of the Mumbai marathon said after he borrowed a pair of Vaporflys from another elite runner.

Athletes sponsored by other brands have been spotted running in painted versions of the Vaporfly. The shoes have been disguised to avoid their sponsors seeing them run in trainers from another brand.

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“I would say even to this day they [Nike] are still two years ahead of everyone else,” says the anonymous owner of the Instagram account protosofthegram. The account posts images of athletes running in prototype running trainers, many of which haven't been formally acknowledged by the companies that create them. One recent pair of adidas trainers, which could be its rival to Nike's Vaporflys, appear to contain a very large chunk of foam in the midsole. “Other shoe companies were kind of caught off guard and have now only in the past year woken up,” the account owner, who says they have no connections to any running brand, adds.

Carbon fibre in running shoes isn't totally new, but it’s the combination of plates and soft foams that has surprised shoe manufacturers. The development of running shoes is a lengthy process that can take years for a product to go from design to testing and eventual manufacture. Shoe brands spend millions in developing new technology to enhance their trainers, attempting to make them more comfortable, lighter and ultimately faster than the products of their rivals.

Jens Jakob Andersen, the founder of RunRepeat.com, a website with more than seven million shoe reviews, says the Vaporfly has been popular with runners. But criticises the response of other companies. “Running shoe brands are reactive when they should be proactive,” he says. “Sure, they do file patents every year, and sure, if we compare running shoes today with those ten years ago, there has been some innovation, but innovation is moving at a depressing speed.”

The first competitor to release a shoe similar to Nike's Vaporfly was French brand Hoka One One. Its CarbonX shoe was created for a special running event, where the 50-mile world record was beaten. Since then, Saucony and New Balance have released their own shoes that include carbon fibre plates.

In the last week, two more shoes using carbon have been launched. Under Armour's Hovr Machina uses a blend of the Pebax foam and has what it is calling a propulsive carbon fibre plate.

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Elsewhere, the Brooks running brand has its own proprietary foam and carbon fibre plate that's similar to the design of Nike's. The shoe has been in development since 2017. “At the end of the day, it’s a bit of a shame that people are talking more about the shoes than the athletes,” Nikhil Jain, a senior manager of Brooks told WIRED US. “That's the saddest part of the narrative right now.”

However, given the deadlines put in place by World Athletics, sport brands may need to move up their production and release dates for their athletes to be able to use new shoes in the Olympics. If they don't, Nike's advantage may be bigger.

Elite runners at the 2019 New York Marathon, led by eventual winner Geoffrey Kamworor Getty Images / Michael Molzar / Contributor

Sports' governing bodies can't win. They will always be confronted by companies that create new technologies that push the boundaries of performance and what is acceptable within the rules. Most people accept that sports equipment is constantly evolving, but the question becomes at what point innovation in technology and design undermines human performance, or becomes detrimental to the sport. In 2008, the US Golf Association changed the rules around clubs to ensure that players couldn't hit the ball too far with one drive.

“There's an inherent tensions between sports administrators who set, and from time to time change, the regulations as they pertain to sporting equipment, and the brands who are seeking to innovate in order to improve the performance of the equipment,” Loughborough's Caine says. World Athletics' new rules bring running up to speed with where running shoe technology is. The previous rules were loose and vague, stating that shoes “must not be constructed so as to give athletes any unfair assistance or advantage”.

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The running shoe case is similar to Speedo's controversy as, in both cases, world governing bodies have had to react to changes that have already been made. The records have already been broken; in swimming’s case, where the LZR Racer suits were subsequently banned, athletes can now find it hard to catch up.

The world records set with Nike’s Vaporfly will still stand. But it’s not the end of the story for running. In its statement, World Athletics said that after a review its technical committee found that there is “sufficient evidence to raise concerns that the integrity of the sport might be threatened by the recent developments in shoe technology”. As a result, it is going to further research the shoe technology that has been used so far.

Other sports, including high jump and long jump, have limits on shoe sole height that are designed not to give athletes an unfair advantage over competitors. Athletes have argued that the shoes they run in should be allowed. “They are fair,” Kipchoge, a Nike sponsored athlete, told The Telegraph. “Technology is growing and we can’t deny it – we must go with technology. So for those that are against the shoe, it’s the person who is running, not the shoe.”

American Asics sponsored runner Sara Hall tweeted ahead of World Athletic’s decision that wanting limits to shoe technology doesn’t make people “anti-innovation”. She said: “Innovation is awesome – until it creates a quantum leap in the sport.” Fellow American, and former Nike athlete, Cara Goucher missed out on a place in the Rio 2016 Olympics after finishing fourth in the country's trials. The top three finishers qualified. She has since spoken out about the potential impact that those finishing in front of her, wearing prototype Vaporflys, may have had.

Until more is known about how the individual technological components of running shoes – the carbon fibre plates and foam – impact how runners perform, perhaps the best way to regulate trainers is by their sole height. “I don't think it's a perfect solution, but it's the best one available at this point,” Tucker said before World Athletics' decision was published.

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The rule changes from the athletics body also stated that multiple carbon fibre plates in one shoe can't be used. It has been suggested that Kipchoge's AlphaFly shoes had three different carbon fibre plates. “The challenge there is how do you know that it's in the shoe? You have to x-ray or MRI every shoe at the finish line and then that seems ridiculous,” Tucker says. World Athletics says it can inspect shoes at races if it believes there's any wrongdoing.

For now, the popular Next% remain legal. But future iterations of the shoes, and those that remain secret in the research labs of sport companies, may never be used competitively. Caine adds: “Outsole thickness isn't the variable that matters, it just happens to be the variable that's easy to measure. What matters is the energy return to the individual athlete. That can be varied and tuned – in other words personalised – to a particular athlete. Some athletes would benefit more from that particular technology than other athletes will.”

Matt Burgess is WIRED's deputy digital editor. He tweets from @mattburgess1

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