Carbon-fibre plates in run shoes: what they are, how they work and whether they’re worth the hype

In the quest for ever lighter, more rigid and responsive kit, carbon fibre has long been at the forefront of evolving sports technology. But for triathletes and cyclists, much of its influence has been witnessed with faster bikes.

Now, though, carbon fibre blades have spread to footwear on a mass scale and unless you’ve been living on a Vaporfly-free island for the past 12 months, you’ll be aware of the rumblings-turned-shockwaves that have had a seismic impact on the running industry – and therefore the final leg of triathlon.

Nike (who else?) are the brand in the eye of the storm, with its poster boy Vaporfly shoe polarising audiences who see it as either free market innovation at its finest or about as welcome as the tsetse fly. But why the furore? Are the improvements genuine? Is it really all about carbon fibre plates? Is it fair? And does fairness matter anyway?

This article sets out to try and provide background and answer some of those questions.

What is a carbon plate and how was it developed?

In 2013, Nike started developing its Vaporfly shoe, taking out a bunch of patents as it melded the responsive Pebax foam (thermoplastic polymers that Nike called ZoomX) with a stiff carbon fibre plate in the shoe’s sole.

For many onlookers it was just the next – albeit strange-looking – evolution of run shoes and part of the marketing hype around the brand’s initial Breaking2 attempt (the exhibition on the Italian Grand Prix circuit in May 2017 where Kenyan Eliud Kipchoge wore the Nike Zoom Vaporfly Elite and missed the 2hr mark by 25secs).

How do carbon-plates make you a faster runner?

The technology, as outlined below, is purported to be superior to a traditional running shoe, but even at this later stage its true performance advantage and precise mechanism should be heavily caveated by the need for more testing.

1. On every step, the stiff carbon fibre plate helps the foam compress and expand more quickly, returning more energy to the runner (lab tests put this at over 80% and the highest ever recorded).
2. The carbon fibre plate also helps stabilise the runner’s ankle, reducing rotational force and lessening the workload for the calves, in theory helping runners stay fresher for longer through the race and recover more quickly afterwards.
3. The plate keeps the toes straight, further reducing energy loss.

Nike were not shy about its benefits. In fact, they hammered home the point, calling an early iteration the Vaporfly 4% Flyknit after tests showed it required an average of 4% less energy to run a given pace compared with regular shoes (it would call later editions Next %).

It’s worth pausing to contemplate this claim. Anyone who has run a marathon or appreciates what it entails, can acknowledge that 4% less effort isn’t just nibbling at the edges, it’s a colossal chunk. For a 3hr marathoner, it would lop off minutes over the 26.2miles, something that could otherwise take years of training to accomplish. Plenty of eyebrows were raised. Surely a simple running shoe could not wield this much power?

What was widely accepted at this stage was that you couldn’t miss them, either because of the neon hues of key lime green or blushing pink, or because of the eye-watering $250 price-tag.

What was less observed, and this will only become apparent with time and more data, is whether an enforced change in running style, and thus biomechanical forces, would increase or decrease the risk of injury. Those buying into the barefoot running revolution partially spawned from Christopher MacDougall’s seminal book, Born To Run (Watch his TED Talk here), might remember the effects of changing gait at haste. If not, their calves and Achilles will probably remind them.

Was this something completely new?

Yes… and no. The idea of carbon fibre plates in shoes is far from a novel concept. An imagined conversation between Nike and World Athletics in this excellent piece by Alex Hutchinson for Outside illustrates how both carbon plates and different types of foam have been used previously, such as the Adidas ProPlate or the Adidas Boost foam. In essence, Nike weren’t doing something that hadn’t been tried, and in some cases abandoned, before.

So, this is just rehashed marketing hype then?

Lots of people thought so initially, but then compelling evidence poured forth. On October 12, 2019, Eliud Kipchoge donned the latest iteration, the Alphafly, and finally broke the hallowed 2hr barrier in the INEO 1:59 attempt in Vienna.

Lessons had been learnt from the first attempt. The Kenyan’s pacers were drilled into a new aerodynamic formation, a crowd was present, and the shoes looked chunkier and more space-aged than ever. Kipchoge finished with such aplomb he looked like he was capable of repeating it immediately – and no-one got anywhere near the shoes.

Records started to tumble, from the very next day, in fact, when Paula Radcliffe’s longstanding marathon world best, set way back in London in 2003, was obliterated by Kipchoge’s compatriot Brigid Kosgei in the Chicago marathon in yet another iteration of the shoe.

In truth, athletes had been running faster for months, but the spotlight was now well and truly glaring, at world level, national level, and club level, and each express time was achieved by a runner wearing the Nike brand. It was as if the most accessible of sports, that had evolved in tiny, incremental steps for years, had just taken one giant bound.

More independent non-Nike lab tests were conducted. Some scientific, some less so, including former 800m ace Nick Symmonds of the USA conducting his own treadmill experiment to prove the advantage the shoes gave in lowering heart-rate for the same pace (and even taking a hacksaw to them afterwards)

Seeing market share fall, rival brands started to produce their own carbon fibre models. Hoka, already known for its platform-esque footwear were one of the first off the mark. The brand, that also sponsors the run leg of the Ironman World Championships, were paid a visit by 220’s Matt Baird in issue 374 to look at how it developed its Carbon X model.

Back at the elite end, there were ludicrous reports of athletes wearing Vaporflys but painting over the Nike swoosh because they were signed to different sponsors. The winner of the Mumbai marathon even claimed he lost his own shoes on the flight to India and borrowed his winning Nikes from a rival competitor.

Kara Goucher, a former member of the Nike Oregon Project, and an outspoken critic of its banned head coach Alberto Salazar, explained how she felt she may have lost out in a place in the 2016 Olympic Games after being beaten by Nike-wearing athletes in the US trials. In the 2019 Ironman World Championship, eight of the leading 15 male finishers and the entire women’s podium all wore Vaporfly Next%.

It wasn’t just the record books that were being ripped up. The Vaporflys seem to provide the biggest leap forward in running since a switch from cinder to synthetic tracks. Almost everything we thought we knew about distance running, seemed to be turned on its head.

Times have improved, but so what?

That’s a good question, and defenders of innovation will argue, just that: So what? It’s progress. Stop whining, embrace change or be left behind. The Vaporflys are available to buy, other brands are rushing to produce their own equivalents, and to paint Nike as the villains, when they are trying to innovate – and invest – in the sport is misguided.

Moreover, World Athletics, formerly the IAAF, makes the rules (which triathlon has historically deferred to) and if Nike develops a shoe in breach of said rules, it comes down to governance. In fact, shouldn’t it be Nike’s raison d’etre to push the boundaries of what’s possible? Shouldn’t they be lauded for this engineering breakthrough?

On the flipside (and we’ll come to the rules momentarily), there are a number of issues raised by disgruntled voices that cannot simply be dismissed as intransigence.

One objection is that through the rise of Vaporflys, competitive running is no longer a level playing field nor widely accessible to all – and this strikes at the very essence of the sport. Unlike Formula 1, and even cycling to an extent, they argue that running shouldn’t be an arms race for whoever has the best equipment, but majoritively down to human endeavour. If technology does play a role in performance, it shouldn’t be the differentiating factor, which is what the shoes have become.

They’re also expensive and thus price many out, which is exacerbated if the durability of the shoe is poor and there have been reports of the foam losing responsiveness after a few hundred miles. The performance advantage of the Vaporfly further stuck in the craw of runners who felt to be competitive they were now railroaded into purchasing from a brand they disliked for a myriad of other reasons.

One of the more compelling arguments is that of responders versus non-responders. The eminent sports scientist Ross Tucker makes this point at length in an excellent blog here, but to summarise, Tucker’s argument is that Nike’s shoe tech gives so great an advantage to some athletes over others that it renders training, tactics and nutrition almost inconsequential – and as such, we cannot have faith in what we’re watching.

In a lab test at the University of Colorado, 18 runners were tested for running economy – how much energy is needed to run at a given pace – and found that when wearing the Vaporflys versus the Nike Zoom Streak 6 or Adidas Adios Boost 2, improvements ranged from 1.59% to 6.26% (hence the 4% claim). Tucker argues these margins are so great it distorts any pure physiological comparisons and as the tech improves the discrepancy widens.

But aren’t there laws in place to govern shoe tech so it is fair to all?

You’d think so wouldn’t you? But unfortunately, the rules were so vague (and rarely enforced) that manufacturers have been left to their own devices. One of the few companies to fall foul of them, the now dissolved Spira Footwear, did put springs in its soles, which led to a whole other can of worms and some guerrilla marketing tactics in offering $1million to a Spira shoe-winning runner in the 2006 Boston marathon.

The two laws in questions essentially stated that shoes had to not provide an unfair advantage and must be reasonably available to all. It’s not hard to see the wriggle room in these definitions. What constitutes an unfair advantage is difficult to prove, the ‘being reasonably available’ part was just run over roughshod, with athletes turning out with impunity in prototypes for years.

Despite this, given all the grievances discussed above, World Athletics were now under pressure to do something. But carbon fibre plates were not new. Different types of foam cushioning were not new. Where did they go from here?

So, what has been done about it?

World Athletics tried to draw its line in the sand, but many think the incoming commercial tide will just wash it right away. On 31 January, new regulations were introduced for track spikes and road shoes, which stated that models could not have more than one carbon fibre (or rigid) plate in the midsole and the sole depth for the latter could only be a maximum of 40mm. In trying to clamp down on brands unveiling prototypes on the eve of competition, it also ruled that shoes had to be commercially available for four months, with an April 30 cut-off for new models before the 2020 Olympic Games.

Will the ruling have any effect?

Critics say the ruling has gone nowhere near far enough. As if to illustrate – or rub World Athletics’ noses in it – within a few days of the announcement, Nike revealed its latest Alphafly model at the limit of the World Athletics ruling, sparking rumours of inside knowledge. The new shoes have not been seen in competition yet, but early reports suggest it provides an even greater performance advantage than the Vaporfly.

Criticism from the likes of Tucker, argue that the 40mm sole thickness, known as stack height, simply provides far too much leeway for brands to develop propulsive technology within the midsole, the argument being that the greater the depth the greater the angle available to place the carbon fibre plate, and the greater the energy return.

One spin-off that hasn’t been widely considered is the additional administration and expense that will be caused with brands having new products approved by World Athletics. While it might be small beer for big companies such as Nike to pay fees for certification, it’s yet another barrier for entry for smaller companies looking to break into the notoriously difficult running market.

As for policing the stack height (or counting the number of carbon plates within) that’s a whole other issue, and judging by how unenforceable the original prototype ruling proved, not one that World Athletics will relish.

Where does this leave triathlon?

The International Triathlon Union tends to fall back on the individual disciplines’ regulations when it comes to rulings of this nature. Therefore, the swim guidelines will refer to FINA, the cycling to the UCI, and the run leg to World Athletics. There are some exceptions, notably with triathlon playing fast and loose with UCI-legal terms in cycling, but with run shoes it’s not been a controversial topic, until now. After the World Athletics ruling, the ITU’s technical committee met to decide its direction and whether they would adopt the new rules wholesale. At the time of publishing, they were still deliberating.

What happens next?

This saga is likely to run and run. World Athletics, under pressure to show leadership, has made a tentative ruling by its own admission. Concerned about a repeat of the swimming debacle where revolutionary swimsuits appeared in 2008, swept the board at the Beijing Olympics and were subsequently banned, it’s recommended “further research to be undertaken to establish the true impact of this technology.” A new working group comprising biomechanics specialists and, somewhat controversially, shoe manufacturers, will be set up to review the process.

Don’t expect to hear much from the athletes, though. Largely due to sponsor commitments, few are prepared to speak out about the technology. As for everyone else, as we’ve seen from the discourse to date, runners and triathletes tend to be split on the issue. Those worried about being disqualified from a race should have few fears. No shoe currently in circulation appears to breach the guidelines. If you wish to wear a pair of Vaporflys or any other carbon fibre plate shoe for that matter, it’s down to your own moral judgement and your own bank balance.