at the races Running’s Arms Race: a portrait of the world’s fasest shoes

Running’s Arms Race: a portrait of the world’s fasest shoes


Some refer to it as the carbon shoe craze. Others call it ‘mechanical doping.’

The conversation in running has shifted. No longer is the focus just on performance. It’s centred on footwear. Everyone wants – needs – to know what others race in (or used to race in, or virtual race in, but you get the point). At the core of the discussion: carbon fibre-plated shoes, a relatively new addition to modern footwear.

As companies race towards the fastest shoe on the market, many runners question how far can a shoe go before it’s considered illegal?


Carbon fibre-plated running shoes as we know them first arrived on the scene in 2016. At the time they were merely prototypes and later revealed as the Nike Zoom Vaporfly 4%. The shoe notably features a thick midsole, a carbon fibre plate, and ZoomX, a type of proprietary foam that has a high energy return. The foam is named Pebax. Funny enough, Pebax provides foam cushioning for Mizuno, Asics, Reebok, and others yet brands aren’t able to replicate Nike’s success.

Several generations of the 4% followed, including the Nike Zoom Vaporfly 4% Flyknit (essentially the same shoe), the Vaporfly NEXT%, and the Zoom Alphafly NEXT%. The NEXT% draws its name for having a more than 4% improvement in running economy, a science-backed claim, which makes the shoe so effective, and controversial.

Today, road racing records across the board have reset, both at the Canadian and global level. These records aren’t without controversy: a heated debate has emerged about whether shoes with such an advantage should even be permitted. In fact, recently, Norwegian marathoner Sondre Nordstad Moen broke the 1-hour record in the NEXT%, disqualifying his result from the record books because the shoes have too much cushioning for a track event.

While the summer road racing circuits are on pause because of coronavirus, now’s a good time to nerd out by diving deep into the carbon fibre obsession.

Why carbon fibre?

Carbon fibre is a simplistic way to group shoes together. But, it’s not just the carbon fibre plate that sets the 4% and NEXT% apart. Three factors contribute to performance: an embedded carbon fibre plate, midsole foam, and midsole thickness.

Recent studies suggest that, contrary to popular belief, the energy-returning foam is more beneficial than the carbon fibre plate. The plate is believed to have some effect: it reduces energy loss by keeping the first metatarsal phalangeal joint and ankle stable. The carbon fibre plate also adds a level of rigidness to the shoe. Without the plate, the cushioning would be far too soft, significantly reducing the life of an already fragile shoe.

In a marathon, half-marathon, or for any distance for that matter, minimizing energy loss in each stride is critical.

What currently exists?

Many racing flats with carbon fibre plates exist, although brands lag behind Nike in releasing their own versions. Hoka One One has the Evo Carbon Rocket. New Balance has the FuelCell TC. Saucony has the Endorphin Pro and has a claimed 88% energy return. Brooks has the Hyperion Elite. Asics has its MetaRacer. On has the Cloudboom.

Additionally, Adidas worked on their own prototype, equipping their sponsored athletes with a densely-cushioned prototype at January’s Houston Marathon. That shoe turns out to be the adidas adizero Pro.

However, it’s Nike’s green and pink fluorescent NEXT% colourways that dominate start lines. And, when we return to in-person racing, it will likely be the Black Electric Green (or White/Jade Aura/Flash Crimson) of the Alphafly that stand out on start lines. This is in part because Nike was first to market. And the Vaporfly-line is well-known to make runners faster. A New York Times study analyzing more than a million marathons and half-marathons since 2014 found that runners who race in the Nike Vaporfly 4% and NEXT% run 4 to 5 percent faster versus a runner who doesn’t. Additionally, the runners in the 4% and NEXT% are thought to 2 to 3 percent faster than runners in the next-fastest shoe.

Clearly, not all shoes are created equal. Compare two runners of equal fitness. One in the NEXT%, and the other in the next-fastest shoe. They would not finish even remotely close to each other in a marathon, all else being equal.

Historically, the Vaporfly 4% sold out in minutes in retail. Then, shops consistently replenished NEXT% inventory to keep up with the sky-rocketing demand. Now, the Alphafly NEXT% is just as hard, if not more difficult to buy. Currently, the shoe is sold out at, and a few pairs exist via the sneakerhead online marketplace called StockX.

2017 New York City Marathon champion Shalane Flanagan in 2018 called the Vaporfly 4% “hands-down the greatest gift Nike has ever given me.”

To reiterate, these shoes are not that new. Brands had years to catch up. So, why haven’t they?

Kara Anastasiadis, associate VP of purchasing for FGL Sports (a Canadian retail brand that operates Sportchek), says: “Each brand has its own energy return technologies that they are committed to, invested in and have improved over time. Some of the hesitation comes from not wanting the perception or appearance as a “me too” brand. However, as Nike continues to develop this relatively new technology you will see other brands start to incorporate.”

Brands that do have carbon fibre plates on the market right now don’t use a comparable foam, or, if they do, don’t use the same amount (40+ mm) as Nike’s shoes.


Prior to this year, World Athletics (formerly the IAAF) rules stated that shoes “must be reasonably available to all in the spirit of the universality of athletics and must not be constructed so as to give athletes any unfair assistance or advantage.” So whether Nike’s 4-5% improvement in running economy is considered an unfair advantage is debatable. Shoes being readily available is also questionable as many sponsored athletes race in prototypes which by definition is a preliminary model of a product with limited access.

Many runners, mostly non-Nike athletes, and select leading experts call for stricter regulations. Some recommend a limit to a shoe’s midsole thickness, or a variant. The fact that Eliud Kipchoge wore the Alphafly—at the time a science experiment of a shoe which had three plates and four fluid-filled chambers according to patents—during his 1:59:40 marathon adds fuel to the fire. (Speaking of science, the upper part of the Alphafly is called the AtomKnit.)

It appears the world governing body listened to these complaints.

In a highly-anticipated decision, World Athletics earlier this year amended its rules governing footwear adding that effective April 30, “any shoe must have been available for purchase by any athlete on the open retail market (online or in store) for a period of four months before it can be used in competition.” Further, they state that “if a shoe is not openly available to all then it will be deemed a prototype and use of it in competition will not be permitted.” The shoe must also not exceed a midsole of 40 mm, and contain no more than one carbon fibre plate.

In a fascinating twist of events, Nike around the same time unveiled the consumer version of Kipchoge’s marathon shoe, officially calling it the Nike Air Zoom Alphafly NEXT%. The shoe skirts legality based on World Athletics’ verbiage including two airpods in the forefoot, the same ZoomX foam in the heel as the NEXT%, and one carbon fibre plate. The stack height is conveniently just shy of the limit.

On the other side of the argument, it can be argued that sport thrives on technological innovation. Brands push the envelope to have the best (and in running, best = fastest) product on the market. It’s a discussion that is certainly not over, and perhaps will become even more fascinating as Tokyo 2021 approaches.

One thing is for sure: the footwear race just got a whole lot more interesting.