Nike’s Speedy Shoes

Last week, I took delivery of a pair of Nike ZoomX Vaporfly Next% running shoes. Since its inception a few years ago, the Vaporfly product line has attracted more publicity and attention than any other. The record-setting performance caused wide-spread discussion about every aspect of their design and their use in competition – ultimately resulting in regulation changes.

For anyone who has no idea what I’m talking about, here’s a brief catch up:

• 2016 Rio Olympics: All three medallists in the Men’s Marathon wear a Nike Vaporfly prototype.
• 2017 Project Breaking2: Nike set out to break the 2-hour barrier for the marathon. Eliud Kipchoge leads their assault using a new Vaporfly prototype and finishes in a monumental 2:00:25 – taking minutes off the previous fastest time. (The Breaking2 documentary by Nike & National Geographic is well worth a watch, although it appears to have been moved from YouTube to Disney+).
• 2017: Nike release the Zoom Vaporfly 4% to the public.
• 2018: Kipchoge breaks the official marathon world record (2:01:39) wearing a pair of Vaporflys.
• 2019: 86% of the possible podium spots in the six most prestigious marathons are taken by athletes running in Vaporflys.
• 2019: Nike release an updated model, the ZoomX Vaporfly Next%.
• 2019 INEOS 1:59 Challenge: Eliud Kipchoge again targets the 2-hour marathon using a new prototype shoe – the Nike Air Zoom Alphafly Next%. He sets a time of 1:59:40.
• 2020: The prototype shoe used in the INEOS 1:59 Challenge is banned by the IAAF. Nike release a regulation legal version to the public.

The Nike ZoomX Vaporfly Next% and Zoom Alphafly Next% remain as Nike’s top road-running shoes. Although competing brands have closed the gap, Nike’s are still known to be the fastest available, and see amateurs and professionals tear up their personal best times alike.

As expected, upon their release, the shoes caught the eye of a lot of people. They had been developed largely from scratch with Nike using new materials in all parts of the shoe, and including a carbon fibre plate within the sole.

However, not all the attention they gathered was positive, and many people raised concerns. Although some of these issues were legitimate, many of them were based on an imperfect understanding of what made the shoes fast. That is what I’m hoping to explain in this post.

Absolutely nobody asked for this, but I spent the majority of a training run on the weekend thinking about this and I haven’t posted anything on here in a while… so why not?

A couple of things to note:

• I have not actually worn the shoes yet. This is not going to be a review. There are plenty of those already on the internet in various forms. The shoes have been proven to be faster in a number of independent studies so my opinion would be fairly inconsequential. Instead, my aim is to dispel some misconceptions surrounding them, and respond to some of the critics.
• Last year, as part of my engineering degree, I completed a short coursework assignment on this topic. This blog is not going to be written like an academic paper, because that would be super boring. As such, I am not going to include references right now. However, at the bottom you will find a link to the coursework I did, which is complete with citations and should cover any claims I make here. If you would like to call bullshit on any of my comments in this post, please read that first… and please do not retort with a link to a Buzzfeed article that says it’s like wearing rockets on your feet – that does not count as a credible source.

Springs for shoes!

First things first – the carbon fibre plate is NOT a spring.

When the shoes were first released, the carbon plate was the hot discussion topic in every article. When you think of the components necessary in a running shoe, a carbon plate is typically not one of them. I think this is what caused people to start speculating so violently. I’m assuming the conversations and thought processes went something like this…

I did the same thing. However, when I was researching for my dissertation (also related to running technology) I followed a trail of journal papers that showed this whole thought process to be wrong. The carbon plate is in fact there to increase the stiffness of the shoe.

What does that mean? Well, compared to other running shoes, the Vaporflys are a lot less bendy.

It may seem counter-intuitive, but through altering the ankle joint moment, this actually makes running more efficient… stay with me.

Imagine that you have to loosen an extremely stubborn bolt, you’ll have a much easier time if you’re able to use a spanner with a long handle.

Now, to apply this to a runner, think of the ankle joint as the bolt you’re loosening, the foot as the spanner handle, and the toes (where you’re pushing off from) as the hand turning the spanner. Loosening the bolt is easier with a longer and stiffer lever, just as pushing off from your toes is more efficient with a longer and stiffer lever.

This analogy is very oversimplified, but hopefully it illustrates the principle at play. The carbon plate stops the toes flexing, thereby increasing the effective length and stiffness of the foot.

The reality is that longer and stiffer does not necessarily equal better (just ask any bloke past retirement) – there is an optimum for each of these factors*. However, I don’t think it’s necessary to get bogged down in the biomechanics, nor the influence these changes have on ground contact time. My main hope is that this serves to dispel the myths that the carbon plate acts as a spring.

*(The optimal stiffness of the shoe depends on each individual runner and is largely related to their weight. Nike have somewhat addressed this by tweaking the stiffness based on shoe size – given that people with larger feet are typically going to be heavier).

What makes them so fast?

There are a number of contributing factors when it comes to this question, and the carbon plate and increased stiffness is one of them. They are also very lightweight, and, apparently, more aerodynamic than previous models. However, the most significant technological development lies in the foam.

The ZoomX foam, of which there is a lot (look at the size of the sole compared to predecessors), has a much greater energy return than any of its competitors. Basically, when you go to push off at the end of a stride, the foam bounces back more than others. Progress has been made in this regard by companies such as Adidas, New Balance, and Hoka since I researched it in detail, but Nike are still leading the charge. This foam is the main reason records tumbled.

Springs for shoes! – Part 2

OK so, even if the carbon plate doesn’t act as a spring, it sounds like the foam does?

Yes and no.

This is another counter-intuitive point, but in short, running on springs would NOT make you faster.

Let me reiterate that… even if the carbon plate did act as a spring, it would not make you run faster. And by the same logic, in my opinion, would therefore not be cheating (a number of people called for the shoes to be banned).

This is tricky to visualise. I found myself exploring a rather deep YouTube rabbit-hole trying to get my head around this. The science relates to the influence on ground-contact time but saying it like that doesn’t really help anyone. For whatever reason, if you picture somebody running around with springs for shoes, it’s difficult to imagine that they’ll actually run slower.

An easier way to think about it, is to scale it up and instead imagine that you’re running on a really long trampoline. The springs are now on the floor rather than your feet, but the principle is the same.

As you run, within each individual stride, you will be able to travel further and faster. However, each of these strides takes so much longer since the floor retracts underneath you before springing back up. Essentially, every time your foot touches the floor, you have to pause while the trampoline does its thing before carrying on. To further the point, it helped me to visualise someone sprinting along a solid surface next to me – they would win.

It doesn’t help that if you Google whether springy shoes are faster, you get met with mixed and ambiguous results. Firstly, there is one legitimate paper (here) that states springs could be used to allow humans to run faster. However, on closer inspection the proposed mechanism is an exoskeleton that covers the whole leg, and not just the shoe.

In contrast to this, you can find YouTube videos such as this one: Fastest Man in the World. It features a highly delusional man who’s invested a lot of time and money in creating his own springy boots. He claims he’s super-fast. Spoiler alert – he is not.

The comments section underneath is filled with others willing to believe his unsubstantiated claims and they’re either hilarious or painful to read depending on how you look at it (yes, I see the irony given that I’ve not included any references here).

What does this mean? If you had springs on your shoes, you would indeed cover more distance with each stride, but each time you hit the ground, your foot would stay on the floor for longer whilst the springs compress and decompress – to such an extent that you would actually slow down overall.

So, to summarise…

Carbon plate for stiffness + energy efficient foam = fast. In my oversimplified world, it’s that simple!

With the above in mind, I would like to explicitly say that I do not believe these shoes represent cheating on any level. Yes, the ZoomX foam is ‘springy’, but that’s also the case with foam used in other shoes, just to a lesser extent. Whichever way you spin it, the energy used to propel the athlete forward comes exclusively from the runner. The shoes make runners more efficient, they do not provide extra power

So, there’s no problem at all?

Not exactly. In reality, whilst using the shoes should not provoke cries of cheating, the existence of them creates a number of potential issues. Ones that can’t really be argued with facts and lab studies, but are more based on opinion. For example, athletes sponsored by shoe brands other than Nike, suddenly found themselves at a distinct disadvantage. Whilst use of the Vaporflys should not be considered cheating, where do you draw the line before the competition becomes irrelevant? How does this situation compare to the Speedo LZR swimsuits that were banned by FINA in 2009?

When I sat down to write this post, I had it in mind to talk mainly about these points, rather than write a hopefully-not-too-boring explanation about the workings of a shoe… I thought my answers to the harder questions needed context if I was to justify them, and now here we are. I’ve spent the majority of my rest day writing this and drawing stick figures, so the other stuff will have to wait.

That’s all for now. I’m still living in Girona. The race season has started. Things have been going pretty well.

Thanks for coming.

Tom : )

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Here is the link to the university assignment that I mentioned:

Disclaimer: I really enjoyed researching the topic, but this assignment pissed me off. Everyone on the course was given a different essay title by a number of different lecturers. The pretentious dick I got stuck with never responded to any of my emails and COVID was happening so I couldn’t hunt him down in person. All I knew was that I had a 2000-word limit for a topic I could have written 10,000 about. I had no reference for the criteria I was to be marked against, and therefore very little idea of what to expand in more detail and what to leave out. All I’m saying is, if you choose to read it, please don’t judge the piece of work at face value!