Rai Benjamin of the US may have won the silver medal for Men’s 400-metre hurdles at the Tokyo Olympics this week, but he didn’t escape the controversy surrounding his shoes, and whether they created an unfair advantage.
Despite breaking a world record himself, gold medallist Karsten Warholm of Norway took aim at Benjamin about his “super shoes”, developed by footwear giant Nike.
“If you put a trampoline there, I think it’s bull***t,” said Warholm. “I think it takes credibility away from our sport. I don’t see why you should put anything beneath a sprinting shoe.”
The controversy centres on whether Benjamin cheated by essentially “tech-doping”, using a shoe technology that gave him a unique advantage – namely, a carbon plate insert in the sole that acts as a kind of lever to propel a runner, and new foam padding underfoot that appears to spring back after it is compressed.
Ironically, Warholm wore his own specialised shoes.
“What I can say about the shoes that I’ve been developing in a collaboration between Puma and the Mercedes Formula One team is that we’re trying to make them as credible as they can be,” Warholm said.
“Yes, we have the carbon plate (insert), but we have tried to make it as thin as possible because that’s the way that I would like to do it.
“Of course, technology will always be there, but I also want to keep it down to a level where we can actually compare results. That’s important.”
Released in 2017, another shoe garnered a similar type of reaction. The Nike Vaporfly 4% was widely criticised as giving an unfair advantage to distance runners, and a 2021 study confirmed that they reduced marathon run times by around 2%.
With so much shoe technology, you’d think there’d be good science to back it. Is there?
How are shoes regulated in athletics competitions?
In athletics, shoes are a huge deal. Technology is always evolving, so new shoes with their innovations frequently hit the market. But for shoes to be accepted in official competition, they must adhere to regulations set out by World Athletics (formerly known as the International Association of Athletics Federations).
World Athletics claims that the spirit of athletics regulations is to promote fairness and accessibility, so they assess each competition shoe to make sure it won’t give an unfair advantage. While the rules are in black and white, they are constantly evolving to keep pace with the technological advancements.
According to the rules, “Athletes may compete barefoot or with footwear on one or both feet. The purpose of shoes for competition is to give protection and stability to the feet and a firm grip on the ground. They must not give athletes any unfair assistance or advantage. Any type of shoe must be reasonably available to all in the spirit of the universality of athletics.”
Each athlete must fill out a shoe declaration form identifying the shoe company, model and any orthotic additions for medical reasons; any shoe models in development are prohibited. If a referee believes a shoe has given an unfair advantage to an athlete during a competition, they can confiscate them immediately after for reassessment.
There are two parts of the shoes that raise the most suspicion – the soles and the spikes. Nike’s “super spikes” are under particular scrutiny right now.
“When I was told about [records broken by athletes who wore the shoes] I couldn’t believe that this is what we have gone to, you know what I mean?” Usain Bolt told Reuters in an interview from Jamaica. “That we are really adjusting the spikes to a level where it’s now giving athletes an advantage to run even faster.
“It’s weird and unfair for a lot of athletes because I know that in the past they [shoe companies] actually tried and the governing body said, ‘No, you can’t change the spike’, so to know that now they are actually doing it, it’s laughable.”
Spikes are obviously allowed for track athletes, and Nike aren’t the only manufacturers to add them to shoes, but they must meet certain specifications: precise rules detail the acceptable length, diameter and number of spikes allowed.
First of all, they must be no longer than 9mm, except for those used in high jump competition, where they can be up to 12mm. In terms of diameter, a spike must also “fit through a square sided 4mm gauge” for at least half its length.
Second, “Any number of spikes up to 11 may be used but the number of spike positions shall not exceed 11.”
Nike has developed multiple spiked shoes, but the two controversial super spike models are the Air Zoom Victory and the Zoom X Dragonfly, both of which are approved shoes for all events on World Athletics’ Shoe Compliance List.
There are also specific regulations for the soles of the shoes. Thickness and inserts regulations vary across events, where long distance events are allowed thicker soles.
The Vaporfly 4% shoes have thicker, springy soles and are currently only approved in road – long distance – events.
Any grooves or ridges on the bottom of the shoe must be made out of the same material as the sole, which “must not contain more than one rigid plate or blade made from carbon fibre or another material with similar properties or producing similar effects, whether that plate runs the full length of the shoe or only part of the length of the shoe; and may contain one additional rigid plate or other mechanism only where used solely to attach spikes to the outer underside of the shoe.”
What are super spikes made of?
The ZoomX Dragonfly foam and plate are made from Polyether block amide (PEBA), which is a flexible polymer with high elasticity. They are light, compressible and resilient, which means they bounce back into shape when pressure is applied, essentially returning energy to the athlete.
The Air Zoom Victory also uses PEBA foam and a carbon-fibre plate that curves up at the toes. Carbon-fibre is claimed to keep the toes straight and stabilise the runners ankle, reducing the force of impact on the calves. Overall, this should help the athletes put less effort into the run, which can be used to decrease run times.
However, a study from 2014 found the biomechanical response to plates was unique to individual athletes. The researchers found that some athletes changed the angle of their ankles as they hit the ground, which may reduce the force of impact and stabilise their legs, but this varied between runners. The study was also performed on distance runners who may not represent sprinters and hurdlers.
A 2017 study found that muscle effort and metabolic cost – that is, how much oxygen is being used in cells – were slightly lower when wearing stiff soles, because angular momentum was lower in the joint between the foot and toes. But it only helped if it didn’t disrupt natural joint movements, which depended on the individuality of the runner.
This was iterated again in a 2020 study: plates didn’t alter the body’s movement enough to reduce metabolic cost and had no effect on running economy. However, sprinting is also an anaerobic activity that doesn’t rely on metabolising oxygen, and so this study may not represent the mechanics of sprinting.
The spikes themselves are designed for either sprints or long distances. For sprints, spikes are put at the front, not the back, because sprint athletes spend more time on their toes than on their heels. In this case, the super spikes are light but stiff to provide grip, even when it’s raining.
Victories and Dragonflies have 6 stainless steel spikes to take the force of the foot as it falls, supposedly preventing energy loss. Generally, more spikes can take more force, which is why World Athletics has a limit on them.
But, like the carbon-fibre plates, these, too may have highly individual effects depending on the athlete.
However, there isn’t any publicly available scientific evidence that suggests the plate, plastics or spikes make enough of a difference to be considered an unfair advantage.
Are they really better than other shoes?
This question can’t be answered yet, as all the data is either held by Nike, or is purely anecdotal.
Unlike Vaporflys, the two Nike super spike models haven’t undergone the same types of scientific study, so it’s unknown whether they really give a statistical advantage.
To prove that the shoes were giving an unfair advantage, they would need to be subjected to short, medium and/or long-distance time trials with a large sample size of professional (and preferably amateur, too) athletes. Professional athletes are useful in this situation because they generally have records of previous runs.
Most importantly, the study should not be funded by a private sporting company, as that can add a bias to the results by driving the research in a specific direction and confirming the wrong questions.
Such a study would provide evidence as to whether the shoes were shaving time off a run, but it hasn’t happened yet.
A study like this could also determine whether the pandemic has skewed recent running results. Anecdotally, record runs may be the result of having more time to train, although some athletes will have lost training time depending on restrictions in their country.
Likewise, Nike-sponsored athletes who wear the shoes not yet released to the public are already elite athletes, and might have an extra advantage because they can train with Nike sponsored coaches and facilities. This means studied presented by Nike may have an implicit bias, another pointer to the necessity of independent studies.
So why hasn’t the research into super spikes happened?
There are multiple reasons why research isn’t conducted, but the big one is often the same: money.
Research is less likely to be funded if it only benefits a few people. Top-level track shoes are generally only used by elite athletes, so there aren’t substantial public health issues in play.
On the other hand, road shoes for long distance are more commonly used by amateur athletes, or people who run as part of an everyday fitness regime. These may be researched by private companies, like Nike, to determine safety, performance and marketability. In this case, a company may choose not to disclose or publish information they wish to keep private – for example to prevent a rival company adopting the technology. Alternatively, they may choose to release research even when an implicit bias is present, in order to present only the most beneficial results for the company – for example, only publishing results that favour their product over competitor’s.
While the research conducted by sports technology companies is instrumental to technological advancement, the results must still be independently scrutinised for genuine validity.
The second reason is that anaerobic performance is notoriously hard to measure, as pointed out by a preprint article by Healey et al. This is because the body switches from using oxygen to carbohydrates to fuel its movement when sprinting, so it’s a lot harder to measure when metabolism isn’t constant.
Beyond this, the performance of shoes varies between sprint and long-distance running, and much of the literature is based on long distance and may not translate to sprints. This means the most feasible studies involve time-trials, which could show whether the shoes make a difference, but not show how.
When it comes down to it, without accurate, preferably independent, science, we can’t know whether the shoes truly make a difference.