They Can Get You Disqualified, But Are Thicker Shoes Faster?

The recent disqualification of the Derara Hurisa from the Vienna marathon was the first high-profile disqualification of an athlete for a violation of the new footwear rules from World Athletics on the roads. Hurisa’s shoe — the Adidas Adizero Prime X — had all the ingredients of a super shoe: better foam with rigid pieces embedded — with one distinctive quality: It was thicker than the current regulations allow.

This revisits the classic question around the rules: Are thicker shoes faster?

The answer is nuanced, but it boils down to this: Faster shoes are thick, but thicker shoes aren’t necessarily faster.

The shoes of today that are helping athletes run faster do so from the interaction between more perfect foam — lighter, softer, and more resilient polymers — and rigid architectural pieces — curved and embedded stiff plates and rods. In 2020, World Athletics put regulations in place to manage the potential performance advantages by limiting the thickness of the shoes and the extent of those architectural features. For a detailed look at the rationale for the sole thickness regulation strategy, see: Why Limit Sole Thickness In Running Shoes?

Benefits of a Thicker Sole

Adding more of the foam to the bottom of runners’ feet is likely to be beneficial for two reasons.

First, its greater compliance (softer cushioning) and its higher resiliency (greater energy return) help the runner to run more efficiently by wasting less energy step-to-step and recycling more energy underfoot. If the foam is functioning like a spring, a longer and longer softer, more perfect spring can store more and more energy and give more and more of it back.

Second, the foam provides a three-dimensional matrix within which to put the aforementioned rigid pieces. The specific mechanisms of how and the extent to which the rigid pieces (e.g., curved carbon fiber plates or rods) are beneficial are still being elucidated, but it’s likely helping the body both use the benefits of that foam — the foam on its own it may be too soft or unstable — and and by subtly manipulating the runner’s mechanics to more efficiently move through foot strike. These benefits are great, in theory, as they allow us to augment the runner’s legs with elements that ostensibly function better than the human body’s own elastic structures without fatiguing.

Costs of a Thicker Sole

However, shoe optimization is not a simple exercise of maximization or minimization of a particular design feature. At some point adding a lot more of that foam will become detrimental.

The first and most obvious reason is weight. One of the canonical heuristics in shoe design is that a 100g addition to a shoe decreases running economy by 1%. More foam means more mass, and at some point the cost will literally outweigh the benefit.

The second is the issue of stability, which itself is multifactorial. As a shoe gets thicker and thicker, the runner has to use more energy and muscular control to stay upright, and it invites greater and greater risk of catastrophe (i.e., falling or injury — both of which will dock more than a few percentage points from the runner’s efficiency). This would be an issue in straight-line or treadmill running, but it would be further exacerbated by deviations that a runner would experience on most road courses, e.g., turns, uneven footing, etc.

Finally, even if the shoe’s foam or architecture affords more perfect spring-like mechanics, the runner may not necessarily be able to fully use those spring-like structures within the realm of the forces that a runner typically generates or with what current materials allow.

Balancing Those Costs and Benefits

So, to make an advantageous shoe, using more foam than old racing flats has been shown to be beneficial, for the aforementioned reasons. The favorable mechanical qualities of new foams, coupled with their lighter weight, shifts that optimality of how much is beneficial. Moreover, the thicker and thicker the sole, the more and more design space there is to configure those rigid elements to be more beneficial — a curved, embedded plate, à la the Vaporfly, is going to be better than a flat plate, especially if that flat plate had to be on the top or bottom of the shoe.

A 32 or 33 mm shoe with next-generation foam and a more tailored rigid piece embedded within it will allow for engineering that more fully harnesses the potential advantages of the technology, and would likely further advance the benefit — which is what was demonstrated with the early iterations of the Vaporfly. It consistently enhanced runners’ efficiency in well-controlled studies. The 36-40 mm shoes that are on the feet of most athletes at major marathons now may take it further. The point at which those benefits start to be overwhelmed by the cost of a higher shoe is likely highly individualized and has yet to be determined.

So Were Hurisa’s 50mm Shoes a Performance Advantage?

Maybe, but maybe not. The addition of more of that better foam might indeed be beneficial for him. Again, it adds more of a synthetic, highly resilient elastic element to the leg that does not fatigue. However, the shoe weighs 57 g more than Adidas’s current legal super shoe (the Adios Pro 2) and 85 g more than the Nike Vaporfly Next%. So, its benefit must overcome at least a 0.5% loss in efficiency.

Furthermore, it has to be stable enough for him to safely run without wasting more energy to stay upright. That’s not trivial moving at 3 minutes per km (under 5 minutes per mile) on 5 cm-high shoes.

There are even more unanswered or unexplored prognostications about how the shoes may or may not be beneficial. These include the extent to which training in them may help, or how the benefits or detriments might evolve through the fatigue of a race. So, we’re left with an unsatisfying question mark.

Why Limiting Shoes Is Good for the Sport

The answer to that question of whether they were an advantage — “we don’t know” — is one of the primary reasons why regulations around shoe technology have an important place in our sport. Rules create a framework in which we can understand and appreciate performances.

The advocation to regulate shoes based on their thickness was not a suggestion that thick shoes are always beneficial, but that more and more substantially beneficial shoes are likely going to be thicker. Limiting the thickness of the shoes creates an eventual ceiling for the extent to which they can be advantageous.

Without that, performances in distinctive new technology will always invite questions of the extent to which the equipment played a role, obscuring the athlete. Derara Hurisa beat Leonard Langat by 3 seconds. Had he been in regulation shoes, would the conversation have been about the great race that happened at the Vienna marathon, or perhaps the commanding victory of Leonard Langat, or maybe the more commanding victory of Hurisa? We don’t know. Instead, we’re asking, “Was it the shoes?”

This is great in the short-term for the manufacturers of those shoes and for the shoe geeks among us, but are there longer term consequences of perpetually shifting conversations away from the athletes and constantly shaking up the understanding of the physicality of performances? We don’t know the answer to that either, but the rules as they stand now have seemingly and effectively shifted conversations back to the races over the past year and a half. Importantly, they’ve also allowed us to build out an understanding of what good and great performances are in the new footwear without perpetually confounding that perception. I’m excited for the scientific exploration of these footwear characteristics — thicker soles and their potential optimizations — but for now, I’m also excited to enjoy athletes racing on consistent footing.

Geoffrey Burns, Ph.D., is a postdoctoral research fellow at the University of Michigan. He studies running, biomechanics, and sport performance. He also competes internationally in ultramarathons.