The Nike Vaporfly 4%, Next%, and now the Alphafly Next%; the world’s most controversial line of shoes have made running popular enough that it actually got on the news – and not just because of the Olympics. But, what made these shoes so controversial? Was it the carbon fiber plate, the new specialized foam, or how it aided Eliud Kipchoge in accomplishing the once-thought impossible feat of a sub two-hour marathon? Well, today, with the power of SCIENCE, and hours, nay, days of research I have found the answer.

For a brief moment, let’s recall an earlier post about the history and evolution of running shoes, in which we learned that Nike popularized an increase in the cushioning of shoes, especially in the back heel, with the Nike Cortez. This became a trend in daily trainers, with running shoes becoming higher in volume and overall cushioning to absorb more of the initial impact of the foot into the ground. However, racing shoes largely retained a more minimal design, favoring less cushion and less weight. This continued right up until 2016, when the Nike Vaporfly came onto the scene. 

Nike’s goal for the Vaporfly was to design a shoe that was capable of aiding a select few runners in breaking the two-hour barrier in the marathon. To do this, the shoe needed to do something a little different than the typical racing shoe. Rather than continuing to make minimalist racing flats, Nike went with a high-cushion design using a new material other than the usual EVA and TPU compounds found in trainers and racers. This new foam was called Polyether block amide, more commonly known as Pebax.

Traditional elastomers (better known as foam), are limited when it comes to energy return – how much kinetic energy from impact pushes you back off the ground, as opposed to being lost to the ground as heat. EVA foam tends to return 50% to 60% of this kinetic energy, while TPU performs slightly better, with a range of 70% to mid-80%. It’s even higher for Pebax foam – the Nike Vaporfly 4% was so named because it provided 4% greater energy return compared to the other leading racing shoes on the market at the time.

Energy return is tested and calculated using a series of mechanical tests, in which a shoe last is fitted to a servo hydraulic compressor system that applies a set force of 2000 N. The effects of force from 0 N to 2000 N are recorded over 185ms to observe the deformation of the shoes – below are some of the charts and graphs regarding each of the models and their overall deformation over amount of force.

As you can see, there are two curves that meet at the point where 2000 N of force are applied. Those two curves represent the loading (upper curve) and unloading (lower) curves, and the area in between represents the energy lost as heat. The Nike Vaporfly (labeled in this experiment as NP) has a noticeably larger and longer deformation curve that also has a smaller area compared to the other two test models.

Here, we see the two most critical points in shoe design: compliance and resilience. Compliance refers to the amount of compression that occurs when a certain amount of force is applied, which tells us how soft a shoe is. Resilience is the percent of mechanical energy that can be stored and later returned, and is the reason for the carbon fiber plate now ubiquitous in racing shoes – it has been found to decrease the mechanical energetic cost of running by ~1% simply because of its stiffness.

So, case closed, it’s the carbon fiber plate that is making these shoes so phenomenal for running! Not exactly. Let’s go back to the two main factors of shoe design – compliance and resilience. If a shoe were to be completely resilient and not have a lot of compliance (or softness), then the curvature of the Force and Deformation chart would have very little deformation and bend, meaning very little trapped mechanical energy. When the shoe attempts to return that energy, the secondary unloading curve will be a lot lower, with a huge amount of mechanical energy lost as heat. So, the carbon fiber plate isn’t the sole reason for the shoe performing as well as it does. What does aid in the performance of the shoe is the use of the Pebax foam and its high compliance, which traps a significant amount of mechanical energy through its high compression and bounciness*. The carbon fiber plate provides resilience, which makes it easier and much smoother to release that trapped mechanical/kinetic energy, and is why the bottom unloading curve is so close to the upper loading curve.

Although the Vaporfly 4% was something new and exciting, that wasn’t the shoe that really set these high-cushion, carbon fiber-plated shoes apart from the rest – and neither was the Vaporfly Next%. If anything, it was the Vienna Nike Alphafly Next% which was used by Eliud Kipchoge to break the 2-hour marathon mark, setting off a huge wave of controversy that prompted new regulations regarding racing shoes. Racing shoes were ultimately limited to not having greater than a 40mm stack height and not having more than one plate. These were implemented because of some of the designs that Nike had for the Alphafly Next%, which were ultimately never released commercially. The shoes that Kipchoge wore during the sub two-hour marathon were speculated to have three different carbon plates (some not full-length plates) and possibly two sets of Zoom air units in the forefoot to provide extra propulsion and higher energy return. The Zoom air units provide an effect similar to a bouncy ball compressing, shifting the way in which compression affects the ball and putting an even higher spring in the back to propel the ball forward (see below).

The compliance of Pebax foam trapping mechanical energy and the resilience of the carbon fiber plate providing an explosive output of that energy, plus a little extra pop from the Zoom units, combine to create a whole new beast of a shoe that can make running feel like cheating.

However, I do not believe, despite trying out the shoe for a brief moment and distance, that it is cheating – I think the boundaries set by the World Athletics are more than fair as a limit and allow companies to push forward and innovate fairly, even for smaller companies. When it comes to the Vaporfly Next% and the new Alphafly Next%, a lot of what makes the shoe isn’t complete rocket science or something that involves magic and finding a unicorn; in reality, it’s just reapplying a lot of simple physics we already know and using them to make something even better.

*Bounciness come from a high coefficient of restitution, where a number indicates how much kinetic energy remains after a collision of two objects. If the coefficient is high (very close to 1.00 out of 0-1) it means that very little kinetic energy was lost during the collision.