At this point, most people believe that Vaporflies increase race performance, although there are still some skeptics. There’s even less consensus about how the Vaporflies work. This list is my attempt to put all the theories of how Vaporflies work in one place, whether the theories are plausible or not. Personally, I’m convinced Vaporflies work and I think they’re great shoes, but understanding how they work is especially important if you hate them and think they should be banned. If you get World Athletics to ban shoes for something that ultimately has little to no effect (leading to a rule against carbon plates when it’s the stack height that makes a difference, or vice versa), your efforts will do more harm than good. And some things simply can’t be banned—are you going to make a rule against shoes with an outsized placebo effect?
So here’s my list, organized by category. I think some of these theories are clearly false (such as 0.1). My personal vote is for 5.1/5.2, but I could be wrong. Some theories have logical implications (like if you think it’s all about the foam, then a lightweight Pegasus Turbo should be just as fast; if you think it’s all about the carbon plate, then the Zoom Fly should be just as fast). Feel free to agree, disagree, add your own theories, or discus.
0 Null theories
0.1 Vaporflies don’t make you faster. Everything is just a statistical artifact. People would be running the same times in Adidas Adios or Nike Streaks.
0.2 Vaporflies don’t make you faster. Nike just sponsors the fastest runners.
0.3 Vaporflies don’t make you faster. Instead, runners who can afford them are more serious about running.
0.4 Vaporflies make you faster, but it’s just the placebo effect. The high price creates intense buy-in to the myth by the consumer.
0.5 Vaporflies make you faster because they’re so expensive and have the reputation for low durability, so everyone is running in almost new shoes that feel springier and more cushioned than their regular shoes¬—as new shoes almost always are.
1 Carbon plate theories
1.1 The carbon plate acts as a spring, propelling the runner faster by storing and releasing energy when it bends. That’s why the Zoom Fly, Carbon Rocket and other shoes with carbon plates are just as good.
1.2 The carbon plate acts like a spring, bending and recoiling so that the arch of the foot doesn’t have to, thus saving the runner energy and physical wear and tear. Carbon plates in other shoes bring a similar advantage.
1.3 The carbon plate isn’t a spring. Instead, it acts like a lever, using some of the force of toe-off to propel the runner upwards, relieving the muscles and tendons that would otherwise be required.
1.4 The carbon plate isn’t a spring. It’s stiff, so it enforces toe-off from all five toes rather than just one or two, distributing the muscle strain more evenly across the toes, preventing fatigue, and promoting a more efficient running form.
2 Weight theories
2.1 Vaporflies make you faster, just like any other lightweight racing shoe will make you faster. Besides the light weight, everything else is smoke and mirrors. That’s why the Zoom Fly isn’t as effective.
3 Foam theories
3.1 The Vaporfly isn’t any faster, but its soft foam makes recovery from long workouts and races easier.
3.2 The ZoomX foam is better tuned than Boost or EVA. Just as every spring as a particular frequency for its oscillations, ZoomX absorbs and releases energy at a particular frequency. While a very long spring of 1 oscillation/second wouldn’t be useful in a running shoe, a rate of around 90 oscillations/second might. Nike has tuned ZoomX to a frequency that is a good match for most runners. For this reason, some runners benefit from the Vaporflies more than others.
3.3 The ZoomX foam is softer and therefore more effective than other foams at preventing cumulative muscle damage, which is especially important in marathons. The Pegasus Turbos would be equally effective, but they’re heavier.
3.4 The ZoomX foam acts as a spring by storing and releasing energy, like Boost or EVA, but better. The high heel stack gives an advantage to heel strikers. The carbon plate only adds stability to the platform.
4 Stack height theories
4.1 The stack height in itself doesn’t make a difference. It’s just space to add foam and a carbon plate.
4.2 The high stack height effectively increases runners’ leg length and stride length, allowing them to cover a longer distance with each stride at low metabolic cost, much like running in lightweight Hokas.
5 Combination theories
5.1 The carbon plate doesn’t act as a spring. Instead, it spreads the force of the foot strike evenly across the whole midsole. The whole foam bed stores and returns energy to the whole foot, instead of only the point of highest impact force. This prevents cumulative damage to the foot structures and lower leg due to high impact forces and achieves more efficient energy return than other shoes. The Zoom Fly foam is not soft enough for this effect to occur.
5.2 One of the above theories is the primary explanation, with some of the others also contributing to runners’ increased performance.
5.3 Several of the above theories are correct. It isn’t any one thing, but a combination of several relatively minor things that makes runners faster.