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In the beginning was the VaporFly, Nike’s magical new shoe that let people run faster than ever before. And runners saw that it was good — perhaps too good. Then came others, one or two cousins at first, and now a whole clan of shoes that share familiar characteristics and let runners from diverse tribes compete on an equal footing, or close.

Now with a super shoe from every major brand and constraining their construction, it would seem we should understand what makes them magical. But scientists continue to debate what elements of the shoes contribute most, and try to parse the exact source of their performance-enhancing effect. The answers have implications for how they are regulated, and, more importantly for most of us, how we find the best pair for our training and racing.

The Recipe

The basic ingredients for a super shoe are well known. “There’s a recipe here that all of these shoes are following: A very compliant, very resilient foam, lightweight, with rigid architecture within it,” says Geoff Burns, ultramarathoner and research fellow in biomechanics at the University of Michigan. That “rigid architecture” comes in the form of a stiff, curved rocker plate that runs through the middle of the thick foam midsole. Neither thick stack heights (think HOKA), nor resilient “energy-return” foams (think Boost), nor rockers (HOKA again, and clones), nor plates (think track spikes or the On speedboard) are new — but the specific materials and combination revealed in the Nike Vaporfly was new, as was its laboratory-proven ability to reduce the energy cost of running by 4%, making all previous racing shoes instantly obsolete.

In the Vaporfly, the foam was a new compound made out of Pebax, one that returns more energy — or, more accurately, loses less relative energy — than any other foam to date. The Vaporfly’s plate was made of carbon fiber and had a unique, dramatically curved S-shape. In the ensuing years, other companies have tinkered with different ingredients in variations on the same theme. But it remains unclear whether the magic comes from the foam, the plate, specific characteristics of either, or some combination of effects we haven’t figured out yet.

Pieces of the Puzzle

In the past two years, multiple studies have given us small pieces of the performance-enhancing puzzle.

In 2018, researchers at the University of Colorado compared the new shoes to two traditional racing flats, and concluded that, while substantial energy was stored and returned by the resilient foam, the plate’s contribution was miniscule (50 times less). The ’s key takeaway, widely reported, was that the “bulk of energy saved by Nike Vaporfly 4% comes from softer foam midsole, not carbon fiber plate.”

The researchers didn’t say, however, that the plate was peripheral — it just wasn’t acting like a spring and returning energy. They noted that in addition to the superior energy storage and return of the mid-sole foam, the metabolic savings of the shoes appeared to come from “the clever lever effects of the carbon-fiber plate on the ankle joint mechanics” and its stiffening effects on the ball of the foot. In sum, said lead author Wouter Hoogkamer, then a postdoctoral researcher, “We found that it is not one magic thing that explains the metabolic savings in this shoe, but rather a combination of a whole bunch of biomechanical factors related to the foam and the plate.”

Adding to the knowledge about the plate, at the bi-annual Footwear Biomechanics Symposium in July 2019, a Nike team presented some of the research they did during the design of the VaporFly 4%. The swoosh designers prepared four prototypes, identical except for the curvature of the embedded plate, which ranged from flat to extremely bent, then compared their performance. The research showed that advantageous mechanical changes — at both toe joint and ankle — increased with the curvature of the plate, and were optimized with the extremely curved shape (a shape which, not coincidentally, they ). While this research revealed the importance of the plate’s curve, it didn’t shed much light on its role and importance relative to the foam on the overall performance-enhancing effect of the sole.

Last summer, esteemed biomechanics researcher Benno Nigg published two in scientific journals in which he argued that the rigid, curved plate was actually the key, and could, if optimized, increase performance by up to 6%, while the foam, no matter how bouncy, only accounts for 1%. His reasoning is that energy return, from either foam or plate, is largely irrelevant for distance-running performance as it doesn’t get returned at the right time or place to help push-off. But, Nigg says, looking at energy-return missed the point of the plate: “The storage of energy doesn’t have to do anything with that plate. Storage of energy means you can bend, then bounce back — that doesn’t have anything to do with it.” Nigg’s hypothesis says that instead of spring-like effects, this stiff curved plate is doing something new, namely a “teeter-totter” effect: Force directed downward on the toe is transferred, via the fulcrum of the curved plate, upward on the heel, enhancing propulsion without requiring muscular energy.

Other scientists were respectful of the ideas, but skeptical that the teeter-totter effect is actually primary in this puzzle — partly because Nigg’s conclusions were based on modeling, not research, and partly because it seems overly simplistic and doesn’t appear to account for the combined interaction of the novel materials in these shoes.

“I would not, based on what I’ve seen so far, say that the teeter-totter is the most dominant factor,” says Hoogkamer, now Assistant Professor of Kinesiology at the University of Massachusetts, Amherst. “For me, it is still the foam and its energy return that is dominant. I’m not going to say that the teeter-totter is not a thing; I think that it makes some sense. But it is more a complicated story and we need to look at all the other things.”

Burns says, “Whether or not there is a teeter-totter mechanism —  and there may be something to that effect— A) it’s definitely not 6% in a shoe. And B) it’s probably not even the lion’s share of the benefits.”

Recent soon to be published by Hoogkamer and colleagues further clarifies — or complicates — the issue. The UMass researchers made multiple cuts in the plates of some Nikes so that they would bend freely both up and down, then compared their performance to intact shoes. Surprisingly, while they observed some differences in the mechanics around the toe joint, ankle mechanics and, most telling, running economy were essentially the same when running in the cut-plate shoes as in the intact ones.  This new research seems to indicate that the plate is acting neither as a spring nor a lever. “Our new data suggests that it’s not really doing that much,” says Hoogkamer.

However, Laura Healey, first author of the study, points out: “It is important to keep in mind that we didn’t fully remove the plate, so we can’t completely say its overall effect is negligible. It is possible it still may have been contributing to the overall distribution of pressure while running, or helping maintain the overall geometry of the shoe, for example.”

What Do We Know?

While the scientists may disagree on what element is most important, revealing how much they, and we, still don’t know about how the shoes work, they all agree that it’s complex and interconnected. In fact, trying to isolate the effect of the ingredients may be asking the wrong question.

“It’s entirely reductionist to try and isolate the effect of a single factor and tie it to performance,” says Burns. “Because any of those things, whether it’s the mid-sole material, the thickness, the longitudinal bending stiffness, the weight of the shoe, the curvature within the plate or placement within the mid-sole. All of those play off each other: You change one and it changes the optimality of the others… it’s just a complex movement and highlighting any one aspect of it just might be a fool’s errand.”

The total effect could, in fact, be greater than the sum of the parts. “It may be that 1+2=4,” says physiology professor Rodger Kram, who led several University of Colorado studies on the Vaporflys. “Maybe that there’s a synergistic effect between the foam and the plate that we don’t understand.”

The consensus among most of the scientists is that neither the foam nor the plate can work independently. “The foam is kind of the key that opened the door, but in order to realize the benefits of that foam, you need another architectural piece in it,” says Burns. “The presence of the plate could be something that better orchestrates the compression and decompression of that foam… to essentially move your foot through the length of the shoe quickly and efficiently.”

“The plate’s purpose is to give some integrity to all that foam,” Iain Hunter, biomechanics researcher at BYU, told Brian Metzler on a . “They’re so high off the ground, they feel a bit unstable. That plate in there helps with the integrity of the foam, keeping it so you can run without wobbling.”

Kram says: “The plate is mostly acting as kind of a glue or a stabilizer… it’s the coherence of the plate that keeps the foam from becoming a jelly donut.”

Trying to separate the effect of one part is rather like arguing whether it is the muscles or the bones in your arm that lifts a weight: they are part of a system that doesn’t work as independent parts. And, until someone comes up with better materials or an entirely new design, it seems we need all ingredients in the recipe to make the magic.

Given this complexity, the World Athletics ruling limiting stack heights, while arbitrary, seems appropriate as it constrains how much you can put into the midsole without trying to isolate which ingredients you use. Burns elaborated on the arguments for the ruling in .

So What?

In the end, as runners we want to know what all this research and debate means for us. Perhaps the greatest take-away is that these shoes and their effects on your stride are still somewhat mysterious. “The complexity of this speaks to the infinite complexity of the human body,” Burns says.

Because the total effect of the super shoes is complex, how the shoes interact with each unique runner is as well. continue to point out that the benefits these shoes provide differ from individual to individual.

This individual response becomes very apparent when you start trying on the various models: Each one has a distinct bounce and balance, a unique roll and ride — and each runner feels and reacts to these characteristics uniquely. As an example, when we had several editors and writers , each of us was adamant that a different one stood out above the others, and that the others’ favorites were less than perfect, even subpar.

“Some are right for you and some are not right for you,” says Nigg. “Because each athlete has a different foot, so the stiffness of the foot is different, the joints are different.” Each runner has to find the one that works for your physiology and your stride.

“People react differently to everything,” Kram says. So you can’t depend on what someone else likes or thinks is best. “The stupidest thing you can do in choosing a running shoe is to ask what your friend wears. I would never ask my friend, “What eyeglasses do you wear,” and then wear his glasses.

Unfortunately, unlike eyeglasses, you can’t yet go to your local running store and get a reliable prescription for what is best for you.  If you’re in Memphis, however, and willing to part with an hour and $100, you can get a “” from biomechanist Max Paquette at the University of Memphis Human Performance Center. Paquette says they’ll put you on a treadmill equipped to measure your metabolic data and running economy, and have you run 4–6 minutes at your goal race pace in up to four different shoes in “mirror” order: shoe A, B, C, then C, B, A. Paquette started offering the test in late 2019 in the wake of all the shoe research and debate, which he found mostly irrelevant for the individual runner.“If you’re Jonathan and you want to know what shoes are best, it doesn’t do you much justice to read some papers about it,” he says, “You have to figure out what shoe works best for you.”

Any lab that can measure V02 max could do this, although Paquette is unaware of anyone else offering such a shoe-comparison service. Lacking access to such a lab, he recommends following a similar test protocol yourself with multiple shoes. Running on a “pretty stiff treadmill” and carefully keeping all variables except the shoes as consistent as possible, you can compare chest-strap measured heart rate at the same pace in each shoe.

Kram recommends performing this test on a track, and limiting the comparison to two shoes. After you’re warmed up, run a mile at marathon pace twice in each shoe, following the mirror A, B, B, A schedule, with 5 minute rest between tests. If you want to be more precise, a week later do B, A, A, B. You’ll need to keep your pace strictly consistent by noting your splits every 100m — GPS is notoriously imprecise, especially on the oval. When finished, Kram says, “Average the heart rate during the last 2 minutes of each trial, then average all the As and all the Bs.” Compare the final numbers to each other — “obviously lower heart rate is better.”

For more experienced runners, Paquette says rating changes in perceived effort between shoes might be even better than comparing heart rates, which can vary for reasons other than running effort. Rating perceived effort might also be more doable in the context of a running store, where they’re unlikely to let you run in shoes multiple times and monopolize their test treadmill for an hour — or take them out to a track.

While it’s maddeningly imprecise for numbers-driven runners, one of the best we have right now may be to trust the complexity of your body to tell you what feels right — something Nigg calls the “. Comfort, in this context, is more than “ah” when you step into them. You have to run in the shoes, at the pace you’re going to be using them, and note the one in which everything falls into place. You land where you expect to land, the foam cushions and supports just right so that you hardly touch the ground, the rocker drops away at the exact spot that makes you feel fast but not falling, you fly off the toe — and discover you’re running faster than ever with less effort. You’ve found your magic.