THE FASTEST MARATHON SHOE EVER

THERE ARE FEW SECRETS SURROUNDING NIKE'S ALPHAFLY, YET SCIENTISTS STILL STRUGGLE TO EXPLAIN WHY THE SUPER SHOE IS SO SUPER.

- JOHN TYLER ALLEN

WHEN THE PUBLIC first noticed Nike's new Alphafly, it didn't look like much: In August 2018 the French Instagram account @RUN'IX posted a photo of Eliud Kipchoge flanked by smiling runners. There, atop the red Kenyan soil, Kipchoge is wearing a nondescript black upper attached to a chunky slab of what looked like the Vaporfly's ZoomX foam. This was the first spark of speculation on how Nike planned to follow up on its world-beating Vaporfly, though Kipchoge had already been testing prototypes since January.

In the two years prior, Nike's new Vaporfly, unique for its thick, bouncy Pebaxbased foam midsole and carbon-fiber plate, had upended the less-is-more maxim governing racing flats by winning-sometimes sweeping-nearly every major marathon it entered. And in the month after those first mysterious photos, Kipchoge, wearing Vaporflys, would claim his first marathon world record in 2:01:39 at the 2018 Berlin Marathon.

More photos of the new prototype, midsole seemingly (bewilderingly) thicker than the Vaporfly's, surfaced every few months until, in October 2019, in Vienna, Kipchoge wore a polished version of the still-unnamed shoe to clock the first sub-two-hour marathon (albeit not an official record) at the Ineos 1:59 Challenge. The following June, the Air Zoom Alphafly NEXT% was released-and, last year, was updated as the Alphafly 2. Meanwhile, Kipchoge has used Alphaflys to lower his official world record to 2:01:09 and claim his second Olympic gold medal in the marathon.

The Alphafly, with its air chambers, is an obvious evolution from the Vaporfly. And it retains the mechanically familiar block of responsive foam stabilized by a heel-to-toe carbon-fiber plate. That's no longer a mystery. But how, exactly, does that system translate physiologically to the fastest marathon ever run? Biomechanists are still figuring that out.