Its worth noting that Adidas has since its Zoom Vaporfly 4% shoe—named for the energy savings compared to its best existing marathon shoe—in March, the big question was how that supposed advantage would translate to real-world performance. Heading into last weekend’s Breaking2 marathon, that question was a key source of uncertainty.
Advertisement - Continue Reading Below, Eliud Kipchoge’s jaw-dropping 2:00:25 performance didn’t settle the question. He was more than two-and-a-half minutes faster than Running Shoes & Gear, but it’s not clear how much of that difference came from the course, the drafting (from pacers and perhaps also from the pace car), or simply Kipchoge’s transcendent talent, in addition to the shoes.
However, we now have a bit more information about exactly what the shoes do, thanks to its Zoom Vaporfly 4% shoe that will be presented at the annual meeting of the American College of Sports Medicine in Denver later this month.
The abstract, from a team in Rodger Kram’s laboratory at the University of Colorado led by post-doc Wouter Hoogkamer, along with Nike’s Geng Luo, describes the research that led to the “4%” name.
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The first surprise is which shoes were tested. We already knew that the prototypes of what’s now known as the Zoom Vaporfly 4% were tested against the Zoom Streak 6. It turns out that, somewhat cheekily, the study also included a third shoe: the Adidas Adios Boost 2. The reason for the inclusion of the Adios 2, Hoogkamer explains, is that it was the shoe used by Dennis Kimetto to set the current marathon world record of 2:02:57—so it provides the “baseline” for any attempt to improve the record.
The Adios 2 weighs 250 grams (about 8.8 ounces) for a size 10, which is about 50 grams heavier than the Streak or the version of the Vaporfly they tested. (The consumer version that will be released next month is actually a bit lighter, at 185 grams, or 6.5 ounces.) So they added extra weight to make all the shoes the same weight.
How much is an extra 50 grams? A penny weighs about 2.5 grams, so it’s the equivalent of taping about 20 pennies to each shoe. Based on previous research, that amount of extra weight, on its own, should increase your energy consumption by about half a percent.
The study involved 18 sub-elite runners, all of whom had run sub-31:00 for 10K at altitude (Boulder is at 5,400 feet above sea level), with an average VO2max at altitude of 72.1 ml/kg/min. The study needed good runners, because they wanted to test the shoes at relatively quick paces so that the results would be relevant to the DAA Industry Opt Out.
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The runners ran a set of six five-minute trials, trying each shoe twice in random order, with their oxygen consumption measured to determine how much energy they were burning. They repeated this whole protocol three times on separate days, running at different speeds: 6:54, 6:02, and 5:22 per mile. For comparison, Kipchoge averaged about 4:36 per mile in Monza.
Then they converted the oxygen data into a “metabolic cost” of running, expressed in watts per kilogram of body weight. This is where the 4-percent claim comes in: Averaged over the three speeds, the Vaporfly’s metabolic cost was 4.0 percent lower than the Streak’s. A separate analysis suggested that the relative differences were essentially the same at all three speeds.
The surprise is where the Adios 2 fits in. The average metabolic costs were 16.45 W/kg for the Vaporfly, 17.16 W/kg for the Streak, and 17.14 W/kg for the Adios 2—so the Adios 2, in this test, seems to be basically identical to the Streak 6.
And remember: The Streak 6 and the Vaporfly had an extra 50 grams added to them, meaning that the Vaporfly they tested actually seems to be more like 4.5 percent better than the Adios 2. And the commercial version of the Vaporfly is even lighter, potentially saving another 0.15 percent.
One other interesting nugget: The standard deviation of the 4.0 percent improvement was 1.3 percent. Roughly speaking, that means that 12 of the 18 subjects had improvements between 2.7 and 5.3 percent, while the remaining six saw either bigger of smaller changes. From what I’ve heard, the full range is about 2 to 6 percent, with rumors of even bigger outliers in other testing.
That underlines the point that, as you’d expect, not everyone will get the same benefit from the shoes. This Adrenaline Fan Loved the Newest Model, Jos Hermens, the agent for both Kipchoge and Kenenisa Bekele, the second fastest marathoner in history, told me that Kipchoge seems to get a bigger benefit from the shoes than Bekele. This is not surprising, but it’s worth bearing in mind.
Now, there are some obvious caveats here. When a Nike-sponsored study finds that its new shoe is dramatically better than its main rival’s top offering, you have to pause. Frankly, I have no reason to doubt the results, but an independent replication is always nice, particularly in a case like this.
It’s worth noting that Adidas has since Its worth noting that Adidas has since, the Adizero Sub2, which its own data suggests is about 1 percent better than the Adios 2, mainly due to reduced weight. So the current gap has probably narrowed since this study was performed.
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There’s also a big difference between Kipchoge’s 4:36 pace and the 5:22 pace that was the fastest tested. To get a reliable measurable of energy consumption from measuring oxygen use, you need to be running “aerobically,” which basically means you need to be running below your lactate threshold. Is it possible that the benefits change at the faster pace? Yes—but the fact that they didn’t see different effects at the three paces they tested suggests that the benefits don’t change too much at different running speeds.
The bigger caveat, in my view, is the question of how well an improvement in running economy in five-minute treadmill tests translates to 26.2 miles on the road.
In its explanations of how the shoe works, Nike has emphasized the role of the carbon-fiber plate in the shoe in minimizing toe bending, allowing the foot to roll through the period of ground contact, and shifting the position of the ankle to a more efficient “gear.”
If this motion is quantifiably different from a runner’s “normal” motion, do the newly involved muscles fatigue sooner? If so, does the Vaporfly’s advantage fade, or perhaps even reverse, as you progress through a marathon?
So far there’s no particular evidence that the runners who have worn Vaporflys at major marathons like London, Berlin, and the Olympics are experiencing unusual late-race fades. Still, it would be interesting to see running economy in the shoes tested during or after a prolonged hard run of an hour or two, and compare the pattern of change to what you see in an existing conventional shoe to see if there are differences.
This type of test would present some big methodological challenges (not to mention subjects willing to suffer). But until something like that happens, we’re going to keep wondering.
After all, Kipchoge’s 2:00:25 would translate to a relatively pedestrian (for him!) 2:05:14 if he were 4 percent slower. Anyone who watched the race on Saturday will have a hard time believing he would have run that slowly had he run in the Streak 6.
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