Nike didn’t tell
Rodger Kram and Wouter Hoogkamer much about the prototype
shoe at first. “We knew it had a new foam, called
Pebax, and they showed us the carbon [fiber] plate,”
recalls Kram. The exercise physiologist and longtime
director of the University of Colorado Boulder’s
Locomotion Lab, and Hoogkamer, then a post-doctoral
researcher there, were writing a journal paper, published
in March 2017, detailing how an elite runner might
break the mythical two-hour marathon barrier. One
factor they detailed was shoe design, and the two
had been given what would become the ZoomX Vaporfly
4% to test.
Explaining how shoe design might help break two hours
was tricky, says Kram, because while they had already
completed testing on the new shoe, they couldn’t
include their non-peer-reviewed data in a peer-reviewed
journal. So they focused on the weight of the foam
midsole, which, based on previous studies, they estimated
could improve an athlete’s running economy by
around one percent.
“The world record
at that time [2:02:57 by Kenyan Dennis Kimetto in
2014] was in the Adidas Boost shoe,” says Hoogkamer,
now a professor at the University of Massachusetts
Amherst. However, he adds, “we didn’t
say you can make the foam better; we said, ‘Boost
is pretty heavy, so maybe you can make it lighter.’”
As for the Vaporfly testing
itself, Kram and Hoogkamer approached it with one
simple question: Was it faster?
In May of the following
year, Nike provided a teaser of an answer with Breaking2,
where Eliud Kipchoge of Kenya came within 25 seconds
of the two-hour mark on the Formula One Autodromo
Nationale track in Monza, Italy. Even unsuccessful,
it was a striking improvement over Kimetto’s
record, and running fans debated what was responsible
for the two percent time drop—--the dead-flat
track, the pacers, the massive car-mounted clock that
also functioned as a draft vehicle, or the prototype
shoe Kipchoge wore.
Hoogkamer and Kram’s
next study, published that fall with the innocuous
title “A Comparison of the Energetic Cost of
Running in Marathon Racing Shoes,” offered a
more definitive assessment. The prototype shoe, compared
with two top existing models—Nike’s Zoom
Streak 6 and the Adidas Adizero Adios Boost 2 that
Kimetto used in 2014—reduced the metabolic cost
of running by four percent. That savings, Hoogkamer
and Kram calculated, “should translate to ~3.4
percent improvement in running velocity at marathon
world record pace (20.59 km/h).” What’s
more, because they normalized shoe weight across all
three test samples, the benefit they found wasn’t
from lighter weight.
The study was instantly
controversial. It was funded by Nike, for starters
(Kram is also a paid consultant), and two Nike employees,
Geng Luo and Emily Farina, were listed as coauthors.
Subsequent studies not associated with Nike have validated
the results, although sample sizes have all been small
and the resulting effect varied.
Immediately, people questioned
the role of the carbon fiber plate. “Any device
inserted into the shoe, and which purports to add
to energy return…should be banned,” wrote
exercise physiologist Ross Tucker on his Science of
Sport blog. Tucker maintains that the plate is essentially
a spring, pointing out that this is how Nike refers
to a carbon plate in a previous patent.
Lost in the clamor over
the benefit of the carbon plate was the role of the
foam itself. In both their initial study and an early
2018 follow-up on the biomechanics involved, Hoogkamer
and Kram pointed to one major contributor to the improvements
in running economy: “For now, the elastic properties
of the Nike Prototype shoes provide the best explanation
for the metabolic energy savings.” In plain
English: The new foam was noticeably springier, which
made runners faster.
This foam, which Nike
branded ZoomX, helped usher in a new era in shoe design
and materials, especially midsole foam. Although the
race for a breakthrough foam started in earnest in
2013 with Adidas’s first Boost-equipped shoe,
Nike’s bold performance claims around the Vaporfly
4% and Next%, right down to the names, have changed
the conversation. “When we started doing our
outsoles, our focus was reinventing cushioning,”
says Olivier Bernhard, cofounder of On Running. “Everyone
you talk to nowadays, it’s about durability
and resilience, rebound.”
As shoemakers continue
to try to improve performance, and research accumulates
showing a modest but undeniable benefit to next-generation
materials, the foam wars have only gotten more heated.
Recreational runner Andy
Jacques-Mayne is well aware of the shift in conversation.
Jacques-Mayne is a former pro cyclist, a sport that
prizes attention to technical detail and the marginal-gains
ethos of seeking even the smallest improvements in
efficiency. On paper, he’s a prototypical Vaporfly
buyer, noting that “from a performance standpoint,
buying shoes that supposedly offer free speed sounds
great,” although that’s far from his only
consideration. But for Jacques-Mayne, like a lot of
runners, foam technology is an afterthought.
For almost 50 years,
the primary technology in running shoes, midsole foam,
has hardly changed. From Bill Bowerman’s original
Nike Cortez to most of today’s shoes, the material
of choice is EVA, or ethylene vinyl acetate. Only
recently has that started to change.
EVA makes an ideal midsole:
in foam form it’s exceptionally light and provides
great cushioning. Crucially, it’s also cheap,
and it’s easy to make into a foam. And with
decades of experience, shoe companies and the foam
manufacturers who turn the raw EVA into sheets of
midsole foam or compressed midsole blanks have learned
little tweaks to the formula and manufacturing that
offer a seemingly endless amount of variation to how
a shoe feels in stride.
But there are drawbacks:
EVA’s performance is highly dependent on temperature;
in hot weather it tends to feel like a marshmallow,
while in cold temps the midsole hardens and loses
its cushioning. It has a relatively high compression
set, which means it packs out and permanently loses
its cushioning quickly. And while it offers good cushioning,
it isn’t great at energy return, where a sole
is compressed and then rebounds, returning some of
the force back to the runner.
While foam suppliers
can tweak EVA’s performance, they can’t
fully overcome those basic limitations because they’re
baked into its chemical composition. EVA is what’s
called a random copolymer, says Polymer Solutions
founder Jim Rancourt, Ph.D., a materials scientist
with 40 years of experience in plastics, and a longtime
runner. Ethylene is a hard plastic used in products
like milk jugs. Vinyl acetate is basically Elmer’s
glue. Combine them, and you can get a solid-but-spongy
product tuned by the ratio of each monomer. But that’s
about all you can control.
“The analogy I
use is Lego blocks,” Rancourt says. Say ethylene
(the hard component) is blue blocks, and vinyl acetate
(the soft component) is yellow. A shoemaker can ask
a foam supplier to adjust the amount of each in the
blend, but unlike other materials, EVA’s blue
and yellow blocks will connect in a random pattern.
That basic ratio is a crude way to control properties
like softness. Another method is to vary the density
of the foam by putting less or more into a mold, but
less dense foams generally lose their rebound characteristics
more quickly.
The final option is to
blend EVA with other materials, or just use something
different altogether. Shoemakers have for years tried
to find alternatives, all the way back to Nike’s
original Air Tailwind and its air cavity in 1978.
In the 1980s, says Spencer White, vice president of
human performance for Saucony, that company experimented
with a material from DuPont called Hytrel. “It
was just as bouncy and lively as the foams we’re
working with now, but we never figured out how to
make it inexpensive enough to make a whole midsole
out of it,” he says.
By 2007, the German chemical
company BASF had found a way to puff, or expand, thermoplastic
polyurethane (TPU) into little spongy pellets that
sprang back quickly when compressed. They called the
product Infinergy but struggled to identify an application
for it—that is, until they figured out how to
fuse the pellets into a single layer of foam, and
signed an exclusive footwear license to Adidas to
use it as Boost, which debuted in 2013.
But perhaps nothing has
been as revolutionary as a plastic developed by the
multinational chemical company Arkema, called Pebax.
Like a lot of plastics,
Pebax has been around for decades. In solid form,
it adds flexibility to rigid ski boot shells, and
it’s used in heart catheters. But the foam form
wasn’t patented until 2004, and Nike and Reebok
were the first to adapt it for running shoes.
Pebax is a brand-name
play on the generic name: polyether block amide (PEBA).
Like TPU, it’s what’s called a block copolymer,
a definition that’s essential to its advantages.
To go back to Rancourt’s Lego analogy, Pebax
is also made of blocks. But instead of being randomly
arranged, Arkema can actually join chains of blocks
of a single color, and vary the length of those chains,
alternating hard and soft sections precisely to control
the amount of cushioning and rebound.
So as a product, Pebax
isn’t one specific thing, says Kevin Hanrahan,
chief marketing officer for Arkema’s technical
polymers unit. “It’s a range from very
rigid to very soft, and you can get the energy-return
properties of a polyether without compromising softness.”
That’s also technically the case for TPU or
any other block copolymer, says Rancourt. But there
are certain advantages to some over others. Weight,
for instance: Arkema says Pebax is as much as 20 percent
lighter than TPU-based foams like Boost. TPU, in turn,
has the advantage of long life, since it has a lower
compression set than Pebax or EVA. And both TPU and
Pebax offer consistent performance in a wide temperature
range. But maybe the signature advantage among these
foams is Pebax’s rebound, or energy return.
“Energy return”
is a bit of a misnomer; it’s more like “less
energy lost.” Any foam midsole, when compressed
under load, will store a certain amount of energy
on a footstrike that rebounds back when the runner
pushes off. Shoes with midsoles based on EVA foams
have a range of energy return depending on how they’re
formulated, but generally, they top out around 65
to 70 percent. TPUs like Boost are better, at 70 to
76 percent. But the two best shoes Runner’s
World has ever tested in terms of energy return are
the Vaporfly 4% and 4% Flyknit, which offer as much
as 82 percent return in deflection and rebound testing.
(Kram and Hoogkamer measure it even higher, at 87
percent.)
Why does energy return
matter? Iain Hunter, Ph.D., a professor of exercise
science at Brigham Young University, found a clue
that he wrote about in a June 2019 study in the Journal
of Sports Sciences, where he attempted to replicate
Hoogkamer and Kram’s 2018 metabolic cost comparison
study. Both studies found runners had a longer stride
in the Vaporfly than other shoes, which makes sense
from an increased efficiency perspective: The farther
you travel per stride, the faster you’ll go
at a given cadence.But Hunter also measured a few
parameters that the CU study didn’t, like vertical
oscillation—essentially how high the runner
is bouncing with each stride. There, Hunter found
that the Vaporfly led runners to bounce higher, which
normally would correlate with lower running economy,
“unless it’s not the body that’s
generating the energy to make that happen,”
Hunter says. His theory: The Vaporfly sole’s
bounciness returns more of the energy to the runner
than other foams, which produces a longer stride for
a given amount of ground time.
Hoogkamer also thinks
that because Pebax is a softer-feeling material than
the TPU-based Boost, there’s more compression
to begin with, leading to an overall higher rebound.
“The actual joules, or energy units, stored
in the compression phase are a lot higher because
it’s such a soft shoe,” he says.
The CU study found slightly
higher peak forces on impact for the Vaporfly (the
opposite of the BYU study), but because the shoe is
so soft, Hoogkamer theorizes that it actually feels
less harsh to the runner, which enables the runner
to take longer strides. That may matter over the course
of a season as much as during a single race. At the
July 2019 Footwear Biomechanics Conference, Nike research
scientist Brett Kirby presented data showing that
runners wearing the Vaporfly sustained less muscle
damage than a control group wearing other shoes. Reduced
muscle soreness suggests they could sustain higher
training loads.
Whatever the mechanism,
it’s undeniably effective. In addition to the
CU and BYU studies, Kyle Barnes, Ph.D., a professor
of exercise science at Grand Valley State, found similar
running economy changes in a comparison with the Adizero
Adios Boost 3. And in a messier but far larger sample
size, the New York Times’s Upshot examined roughly
half a million marathon times from public data like
Strava and concluded that, on average, runners wearing
the Vaporfly ran three to four percent faster than
similar runners in most other shoes, and one percent
faster than the next fastest shoe, Nike’s Zoom
Streak. (The authors noted that “the analysis
suggests that, in a race between two marathoners of
the same ability, a runner wearing Vaporflys would
have a real advantage.”)
As a result of all the
data, and Nike’s aggressive marketing, shoe
companies are racing to develop faster foams and smarter
ways to use them. White says Saucony has tested dozens
of foams over the years, and almost used Boost before
Adidas snapped up the exclusive. On Running has its
CloudTec system and new foam formulations like Helion.
Reebok uses Pebax in its Floatride midsole. Under
Armour uses a TPU foam called Infuse in its Hovr line.
Skechers’s Hyper Burst foam is a CO2-infused
EVA, and the DNA AMP midsole in the Brooks Levitate
is another polyurethane foam from BASF called Elastopan.
Pebax isn’t even the only PEBA out there; a
similar product, called Vestamid, is available from
the German chemical company Evonik. And Adidas, which
has been a little quiet on the foam front besides
tweaked versions of its midsole tech like Boost HD,
is rumored to be working on a new shoe that was supposedly
tested at the Berlin Marathon, and which may be its
answer to Pebax.
Most runners within range
of the hallowed three-hour marathon mark, if offered
a simple equipment choice to knock six minutes off
their time, would probably take it in an instant.
But two of the downsides of Pebax foam are its cost
and its high compression set. The Vaporfly 4% and
Next% shoes are $250—a lot of cash for a shoe
that may only last a handful of races. That’s
significantly more than most runners are willing to
pay. “The bulk of the market has crept up a
little but [the average price] is still at $120 to
$130,” says Saucony’s White. “In
the big races you’ll see a lot of high-end shoes
at the front, but it falls off pretty quickly after
that.”
Beyond the cost issue,
runners become loyal to shoes for many reasons, like
a shape that properly fits their feet and the way
the design supports their stride and biomechanics.
A 2015 meta--review of studies of running shoes and
injuries, by University of Calgary exercise scientist
Benno Nigg, Ph.D. suggested that runners “intuitively
select a comfortable product using their own comfort
filter that allows them to remain in the preferred
movement path.” Foam, especially the new supersoft
and bouncy stuff, seems to fit right into that ideal.
But it’s an open question as to how much runners
actually care about it, whether it’s called
ZoomX or Boost or whatever. “I suspect most
runners have no idea what we’re talking about,”
says Saucony’s White. “The majority just
want a good shoe that feels great.”
There’s also the
still-evolving discussion of what provides the performance
benefit, including the exact foam formulation, which
are often variations on the base material. Nike’s
version of Pebax provides excellent energy return,
for instance, but Reebok’s Pebax-based Floatride
foam, used in several of its top shoes, returns values
in the RW tests that you’d associate with a
quality EVA shoe, but below Boost or ZoomX.
And we don’t yet
know how much of the improved running economy lies
in other aspects of shoe design, like air pods or
carbon fiber plates, and whether that’s because
those elements act as propulsive springs, or if they
merely stabilize soft foams to maximize energy return.
In another presentation at the Footwear Biomechanics
Conference, Emily Farina of Nike’s Sports Research
Lab showed data suggesting that, depending on its
shape, the carbon plate in the Vaporfly might be responsible
for more of the running economy improvement than researchers
had thought, although the exact mechanism wasn’t
explained.
If Nike, which has a
division devoted to exploring questions like this,
knows any more, it isn’t saying. The company
declined multiple requests to make Farina and other
Sports Research Lab staff available for interviews,
but e-mailed back a one-line statement: “The
magic of the Nike ZoomX Vaporfly Next% and the Nike
Zoom Vaporfly 4% is in the full system and how all
the elements work together, not in any single ingredient.”
That sounds like bland
corporate jargon, but there may be more to it. Among
Nike’s various patents around the Vaporfly is
a pending application from 2018 titled “Stacked
Cushioning Arrangement for Sole Structure” which
looks similar to Kipchoge’s shoes from the INEOS
1:59 Challenge. The patent details a shoe design with
several distinct layers of foam, as many as three
carbon fiber plates, and two pairs of fluid-filled
forefoot cushioning chambers sandwiched between the
plates. So it seems pretty clear that the plate (or
plates) matter.
But foam absolutely matters
as well, even if most runners don’t pay that
much attention to it. (It doesn’t help that
shoe companies insist on hype-y names like ZoomX or
Boost or Helion and liberal use of marketing word
salad to describe technologies they didn’t invent.)
Perhaps the biggest reason foam matters is the trickle
down. Pebax is expensive now, but costs may come down
as foam midsole suppliers learn more effective production
techniques. And while a “halo product”
like the Vaporfly is good marketing now, Nike may
ultimately choose to spread its R&D costs across
more models, at lower prices.
EVA, for all its drawbacks,
isn’t going away: It’s still light, cheap,
and easy to tweak. On Running’s new Helion foam
is an EVA-based product, says Bernhard, who adds that
in the beginning, the company used a mix of EVA and
polyurethane foams but in testing, they found that
EVA worked best with its sole structure.
On Running’s experience
suggests that future breakthroughs may come as much
from structural elements like sole designs or plates
as from new foams, although Rancourt says other block
copolymers are out there that might provide similar
performance characteristics to Pebax.
For all its promise,
Pebax hasn’t taken over running yet. Absent
broader availability of models like the Vaporfly,
or a breakthrough that provides significant price
drops, it may not. Nike itself seems to acknowledge
this with shoes like the race-oriented Zoom Fly 3,
a near-clone of the Vaporfly down to the carbon plate,
but made with its React foam. That shoe costs $90
less.
Despite its competitors’
efforts—and calls from non-Nike athletes for
equipment regulations—Nike held its lead in
the foam wars through 2019 with Vaporflys winning
the Berlin, New York, and Chicago marathons, while
Kipchoge broke the two-hour barrier with his latest
prototype. But ultimately, runners and not shoe companies
will decide the fate of shoe technology like the Vaporfly,
and those considerations come down not just to performance,
but fit and even philosophical approach to the sport.
Mindful of the importance
of on-bike form to his cycling career, Jacques-Mayne
says he pays close attention to his biomechanics and
stride (midfoot, with a touch of heel). He’s
run in Hoka One One, Adidas, and Nike, among other
brands, but trains in New Balance’s 880, for
its neutral support and “just right” amount
of padding.
The Vaporfly wasn’t
available when he got into running, so it wasn’t
among the shoes he tried when searching for a fit.
But even now, he adds, he’s more interested
in gains from improved training and stride mechanics.
“I was a novice runner with a huge motor, so
growing the mechanics and muscles to unlock speed
was way more important,” he says. And he’s
a little wary of improvement that isn’t earned.
While the idea of knocking four percent off his PR
sounds great, he says, he hasn’t felt the need
to try Vaporflys.
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