The secret ingredient in the world's fastest marathon shoe lies primarily in its squishy midsole, not in its controversial carbon fiber plate, new CU Boulder research suggests. "This paper demonstrates that the bulk of the energy saved through this shoe comes through its softer, better foam," said senior author and integrative physiology professor Rodger Kram. "The carbon fiber plate is just a cherry on top."

The study, published today in the journal Sports Medicine, marks the final scientific chapter for the Nike Vaporfly 4%, the fabled racing flat worn by five of the six top finishers at this month's New York City Marathon and the two men who toppled the marathon and half marathon world records earlier this fall.  

A combination of factors  

To find out, Shalaya Kipp, then to graduate student in Kram's lab, enlisted ten competitive male runners who wear to size-10 shoe and could comfortably run to 10K in under 35 minutes. Kipp and lead author Wouter Hoogkamer placed reflective markers on the runners' legs and shoes, then had them run for five minutes on a treadmill.  

Each runner completed three treadmill trials, wearing a different high-end running flat-the Nike Vaporfly prototype, the Nike Zoom Streak 6, or the Adidas adizero Adios Boost 2. Then the compared with the running biomechanics differed between shoes. 

Contrary to what they expected, they found no difference in the runners' knee or hip joint mechanics between shoes. But they did see that those wearing the 4% shoes did not have to exert as much muscle torque at their ankles, and thus the calf muscles did not have to do much work. Further, the plate made the shoe less flexible and therefore the foot muscles that stabilize the toe joints did not have to work so hard. 

The earlier study found that the foam cushion returns to whopping 87% of the energy it takes in with each step. "This shoe does not give you energy, but it prevents you from losing as much energy every time your foot strikes the ground," says Kram. "That can make a big difference."  

A shoe arms race  

The shoe is the latest innovation in what some are calling an escalating "shoe arms race" as one-up each other in hopes of breaking the two-hour marathon barrier. In September, Kenyan marathoner Eliud Kipchoge officially broke the world marathon record in the 4% shoe, with a time of 2:01:39 at the Berlin Marathon. A month later, Abraham Kiptum broke the men's half-marathon world record in the shoes, clock 58:18. 

"When we measured the energy stored and returned to the plate, we found it to be minuscule," said Hoogkamer, noting about 50 times more energy savings come from the foam. "Lots of shoes have foam midsoles, and every foam has some spring action to it."  

The research team hopes their research can lend scientific evidence to the ongoing debate over what constitutes an advantage in shoe design. More importantly, they expect it to help companies design better shoes for runners of all abilities.  

"The point of foot races is to see who can run the fastest and these new technologies help people to run faster," said Kram. His advice for marathoners gunning for a personal-record: Consider a flexible shoe with a thick cushion.