A new journal article has reported, for the first time, on the durability of running shoe super foams (called PEBA, for Polyether Block Amide) vs more traditional shoe foams made of EVA (Ethyl Vinyl Acetate).
The article, titled “Influence of Different Midsole Foam in Advanced Footwear Technology Use on Running Economy and Biomechanics in Trained Runners”1Víctor Rodrigo-Carranza, Wouter Hoogkamer, José María González-Ravé, Muñoz, S., Carmen, D., A. Martínez Romero, & González-Mohíno, F. (2023). Influence of different midsole foam in advanced footwear technology use on running economy and biomechanics in trained runners. Scandinavian Journal of Medicine & Science in Sports. https://doi.org/10.1111/sms.14526 was published in the Scandinavian Journal of Medicine & Science in Sports.
The research was carried out by a running and sports performance group at the University of Castilla-La Mancha in Toledo, Spain.
First author Victor Rodrigo-Carranza is now a post-doc student at the University of Massachusetts, where he’s working under veteran super-shoe biomechanist Wouter Hoogkamer, PhD.
The experimental protocol paired four pairs of shoes against each other. All weighed about the same and were manufactured by the ON running shoe company. However, ON had no other role in the project or its results and did not fund the research.
The four shoes were:
- New PEBA shoes (super foam) with a carbon plate
- New EVA shoes (traditional midsole foam) with a carbon plate
- The same shoe as condition 1, but after 450 km (270 miles) of running wear on asphalt surfaces
- The same shoes as condition 2, but after 450 km of wear
The shoes were worn by 22 experienced male runners. The study team measured each runner’s Running Economy (RE) in each of the four shoes.
The Bottom Line, In Brief
The subjects’ RE was highest, by 1.8%, when they wore the new PEBA shoes vs the new EVA shoes. However, subjects’ RE dropped by 2.28% in the worn PEBA shoes vs the new PEBA shoes.
On the contrary, there was essentially no change in their running economy in the worn vs new EVA shoes.
This was the first study ever to compare running economy in new vs well-worn running shoes.
Conclusion: “There is a clear running economy advantage of incorporating PEBA versus EVA when the models are new. However, after 450km of use, the PEBA and EVA shoes had similar RE.”
How The Study Was Organized: Subjects And Methods
The subjects were all male runners who had been running for at least 6 months without injury. They were capable of running at least 13 km/hour (4:36/km pace; or 7:25/mi) without producing excessive lactate.
All wore USA men’s size 9 shoes, and all were accustomed to running in carbon-plate super shoes.
In the experimental protocol, all subjects ran on a laboratory treadmill in the four different pairs of shoes. Their treadmill run took 56 minutes as follows:
Warm up for 10 minutes, run 4 minutes at 13 km/hour pace in shoe 1; rest for 10 minutes, run 4 minutes at 13 km/hr pace in shoe 2; etc.
This process continued until each subject had run in all four pairs of shoes. The order of the shoes was randomized for each runner throughout the testing.
All shoes weighed nearly the same. The weights varied only from 226 grams to 234 grams. All shoes had a stack height of 38 or 39 mm (the World Athletics limit is 40 mm), and a heel-toe “drop” of 8 or 9 mm.
The primary measure recorded was the RE of each runner in each pair of shoes. The researchers also noted the following variables for each runner: contact time, step frequency, step length, flight time, leg stiffness, and vertical oscillation.
In addition, all runners were asked to subjectively assess their perceived effort, comfort, and muscle soreness in each of the four pairs of shoes.
Interesting Notes About The 450 km Of Shoe Wear
Someone had to wear the shoes for 450 km before the experiment could begin. That someone also had to maintain the shoes in tip-top condition.
Why? If the shoe-testing subjects had been able to tell “new” shoes from “worn” shoes, this would likely have influenced their subjective evaluations of the shoes, and perhaps even the physiological and biomechanical measures.
They almost certainly would have reported that the new shoes were fresher feeling than the old shoes.
First author Victor Rodrigo-Carranza did the shoe-wearing himself. He is 26, and a runner since high school with a best half marathon time of 1:23.
It took him nearly four months to achieve the 540 miles of running (270 in each of two shoes). He ran five times a week, logging 6 to 7 miles with each run.
He was particularly careful to keep the worn shoes as clean as possible. “That was a big concern for me,” he says. “I tried to do all the kilometers on asphalt and used the utmost care to take care of the shoes, especially the uppers. I can guarantee that the subjects did not identify any differences between the conditions with the naked eye.”
Main Results: What The Experiment Proved
All runners finished their 56-minute treadmill effort with a modest lactate level, indicating that they had not overly extended themselves. Their respiratory-exchange ratio likewise remained at a level indicating a modest but not excessive effort.
All runners were more economical in new PEBA shoes vs worn PEBA shoes. Surprisingly, in the EVA shoes, several runners actually exhibited greater economy in the worn shoes.
Many biomechanical variables did not change between conditions. However, there were significant differences in step length, step frequency, and contact time.
Generally, strides were longer in PEBA shoes, and step frequency increased in worn EVA shoes. Heart rate did not change between conditions, nor did perceived exertion.
Why Did Things Happen As They Did?
PEBA shoes are definitely lighter than EVA midsoles and provide greater energy return when measured with standardized machine testing. However, running can not be evaluated with a simple, static machine test, because runners are constantly moving.
Running smoothly requires great timing and coordination. As the paper states: “Energy return sometimes occurs at the wrong time, frequency, location, and direction during running, thereby compromising the ultimate effect on performance.”
In addition, PEBA’s fantastic energy return and lightness can have negative effects. Some runners are non-responders in PEBA shoes; they get slower, not faster.
Also, because PEBA is less dense than EVA, the “wrinkling between air cells” could be greater with increasing usage. This would lead to more compression of the foam–and hence faster loss of resilience (“energy return”).
This appears to be exactly what happened in the present experiment. The denser, heavier EVA held up better after 450 km.
Runner Take Home Message
In the Perspectives section of the paper, the authors state: “These results generate important new knowledge for the footwear industry.”
They suggest that shoe companies should manufacture two different types of super shoes: one with stiff plates embedded in lightweight PEBA foam; and a second with similar plates embedded in denser EVA foam.
The first, when brand new, would be superior for top racing efforts. The second would have greater durability and presumably cost less. This would make them the better choice for regular use over many months by hard-training runners.
The authors also note that the important question of injury risk was not evaluated by their study. While they did observe “minor spatiotemporal modifications on running biomechanics” between shoe conditions,” they did not investigate injury outcomes per se.
They encourage shoe companies and other researchers to do additional research to determine if there are different injury risks in the two types of shoes.
Questions And Answers With The Researchers
I asked Victor Rodrigo-Carranza a number of questions about the paper, all of which he answered fully. Since Wouter Hoogkamer is the senior researcher, I asked him for an overall observation. Let’s start with him.
MH: What do you believe is significant about the new paper?
W Hoogkamer: I think this is important information for the running community to have. It clearly shows that, after 450 km, the PEBA shoes don’t perform as well as right out of the box. It would be great if this inspired shoe manufacturers to improve their foam compositions to last longer. Unfortunately, recent trends seem to suggest that this might not be on their radar.
[Author’s note: This seems an apparent reaction to Adidas and other companies recently releasing shoes that are super light (and maybe super fast) but not long-lasting, by the companies’ own admissions.]
I would like to add that this study was done only by male runners. I am curious if women runners, who are less heavy on average, would experience the same loss of running economy. Or perhaps less.
MH: Victor, what was the primary reason you performed this experiment? What was your goal?
V Rodrigo-Carranza: There are many different models of super shoes on the market today. But there was no previous scientific evidence about what happens to the shoes if runners do many miles in them.
We had the hypothesis that, because of the characteristics of the lightweight foams, super shoes might wear out faster than previous EVA midsole shoes. That would change the recommendations often given to runners about how long to wear their shoes.
[Author’s note: Runners are often advised to buy new shoes after running 400 to 500 miles in their current shoes.]
Therefore we wanted to compare the two midsole materials–the traditional EVA (for which there is a lot of evidence) and the newer PEBA (for which there were no scientific results).
MH: What do you think is the most important information revealed by your paper?
V Rodrigo-Carranza: We provide very practical information for shoe companies and also for runners. They’ll be able to make informed decisions about what shoes to use when accumulating more training volume, and which shoes to wear for racing.
MH: What do you think the results might have been if you had tested the shoes after, say, 600k? What would have changed?
V Rodrigo-Carranza: Very good question. We didn’t have time for further testing, but it would have been interesting to test every 200 km until 1000 km.
At 450 km, the EVA shoe did not suffer much wear, but previous studies have shown significant EVA wear at 600 km. So we could expect the same result, although the carbon fiber plate may change the story. Future studies should evaluate these questions.
MH: Be honest with us, Victor. You ran a lot of miles in these shoes. Did you have a personal preference for one foam over the other?
V Rodrigo-Carranza: Yes, I felt more comfortable in the PEBA shoes, both new and worn. However, there was a lot of personal variability in the subjective preferences of our subjects. Every runner has his or her own favorite.
