The Science of PEBAX Shoes and Carbon Fiber Plates: What We Know Now
Review by Chief Editor and Founder Matthew Klein
In April of 2020, we posted a critical review diving into the research and evidence around the quickly emerging carbon fiber-plated and/or PEBAX-based footwear. We discussed several things including early thoughts on carbon plates, newer foams, geometries, interactions between them, and most importantly how much we do not know about what was truly going on. In a short two years, there has been a large amount of research into these topics. While there are still significant holes in our understanding due to research being so far behind current industry trends and development, we decided to create an update to address this new information. As this is an update, those wanting the original research should check out the original article HERE so we can focus on all the new research since that time.
Then and Now: Updated Research on Super Shoes
The original studies on the Nike Vaporfly Next% demonstrated clear economic improvements over traditional racing shoes (Hoogkamer et al., 2019; Hunter et al., 2019). However, original testing of these shoes were done with elite athletes and all known research was done on flat treadmills. Several questions rose out of this, including whether or not these shoes would still benefit slower runners, who/how many people actually would respond to this type of footwear and whether they would still be beneficial on hilly roads. The answers to both these questions were addressed in three separate studies.
A study with Dustin Joubert on its team found that while slower runners running at slower speeds still experienced some benefit with super shoes (specifically the Nike Vaporfly Next% 2), it was far less than previous research suggesting 3-4% economy improvements (Dominy & Joubert, 2022). The improvements were 0.9-1.4% and while they are technically better, there are other factors to consider over the longer periods of time these athletes will be on their feet. Given the far smaller magnitudes of improvement in economy, runners going above 4 hours may want to consider other important factors like comfort instead of shoes with economy improvements. Although we have absolutely no evidence on them yet, this is where the new super trainers (New Balance Fuelcell SC Trainer, Asics Superblast, Adidas Prime X) may come in.
Following up on whether slower runners would also benefit from these shoes, additional research has looked into whether the majority of recreational runners will actually experience the 4% economy improvements seen in the Vaporfly studies. Given that the testing and research was done on those with fairly fast racing times, it was uncertain whether the subject testing pool was too narrow. Heyde et al. (2022) studied a group of 32 recreational runners and found that while the group average was 4%, the variability in response was high. This response was suggested to occur in only 25% of recreational runners, with others have greater or lesser responses. This further suggests that there will be those who respond to these shoes and those who do not, further emphasizing our continued concept that shoes are tools. Different people will need different tools to optimize their unique biomechanics, which may or may not respond to these footwear types.
A study on up and downhill in Nike Vaporflys was performed by Whiting et al (2022). They found that while there were economy improvements compared to a control shoe (Nike Streak), it was less than reported for the economy improvements on flat/level ground. The study suggested that while these new super shoes may provide significant benefit on certain courses, their overall benefit would be reduced on hilly courses. This would not be a major issue for larger road marathons, but suggests that super shoe development on trails may need to come from a different angle to maintain the same improvements seen on road.
Carbon Fiber Plates: Important, but Still Not the Sole Factor
We received a large number of angry emails and messages over our quoting of older research that stated that the overhyped carbon plates contributed at most 1% economy improvements and were negligible compared to the improvement from newer foams. However, newer research has continued to tell the same story. This has continued to be the case, with researchers cutting the plate in vaporflys and finding no changes in economy improvements (Healy & Hoogkamer, 2022). This does not mean the plates are useless, but that their mechanism is different than often advertised. Thus far, evidence suggests that the plates stabilize the foams and help facilitate motion, rather than return it (Agresta et al., 2022; Healy & Hoogkamer et al., 2022). On a runner-specific scale, Dustin Joubert was able to compare a variety of carbon fiber-plated racing shoes and found only the shoes with special foams seemed to make a major difference in running economy compared to a non-plated control (Joubert & Jones, 2022). The carbon plated only racing shoes had little difference if any from the non-plated control, again suggesting the importance of the foams.
To follow up on the Mcleod et al., (2020) study on certain people responding to different levels of longitudinal bending stiffness, it appears that in order for people to benefit from plates, the flex point needs to line up with the 1st MTP joint (Agresta et al., 2022; Hébert-Losier & Pamment, 2022; Ortega et al., 2021). This would make sense given that this is a major pivot point for the body as it transitions from the support to the propulsive phase of running gait. Additional studies have explored plate location and plate shape. Studies looking at top-loaded or insole-based carbon plate versus embedded plates found that the embedded plates improved economy with significant differences while the insole-based and top-loaded plates did not (Ortega et al., 2021). Curved plates, much like the ones that are seen in the more elite racing shoes on the market, have also been found to facilitate motion as they tend to line up with the metatarsophalangeal joint axises more efficiently (Agresta et al., 2022; Hébert-Losier & Pamment, 2022; Nigg et al., 2021; Ortega et al., 2021).
Thus, while the plates may not be contributing the most to the major economy and performance improvements seen in the top racing models, they are still important. That importance is based heavily on their location, shape and geometry within the shoe.
Foam/Midsole Material: This Still Does Very Much Matter
While there have been a large number of studies on plates and racing shoes with plates, few studies have examined these new midsole materials in isolation. We know from the above mentioned studies that the foams play some of the most important factors in shoes that have better economy and performance improvements (Joubert & Jones, 2022). In Joubert's study, PEBAX, specifically ZoomX and what we suspect to be a nylon based PEBAX foam (FF Turbo in the Metaspeed Sky) had the greatest economy improvements of all the shoes tested. We know that cutting the plate in the elite most racing shoes actually does little to change the economy improvements, again suggesting the energy returning capabilities from these foams play a large part in the economy improvements (Healey and Hoogkamer, 2022). Given what we know about plates having larger economy influences at higher speeds, the foams may also being playing the biggest role in economy improvements in runners using them at slower speeds (Dominy & Joubert, 2022; Nigg et al., 2021; Ortega et al., 2021).
While further research is needed on how the plates and foams work together, evidence continues to suggest that the midsole foam type is the most important factor in the economy improvements of the top racing shoes. However, just because a foam has a certain name does not mean the density and make-up are exactly the same. Several companies are experimenting with infused and modified PEBAX foams, such as Puma's bio based Nitro Elite in the Fast-R. Research has been done on this shoe and it was found that the energy return was far better in the forefoot based Nitro Elite foam compared to the Zoom X forefoot on the Nike Vaporfly Next% (Kisewetter et al., 2022). There are several other factors involved, including geometry that could not be controlled, but further evidence is needed comparing the expanding foam types.
Based on the initial evidence, PEBAX foams are still superior to all the others available on the market.
Kinematics and Kinetic Changes
Given the large variations in geometries and foam types between the shoes, biomechanical studies have not been as common as the economy studies. Of those that have come out, much of the evidence has seemed to match the original data from Hoogkamer's original vaporfly study. Runners have been shown to mostly exhibit biomechanical changes at the ankle, with decreased work and dorsiflexion angles being a common change (Hébert-Losier et al., 2022; Hébert-Losier & Pamment, 2022; Hoogkamer et al., 2019). Longer flight times, great vertical displacement and decreased hip range of motion during stance phase were also seen in recreational runners at submaximal speeds (Hébert-Losier et al., 2022). In more elite runners, the biomechanical responses were often not different in the elite most athletes or demonstrated that athletes adapted their mechanics individually to the shoe (Hata et al., 2022; Kisewetter et al., 2022). This continues to suggest that individual responses are variable, with the great changes occuring at the ankle joint (Hébert-Losier et al., 2022; Hébert-Losier & Pamment, 2022; Hoogkamer et al., 2019).
Correct Execution of Super Shoes
Similar to this section in the previous review, the most important factor seems to be the type of foam used. PEBAX specifically seems to have the best energy return of the foams on the market. This needs to be combined with a curved plate or stiffening agent embedded in the midsole with a large stack height of PEBAX foam and a lighter weight. What effect the geometry of the sole plays into this we can only guess at, but with the stiff midsoles and tall stack heights, a strategically rockered sole is a must. Thus, running shoe companies debuting shoes without PEBAX or midsole foams with extremely high energy return will continue to not be able to compete with the top footwear regardless of whether their shoe has a carbon plate or not. This is not only suggested by this review, but has been seen by research studies comparing these models (Joubert & Jones, 2022). Shoes like the Hoka Carbon X, Brooks Hyperion Elite and others that only feature a plate and not a PEBAX foam will not be able to come close to the top shoes on the market like the Nike Alphafly, Nike Vaporfly and the ASICS Metaspeed Sky.
Editor's note: we are aware of some shoes under development that provide higher energy return than these models, but will have to wait and see how they perform in independent research studies.
Conclusion
Despite the update in information here, there is still a great deal we do not know. Further still, the research has had difficulty keeping up with the advancements seen by running companies. We do know that the foams are one of the most important characteristics, but the true magic is understanding how all the different pieces work together. We have seen several models that are testing well in research and we can make some educated guesses on why they do so much better than their competition. The true understanding of these interactions though may never be provided to the public given proprietary nature of shoe design. That level of research may never be possible or public given the financial demands of creating your own running shoes. However, even though the research is behind the development of new shoe models, we must continue asking questions about the mechanisms behind these things and how they are impacting our bodies. The greatest unknown is the impact they may have on injury rates and types, of which there is no research or information currently. That has been left up to clinicians and runners to determine, although hopefully someone will investigate this soon.
Read Part One by Clicking Here
Editor's Note: This update is current as of 11/9/22 and we will continue to update this. As always, the views presented on this website belong to myself or the selected few who contribute to these posts. This website should not and does not serve as a replacement for seeking medical care. If you are currently injured or concerned about an injury, please see your local running physical therapist. If you are in the Los Angeles area, I am currently taking clients for running evaluations in person or via telehealth.
The above post and all writings are my own and property of Matthew Klein PT DPT PhD(c) OCS GCS FAAOMPT. This is part of a larger series of work for my PhD. Reproduction of this work without explicit permission will result in full pursuit by the law for copyright violation.
References
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Dominy, Trace A. and Joubert, Dustin P. (2022) "Effects of a Carbon-Plated Racing Shoe on Running Economy at Slower Running Speeds," International Journal of Exercise Science: Conference Proceedings: Vol. 2: Iss. 14, Article 15.
Hata, K., Noro, H., Takeshita, T., Yamazaki, Y., & Yanagiya, T. (2022). Leg stiffness during running in highly cushioned shoes with a carbon-fiber plate and traditional shoes. Gait & Posture, 95, 9-14.
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