Physical Therapists Using Clinical Analysis To Discuss The Art And Science Behind Running and The Stuff We Put On Our Feet

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An Evidence Based Review of PEBAX and Carbon Fiber Shoes
By Chief Editor Matt Klein

In 2016, the Nike Vaporfly, a unique running shoe with a combination of a super soft responsive foam called polyether block amide (PEBA) foam and a full-length carbon fiber plate, was introduced to the market. Since that time, many individuals, both in research labs and on the roads, tracks and trails, have confirmed its potential for performance improvement. Multiple world records have been set by individuals wearing this type of shoe and recently the statistically unlikely men's sub 2 hour marathon was run on a closed course in the newest version of this shoe (as of writing this) called the Nike Zoom Alphafly Next%. Additionally, the women's statistically impossible sub 2:15 marathon was destroyed in 2019 by an individual wearing an older version of this shoe type (Angus, 2019). While most of the research has been done on the Nike Vaporfly, many other major running companies have begun to release their similarly designed shoes to market as as new rules regarding prototypes and specifically designed footwear set by the International Association of Athletics Federation (IAAF) to allow their shoes to be used at the 2021 Summer Olympics. Saucony released the Endorphin Pro (review), Brooks released the Hyperion Elite (review soon), Adidas is releasing the Adizero Pro, New Balance has the Fuel Cell RC on the way, Asics with the Metaracer, Mizuno is working on something (review soon), Hoka has the Rocket X, Carbon X (REVIEW) and Carbon Rocket, and Skechers has the GOrun Speed Elite Hyper (REVIEW). The problem with all of this is that both the majority of people and companies are focused on the wrong components of the shoe when it comes to performance. Let's talk about the key characteristics of this type of footwear, what is important and why only a few companies have actually done this correctly (hint: Nike and Saucony are currently the only two companies to have shoes on the market that correctly fit the criteria of being super shoes).

Editor's Note: This Literature Review is based off the current research available as of 4/5/2020. I cannot comment of research that is not public. Many companies have likely done their own studies, (which are inherently biased), however this research is not public information thus is not included in this review. The concept of performance is FAR more complicated than a foam or plate as I am sure we will see in the coming years with more research. Again, this is based off of the current research available at this time. More will come. 

Image from Pebax Powered

Footwear Characteristics & Research

The key characteristics of this new footwear type include a PEBA midsole foam for shock absorption and rebound, usually a tall midsole/maximalist design (not always) and a full length carbon fiber plate for stability and propulsion (Burns & Tam, 2019). Most of the literature on the performance characteristics of this type of footwear are focused on running economy. Running economy is defined as the "energetic cost of running at a specific velocity" (Hoogkamer et al., 2018). Runners with better running economies may be more likely to maintain faster running speeds for longer periods than compared to those with inferior running economies (Daniels & Daniels, 1992). The current research has demonstrated that these types of shoes that include the PEBA foams and carbon fiber plates improve running economy in most runners from 2% to as much as 10% (Hoogkamer et al., 2019). Further testing has demonstrated the superiority of these shoes to older models in energy return and efficiency, with one of the most significant studies demonstrating a significant difference between the original Nike Vaporfly, Nike Streak and Adidas Adios Boost 2 (Barnes & Kildling, 2019; Hoogkamer et al., 2019). Almost every measure associated with performance has been shown to improve with this footwear type compared to other racing shoes including efficiency, oxygen consumption, stride length, center of mass oscillation, plantar flexion velocity and more (Hunter et al., 2019).

In regards to the maximalist design, a few features may help with fatigue resistance but have not been shown to improve speed or efficiency. There is research demonstrating that rockered sole designs decrease the work required at the ankle and foot (Ogsten, 2019). This works by mimicking and replacing the rockers of the foot and ankle with the curved midsole (Sobani et al., 2014). Further research has demonstrated decreased load into the forefoot with a rockered shoe (Brown, 2004), however there is a retribution of all this work into the knee and hip (Sinclair et al., 2016).

Brooks launch new running shoe with a carbon fibre plate
Image from Runners World

Carbon Fiber Plates

The greatest amount of attention outside of the research world has focused on the carbon fiber plates. Despite what one would logically think, the carbon fiber plates have been shown to only improve running economy and performance by 1% (Roy & Stefanyshyn, 2006). Even then, research from the same lab the Jared Ward is part of has suggested that the optimal stiffness of the footwear and these plates are completely subject dependent (Mcleod et al., 2020). That means the optimal stiffness required by each person of the shoe and plate varies depending on their individual biomechanics (and other factors). So despite what everyone gets excited about, the carbon fiber plates play a very small part in the performance improvements as demonstrated by the currently available research.

Is it the shoes? A proposal to regulate footwear in road running ...
Image from BMJ Blog and Burns & Tam (2019)

Foam/Midsole Material

The research has repeatedly demonstrated that the PEBA foams have the greatest influence on running economy and contribution to performance improvements over any of the other components (Hoogkamer et al., 2019). While the carbon fiber plates have been shown to improve economy by 1%, the PEBA foams have been shown to improve economy by anywhere from 4-6% and sometimes greater depending on the individual (Roy & Stefanyshn, 2006; Barner et al., 2019; Hoogkamer et al., 2019). The mechanisms behind this include improved energy return as well as decreasing fatigue over longer efforts. The use of PEBA foams therefore is one of the most important elements of design in this footwear category, which means that most companies have missed the mark in their designs.


Correct Execution

As mentioned above, the most significant factor for most subjects appears to the use of PEBA foams in this footwear type. The benefits again appear to be from both superior energy return as well as reducing fatigue over longer distances. A common note among athletes is decreased muscle soreness and fatigue following hard training sessions or racing compared to other footwear. Thus most companies have been executing this incorrectly when claiming their shoes are "super shoes."  The only two companies that can truly claim this with models available to the public are Nike with the Vaporfly and Alphafly models and Saucony with the Endorphin Pro. Every other company has used non-PEBA midsoles with carbon fiber plates, meaning they are missing out on the true factor that improves performance. The carbon fiber plates are awesome and stabilize the PEBA foam well, but by themselves do little. The use of carbon fiber plates isn't even new as Adidas was using them almost two decades ago in their racing shoes.

It should be noted that there there will be other foams that match PEBA foams. This has yet to be demonstrated via peer-reviewed research yet, but some of the key characteristics of this foam is the high resilience due to an optimal durometer (which is a measure of material hardness) and keeping weight down as much as possible. The Vaporfly and Endorphin Pro have a a very specific durometer with a large amount of midsole while maintaining a low weight for the amount of shoe present. While both use PEBA, one cannot say that there will not be other foams that have similar properties. The New Balance RC uses FuelCell as the midsole, which is a nitrogen infused compound.  The Brooks Hyperion Elite 2 is rumored to feature DNA flash, which is also a nitrogen infused material. There is no independent research that I am aware of on the performance benefits of nitrogen infused soles, however if the weight can be kept low enough and there is enough resilience in the sole to provide energy return a fatigue resistance, this may be a comparable foam. For now, research still needs to be done.

So most companies are getting this wrong. Brooks with the DNA zero is not a PEBA foam as far as I am aware and although it is firm and stable, is likely not contributing to shock absorption and returning energy. I am hoping that with the upcoming Hyperion Elite 2 that this is rectified with a change to DNA Flash, although as I said before, nitrogen injected foams have yet to verified as improving energy return and fatigue via independent labs. The entire Hoka carbon fiber line has gotten this wrong as they continue to use EVA cushioning with a carbon fiber plate, meaning that the only thing improving running economy is the plate, which as we discussed only contributes 1%. I had high hopes for the Rocket X, but word on the street is that it is EVA based as well (hoping that is wrong). As cool as they look, the Adidas Adizero Pro also misses this as previous research has demonstrated the superiority of PEBA foams to TPU foams (Barnes & Kildling, 2019; Hoogkamer et al., 2019). The Adizero Pro features a carbon fiber plate, Boost and Lightstrike. From my recent review of the Adios 5, Lightstrike is certainly not performance enhancing, but hopefully the design makes for a lightweight fast racing flat. It however will not qualify into this category. Asics and their metaracer, which had my hopes up for so long, also may miss the mark as the only significant factor is the plate. The use of a cellulose based Flytefoam is interesting, but again has no research behind it. The fact that it sits fairly low to the ground and has a more traditional stack height also misses the benefit of a more rockered sole. The Skechers GOrun Speed Elite Hyper, although my favorite, also does not fit into this category. It features a carbon fiber H-plate and like the metaracer sits at a traditional stack height. As much as I love HYPER (CO2 infused EVA), it does not qualify and like Flytefoam, FuelCell, Boost, Lightstrike and DNA Zero, has no research behind them to demonstrate anything close to any kind of economy improvement.

I am not meaning to downplay certain foams. FuelCell, the new Flytefoam, DNA Flash, Hyper and more just do not have an independent research behind them to suggest such massive economy improvements. It is not enough for a company to tell you they did research, which is highly suspect given the strong risk of bias and not likely to be released. Independent research has to be done to confirm that these foams can match what has already been researched on PEBAX foams.

Ironically, Reebok has been using a PEBAX foam in a few of its shoes for some time (Floatride Run, Floatride Run Fast and the Run Fast Pro). However they have yet to pair it with a carbon fiber plate and maximalist design. I hope they can come through where their fellow company Adidas has missed.


It Still Isn't That Straightforward....

Despite all the evidence and research I have laid out above, it should be noted that only certain people seem to get benefits from the foams, carbon fiber plates and other technologies associated with the shoes. Research has come out just recently that the actual benefit derived from each subject varies greatly based on individual factors (Herbert-Losier et al., 2020). These performance changes can be as great as 10% (or more) in some people and for others may actually be detrimental. So as much as we talk about the percentages gleamed from the research suggesting how much certain elements contribute to changes in economy, remember that each person is unique in that effect.

Additionally, the research I have listed above is still somewhat limited in understanding all the factors that may improve running economy. Running companies have done plenty of research on carbon fiber plates, yet have not released it to the public. What research has been done focuses heavily on the plates and foam, yet any researcher will tell you that trying to dumb things down to one or two variables is foolish. Far more needs to be done on understand all the factors that contribute to an optimal racing shoe.

Nike and Brooks Dominate the 2020 Olympic Marathon Trials
Image from Jeff Dengate's Runners World Article

To summarize the above, although the public and companies are excited about carbon fiber plates, based on the current literature they provide only a small part of what makes these shoes special. The majority of the performance and economy improvements come from the foams and this has been repeatedly shown when compared to other traditional racing shoes, TPU foams, track spikes and even other shoes with carbon fiber plates (Roy & Stefanyshn, 2006; Barner et al., 2019; Hoogkamer et al., 2019; Mcleod et al., 2020). Thus far, only a few shoes on the market truly deserve to be called super shoes at this time. These again are the Nike Vaporfly/Alphafly series and the Saucony Endorphin Pro. A few others not available to the market at this time (and a few from companies still in wear testing phases) do meet this strict criteria, but every other company has fallen short with their current offerings. One cannot use evidence of "so and so won this race." Yes, Des Linden won the Boston Marathon in horrible conditions in a pair of Hyperion Elite. That is what we call an outlier due to extraneous variables (in this case called covariates), ie weather. Yes, the women's winner of the 2020 Olympic Marathon Trials was wearing the Hoka Rocket X. That's called being a great athlete. Further research must be done on other foams to see if they reach the superiority of PEBA foams. At some point some other superior foam will come along, but that day has not arrived.

I have written this not as a way to talk down to companies, but as a way to raise awareness about what actually improves economy and performance. Marketing is great, but we as researchers consistently try to find the truth. I hope through this literature review that you find some additional information that helps you understand how truly complex this is. The above information is taken from many of my papers that I have been using in my PhD work. I hope you found it useful.

Thanks for reading!

The above post and all writings are my own and property of Matthew Klein PT DPT OCS 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.

Editor's Note: 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.

Doctor of Physical Therapy
Board Certified Orthopedic Clinical Specialist
PhD Student Movement Science and Rehabilitation
Fellow of the American Academy of Orthopedic Manual Physical Therapists
***Disclaimer My views are based on my extensive history in the footwear industry and years testing and developing footwear. If you are a footwear rep looking for footwear reviews or consultations on development, we are currently looking to partner with companies to assist, discuss and promote footwear models. Partnership will not affect the honesty of our reviews.

A WORD OF THANKS: This post was inspired by many of the people who have shaped my understanding and knowledge, including Simon Bartold, Craig Payne, Benno Nigg and many more. Thanks to Jeff Dengate, Alex Hutchinson and many more who continue to inspire me to write and stay hungry for more.


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2. Barnes, K. & Kilding A. (2019). A randomized crossover study investigating the running economy of highly-trained male and female distance runners in marathon racing sheos veruss track spikes. Sports Medicine, 49(2): 331-342.

3.. Brown, D., Wertsch, J., Harris, G., Klein, J., Janisse, D. (2004).  Effect of rocker soles on plantar pressure.  Archives of Physical Medicine and Rehabilitation, 85(1): 81-86.

4. Daniels, J., Daniels, N. (1992). Running economy of elite male and elite female runners. Medicine and Science in Sports & Exercise, 24(4): 483-489.

5. Hebert-Losier, K., Finlayson, S., Driller, M., Dubois, B., Esculier J., & Beaven, C. (2020). Evidence of variable performance responses to the Nike 4% shoe: Definitely not a game-changer for all recreational runners. (Pre-Print).

6. Hoogkamer, W., Kipp, S. & Kram, R. (2019). The biomechanics of competitive male runners in three marathon racing shoes: a randomized crossover study. Sports Medicine, 49(1), 133-143

7. Hoogkamer, W., Kipp, S. & Kram, R. (2019). The biomechanics of competitive male runners in three marathon racing shoes: a randomized crossover study. Sports Medicine, 49(1), 133-143.

8. Hunter, I., Mcleod, A., Valentine, D., Low, T., Ward, J., Hager, R. (2019). .Running economy, mechanics and marathon racing shoes. Journal of Sports Sciences, 1-7.

9. Mcleod, A., Bruening, D., Johnson, A., Ward, J., Hunter, I. (2020). Improving running economy through altered shoe bending stiffness across speeds. Footwear Science, 1-11.

10. Ogston, J. (2019). Comparison of in-shoe plantar loading forces between minimalist and maximalist cushion running shoes. Footwear Science, 11(1), 55-61.

11. Roy, J. & Stefanyshyn, D. (2006). Shoe midsole longitudinal bending stiffness and running economy, joint energy and EMG. Medicine & Science in Sports & Exercise: 38(3), 562-569.

12. Sinclair, J., Richards, J., Selfe, J., Fau-Goodwin, J., & Shore, H. (2016). The influence of minimalist and maximalist footwear on patellofemoral kinetics during running. Journal of Applied Biomechanics, 32(4), 359-364.

13. Sobhani, S., Bredeweg, S., Dekker, R., Kluitenberg, B., Heuvel, E., Hijmans, J., Postema, K.  (2014).  Rocker shoe, minimalist shoe and standard running shoe: A comparison of running economy.  Journal of Science and Medicine in Sport, 17(3): 312-316.

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