Advances in Midsole Technology in Athletic Shoes and Performance Impact
There has been a dizzying array of advancements in athletic shoe midsole technologies such as foams, higher sidewalls, larger sole flares, and unique midsole geometries and sculpting. These authors summarize recent midsole technologies to better educate patients to pick the right shoe for their foot type, comfort level, pathology, and loading pattern; discuss possible benefits of shoe rotations; and help achieve better adherence with the treatment plan.
Getting properly fit every year at a specialty footwear store can make a huge difference in successfully finding the right comfort and support in a shoe. Once the patient has found that first shoe that works for them, they can consider carefully building a rotation of shoes that suit their mechanics while offering distinctly different mechanical features to provide different loading patterns for different types of activities or surfaces, in our experience, this rotation may help the following:
- prevent overuse injury
- strengthen the body for various impact types, rates, intensity, and duration
- improve the versatility of the patient during prolonged standing (switching halfway during a work shift) and running (speed, recovery, trail, rain)
What You Should Know About Midsole Foams, Plates, and Other Materials
It is important to remember that each person can react differently to a type of foam and/or stability device—and even the shoe’s platform width—so one should recommend shoes on a patient-by-patient basis. Education of the patient is important when they are selecting athletic footwear: the role of active deliberate control (prehab, balanced strengthening, range of motion, coordination, training, technique) of their own running mechanics, rather than relying on the passive mechanics offered by the shoe (eg, participants demonstrated higher impact loading rates with softer shoes).1
Before the advent of fancier foams, in 1975 Brooks Vantage was the first shoe to feature ethylene vinyl acetate (EVA).2 EVA is a copolymer of ethylene and vinyl acetate and is a type of “elastic polymer” or “elastomer.” At the time, EVA was considered revolutionary cushioning material. A few years later, compression-molded EVA added durability to cushioning called the Phylon in the Nike Cortez and then in 1979, Nike introduced a hidden pocket of pressurized air called Air cushioning. Fast forward to 2024 and we now see a push towards polyether block amide (PEBA) by many of the top athletic footwear brands.
What Is PEBA 2.0?
This has been the year of PEBA 2.0 when many footwear companies have highlighted their version of PEBA or a PEBA blend in their midsoles. PEBA has been around since its patenting in 2004 so it is technically not a few foam. What is new is the number of brands using it now in their midsoles.
PEBA is a high-performance thermoplastic elastomer with better energy return and responsiveness. It is also lighter and often more resilient compared to traditional foams. PEBA claims to reduce foot pressure and can perform over a wider temperature range than traditional foams.3 Pebax is a trademarked version of PEBA. When “super shoes” first became available in 2017, PEBA was only found in the highest-end super shoes with carbon fiber plates, but now we have seen footwear companies adding the material (often as a PEBA blend) to their everyday running shoe lineup.
We have noted PEBA combined with nitrogen in the case of some Mizuno midsoles (Neo Vista), Puma (Nitro Elite, Nitro Deviate), and Mount to Coast, while other brands combined PEBA with their proprietary EVA midsole foams: Asics (Superblast 2), New Balance (FuelCell Rebel v4 and Fresh Foam X Balos v1), Saucony (Endorphin Speed, Tempus and Endorphin Pro), On Running (Cloudmonster Hyper, Cloudboom Echo 3).
PEBA-based midsoles are often softer than traditional forms. There are several clinical questions and concepts to ponder, though. Do they help reduce fatigue? Reduction of forefoot pressure may help metatarsalgia. Softer midsoles can be helpful for arthritis. Softer midsoles can affect overall stability. Really soft shoes may not be ideal for a patient with severe balance issues.
When is a soft shoe contraindicated in a runner with an injury? Shoes with high energy return can potentially amplify imbalances. Some of these PEBA-based midsole foams in racing shoes (designed for light weight and energy return) may break down more quickly than standard EVA. Therefore, it is important for the athlete to monitor shoe wear, including anticipating that some shoes with PEBA may experience a shorter lifespan, including premature uneven wear.4
In a further effort to reduce weight, manufacturers may also choose to use a thinner layer or partial coverage of outsole material, which may also result in lower durability. In our observation, the pros and cons of midsoles are based on runners, leaving us to extrapolate data. The patient who spends a lot of time standing and walking should be fit in a specialty running
shoe store. This web article5 lists good quality daily training running shoes as the best walking shoes, prioritizing fit, cushioning, durability, and if needed, style. Heel to toe drop and cushioning/support will be individual. Typically, people selecting a walking shoe should be discouraged from opting for high performance running shoes focused on light weight and dynamic response. In discussing the treatment plan with your patients, you should discuss whether you want them to look for shoes with a softer or firmer midsole. Since the entire body is interconnected keep in mind that super-soft shoes can make not only the foot but also the hips work harder, and also create more work for the entire kinetic chain. You should also factor in a person’s weight as very soft shoes can be challenging for heavier people because they can lead to instability and injury. This is due to the body’s musculoskeletal system trying to compensate for the soft ride.
Softer shoes can cause the body to respond differently and potentially alter biomechanics. Highly cushioned shoes can make the legs stiffer when landing. This can increase impact loading, which in turn can increase the risk of injury to the joints and tissues. This is more noticeable at faster running speeds.
However, is this as big a deal for walkers or folks just trying to get around dealing with an injury? Softer shoes can cause the legs to use more muscle energy to create propulsion.
Softer shoes can change the spring-like mechanics of walking or running, which can increase impact loading which might not be ideal for those lacking range of motion in the ankle and Achilles tendonitis. According to the Doctors of Running, those who need less joint excursion may do better with a softer shoe while those that need to shock absorb more may do better with a firmer shoe.6
Midsole Geometries and Sculpting and Higher Sidewalls
In recent years, many footwear manufacturers have constructed shoes with various sculpting patterns that either control or release particular areas in the midsole, lattice structures, and angular molding not only to save weight but to maximize cushioning in high-impact areas. These include New Balance FuelCell Rebel v4, and Adidas 4D shoes.
We have also noticed that many brands are moving towards higher sidewalls in their shoes as a way to help guide the foot. People with more sensitive feet, however, may get rubbing from the sidewalls if they are not the right width.
Impact of Sole Flares and Wider Platforms
Some shoe models have incorporated a larger sole flare, which can lead to issues like increased ankle instability, potential for excessive pronation, and an awkward gait due to the larger lever arm it creates, particularly for runners who land on the outside of their heel. The farther posterior this flare travels, the harder your anterior tibialis and posterior tibialis will have to work to eccentrically lower your foot into a pronated position.
Wider platforms are often coupled with taller stack heights can help with stability but those patients with narrow gait patterns, stepping on toes, and kicking medial ankles may not want to use shoes with wider platforms.
To Plate or Not to Plate
Puma recently released its jaw-dropping Fast-RB Nitro Elite, which features a full-length separate carbon fiber plate at the top of the midsole and two more thermoplastic polyurethane (TPU) plastic plates under the forefoot, and a massive 58 mm/46 mm midsole stack. The midsole is also decoupled, so the heel and forefoot sections are not joined together by foam, just the plate, and the heel does not touch the ground. Many plated shoes feature midsole voids under the midfoot area to increase stored energy.
In a 2023 article, Dr. Jeffrey Sankoff discussed the additional strain on the Achilles by plated shoes. “With decreased bending, the push off point for the foot moves forward and more strain is placed on the Achilles tendon to lever this longer platform around the ankle joint and propel the body forward.”7
It would be beneficial to have a brief discussion with your more active patients who may want to start using plated shoes and let them know the benefits and potential injuries like bone stress injuries when using a plated shoe for all of their training.8–11 The introduction of carbon fiber plate footwear into athletics should be careful and gradual on an individual basis to reduce risk of injury due to altered foot and ankle mechanics. Some articles have discussed the benefits of building/employing a shoe rotation with plate shoes as well as shoes with different heel-to-toe drops.12 Multiple shoe use and participation in other sports are strategies potentially leading to a variation of the load applied to the musculoskeletal system. Therefore, there are potential benefits of building/employing a shoe rotation with plate shoes as well as shoes with different heel-to-toe drops to decrease the incidence of running related injuries, due to the repetitive overuse that could result from always wearing the same pair of shoes. The most recent sources that mention shoe rotations in terms of different types of runs in a week or training block are in running magazines, webpages, and YouTube videos.13,14
More Than Just Carbon Plates: Rods and Different Materials
While most footwear companies have gone with a carbon fiber plate, Adidas introduced carbon fiber infused rods in some of their models like the Adios Pro 3 and Takumi Sen 10 (called the Energy Rod system), which placed them in the midsole to stabilize the foam and line up with the metatarsal bones in the foot. Some articles have noted these types of rods can offer more flexibility in design and can be better tailored to specific foot movements, while still providing the same energy return benefit as a plate. The intent of the energy rod is to emulate the metatarsal bones of the foot, said Handy, giving the shoe a much more natural gait than a big carbon plate.15 While independent studies have verified that the Nike Vaporfly does make runners more efficient, it appears to be the combination of the carbon-fiber plate and the ZoomX foam, and not one or the other alone. Again, it may come down to individual foot mechanics and preferences.
In an effort to bring plates to more manageable price points, footwear companies have added TPU (New Balance, Saucony, On Running, Hoka One One), glass fiber plates (Mizuno Neo Vista), and polypropylene (On Running) sandwiched in between midsoles of different levels of cushioning.
Shoe Discussions Can Make a Huge Difference
When we distill all this new technology into a clinical setting, discussing which shoe technologies and characteristics will benefit your patient’s particular issue will go a long way in helping with their overall treatment as well as their comfort.
Do you want patients to get a softer or more responsive midsole? Do you want them to purchase a shoe with a plate and if so, what kind of plate, or do you want them to avoid plated shoes? Should they look for a shoe with a wider or narrower platform? Would shoe rotations help in their treatment plan, especially for your aging patients or those on their feet all day long for work? We have noted that many in the podiatric community are fans of this because shoe rotations provide a variety of loading patterns and may prevent overuse injury and strengthen body for various impact types.
Ultimately, the most comprehensive discussions and all of the best technology won’t do your patients any good if they aren’t in a shoe of the right type, size, and width. We often see folks who do not match their foot shape to the shoe shape, which will cause all sorts of foot pathologies or people needing a wider width who just go up a full size, which does not allow the foot to bend where the shoe bends. Shoes have become so high-tech that being properly fitted and measured at a specialty store whose staff is knowledgeable about the fit and feel characteristics of each brand is crucial in helping patients with their treatment.
Tim Dutra, DPM, is an Assistant Professor at Samuel Merritt University College of Podiatric Medicine. He is a Podiatric Team Physician at the University of California, Berkeley. He is the Clinical Director of Special Olympics Healthy Athletes Fit Feet. Dr. Dutra is the Past President of the American Academy of Podiatric Sports Medicine.
Stephen Hill, PhD, is an Associate Professor and Manager of Motion Analysis Research Center at Samuel Merritt University.
Jennifer Wong, CPed, is the Certified Corporate Wellness Specialist & Business Development Rep at New Balance.
References
1. Sinclair J, Mcgrath R, Brook O, Taylor PJ, Dillon S. Influence of footwear designed to boost energy return on running economy in comparison to a conventional running shoe. J Sports Sci. 2016;34(11):1094-1098. doi:10.1080/02640414.2015.1088961
2. Brooks Running. Our history. Accessed Oct. 30, 2024.
3. Tuntun. Application of PEBA in sports shoe soles. Published Aug. 25, 2024. Accessed Oct. 30, 2024.
4. Kirby KA. Running Super shoes: truly super, or just hype? Podiatry Today. 2022; 35(5):40-46.
5. Luff C. The 10 best cushioned shoes for walking, expert-tested. Very Well Fit. Published April 11, 2024. Accessed Nov. 11, 2024.
6. Doctors of Running. Beginner’s guide to walking, and the best running shoes for walking. Accessed Oct. 30, 2024.
7. Sankoff J. New study examines how running shoes can improve (or worsen) biomechanics. Triathlete. Published Nov. 20, 2023. Accessed Nov. 11, 2024.
8. Tenforde A, Hoenig T, Saxena A, Hollander K. Bone stress injuries in runners using carbon fiber plate footwear. Sports Med. 2023;53(8):1499-1505. doi:10.1007/s40279-023-01818-z
9. Mai P, Robertz L, Robbin J, et al. Towards functionally individualised designed footwear recommendation for overuse injury prevention: a scoping review. BMC Sports Sci Med Rehabil. 2023;15(1):152. Published 2023 Nov 11. doi:10.1186/s13102-023-00760-x
10. Sun X, Lam WK, Zhang X, Wang J, Fu W. Systematic Review of the Role of Footwear Constructions in Running Biomechanics: Implications for Running-Related Injury and Performance. J Sports Sci Med. 2020;19(1):20-37. Published 2020 Feb 24.
11. Malisoux L, Ramesh J, Mann R, Seil R, Urhausen A, Theisen D. Can parallel use of different running shoes decrease running-related injury risk?. Scand J Med Sci Sports. 2015;25(1):110-115. doi:10.1111/sms.12154
12. Ball A. How to build a running shoe rotation. Runners World. Published April 25, 2023. Accessed Nov. 11, 2024.
13. Davis M. A marathon training shoe rotation for fall 2024. Accessed Nov. 11, 2024.
14. Burgess M. The science behind Adidas’s new world-record running shoe. Wired. Published Sept. 14, 2020. Accessed Nov. 11, 2024.
