Our previous article, makes a persuasive case for the performance benefits of super-shoes; in this article we’ll look at the engineering innovations behind them.
Super-shoes typically have the following characteristics:
- A thick, lightweight, high-energy-return foam midsole (the layer of material between the inner and outer soles of a shoe).
- A shaped stiffening device consisting of a semi-rigid plate or rods embedded within the midsole.
- Some shoes (e.g. Nike’s Alphafly) additionally incorporate air cushioning units.
We will look at each of these features in turn.
The requirements of foam midsoles:
As horizontal velocity is quite constant, we can approximate running as a series of alternating vertical hops; for each hop the body must change its vertical velocity (accelerate) from downwards to upwards as it collides with the floor. These collisions can cause large forces during running which may lead to discomfort and injury; a running shoe can reduce these forces by providing cushioning. Cushioning works by introducing a compressible, spring-like structure between colliding rigid objects; this structure deforms during contact, increasing the time of the collision and thus reducing the accelerations and forces. During running, most cushioning is provided by flexing the knee on impact, but this can’t reduce the acceleration of the foot and lower leg as they are below the knee. For runners who land on their forefoot, ankle flexion can cushion these accelerations, but for heel-strikers (who form the majority of runners) this cushioning has to be provided by compression of the midsole.
The amount a foam compresses for a given load is called its compliance (the change in thickness for a given force) and is the inverse of its stiffness. The lower the stiffness / higher the compliance, the greater the reduction in force, so to reduce forces significantly the midsole must have a high compliance / low stiffness. A high compliance midsole will compress considerably under load and so must be thick to prevent it bottoming out and becoming stiff. World Athletics (the international governing body) introduced rules in 2021 restricting the thickness of soles (midsole, outsole and insole) to 40 mm. Shoes with thicker soles are available but are not legal for competition.
Mechanical work is done on the midsole as it compresses when the runner loads it; some of this work is stored as elastic energy within the midsole. When the force is removed the midsole returns to its original shape, returning some of this energy. Any energy that is not returned is lost as heat within the foam. The percentage of energy returned by a foam is called its resilience, and this energy may be beneficial in assisting the runner if it is returned at the right time in the running cycle, so a foam that has a higher resilience may contribute to more efficient running.
Density is a further important performance factor as a low-density foam can be used to make a thick midsole without adding significant additional weight to a shoe. One additional factor that does not affect performance but is important for sustainability is durability – the ability of the foam to withstand multiple impacts without degradation. Running shoes might typically be expected to last at least 500 km without significant degradation, and this would amount to about 250 000 impacts per shoe. An extreme minimum requirement would be that a foam must last for the duration of one race without noticeable degradation in performance.
Foam technology used in insoles:
Historically, running shoes used foam rubber as a midsole material[1] until Brooks released the Villanova shoe with an EVA (ethylene vinyl acetate) midsole in 1975. EVA is a thermoplastic elastomer (TPE) (where thermoplastic means softening when heated and elastomer means an elastic material composed of long polymer chains). EVA is lightweight, highly cushioned, low cost and simple to cast into a midsole, so it quickly came to dominate the running shoe market and continued to do so until 2013 when Adidas introduced Boost midsole foam. Boost is a foam manufactured by BASF from small pellets of thermoplastic polyurethane (TPU) combined together using heat. Boost demonstrated an improved energy return (resilience) compared to EVA of 76% vs 66%[2] and although it was slightly heavier than EVA, runners with Boost-equipped shoes achieved several world records. This led other manufacturers to develop their own competitor foams and there are currently a bewildering range of foams with brand names such as ZoomX (Nike), Lightstrike Pro (Adidas), PWRRUN (Saucony) and FlyteFoam (ASICS). These new foams often outperform Boost foam: for example, ZoomX foam has a resilience of 87% compared to Boost’s 76% as well as being less dense.
The impressive performance of modern foams is due to several innovations that improve energy return and reduce density compared to traditional EVA.
Use of PEBA and similar materials:
In 2016 Nike athletes filled the men’s podium at the Olympic marathon wearing prototype shoes with a secret new foam. The shoe was released to the public in 2017 as the Zoom VaporFly 4%, with the foam trademarked as ZoomX. This foam is made from a base plastic called polyether block amide (PEBA), a high performance TPE consisting of soft, rubbery, polyether units joined together by hard polyamide crosslinks, allowing the plastic’s properties to be considerably tuned. Compared to other TPEs, PEBA has a lower density, higher energy return, and a wider range of operable temperatures, meaning that it is commonly used for high-performance sports equipment such as football and ski-boots. A type of PEBA with the trademark Pebax is made by the French company Arkema; a specialist UK foam manufacturer called Zotefoams found a way to manufacture Pebax into a foam, releasing it for the sports and leisure market in 2010. This material has very favourable properties for a running shoe having energy return of over 85% combined with low stiffness (meaning more cushioning) and a low density. In 2017 Zotefoams announced a strategic partnership to supply high-performance foam materials exclusively to Nike within the footwear industry, leading to the development of ZoomX foam. Other companies also use PEBA foams such as Saucony (PWRRUN), ASICS (FlyteFoam Blast Turbo) and On (Helion HF). Different chemistries and foaming methods mean that super-foam characteristics differ between manufacturers and may even vary within a manufacturer for different models and generations of shoes.
Foaming with a supercritical fluid:
A supercritical fluid is a highly compressed fluid that exhibits both the properties of a gas and a liquid, allowing multiple specialist applications including removing caffeine from coffee beans. In order to produce a foam, supercritical carbon-dioxide or nitrogen are used as the foaming agent and are pumped into a liquid plastic within a mould. The fluid then expands as a gas once the pressure reduces, forming miniature bubbles inside the foam which make it lighter, softer and increase its energy return. By carefully controlling pressure and temperature, the behaviour of the supercritical fluid can be manipulated allowing precise control of the bubbles within the foam structure. Foams made from supercritical nitrogen are considered to have better properties than those made from supercritical carbon-dioxide, but require more extreme conditions to achieve supercriticality, making these foams harder to manufacture and increasing their price.
Blends of polymers:
Amide bonds allow multiple types of base elastomer units to be combined, so for example PEBA and EVA can be combined to control material properties and costs. Puma’s Nitro Elite foam is an example of a combination of PEBA and EVA. This approach allows the performance of low-cost foams such as EVA to be improved to approach that of super-foams.
Carbon plates:
Although popular media often report that the addition of carbon plates is the biggest innovation in super-shoes, they are not novel. Michigan State University researched the use of running shoes incorporating carbon plates in the 1980s, and this technology was released by Brooks as the Fusion shoe. Adidas subsequently worked with the University of Calgary’s Human Performance Lab to investigate the use of carbon plates, introducing a curved carbon fibre ProPlate in the Adidas Equipment Gazelle Pro Plate released in 2001, and then in 2003 a light-weight performance shoe, the 225 grams Adistar Competition. Roy & Stefanyshyn published a paper in 2006 showing that stiffening a shoe using a carbon-fibre plate improved running economy by approximately one per cent[3], and that a shoe with a moderate stiffness was more economical than the stiffest shoe. This small performance advantage was presumably insufficient to justify continuing with an expensive technology and carbon plates were not further used in running shoes until Nike (working with ex Human Performance Lab researchers) introduced a highly curved version in the 2017 Zoom VaporFly. In 2020 Adidas themselves re-introduced carbon-fibre stiffening elements in the form of EnergyRods. These are five carbon-infused rods, mimicking the metatarsal bones of the foot and connecting to a carbon-fibre heel plate all encased in a LightstrikePro foam. Adidas claim that the additional freedom of movement of the separate EnergyRods leads to a more natural gait; Tigist Assefa wore a pair of Adizero Adios Evo Pro 1 shoes containing EnergyRods when she broke the women’s marathon world record by over two minutes in Berlin in 2023.
Contrary to popular belief, the carbon fibre plates may not primarily function as energy storage devices. The idea behind Roy and Stefanyshyn’s work was not to act as a spring but rather to reduce bending of the foot, particularly at the metatarsophalangeal joint, by adding external stiffening to reduce the angle that this joint moved through at push-off. If a plate were to act as an linear spring it would need to deform to be able to store elastic energy, and this would require it to be fixed at one or more points within the shoe and free to more elsewhere; this is unlikely to occur given the way plates are embedded within midsoles. Even if a plate could deform to act as a spring, it would store much less energy than the foam midsole. This is because when springs act in series (as the plate and foam “springs” would have to do for linear displacement), the energy stored in each is inversely proportional to the stiffness of each spring. As the plate is very much stiffer than the foam, it will displace a much smaller distance for the same force and hence store much less energy. If the plate were to act as a torsion spring and bend (as might occur as the foot rolls forwards at the end of stance) then this bending would occur in parallel with the foam midsole, and the energy storage in the plate would be much higher than in the foam.
Air cushioning devices:
Eliud Kipchoge wore an unnamed shoe containing two air cushioning units in each forefoot when he broke 2 hours for the marathon distance in 2019 in Vienna. The following year the shoe was released to the public as the Air Zoom Alphafly Next%. Kipchoge wore an updated version when he retained his Marathon Olympic title in 2021 and lowered the official Marathon world record in Berlin in 2022. Air units are also used as forefoot cushioning in sprint and middle distance track shoes such as the Nike Air Zoom Victory and Maxfly spikes which won both men’s and women’s 100 m gold medals at Tokyo. However, as with carbon plates, there is nothing new about using air as a cushioning system. The Finnish company Karhu marketed the ChampionAir running shoe in 1977 with a patented air cushion under the heel. Nike’s Air Tailwind shoes were released in 1979 with hidden air suspension units. These air units became a visible and iconic fashion feature in Nike’s Air Max 1 training shoe in 1987 and continued to feature in many of Nike’s subsequent running and leisure shoes.
Air units function as a spring in the same way that foams do, with the advantage that their stiffness can be controlled simply by altering the pressure of the gas within them. They are even more efficient than the most resilient of foams, with energy return figures of above 90% according to Nike[4]. However, they are potentially heavier than foam as they require flexible yet tough walls to withstand the large pressures applied. For example, a runner with mass 70 kg (weight 700 N) exerting a force of 6 bodyweights on air units with a cross-sectional area of 50 cm2 will generate a pressure of 0.8 MPa or 8 bar, about 2-4 times the pressure of car or bicycle tyres!
Other features:
Weight: Super shoes are often exceedingly light, with the necessity of having a thick midsole meaning that weight must be lost elsewhere within the shoe. Typically, the uppers and outsoles are constructed of thin materials which may reduce longevity. For example, Adidas’s Adizero Adios Evo Pro 1 shoes weigh just 138 g (US size 9), and the company suggests that rather than being suitable for daily training they are optimised for one single race only. The price of $500 is beyond the budget of all but the most dedicated runner and raises questions about the environmental and ethical consequences of the trend to lightness at all costs. However, unless governing bodies legislate for improved longevity (which seems unlikely) this trend will continue apace.
Aerodynamics: One of Nike’s claims when first launching the VaporFly was that the tapered heel would reduce drag by minimising the wake produced when the shoe moved quickly through the air. Superficially this makes sense, as the foot moves approximately twice the speed of the rest of the body during the swing phase of gait. However, the foot is pointed downwards in mid-swing when it is moving fastest, so the air flows from laces to outsole, not from toe to heel. Other manufacturers haven’t copied Nike’s approach, so this innovation likely has minimal effect on running performance.
Conclusions:
We have seen that there are considerable engineering innovations within super-shoes, but aside from novel foams a lot of these have been previously introduced without much success; it is most likely to be the careful combination of multiple individual innovations that has produced the performance benefits we see with the current generation of super shoes.
In the next article in this Super Shoes blog series, we will consider how these engineering innovations might actually work to allow runners to run faster. For more information about work we do in SERG check out our website, our annual review or our MSc Sports Engineering course.
References:
[1] Cavanagh PR: The Running Shoe Book. Anderson World, Inc., Mountain View, CA, 1980
[2] Hoogkamer, W., Kipp, S., Frank, J.H. et al. A Comparison of the Energetic Cost of Running in Marathon Racing Shoes. Sports Med 48, 1009–1019 (2018)
[3] Roy, J. P. R., & Stefanyshyn, D. J. (2006). Shoe midsole longitudinal bending stiffness and running economy, joint energy, and EMG. Medicine & Science in Sports & Exercise, 38(3), 562-569.
[4] https://www.runnersworld.com/uk/gear/shoes/a30777696/nike-alphafly/
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