Unveiling How Lightweight Running Shoes Lead the Marathon Market

Unveiling How Lightweight Running Shoes Lead the Marathon Market

Abstract

With the popularity of the marathon, lightweight running shoes have enhanced the characteristic of lightness on the basis of shock absorption, support, and comfort. They are favored by marathon enthusiasts and have broad development prospects. The key to achieving lightweight running shoes lies in the reasonable optimization of the midsole material and structure. This article reviews the development process of lightweight running shoe midsoles, focuses on summarizing the innovation of running shoe midsole materials at home and abroad and the research progress of lightweight design of the sole structure, and prospects the research trend of lightweight midsole for marathon running shoes, aiming to provide new ideas for the lightweight and structural innovation design of marathon running shoe midsole materials.

Running can be divided into long - distance running, middle - distance running, and sprinting according to the distance. There are running shoes with different functional characteristics and design concepts for different types of running. For marathon enthusiasts, a pair of lightweight and comfortable running shoes is particularly important. Research shows that under specific running conditions, for every 100g increase in the weight of running shoes, the running time for a 3000m distance will increase by 0.78%. This means that the lighter the running shoes, the higher the running efficiency. From 1908 to the present, the men's marathon world record has been improved from nearly 3 hours to 2 hours, 1 minute, and 39 seconds. In the past hundred years, not only has human physical fitness improved, and training methods and eating habits become more scientific, but advanced sports equipment has also made great contributions to human achievements in the long - distance running field. Especially the design of shock - absorbing running shoes has evolved from heavy to light and from simple to complex. The continuous optimization of their lightweight characteristics has provided runners with a more efficient running experience and a lower risk of injury.

To achieve lightweight running shoes, the part with the greatest design space is the sole. The sole consists of an insole, a midsole, and an outsole. The key to improving the function of the sole lies in the midsole. The reasonable configuration of midsole materials and structures has always been the focus of the development of functional footwear technology. For decades, researchers' studies on lightweight running shoes have mainly followed two trends: the first is the innovation of materials; the second is the exploration of the sole structure design. Among all shoe midsole materials, foamed materials have the best lightweight effect. However, due to the thin wall thickness of their pores, the mechanical properties of shoe midsoles made of them are not ideal, and they are prone to collapse, even becoming "disposable running shoes". For soles with less than ideal support performance, components such as carbon plates can be embedded to improve the mechanical properties of the sole. In addition, 3D printing technology is gradually maturing. Its "additive manufacturing" feature can meet the one - piece molding of complex structures such as porous structures. The bionic shoe midsoles and running shoe midsoles with mechanical metamaterial structures manufactured using 3D printing technology have excellent mechanical strength. Therefore, the popularity of 3D - printed shoe midsoles has been increasing in recent years.

I. Research Progress on Lightweight Shoe Midsole Materials

1.1 Synthetic Materials

Natural rubber is a natural polymer compound mainly composed of polyisoprene. It has good elasticity, durability, impact resistance, efficient heat dispersion, and ductility at low temperatures. The first pair of rubber shoes was brought to the United States in 1820 and exhibited in Boston. However, natural rubber becomes sticky when heated and hard when cooled, making rubber products unstable in practical applications. In the early 19th century, synthetic rubber was invented, which has excellent heat resistance, compression resistance, and wear resistance. In 1839, vulcanized rubber was applied to the shoe - making industry due to its good wear resistance and stability. Subsequently, new - style sports shoes with vulcanized rubber soles and canvas uppers entered the market, marking the birth of modern sports shoes. In the early 20th century, providing new and multifunctional alternative materials became the theme of the development of sports shoe materials. In 1977, Brooks added the emerging EVA (ethylene/vinyl acetate copolymer) material to the Vantage shoe model, marking the beginning of the midsole R & D era. Li et al. used a lightweight, anti - slip, wear - resistant, and well - cushioned elastomer PU (polyurethane) as the raw material to make a low - cost, beautiful, and comfortable running shoe midsole structure. In addition to EVA and PU, elastic polymer materials such as TPU (thermoplastic polyurethane elastomer), PVC (polyvinyl chloride), TPR (thermoplastic rubber), and ABS (acrylonitrile - butadiene - styrene copolymer) are also widely used in running shoe midsole materials, opening up a new situation for the lightweight and functional development of running shoe midsoles.

1.2 Foamed Materials

As is well - known, for materials with the same composition, if the density is smaller, the mass of the same - volume material is lighter. Making the elastomer materials used for running shoe midsoles into foam materials is an efficient lightweight method.

1.2.1 EVA Foamed Materials

Foamed EVA was the first foamed material used in running shoes. Its rebound resilience is generally between 40% and 45%, much better than that of PVC, rubber, and other materials. It also has the characteristics of light weight and easy processing, and is widely used in shoe midsoles. However, foamed EVA - based products have poor aging resistance, flex resistance, elasticity, and wear resistance, which limits its use to some extent. However, blending and modifying EVA with some elastomers, functional fillers, additives, etc. can improve the performance of foamed EVA. Zhu et al. mixed plastics such as EVA with additives such as zinc oxide and stearic acid to make an EVA foamed material with a specific gravity of less than 0.15g/cm³. The running shoe midsole made of it is 30% - 40% lighter than ordinary midsoles. Lei et al. mixed EVA with environmentally friendly recycled rubber and POE elastomers to prepare an EVA composite foamed material with excellent shock - absorbing performance and low density. As a running shoe midsole, it is lightweight and has high rebound. This preparation method using recycled materials is more in line with the concept of sustainable development. Skecher developed the HyperBurst bubble technology midsole material. The marathon running shoe midsole made of it weighs only 181g and has the properties of ultra - lightness, high rebound, and strong wear resistance. HyperBurst uses Skecher's exclusive special - formula EVA material and is physically foamed with supercritical carbon dioxide, becoming the lightest midsole material in Skecher's history.

1.2.2 TPU Foamed Materials

Kharbas et al. conducted a comparative study on the chemical and physical foaming methods of TPU. The research results show that using physical blowing agents can obtain softer foams, and using CO₂ and N₂ as co - blowing agents can obtain foams with lower bulk density, better microstructure, and lower hysteresis loss rate. The microcellular foam materials prepared by supercritical fluid foaming technology have the advantages of small pore size, high pore density, good comprehensive performance, and are more environmentally friendly. Therefore, they are widely used in the preparation of shoe midsole materials. In 2013, adidas used the ETPU material from the BASF Group to make the Boost, a revolutionary ultra - light and shock - absorbing running shoe. ETPU is a "popcorn" shoe midsole material obtained by foaming TPU using supercritical fluid foaming technology. However, due to its overly soft foot - feel and low foam density, resulting in insufficient mechanical strength, it cannot well convert the impact force into the energy for the next step after being impacted, and its performance and application scope are limited to some extent. Therefore, researchers have been actively exploring how to incorporate functional additives into the matrix of foamed materials or optimize the foaming process to prepare polymer composite functional materials with a more uniform foam structure and higher mechanical strength. Zang Meng's research found that introducing small - molecule polar plasticizers such as glyceryl monostearate into TPU can increase the foaming ratio of TPU, increase the pore density, and reduce the pore size. Yu et al. prepared five types of TPU/TPAE composite microspheres with different combinations of TPU, compatibilizers, antioxidants, processing aids, and UV stabilizers, and then foamed the pellet with the highest hardness with supercritical CO₂ to obtain a TPU/TPAE composite foam material. The TPU/TPAE composite foam composed of aminated polyether polyol, nylon 6 (hard segment), and polyetheramine D400 (soft segment) has excellent rebound resilience, high elongation, good hardness and tensile strength, and small micropore size.

1.2.3 PEBA Foamed Materials

PEBA elastomer is a high - performance thermoplastic polymer, a block copolymer composed of rigid polyamide blocks and soft polyether blocks. It is the latest ultra - light running shoe midsole material developed in recent years. This material is 20% lighter than many low - density polymers, and its rebound rate can reach about 70% without affecting its firmness. By improving the foaming process, the rebound rate and volume - mass ratio of its foamed material can be further improved. Xu et al. studied the effects of foaming temperature and CO₂ pressure on the PE - BA foam structure and found that a higher expansion ratio and larger pore size can enhance its elasticity and reduce the energy loss coefficient. The PEBA foam with an expansion ratio of up to 24.5 times has a ball - drop rebound rate of up to 80%. In 2017, NIKE used PEBA as the raw material to obtain an ultra - light foamed shoe midsole through supercritical foaming technology. Since then, domestic brands such as Li Ning, Anta, Xtep, and 361° have successively adopted PEBA materials from companies such as Arkema and Ube to make ultra - light shoe midsoles through the supercritical foaming process and launched a series of running shoe products, such as the Li Ning and Xtep 160 series products, which have received positive feedback from the market.

1.2.4 Other Foamed Materials

With the continuous progress and innovation of foaming technology, research on foamed materials for running shoe midsoles has always maintained a high level of interest. For example, the P4U/D30 material is a shear - thickening fluid composed of an oily liquid lubricant and a polymer suspended in it. It has a unique "adaptive" characteristic, being soft when encountering softness and rigid when encountering rigidity. Peak used this material as the shoe midsole to invent the "TAICHI1.0" running shoe. When running at a low speed, it can provide a soft and comfortable foot - feel for the feet. As the running speed increases, the elasticity of the P4U midsole also increases to meet the needs for rebound and support during exercise. Another example is the TPEE material, which is a linear block copolymer containing PBT (polybutylene terephthalate) polyester hard segments and aliphatic polyester or polyether soft segments. Due to its combination of the elasticity of rubber and the thermoplasticity of plastic, with adjustable softness and hardness, it can provide excellent dynamic performance and durability for shoe products. It is a new variety that has attracted much attention among thermoplastic elastomers. In 2023, adidas launched the Adizeropro series, using TPEE foamed material to make the Light - strikePro midsole with high rebound and ultra - lightness, improving the foot - feel comfort and rebound. The overall weight of the size 42 Adizeropro3 is only 215g. However, although the currently invented foamed materials have significant advantages in lightweight, in future research and development, how to reduce costs, improve production efficiency, and achieve the application of environmentally friendly materials while ensuring performance will become an important research direction in the field of foamed materials for running shoe midsoles.

II. Research Progress on Lightweight Design of Shoe Midsole Structure

Using supercritical foaming technology to prepare high - performance polymer elastomers into elastic foam materials is currently the best way to achieve material lightweight. However, the problem of insufficient mechanical strength is still difficult to completely overcome. Therefore, incorporating lightweight design into the structure design of marathon running shoe midsoles to make them have good shock - absorption and support performance has become another major trend in lightweight development. The following mainly describes the research on the innovative design of lightweight structure of marathon running shoe midsoles.

2.1 Optimization of Sole Components

Generally speaking, choosing a softer material and a thicker midsole can provide better shock absorption. However, this will lead to ankle inversion, an increased risk of sports injuries, and reduced sports stability. This problem highlights the importance of the reasonable configuration of the marathon running shoe midsole structure and materials. The predecessor of the NIKE company, the Blue Ribbon Sports Company, cooperated with the Bowerman Company, breaking through the traditional concept of using lightweight materials. They improved the structure of sports shoes by removing the material from the relatively less - stressed arch area, separating the front and heel parts of the sole, and setting the thickness to 6mm to achieve the goal of lightweight. However, the shock - absorption effect of the sole was not ideal. The method of reducing the midsole volume to achieve lightweight running shoes is still used today, such as the Li Ning Longque and Anta C001. In addition, the method of equipping the sole with functional components to achieve lightweight and adjust the sole performance has also been proven to be effective, such as air - cushioned shoes and carbon - plated shoes. These innovative designs not only provide athletes with better comfort and foot support and rebound effects but also improve their sports performance.

2.1.1 Air - Cushioned Shoes

In the early 20th century, the famous American sports shoe manufacturer Spalding invented the sole structure design with inner and outer air chambers. The central air chamber acts as a shock absorber due to its lower pressure, while the surrounding air chambers provide support and stability due to their higher pressure. This design that reduces material use makes sports shoes lighter. After trying to reduce the midsole volume, NIKE released the first air - cushioned shoe, the Tailwind, with a shock - absorbing air cushion (Air) at the heel. In 1985, in order to make jogging shoes lighter, NIKE sports shoe designers combined the advantages of the Tailwind air cushion and the Pegasus dot - shaped air cushion to invent the Epic air cushion. This innovative design significantly improved the elasticity of jogging shoes. At the same time, a third material was introduced, achieving an overall improvement in the shock - absorption, elasticity, and stability of sports shoes. The design of air - cushioned shoes has been continuously updated and is still in use today, becoming a very representative running shoe of NIKE.

2.1.2 Carbon - Plated Shoes

Carbon fiber plates are advanced composite materials that are valued for their lightness, toughness, elasticity, and excellent fatigue resistance. On the basis of the ultra - light foamed midsole, embedding a shoveled - shaped carbon fiber plate with a certain curvature can provide appropriate propulsion for runners during running and compensate for the lack of stability of the ultra - light foamed material. For example, in the NIKEVapor4%, the first super - running shoe launched by NIKE, Whiting et al. found that the NIKEVaporfly4% marathon running shoe with an embedded carbon plate can effectively improve running efficiency compared with traditional running shoes. Subsequently, adidas launched the adidasAdizeropro series, embedding the EnergyRods2.0 claw - shaped carbon fiber propulsion system. While significantly reducing the weight of the shoes, it also significantly improved the performance of the racing running shoes. In 2023, the Ethiopian athlete TigstAssefa broke the women's marathon world record with a time of 2 hours, 11 minutes, and 53 seconds while wearing the AdizeroAdiosProEvol. Domestic carbon - plated shoes such as the Li Ning Feidian 4Ultra, Xtep 160X5.0, and Jordan Feiying PB2.0 have also become outstanding products in the running shoe market due to their excellent performance, comfort, and lightweight.

2.2 Bionic Shoe Midsoles

Inspired by natural biological materials, people have conducted extensive research on bionic design, promoting the development of various shock - absorbing midsole structures. Inspired by the cat's paw pad and the Triply Periodic Minimal Surface (TPMS), Xiao et al. designed and manufactured a new bionic shock - absorbing shoe midsole through additive manufacturing. Grooves were designed on the metatarsal, toe, and heel parts to mimic the shape of the cat's paw pad and filled with a non - Newtonian fluid STF, inspired by the fat tissue in the cat's paw pad, which can dissipate more impact energy and provide better protection for the human lower limbs when subjected to severe impacts. At the same time, a TPMS porous structure with Gyroid and Diamond topological structures was selected as the matrix to mimic the porous framework of the cat's paw. Compared with the standard shoe midsole, the weight of this cat - paw bionic shoe midsole is reduced by 50%, and it can reduce the peak vertical ground reaction force by 15.5%. Prajapati et al. were inspired by sea urchins and used PU foam and TPU to additive - manufacture a lightweight closed - cell foam - filled lattice - structured composite material. Using TPU as the structural framework achieved high mechanical performance of the structure, and filling with PU foam improved the energy absorption of the structure and achieved lightweight. The load - deformation response of this structure shows excellent stiffness, energy dissipation, and shock - absorption characteristics, making it suitable for shoe midsole composite materials. The honeycomb structure, as one of the most highly - regarded structures in the field of bionic materials, has been favored by many sports shoe brands. New Balance launched the sports shoe ZanteGenerate in 2016. By using the DuraForm process to sinter TPU powder, an elastic midsole with an irregular honeycomb - like structure was printed layer by layer. The weight of a single shoe is about 200g, meeting the standard of lightweight running shoes.

From 2017 to 2018, the domestic brand Peak successively released the Future2.0 and Future3.0 running shoes. Through the SLS technology in 3D printing to sinter TPU powder, a honeycomb - like bionic shoe midsole with excellent rebound and softness was made. The weight of a single Future3.0 shoe is nearly 80g lighter than that of the Future2.0. The design inspiration of bionic shoe midsoles comes from the ingenious structures in nature. These designs mainly aim to improve functionality, especially shock - absorption performance, while trying to balance the lightweight requirement. However, this requirement is often restricted by the properties of the bionic structure, so that while achieving extreme shock - absorption, the lightweight effect is limited to some extent.

2.3 Lattice - Structured Shoe Midsoles

The development of materials science and manufacturing technology has inspired the design of mechanical metamaterial additive manufacturing. Mechanical metamaterials are artificial materials with unique properties that do not exist in nature. They achieve special mechanical properties by arranging basic structural units at the micro - scale. Their mechanical properties can be determined by changing the unit - cell topological arrangement without changing their chemical or material composition. Among all types of mechanical metamaterials, the lattice - type structure is well - known for its application in stiffness and compression energy absorption, so it is applied to the structure design of shoe midsoles. Mechanical metamaterials that periodically repeat a single cell inside the structure are called lattice metamaterials. Lattice metamaterials have been reported and characterized for their application in mechanical compression energy absorption under static loads. For example, the lattice metamaterial with a body - centered cubic (BCC) structure has excellent energy - absorption and shock - absorption properties. Wang Zhuang studied the effects of different BCC structure parameters on the mechanical properties of the lattice metamaterial composed of them and customized a shoe midsole structure with different structure parameters for the toe and heel parts according to its mechanical property characteristics.

Sood and Wu studied the effects of unit - cell structure design and arrangement on the static and dynamic properties of additively - manufactured polyester elastomer lattice metamaterials and found that there are significant differences in the mechanical properties of lattice materials obtained from different unit - cell structure designs and arrangement methods. They designed four types of lattice materials inspired by the structures of four covalent organic framework compounds: sp⊃2; carbon - conjugated, borate ester, TTF - Ph, and ZIF - 8. All four types of lattice materials have better energy - absorption characteristics than ordinary materials, and different unit - cell structure designs of lattice materials are suitable for different parts of the shoe. Since 2017, adidas has used Carbon's advanced CLIP one - piece photocuring technology combined with EPU40 material to print the midsole of marathon running shoes with a metamaterial structure. The same - type 3D - printed sports shoes that have been released include the Futurecraft4D, AlphaEdge4D, and 4dFWD. The midsole of the 4dFWD can provide a forward thrust to the feet after receiving vertical pressure due to its unique unit - cell topological structure design. Although the substrate used for the 3D - printed lattice - structured shoe midsole has a relatively high density and large mass to ensure sufficient mechanical strength, the lattice material with a lattice structure achieves an ideal lightweight effect due to its hollow and porous characteristics. In addition, targeted design of the lattice topology can provide functionality for the shoe midsole. This method not only achieves the goal of lightweight but also provides the possibility for in - depth research on the design of marathon running shoe midsoles. Especially in recent years, with the development of computers, the application of finite element analysis software has become increasingly mature. Mainstream commercial software such as Abaqus has very powerful finite element analysis functions and mature structural optimization design modules, which greatly promotes the application of structural optimization technology in the unit - cell topological design of lattice materials. Using optimization technology to optimize the unit - cell shape and structure parameters of the shoe midsole lattice structure can greatly reduce the R & D cycle and cost and obtain the theoretically optimal combination of structure parameters in an efficient way.

III. Conclusion and Outlook

Marathon running shoes have developed from nothing, from being targeted at professional athletes to entering the mass consumer market, and from having a single sports protection function to integrating multiple functions such as rebound and racing support. The development and design of shoe midsole materials aim to meet wearing comfort and improve sports performance. From the current research situation, although marathon running shoe midsoles have their own advantages in material update and structural innovation design, they also have their own limitations. Supercritical fluid foaming technology makes the foaming of elastic materials more uniform and dense, enabling them to be extremely light and achieve good shock - absorption and comfort. However, it may affect their biomechanical performance and make them prone to collapse and not durable. The bionic or lattice - structured shoe midsoles manufactured by the additive manufacturing technology - 3D printing have high mechanical strength and good comprehensive mechanical performance, but there are certain limitations in the degree of lightweight. Therefore, there is still a large development space for optimizing the comprehensive performance of running shoe midsoles by designing reasonable structural parameters with the help of modern scientific means. With the development of technology and the innovation of materials, the research and application of lattice - structured shoe midsoles will open up new prospects. Combined with a lightweight and wear - resistant outsole, this new - type midsole can better meet the ergonomic requirements, provide uniform support and shock - absorption effects, effectively reduce the impact force during exercise, and thus significantly improve comfort and performance. At the same time, as a cutting - edge manufacturing technology, the development of 3D printing technology depends on the continuous R & D and innovation of consumables. For example, the emergence of TPEE not only improves the durability and stability of shoes, enabling them to maintain excellent performance even after long - term use, reducing slippage and torsion, and enhancing sports performance, but also conforms to the concept of sustainable development, reducing resource consumption and environmental pollution. Looking forward to the future, we can expect to see lightweight and high - performance running shoe midsole products on the market, which will bring a more excellent wearing experience to long - distance runners, athletes, and outdoor adventurers.