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1. INTRODUCTION
Tennis is one of the most popular sports in the US with more than 24.6 million people playing in 2022 (1). The sport is uniquely played among almost all ages and skill levels due to its inclusive physical nature and positive physical and mental health benefits. Within the sport, tennis shoes are one of the most important pieces of equipment impacting players beyond the tennis racket. A proper tennis shoe should allow for maximum comfort, durability, and stability during rigorous multi-directional foot movements and high impact on grass, clay, and acrylic courts (2).
Tennis was first played in the 14th century in the French Royal Courts with early specialized shoes made from canvas or leather attached to flat rubber soles (2). The Plimsoll method of attaching rubber to canvas revolutionized the tennis shoe in the 19th century with the first shoe marketed as a tennis shoe first appearing in 1931 from Adidas (2). In 1934, Fred Perry won Wimbledon in Dunlop Green Flash Trainers, one of the earliest shoes created with the Plimsoll method. Today, tennis shoe technology has advanced immensely with common materials shifting towards synthetic uppers and advanced rubbers (2).
2. COMPONENTS AND NEEDS OF TENNIS SHOES
2.1 Tennis Shoes vs. Athletic Shoes
Despite often being confused with traditional sneakers, tennis shoes differ greatly from general athletic sneakers in their unique needs catered towards specialized tennis movements. As seen in Figure 1, the multi-directional nature of tennis makes it unlike many other sports and requires shoes to provide effective lateral support, often achieved by including an outrigger, increased shoe weight, and increased sole firmness (3). Figure 2 illustrates some of the unique tennis shoe components that contribute to the lateral support needed in tennis shoes.
Figure 1. Multi-directional Nature of Tennis: Lateral Movement. (5)
Figure 2. Four Unique Elements of Tennis Shoes. (3)
Beyond lateral support, high internal foot movement makes a secure upper fit necessary while constant high impact and abrasion on different court types require shoes to have high durability, shock absorption, and traction through sole tread designs. Shoe flexibility, support, and stability are also important aspects to provide safety and high performance during rapid weight transfer and ankle flexion. Comfort must also be kept in mind through breathability and cushioning. Figure 3 highlights key components which make up the average tennis shoe and contribute to the qualities aforementioned.
Figure 3. Components of a Tennis Shoe. (4)
2.2 Biomechanics of Tennis
High intensity, multi-directional movements in tennis give rise to unique biomechanical phenomena such as sliding, shoe torsion, and high translational and rotational friction. Large ankle flexion and shoe torsion often leads to ankle injuries, one of the most common injuries in the sport (7). Thus, tennis shoes should not have large torsional stiffness, as this leads to high ankle inversion angles and increased risk of ankle sprains (6). Sliding is another biomechanical phenomenon specific to tennis which occurs when one's horizontal force overcomes the maximum static frictional forces (6). Specifically, the friction mechanisms of adhesion and hysteresis occur during this movement. Figure 4 illustrates various breaking motions that lead to sliding.
Figure 4. (A) Forward braking motion and (B) Side braking motion (sidestep). (7)
The sliding phenomenon shown above further illustrates why tennis shoes must provide high durability and stability for player safety and performance. Furthermore, constant translational and rotational friction between shoe soles and court surfaces requires a medium level of traction within shoe soles, as high traction leads to foot fixation and injury while low traction leads to increased slip and injury (8). Different sole tread patterns and materials have the greatest impact on influencing the level of traction achieved by the shoe.
3. PRODUCT EXPLORATION
3.1 General Specifications
The tennis shoe chosen for analysis is the NikeCourt Air Zoom Pro Women’s Hard Court Tennis Shoes which can be seen in Figure 5.
Figure 5. (A) NikeCourt Air Zoom Pro Women’s Hard Court Tennis Shoes (B) Interior Informational Label. (9)
This shoe was made in Indonesia for acrylic hard courts and purchased in 2022. The product listing highlights an internal “Zoom Air Unit” for “enhanced energy return and increased stability” in addition to a mesh upper for breathability (9). From material analysis, this tennis shoe is made of polyester, synthetic rubber (Neoprene), and cellular urethane/polyurethane foam.
3.2 Material Characterization
In order to characterize the material makeup of the Air Zoom Pros, FTIR was mainly utilized. Microscopy was also utilized to visually understand material appearance and knit structure. The specific material breakdown of the shoe components may be seen in Figure 6-7.
Figure 6. (A) Synthetic Rubber Toe Cap (B) Neobon / Sulfur Modified Neoprene Polymer Outsole (C) Melinex 377/200 Polyester Film Shoe Upper with a Raschel Warp Knit Top Layer and 1x1 Rib Knit Bottom Layer.
Figure 7. (A) Gelbond Polyester Film Inner with a Raschel Warp Knit Structure (B) Cellular Urethane / Polyurethane Foam Insole (C) Melinex 377/200 Polyester Film Ankle Liner with a Jersey Knit Structure (D) Melinex 377/200 Polyester Tongue with a Jersey Knit Structure.
The material characterization results seen in Figure 6 and 7 are expected due to the fact that many athletic shoe uppers are made of polyester-based coatings and materials while foams are commonly a form of urethane. Additionally, many soles and toe caps within shoes are made of synthetic rubber like neoprene. These materials often do provide reasonable material properties to fit the needs of athletic footwear applications as shown in Table 1. However, shortcomings of these material properties will be explored further in Section 4.
Table 1. Properties of Polyester, Polyurethane Foam, and Neoprene. (10)(11)(12)(13)(14)(15) (16)(17)
3.3 Material Characterization Methodology
Various methods were utilized to characterize each material. The main method for material identification was fourier transform infrared spectroscopy (FTIR), as this was performed for each sample. Methods including differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and contact angle measurements were also performed for select samples to confirm material identity and properties. Each sample was also imaged through microscopy for further insight into material structure. Most samples were imaged through a digital, hand-held microscope while select samples were imaged with the Keyence VHX Digital Microscope.
Specific data for each sample may be found in Appendix A, B, and C. However, material analysis of the sole of the NikeCourt tennis shoe and a running shoe, which I later compare my tennis shoe to, will be explained for clarity of the characterization process. These samples were chosen due to their use of multiple identification methods.
Figure 8. Pegasus 40 Outsole Material Characterization (A) Microscopy Images (B) FTIR Spectrum (C) DSC Graph Displaying 324.64°C Melting Temperature (D) TGA Graph Displaying 404.74°C Decomposition Temperature.
Figure 8 contains microscopy, FTIR, DSC, and TGA data from the Nike Pegasus 40 Shoe outsole material. According to the FTIR spectrum input into Wiley’s Knowitall Anyware software, the outsole most largely aligned with the spectrum of chlorinated polyethylene. DSC analysis was then conducted to examine the heat flow of the sample. The DSC range for melting temperature shown in Figure 8c was farther than that of polyethylene, 110-130°C, showing that the sample may not purely be polyethylene. This may be due to the fact that the sample was contaminated with other substances due to its previous use on various surfaces. In fact, the Knowitall database had also found spectrum alignment with other materials such as dietary supplements, capsules, and multivitamins. On the other hand, further thermal analysis through TGA data confirms the polyethylene identity of the sample as the decomposition temperature of 404.74°C falls in the polyethylene range of 335-450°C.
Figure 9. NikeCourt Outsole Material Characterization (A) Microscopy Images (B) FTIR Spectrum (C) Water Contact Angle Measurement Displaying Value of 78.30-103.23°.
Fun video of finding the water contact angle.
In terms of characterization of the Women’s NikeCourt tennis shoe, different approaches were utilized such as high quality microscopy with material topography imaging, FTIR, and water contact angle measurements were utilized. The FTIR spectrum most closely matched that of neoprene and 2-(3,4-Epoxycyclohexyl)ethyl-functionalized silica gel which is often used as an abrasion-resistant coating (18). The Keyence VHX Digital Microscope was then utilized to collect water contact angle data in order to confirm the material identity. From this data, the water contact angle was found to be 78.30-103.23° which is considered mostly non-wetting. The functionalized silica gel is known to have relatively low wetting, 331 m2/g specific wetting surface area, further confirming the functionalized silica gel identity (19). This water contact angle also aligns with that of Neoprene, 99.37 ± 1.08°, further confirming the neoprene identity (20).
4. ISSUE ANALYSIS AND RESOLUTION
4.1 Issue Overview
From personal experience playing in the NikeCourt women’s tennis shoes, I found that stability, durability, and comfort were main issues that arose. Lack of stability occurs during lateral movements while lack of comfort is usually felt in high impact movements like serving, volleying, and lunging for a ball. Lack of durability can be seen in all shoe-court interactions.
According to Tennis Warehouse and Tennis Express reviews, many other individuals also shared similar sentiments with people explaining how the shoe has “zero lateral stability,” “wore out,” and is “not durable” (21)(22). One user even claimed rolling their ankle in the shoes (21). Overall, this shoe received an average rating of 3 out of 5 stars which is very low for a functional tennis shoe.
4.2 Stability Issue and Resolution
When playing in the NikeCourt shoes, I often felt foot pain and had slightly over-extended my ankle a few times due to the lack of stability within this shoe. The neoprene sole material may have an impact on this stability due to the fact that it has a 0.5 coefficient of friction according to Table 1. This medium-to-low coefficient of friction value means that the sole has little slip resistance, leading to more instability in lateral movement and risk of injury. However, it seems that the shape of the outsole would be a larger contributor to decreased lateral stability. Specifically, the NikeCourt shoes are relatively thin and have a curved sole which decreases stability as seen in Figure 10.
In order to address this issue, the outrigger should be widened for further lateral stability while the outsole should decrease in height to increase court feel and lower the player’s center of gravity. An increase in firmness and flatness of the sole would also help minimize instability in addition to better traction through an optimized sole tread pattern. These changes may be seen in Figures 10-12.
Figure 10. Design Solution #1: Widening Shoe Outrigger for Increased Lateral Stability, Narrowing Upper for and Elongating Lacing System for More Secure Fit.
Figure 11. Design Solution #2: Flattening Shoe Outsole and Decreasing Outsole Height.
Figure 12. Design Solution #3: Optimizing Tread Pattern for Better Shoe-Court Traction.
The decision to provide a thinner herringbone-like tread pattern on the outer side of the sole was influenced by a study published in MDPI (6). This study examined the biomechanical effects of different tennis sole tread patterns on players, specifically testing the four different patterns shown in Figure 13. Researchers found that both large herringbone patterns and uniformly sized patterns tend to cause higher mechanical coefficients of frictions (CoF) than their counterparts (6). Specifically, the sole pattern D had the largest rotational force while pattern B had the highest risk of ankle injury due to its high internal ankle moment. Thus, pattern C, with less mechanical CoF than pattern A and higher CoF during some movements compared to B and D, acts as the perfect medium to prevent too high or low friction levels. As such, the tread pattern in Figure 12 aims to replicate this thinner outer tread pattern with modifications in shifting herringbone to a diamond pattern.
Figure 13. Different Widths of Outsole Pattern A-D. (6)
4.3 Comfort Issue and Resolution
A second major issue within the NikeCourt tennis shoe is comfort. Due to the lack of arch support and cushioning during high impact movements, I often experience foot and sometimes leg pain. My naturally high foot arches make it especially important for my shoes to provide comfort and support.
Figure 14. Design Solution #4: Increase Insole Thickness and Arch Support. (23)
In order to address this issue, the shoe insole should be increased in height with extra material concentrated under the arch of the foot. This design change may be seen in Figure 14.
4.4 Durability Issue and Resolution
The third major issue found within the NikeCourt tennis shoes is the lack of durability in the shoe upper and outsole. As can be seen in Figure 15, a lack of abrasion resistance in materials causes quick wearing out and deterioration of both outsole and upper materials. Figure 15a shows tread pattern loss on the neoprene outsole while Figure 15b illustrates tearing of the polyester upper after a single high-impact drag along a clay tennis court. Figure 15c illustrates other common sole abrasion locations within tennis shoes.
Figure 15. (A) Toe Cap and Outsole Wearing (B) Polyester Upper Tearing (C) Common Sole Abrasion Locations.
The low durability and abrasion resistance of materials is most likely the cause of this issue. As listed in Table 1, polyester has relatively low abrasion resistance, 15-28 mg/1000 cycles lost in the Taber Abrasion test, and a 12-16% elongation to break. Thus, polyester is not durable enough to resist high court abrasion and constant internal foot impact due to rigorous movements. Additionally, the neoprene outsoles usually have good abrasion resistance, but a lower tensile strength as seen in Table 1. In fact, the tensile strength is almost half of that of polyurethane. Thus, material changes to both the polyester upper and neoprene sole should be made.
In order to ideate methods of increased durability, the successful Nike Pegasus 40 was examined. This shoe, unlike the NikeCourt Zoom Air Pro, has very high consumer ratings, around 4.6 stars, and is praised for comfort, stability, and durability (24)(25). Figures 16 and 17 illustrate the material and structural breakdown of its shoe components.
Figure 16. (A) Polyethylene Foam Outsole (B) Three Layer Upper: Polyester Jersey Knit, Polyester 1x1 Rib Knit, and Polyester Nonwoven Film.
Figure 17. (A) Polyurethane Open-Cell Foam Insole (B) Three Layer Tongue: Polyester 1x1 Rib Knit, Polyurethane Foam, and Polyester Raschel Knit.
From analysis of this Pegasus 40, it seems that many of the upper materials are similar to that of the NikeCourt shoe with main differences in fabric structure while the outsole is made of polyethylene rather than neoprene. Thus, taking inspiration from the Peg 40, the NikeCourt outsole should be changed to polyethylene (PE) due to its higher strength and abrasion resistance (26). A comparison of properties between PE and neoprene may be seen in Table 2.
Table 2. Properties of High Density Polyethylene and Neoprene. (10)(15)(16)(17)(29)
Depending on cost restraints and consumer target group, the sole could also achieve further increased durability and abrasion resistance through creation of a carbon-fiber reinforced PE outsole similar to work published in Chemistry Europe (27) and Applied Research (28). In fact, PE has many benefits over other matrixes and carbon fiber precursor materials due to the fact that PE is widely available, relatively lower cost, and can be easily melt-spun, which is a more cost-efficient and environmentally friendly process than PAN solution-spinning processes (28). However, composites come with the caveat of not being as easily recyclable at product end of life. Thus, sticking with purely polyethylene outsole material may be a better option depending on footwear intentions (28).
The NikeCourt toe cap should also be changed to this polyethylene or polyethylene carbon fiber-reinforced material in addition to the design change of being extended further onto the upper. These changes can be seen in Figure 18.
Figure 18. Material Change #1: Outsole Material Shift to Polyethylene-Carbon Fiber Reinforced Material. Design Change #5: Extension of Toe Cap Onto Upper.
In order to improve shoe upper durability, the polyester material should be switched to nylon to achieve higher abrasion resistance while maintaining a reasonable cost of production. Taking inspiration from the Pegasus 40 shoe, the outer structure should be composed of a two layer knit (jersey and 1x1 rib knit) with an outer polyester film for durability and breathability. These changes may be seen in Figures 19.
Figure 19. Material Change #2: Upper Material Shift to Nylon 6.6. Design Change #6: Shift of Fabric Structure to 2 Layer Jersey and 1x1 Rib Knit with Polyester Nonwoven Film.
4.5 Resolution Summary
Figure 20 illustrates a summary of all material change and design solutions to solve the NikeCourt issues of a lack of stability, durability, and comfort. The lack of stability is resolved through a widened outrigger, more fitted upper, a flattened and shortened outsole, and optimized outsole tread patterns. The lack of comfort is resolved through an increased thickness insole with strategic material placement for heightened arch support. The lack of durability is resolved through material upper shifts from polyester to nylon and outsole material shift from neoprene to polyethylene or carbon fiber-reinforced polyethylene. Increased durability is also achieved through the design changes of an extended toe cap and 2 layer upper knit structure with a polyester top film.
Figure 20. NikeCourt Air Zoom Pro Women’s Hard Court Tennis Shoe Material and Design Change Summary.
5. CONCLUSION
Tennis shoes have an immense impact on player safety, performance, and quality of experience. This influence makes it critical for tennis shoes to properly support and aid players. Advancements in tennis shoe material and design have profoundly impacted members of the tennis community and the industry, providing more diverse and exciting landscapes where players can optimize their game and excel. As technology and design continues to advance, the future promises even more remarkable transformations in the world of tennis shoes, further cementing their role as a cornerstone of the sport.
Beyond the suggested material and design modifications suggested, future exploration can be made in the realm of sensing and performance data within tennis shoes, shoe-psychological impact on the athlete experience, and even the aesthetics influence on performance. Possibilities are endless in the journey of continued evolution and innovation of the beloved tennis shoe.
References
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