Traction for Tennis Shoes on Hard Tennis Courts
Background Research
Within the last decade, tennis has evolved into a faster sport as players continue to find new ways to apply greater amounts of speed and spin to the ball. In answer to the increasing intensification of the game, athletes are adapting their playing style by integrating sliding in their technique.
While sliding has always been a crucial component to the game, it generally only occurred during play on clay or grass courts. Hard courts, which are created by covering concrete with a mixture of silica sand and acrylic paint, are purposefully gritty to prevent slipping (Clark et at, 2012). Due to this, sliding on hard tennis courts was uncommon until high-level players like Novak Djokovic popularized it. Since then, it has been observed that elite players make three out of ten returns during hard court matches while sliding (Crone, 2015).
As balls are being hit harder with more spin applied, players are being forced to move quicker to positions on the court (Gorski, 2016). Studies have actually found that sliding allows players to cover distances and change direction faster than running (Crone, 2015). However, because hard courts are explicitly designed to prevent friction, there is a specific technique to creating the best slide, which can reach up to 5 feet. Sliding on these hard courts applies much greater forces on the body and shoes than softer clay or grass courts. The players who are most successful at sliding have discovered how to apply their force at the correct contact angle to maximize their slide on the hard court’s porous surface. When a player sprints and plants his foot and the speed increases between the shoe and the surface, the shoe’s rubber becomes stiffer which reduces the amount of contact between the shoe and court. This therefore reduces traction force to enable the sliding (Gorski, 2016). The materiality and design of a shoe’s outsole and traction patterns explain why some shoes are better at either providing grip or sliding.
The design goal is to identify an optimal level of shoe traction that provides stability while safely allowing players the ability to slide on hard tennis courts. According to the Clark study which analyzed the tribological interactions at the shoe-surface interface on varying hard tennis courts, as the court’s surface roughness increases, a shoe’s coefficient of traction actually decreases (Clark et al, 2012). This is because the court’s increased roughness actually decreases the surface area and contact interaction between the shoe outsole and surface. This is illustrated in Figure 1 where the top graph (a) has a higher surface roughness than the bottom graph (b) (Clark, 2012). 
Therefore, it is necessary to create a tennis shoe with a rubber outsole pattern and materiality that provides an optimal level of traction to the player without allowing too much or too little traction. Although sliding has become increasingly popular in hard court playing styles, a study reported that 21% of tennis injuries were due to uncontrolled slipping (Beiner and Caluori, 1977). On the other hand, it was also found that injury occurrence was higher on hard court surfaces than softer ones such like clay. This supports the hypothesis that surfaces which do not allow sliding increase the potential to cause injury. This is because the excessive loading forces caused by high surface traction increase joint loading in the knee and ankle which increases injury risk as shoe-surface traction increases. (Clark, 2012).
Therefore, the intent is to create an outsole traction pattern that will allow the wearer of the tennis shoe to slide efficiently at a specific angle while also providing a reasonable amount of stability. To do this, an outsole pattern must be designed using varying parameters such as tread width, frequency, and amplitude to create a certain amount of contact points within the shoe and court’s surface.
Prior Art
- The most notable traction patent is the herringbone pattern because its differing directions respond well to the multidirectional footwork required in tennis.
Justin R. Taylor. Sole Structures That Include Portions with Different Herringbone Traction Pattern Arrangements. Patent Number US 2016/0219980 A1. Date Issued Aug. 4, 2016.
2. The addition of vertical flex grooves in shoe outsoles shown below provides the wearer with more torsional flexibility. These grooves are placed on the sidewall of the outsole and facilitate the twisting direction of many tennis moves. 
Flannery et al. “Article of Footwear with Vertical Grooves”. Patent Number US 8,104,197 B2. Date Issued Jan. 31, 2012.
3. This tread system introduces a pivot stud which is the circular concave cup that sits under the ball of the foot. The pivot stud is designed to facilitate simultaneous flexing and pivoting of the foot during activity.
Jerry D. Stubblefield. “Basketball Shoe Sole. Patent Number”: EP0076313B1. Date Issued Feb. 4, 1982
4. This traction pattern utilizes a sinusoidal wave rather than the herringbone pattern. The sinusoidal pattern is advantageous for court sports because it creates a surface contact ratio of around 40% – 95% depending on sinusoidal arrangement and frequency.
Crowley, II et al. “Footwear Outsole”. Patent Number: US 8,984,773 B2. Date Issued Mar. 24, 2015.
5. This patent uses zoning to differentiate tooling among various areas of the foot to enhance stability in court sports.
Opie, et al. “Court Shoe Cover”. Patent Number: US 2012/0124865 A1. Date Issued May 24, 2012.
References
Gorski, C. (2016, July 25). Sliding On Tennis’ Hard Courts Inspires Awe, Poses Risks. Retrieved May 7, 2020, from https://www.insidescience.org/news/sliding-tennis-hard-courts-inspires-awe-poses-risks
Crone, J. (2015, September 6). How the world’s top tennis players ‘slide to get to the ball’. Retrieved May 7, 2020, from https://www.dailymail.co.uk/news/article-3224084/How-world-s-tennis-players-started-slide-ball-hard-surfaces-s-quicker-running.html
Clarke, James & Carré, Matt & Damm, Loïc & Dixon, Sharon. (2012). Understanding the influence of surface roughness on the tribological interactions at the shoe–surface interface in tennis. Proceedings of the Institution of Mechanical Engineers, Part J: Journal of Engineering Tribology. 226. 636-647. 10.1177/1350650112444694.
Biener K and Caluori P. Tennissportunfa¨lle (Tennis inju-ries). Med Klinik 1977; 72: 754–757.
