Clay and tennis have a long history and one of the most legendary Grand Slams, the French Open, is still played on the red clay of Stade Roland Garros. Clay courts are usually made of crushed brick and are common training surfaces in Europe and Latin America.
Despite limited statistical data, there is a general belief that playing on clay courts causes fewer injuries than playing on hard courts. Some theories suggest that there are lower impact forces on the body when playing on clay due to the sliding motions used when moving on court and the fact that the speed of the game itself is slower.
The Science of Clay Courts – Slower Game
Clay, with its rough surface composition, has a high coefficient of friction, and the reduction in forward motion creates a high vertical bounce. The longer the ball is in the air, the more time a player has to move and react, making clay slow when compared to other surfaces and allows the player a higher reaction/recovery time.
On the other hand, grass hard courts have less friction between the surface and the ball, making those surfaces faster. This means that players have less time to react and, when considering the many changes in directions during a rally, this could lead to potential risks.
Another factor that comes into play when comparing clay and hard courts is the Coefficient of Restitution.
The coefficient of restitution (COR) of two colliding objects is typicallya positive real number between 0.0 and 1.0 representing the ratio of speeds after and before an impact, taken along the line of the impact. Pairs of objects with COR = 1 collide elastically, while objects with COR < 1 collide inelastically.
The vertical COR is higher on clay (0.85) than on hard court (0.80) and grass (0.75). This, combined with the coefficient of friction, means that the bounce height on different surfaces can be drastically different.
Physiological Studies on Clay Courts
When comparing metabolic and functional responses while playing tennis matches on clay and hard courts, we find that:
- Statistical analysis showed that playing time was higher on clay courts than on hard courts, and resting time on clay courts and hard courts was not statistically different.
- The exercise to rest ratio was affected by the interaction between playing time and resting time, showing a longer recovery time per unit of exercise on hard courts than on clay courts.
- Distance ran, average heart rate (HR), and average blood lactate concentration (LA) were significantly higher on clay courts than on hard courts.
- There was a lower fluctuation of Oxygen uptake (VO2) response on clay courts than on hard courts.
Another study looked into the technical, physiological and perceptual responses to training on different court surfaces. During training sessions where total duration, distance covered and stroke volumes were comparable, it was found that:
- There was a significant increase in unforced and forced errors on hard courts.
- Higher heart rate, blood lactate, and rating of perceived exertion (RPE) values were reported on clay courts.
- Player and coach RPE were similar on hard courts were similar. However, coaches tended to underrate the RPE of players on clay courts.
This suggests that training on clay courts is actually more taxing physiologically. There is limited data to show that injuries are more common on hard courts than clay courts and further research investigating overall injury trends as well as sport-specific data is needed to draw more definitive conclusions regarding the effect of artificial playing surfaces on injury rates.