Tennis and the slippy clay blues

In the epic 2011 US Open Final, Novak Djokovic and Rafael Nadal mesmerised tennis fans by sliding on hard court surfaces, a technique that had, for the most part, been reserved solely for clay courts. In this year’s tournament Djokovic appeared to have mastered the technique as he slid effortlessly into the final, only to be beaten by a resilient Andy Murray. This technique requires a heightened on-court control and highlights the competitor’s extreme athleticism. Yet both Djokovic and Nadal threatened to boycott the Madrid Masters should the tournament continue to use the new blue clay surface; complaining that, compared to the traditional red clay, it was too slippery and unsafe. So why is it that the same two players who have achieved an incredible level of control on one surface can struggle so much on another?

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Nadal playing on blue clay at the Madrid Masters

Nadal is widely considered the greatest player of all time on clay (winning 7 out of the last 8 French Opens on the red clay at Roland Garros, Paris) and Djokovic has been in the final of the last 5 grand slam tournaments played on hard courts (winning 4 of them). Both players were in great form prior to the Madrid Masters, having contested the final of the Australian open earlier in the year. Djokovic was the defending champion (when the tournament was played on red clay) and would go on to contest the French Open final against Nadal the following month. Surely the answer can’t boil down to poor form, low confidence, bad luck, or even sour grapes. The reason lies in how traction is developed between their shoes and the playing surface.

Figure 1 is a simple diagram showing the forces developed as a shoe and a surface contact. FA is the force applied by the player, FT is the resultant horizontal traction force and FN is the resultant reactive normal force.

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Figure 1: Forces at the shoe-surface interface.

Traction force is a measure of resistance to horizontal movement. The traction force developed between a shoe and a surface will be in one of two ‘regimes’: (I) a static regime – the shoe is deforming but not moving horizontally on the surface; (II) a dynamic regime – the shoe is moving horizontally on the surface. The traction force between a shoe and a surface can be measured by forcing a loaded shoe over a surface. Figure 2, taken from a test performed with a common tennis shoe on a red clay surface, shows the transition between the static regime (I) and the dynamic regime (II).

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Figure 2: The traction developed between a shoe and a red clay surface. (I) static regime – the traction force reaches a peak (dashed line). (II) dynamic regime – a peak traction limit is reached and then the shoe begins to slip/slide on the surface.

The figure shows how the static regime develops into the dynamic regime after a peak traction force is reached. The peak traction force is the maximum traction force available to the player prior to ‘significant dynamic horizontal movement of the shoe’, i.e. slipping.

The traction force available in the static regime is important. To perform a quick and efficient step the player needs traction when landing and pushing off. For both the landing and push off the player does not want the shoe to slip. However, if they exceed the maximum traction force available, they will.

How does this relate to Rafa and Novak? Any significant difference in surface characteristics will lead to differences in the available traction. The physical make-up of the surface and its response to a moisture controlling treatment were given as reasons why the blue clay of Madrid was different to the traditional red. The famous red clay of Roland Garros in Paris is made up of white limestone, dusted with several millimetres of powdered red brick dust (http://www.itftennis.com/about/grand-slams/roland-garros.aspx). The blue clay of Madrid is different, the iron oxide and other metals are initially extracted from the red brick, which is then baked, crushed, filtered and dyed. Also, the combination of rain and heat in Madrid prior to and during the tournament caused salt, used in a treatment to control surface moisture, to crystallise unexpectedly and interact with the clay. The traction characteristics of a clay surface will largely depend on the strength of cohesive bonds that are formed between the clay particles. It is likely that the crystallised salt will have weakened these bonds; lowering the maximum available traction when compared to red clay. Not what is claimed in the video describing the surface:

Djokovic complained:

When you slide on the red clay you have a feeling you can stop and recover from that step. But here, whatever you do, you are always slipping

This quote suggests that it was slipping when wanting to push-off from the surface that was causing his problems on the blue clay. During a push-off a player demands high traction from the surface. It has been shown, surprisingly, that tennis players ‘use’ clay surfaces more than hard courts by exerting greater traction forces (Damm et al. 2011). Nadal and Djokovic are two especially aggressive players and have a reputation of maximising their ‘use’ of any playing surface. They will be exerting traction forces much closer to the maximum available and, compared to their opponents, are more likely to exceed this maximum and slip. It is their aggressive play and failure to adapt their style in time (unsurprising as the salt treatment was reportedly added between a trial period and the tournament) that resulted in their angst.

If Nadal and Djokovic’s commitment to ‘use’ the playing surface was their downfall on the Madrid clay it is to their advantage on hard courts. What is surprising about their ability to slide on hard courts is that, compared to clay, they have been found to have a significantly higher resistance to dynamic traction (Nigg et al. 2003). Hard courts are coated with a layer of mixed silica sand and acrylic paint that gives them a rough finish similar to that found on special anti-slip surfaces.

The contact between a shoe and a hard court is between a viscoelastic rubber (the shoe) and a solid substrate. The traction force resisting sliding between rubbers and a hard solid surface will be a result of adhesion and hysteresis mechanisms. Adhesion is the process of bonds forming, due to intermolecular forces, between the contacting surfaces – the arising traction force is caused by these bonds breaking. Hysteresis is energy dissipation as the rubber continuously deforms and recovers as it slides over the surface. These mechanisms are well understood and it could be, as speculatively reported, that the elite players are using shoes with specially formulated rubber compounds that optimise adhesion and hysteresis affects to allow sliding. It is also possible that a novel outsole design has been developed for the purpose of sliding on hard courts. However, any shoe modification that reduces traction for sliding would be detrimental to other more common movements that require high traction. It is therefore just as likely that these players have developed their skill through combining extreme athleticism and practise.

Either way, to purposefully slide on such a rough surface requires skilled control of the dynamic traction regime by optimising the force applied (FA) on the surface, it also requires speed. With increasing speed, rubber becomes stiffer, decreasing the contact area and reducing the dynamic traction force. The foot speed generated by the top players may be what separates them from the rest.

The ATP has decided against blue clay for Madrid’s 2013 Masters tournament, but with Nadal’s dominance on clay and Djokovic’s supremacy on hard court, perhaps the organisers in Madrid stumbled upon something missing in men’s elite tennis – a level playing field.

References:

Nigg, B. Injury and performance on tennis surfaces. The effect of tennis surfaces on the game of tennis. 2003.

Damm, L., et al. Modulation of tennis players’ frictional demand according to surface traction characteristics in International Society of Biomechanics Congress XXIII. 2011. Brussells.

 

The Author:

Paris marathon_Eiffel tower_2012James Clarke is a Research Associate at The University of Sheffield whose work focuses on how shoe-surface interactions in sport. His interest was sparked after discussing injury mechanisms with his physio following rupturing a knee ligament when playing football.

James graduated from the University of Sheffield in 2006 with a MEng in Mechanical Engineering, for which he studied artificial sport surface characteristics and injuries. In 2011 he was awarded a PhD that investigated the comfort and performance of football boots. James has recently transferred this knowledge to tennis and hopes his research will have an impact on improving performance and understanding injury risk.

In his spare time James likes to try out the shoes he tests by playing for a local football team and, knees permitting, running a marathon once a year.

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