The effect of normal load force and roughness on the dynamic traction developed at the shoe–surface interface in tennis

James Clarke 0 1 Sharon J. Dixon 0 1 Loic Damm 0 1 Matt J. Carre 0 1 0 S. J. Dixon L. Damm Exeter Biomechanics Research Team, University of Exeter , St Lukes Campus, Heavitree Road, Exeter EX1 2LU, UK 1 J. Clarke M. J. Carre (&) Department of Mechanical Engineering, The University of Sheffield , Mappin Street, Sheffield S1 3JD, UK During tennis-specific movements, such as accelerating and side stepping, the dynamic traction provided by the shoe-surface combination plays an important role in the injury risk and performance of the player. Acrylic hard court tennis surfaces have been reported to have increased injury occurrence, partly caused by increased traction that developed at the shoe-surface interface. Often mechanical test methods used for the testing and categorisation of playing surfaces do not tend to simulate loads occurring during participation on the surface, and thus are unlikely to predict the human response to the surface. A traction testing device, discussed in this paper, has been used to mechanically measure the dynamic traction force between the shoe and the surface under a range of normal loading conditions that are relevant to reallife play. Acrylic hard court tennis surfaces generally have a rough surface topography, due to their sand and acrylic paint mixed top coating. Surface micro-roughness will influence the friction mechanisms present during viscoelastic contacts, as found in footwear-surface interactions. This paper aims to further understand the influence microroughness and normal force has on the dynamic traction that develops at the shoe-surface interface on acrylic hard court tennis surfaces. The micro-roughness and traction of a controlled set of acrylic hard court tennis surfaces have been measured. The relationships between micro-roughness, normal force, and traction force are discussed. 1 Introduction During tennis-specific movements the traction provided by a shoesurface combination will influence a players injury risk and performance [1, 2]. Excessive friction acting between shoe and surface (referred elsewhere in this paper as traction) can lead to injury caused by overloading in the lower extremities [3]. Insufficient static traction can lead to a slip (unwanted movement of the shoe relative to the surface), which will result in a loss of performance or, if the slip is severe, lead to a fall which may cause injury itself [4]. Also, a player may choose to purposefully perform a controlled slide on a tennis surface; a type of movement that is common for clay surfaces, but is becoming increasingly common at elite level on hard courts as well. The success of this type of movement will depend on the dynamic traction developed at the shoe surface interface (the force acting to slow down a shoe moving relative to the surface). The tractional properties of a shoesurface combination must therefore be within an optimal range [5]. Elite tennis is played on grass, clay and acrylic hard court surfaces. Nigg [6] reported clay surfaces to have traction/friction coefficients (ratio of horizontal traction force and normal force) of between 0.5 and 0.7, whereas the other surfaces tested had traction/friction coefficients between 0.8 and 1.2. Clay surfaces have generally been reported to have a lower occurrence of injury than acrylic hard court surfaces [610]. The difference in injury occurrence between surfaces has been partly attributed to the inherent differing styles of play on each surface caused by differences in ball speed and bounce [6]. However, the differing tractional characteristics of the playing surfaces also affect the risk of accidental injury occurrence [11]. This has lead to the hypothesis that surfaces which do not allow sliding increase the potential to cause injury. Despite the understanding that shoesurface traction can influence performance and injury risk there remains a requirement for improved understanding of the tribological interactions at the shoesurface interface in sport [12]. The aim of this paper is to present experimental data and further investigate the influence the applied normal force and surface roughness have on the dynamic traction developed on acrylic hard court tennis surfaces. Understanding the mechanisms of dynamic traction will aid the improvement of playing surfaces and footwear constructions. Once traction mechanisms are understood, surface properties and/or footwear can be effectively changed to maximise performance and/or minimise injury risk. The dynamic traction force will be dependent on the friction mechanisms developed between the footwear and the playing surface. Acrylic hard court tennis surfaces are constructed with a top coating of acrylic paint and silica sand mixture. This gives acrylic hard court surfaces a rough surface topography in comparison to other hard surface/ flooring systems. The roughness of acrylic hard courts will be dependent on the paintsilicasand mixture which has been reported to differ between surface manufacturers [13, 14]. Viscoelastic rubbers are generally used on the outsoles of tennis shoes. In clean, dry conditions, sliding contacts between viscoelastic rubbers and a hard solid substrate will result in a combination of the following friction mechanisms: adhesion and hysteresis [1518]. During interactions where the shoe slides relative to the surface, the micro-roughness of the surface will undoubtedly affect the dynamic traction force developed. During a horizontal sliding event the asperities of the solid substrate will cause cyclic elastic deformation of the viscoelastic rubber material. Internal damping causes energy dissipation during the loading and unloading cycle [16, 17, 19]. This loss is the hysteretic component of the contributing friction mechanisms. If local stresses deform the rubber beyond its elastic limit, it will be unable to recover. This results in tearing of the material and leads to additional friction forces at the interface between rubber and surface. Tearing can result in wear and cause the separation of fragments of material from the rubber, this is termed abrasive wear. Adhesion is the process of junctions forming, due to van der Waals interaction, between the contacting surfaces and the arising friction force is the force required for the junctions to shear [1622]. Adhesion friction is more prevalent when rubber slides over a smooth surface and depends significantly on asperity contact and therefore the loading conditions and the roughness characteristics of the surface the rubber is sliding relative to. On increasingly rough surfaces the contribution of the adhesive component of traction has been found to decrease due to reduced asperity contact [18, 20, 22]. Asperity contact is also dependent on the normal loading applied during the dynamic sliding event [16, 22]. Figure 1 (adapted from [22]) illustrates how as the amplitude of the surface roughness is decreased, under the same normal loading conditions, surface contact will (...truncated)