Sports Eng DOI 10.1007/s12283-017-0228-z ORIGINAL ARTICLE Characterisation of ball degradation events in professional tennis 1,2,3 1 3 1 Ben Lane • Paul Sherratt • Xiao Hu • Andy Harland Ó The Author(s) 2017. This article is published with open access at Springerlink.com Abstract Tennis balls are acknowledged to degrade with appears to influence the distribution of ball speed on impact use and are replaced at regular intervals during professional with the surface or racket, suggesting a surface-specific matches to maintain consistency and uniformity in per- degradation test may be beneficial. As a result of these formance, such that the game is not adversely affected. findings a new test protocol has been proposed, utilising the Balls are subject to the international tennis federation’s in-play data, to define the frequency of impacts and impact (ITF) ball approval process, which includes a degradation conditions to equate to nine games of professional tennis test to ensure a minimum standard of performance. The across the different surfaces. aim of this investigation was to establish if the ITF degradation test can assess ball longevity and rate of Keywords Tennis Á Ball Á Impact Á Hawk-Eye Á Surface Á degradation and determine if there is a need for a new Speed Á Angle Á Degradation degradation test that is more representative of in-play conditions. Ball tracking data from four different profes- sional events, spanning the three major court surfaces, 1 Introduction including both men’s and women’s matches were analysed. The frequency of first serves, second serves, racket impacts Approximately 360 million tennis balls are manufactured and surface impacts were assessed and the corresponding each year [1], with wholesale sales figures in the region of distribution of ball speed and (for surface impacts) impact $92 million in the United States alone in 2015 [2]. It is, angle was determined. Comparison of ball impact fre- therefore, important for tennis brands to be able to produce quency and conditions between in-play data and the ITF consistent, durable products to satisfy consumers world- degradation test indicated the development of a new test, wide. Ball performance and durability is also important to more representative of in-play data, would be advantageous professional players, event organisers and the media to in determining ball longevity and rate of degradation with produce the highest quality tennis to entertain spectators. use. Assessment of data from different surfaces highlighted Tennis balls are subjected to the ball approval process, that grass court subjected the ball to fewer racket and conducted by the International Tennis federation’s (ITF) surface impacts than hard court or clay. In turn, this Technical Commission, to ensure a minimum standard and consistency in ball performance. Balls must be approved annually, a list of which is published by the ITF [3], and the & Ben Lane mass, size, rebound, deformation and durability of the ball [email protected] are scrutinised against the standard tests defined in the ball 1 Sports Technology Institute, Loughborough University, approval specification [4]. Loughborough, Leicestershire LE11 3TU, UK The properties of tennis balls are known to degrade with 2 Institute for Sports Research, Nanyang Technological use [5], consequently professional events outline a ball University, Singapore, Singapore change policy to maintain a consistent level of ball per- 3 School of Materials Science and Engineering, Nanyang formance during matches. It is commonplace that a set of Technological University, Singapore, Singapore six balls are in play at any given time. The first set of balls B. Lane et al. is used for the warm-up and the first seven games, with ball Open (2012–2014) comparing aspects of the men’s and changes occurring every nine games thereafter. women’s game. Lane et al. [10] found an average of 105 Comparison of new and used balls has shown general impacts per ball over nine games of professional tennis on trends in the change in properties of tennis balls. Used balls hard court, of which, 40 were racket impacts and 53 were exhibited increased bounce height, mass reduction and surface impacts, the remainder of which were serves. reduced stiffness [6]. Mass reduction is dominated by a loss Notational analysis techniques have also been used to of felt and has been shown to increase with both impact assess rally length [12] and strokes per game at the 2003 speed and number of impacts [5]. The felt cover has been US Open [13]. Many studies assessing ball impact condi- shown to degrade causing mass reduction and changes in tions during play aimed to measure in-play characteristics fuzziness, in turn affecting the aerodynamic properties of of specific impact scenarios to more realistically replicate a the ball [7, 8]. Furthermore, it has been acknowledged that given scenario in the laboratory [14, 15]. Others aimed to balls may fall out of specification during use if the initial measure a given parameter over time to assess if the nature properties were close to the allowable limits when approval of the game was changing [16, 17]. Choppin et al. [15] tested [9]. measured ball and racket parameters at the 2006 Wim- Changes in material composition (felt and rubber), bledon qualifying tournament to replicate typical values for normal impact forces, contact distance and speeds are all a baseline top spin forehand shot in laboratory testing. Ball relevant factors in tennis ball wear [8] and are determined spin rates have also received much attention, mostly to by a combination of the pre-impact conditions (speed, ensure that spin rates were not greatly increasing over time angle and spin) and the surface interaction between the ball and changing the nature of the game [16, 17]. Typical spin and court. To achieve an accurate representation of how a rates and spin axis for flat, slice and kick serves have also ball degrades during play it would appear necessary to been analysed [18]. simulate these phenomena as close as possible. The ITF durability test fulfils its intended purpose of The ITF durability test was based on the typical ball ensuring that manufactured balls meet the basic quality change policy and sets out to determine if the ball in levels and minimum standards for ITF accreditation. question can withstand the demands of nine games of However, a binary pass–fail test can only assess the degree professional tennis [6]. The test was established based on of degradation against the correlated level of use. This is research into the properties of worn, unworn, new and used ineffective if the level of use is less than or greater than the tennis balls, as well as investigating different methods to correlated value. The ITF test, therefore, is not effective at replicate changes in mass, rebound height and deformation determining degradation of ball performance over time, nor in the laboratory [6, 9]. The test itself is twofold; 20 normal is it able to assess how long a ball remains within speci- impacts at 40 m s-1 against a rigid surface to replicate fication, due to its binary nature. While the ITF test has softening, and 2 min of artificial felt wearing to replicate proven effective for maintaining minimum ball standards, felt loss. Post-wear regime results for mass, size, rebound it is unable to assess degradation in ball performance with and deformation are compared to the initial test results for use, across multiple surfaces and at representative impact the balls in question and a maximum allowable change is speeds and angles. defined [4]. The ball approval process is not surface- A ball degradation protocol is, therefore, needed to specific although it does accommodate for use at altitude determine ball longevity in the modern game. The results and on different paced courts. Slight differences in speci- could then be used to evaluate ball degradation perfor- fication are defined as type 1 (fast), 2 (medium), 3 (slow) mance and influence changes in ball design and material and high-altitude balls designed for use slow, medium and composition. A new effective ball degradation test would fast-paced courts, respectively. Although the approved ball be able to assess how long a ball remains within the list is dominated by type 2 balls (with very few type 1 and approval specification, as well as assess the rate of ball no type 3 approved balls) [3], it is commonplace to see degradation with use. brands market balls as clay specific or suitable for all court surfaces. 1.1 Aims Elements of what a tennis ball endures during its life span in professional tennis have been determined, partic- • Evaluate the current ITF durability test with respect to ularly for matches played on hard court. Impact frequencies ball duress in modern tennis. for serves, racket impacts and surface impacts and corre- • Establish if there is a need for a surface-specific ball sponding distributions for ball speed and surface impact degradation test. angle were assessed by Lane et al. [10]. Similarly, Reid • Propose test conditions to enable ball evaluation for et al. [11] utilised ball tracking data from the Australian each surface type. Characterisation of ball degradation events in professional tennis 2 Methods Surface Normal Pre-Impact Post-Impact Ball tracking data collected by the automatic line-calling Velocity Vector Velocity Vector system (Hawk-Eye Innovations Ltd., Basingstoke, UK) from the following events were analysed: • ATP 250 Thailand Open 2011–2013 [hard court (HC), Pre-Impact Post-Impact indoor, male event]; Angle Angle • ATP 500 Gerry Weber Open 2011–2013 [grass court (GC), outdoor with retractable roof, male event]; Fig. 1 Ball-surface impact schematic showing angles measured • Roland Garros French Open 2011–2014 [clay court, outdoor, male (MC) and female (WC)].