Several researchers have pointed out the importance of physical fitness for tennis performance (Fernandez et al., 2006; Ferrauti et al., 2011; Girard and Millet, 2009; Kovacs, 2006; Kovacs, 2007; Reid and Schneiker, 2008; Roetert et al., 1992). Physical fitness in tennis consists of upper and lower body power, speed, and agility (Kovacs, 2006). In junior tennis also maturation and the relative age can influence tennis performance. In junior tennis, players compete within age categories. Within an age category, differences between players can be a maximum of two years. This difference can lead to biological, physiological, and cognitive differences. Therefore, it is im portant to include the relative age of the players and to analyze the possible effect of relative age (RAE) (Loffing et al., 2010; Ulbricht et al., 2015). Furthermore, when adolescent tennis players are measured in relation to their level of tennis performance, physical maturation should be included as part of the measurements (Kramer et al., 2016a). In tennis, both age and maturation might lead to physical advantages for some players, while not for others, and they may possibly be related to tennis performance.
A recent study has shown that physical fitness is important for tennis performance during adolescence (Ulbricht et al., 2016). Ulbricht and colleagues (2016) measured 755 regional tennis players and 147 national tennis players aged between 11-16 years. They found that serve velocity (radar gun), and upper body power (medicine ball throws; overhead, forehand, and backhand) were predictors for tennis performance in boys and girls. When they compared different performance levels, players with a higher level scored better in serve velocity (ES 0.781.02), upper body power (ES .66-1.04), and tennis-specific endurance (Hit and Turn Tennis Test) (ES .05.95) than lower-level players. Furthermore, a study on junior male tennis players found that, at young ages (1013 years), elite players were faster than sub-elite players; when players became older, however, this advantage for elite players disappeared (Kramer et al., 2016a). In contradiction, a study on Danish elite and non-elite male and female tennis players (aged 10-12 years) did not find any differences in power (i.e. SJ and CMJ) physical fitness between performance levels (Bencke et al., 2002). Elite players were the more talented players selected by trainers and trained around nine hours, while non-elite players were the less talented players who trained around six hours. However, to our knowledge no studies exist within tennis in which physical fitness is related to future success. It is unknown if physical fitness can be used for talent identification and which role it plays in talent-development. Previous studies have shown that physical fitness can be important tennis performance, but none of these studies investigated whether physical fitness can predict future tennis performance.
Earlier studies in tennis found that physical fitness, measured by short-term maximal protocols, was important for tennis performance in junior tennis (Kramer et al., 2016a; Ulbricht et al., 2016). However, to the best of our knowledge, no study has reported so far how physical fitness measured in U13 can predict current and future tennis performance. The added value of monitoring physical fitness needs to be clarified so that talent-development programs can be optimized for junior tennis players and tennis performance can increase. Insight into predicting current and future tennis performance is necessary for improving the development of junior tennis players. Therefore, the aim of this study is to investigate whether age, maturation, or physical fitness can predict current and future tennis performance in junior elite tennis players in U13. The value of current tennis performance (U13) for future tennis performance (U16) is also investigated.
All participants played competitive tennis and were part of the talent-development program of the Royal Dutch Lawn Tennis Association (KNLTB). The players were informed about the procedures of the study before they gave their consent, and permission was given by the trainers and parents. This study met the guidelines for ethical standards for sports medicine research (Harriss and Atkinson, 2009; 2011; 2014) and was approved by the KNLTB. The tests were performed on an indoor hardsurface tennis court. Anthropometrics were measured before the standardized warm-up. The warm-up, executed before the tests, included a shuttle run test, up to stage eight. After the shuttle run test, some acceleration sprints and stretching were executed. After finishing the warmup, the tests were conducted.
The inclusion criterion was that players were part of the talent development program of the KNLTB and had a ranking at the Dutch national ranking list at U13 and U16. The study started with 92 players; however six players did not have a ranking at U16 and were left out the study. So, a total of 86 junior elite tennis players (boys, n = 44; girls, n = 42), who turned 13 (range 11.9-13.2 years) in the year the measurements were taken, were part of this study. All players were top-30 ranked on the Dutch national ranking list at U13, and top-50 at U16. The ranking that was used was the end ranking of the year in which the player turned either 13 or 16. The (physical) measurements from U13 were used to predict ranking at both U13 and U16, therefore one group of players was used and we did not use cross-sectional data. For example, if a player was born in 1996, the year-end ranking for 2012 was used for tennis performance at U16.
Three anthropometric tests were conducted, namely standing height, sitting height, and body mass. One single observer measured standing height, sitting height, and body mass, following standard procedures (Lohman, Roche, and Martorell, 1988). Standing and sitting height were measured to the nearest 0.1 cm with a SECA height tape instrument (Model 206, Seca Instruments Ltd., Hamburg, Germany). Players sat on a table when measuring sitting height. Body mass was measured to the nearest 0.1 kg using a UWE balance (Model ATM B150, Universal Weight Enterprise Co., Ltd., Taiwan). Leg length was calculated as standing height minus sitting height. For calculating age peak height velocity (APHV), the Mirwald method was used (Mirwald et al., 2002), in which the maturation offset is calculated and used to determine APHV.
A total of eight physical tests were conducted, a medicine ball overarm toss and a reverse overarm toss were both measured. Players stood behind a line with feet at shoulder width; they held a medicine ball weighing 1.0 kilogram. Players faced forward for overarm throws and backwards for reverse overarm throws (Berg et al., 2006; Roetert and Ellenbecker, 1998; Stockbrugger and Haennel, 2001). Distance from start position until hitting the floor was measured in meters to two decimal points. Furthermore, overarm ball throw was measured using a ball of 200 grams (diameter of 6.5 centimeters). Players held the ball in their dominant hand (Berg et al., 2006). They positioned their feet as if they were serving and threw the ball overarm as far as possible, while keeping both feet on the floor. Distance from start position until hitting the floor was measured in meters, to two decimal points.
The squat jump (SJ) and countermovement jump with arms (CJMas) were both measured. Players were instructed to position their feet at shoulder width and keep their hands on their hips from start to finish during the SJ (Samozino et al., 2008). The starting position for the SJ was with the knees bent 100 degrees, holding this position for two seconds, and then jumping as high as possible. For the CMJas, a player jumped as high as possible, while bending their knees and using their arms. Electronic measurement was obtained by combining the Muscle Lab with an infrared light mat, on which the player stood (Muscle Lab, Ergotest Technology A.S, Langesund, Norway). The Bosco protocol (Bosco et al., 1983) was used; these tests measured power.
Sprinting five and ten meters was measured. Each player executed a ten-meter straight sprint from a standing start, in which a player stood behind a line with feet apart at shoulder width. Players were allowed to start when ready. Time was measured at five and ten meters. Electronic time measurement was obtained by combining the Muscle Lab with an infrared light mat, on which the player stood (behind a line) before starting (Muscle Lab, Ergotest Technology A.S, Langesund, Norway).
The spider test was executed. A player needed to...