The identification, selection, and development of talented tennis players at early ages can have significant social, professional, and financial implications for the players and their families (Abbott and Collins, 2004; Roetert and Ellenbecker, 2007; Vaeyens et al., 2008). A talented young tennis player can be considered as someone whose performance is better and/or increasing faster than his or her peers at training and competition (Elferink-Gemser et al., 2004). Typically, those players are surrounded by the social and material environmental conditions that increase the likelihood of them reaching the elite level (Hohmann, 2001).
Like most competitive sports, youth tennis competition is divided into age categories based on chronological age as defined by a player's date of birth (Helsen et al., 2005).
In tennis and most sports, the different competition age groups are organized by grouping players born within the same 12-month period. Besides these "within one year" groupings, other age-groupings (e.g. annual age grouping and multiyear age bands) used in sport have been shown to generate different effects (e.g. within-year effects, constituent year effects and constant year effects). It is important to differentiate in-between the outcomes (Schorer et al., 2013), although regardless of the group system used, these cut-off dates are commonly used with the goal of reducing maturational differences and create homogenous competition groups allowing a more sensible coaching and evaluation of the athletes as well as to ensure that there is an equal chance of success and fair competition for all players in youth sports (Helsen et al., 2005; Musch and Grondin, 2001; Schorer et al., 2011).
However, research in different sports has found that athletes born early within the selection year are more likely to be selected for elite teams and talent development programs than those born later in the same year (Augste and Lames, 2011; Delorme and Raspaud, 2009; (Helsen et al., 2012; Mujika et al., 2009). This overrepresentation of relatively older athletes in youth sport has been labeled as the relative age effect (RAE) (Barnsley et al., 1985). Previous studies in tennis documented the existence of RAEs, with a skewed birth date distribution in top junior players as well as in senior players, with more players involved in elite development programs born in the first half of the calendar year (e.g., values ranging from 60 to 86%) than in the second half (Baxter-Jones et al., 1995; Dudink, 1994; Edgar and O'Donoghue, 2005b; Filipcic, 2001; Loffing et al., 2010). While the existence of RAEs has been associated with a loss of potential talent, eliminating these effects has proven challenging (Musch and Grondin, 2001). This appears to be related to difficulties in identifying potential causal mechanisms. One of the most recurrent suggested mechanisms is selection bias. That is, coaches mistakenly grant fewer opportunities (e.g., instruction, access to elite group or team) to relatively younger individuals than should be warranted by their latent ability or talent (Deaner et al., 2013).
In Germany, young talented tennis players are scouted and recruited to join regional and national training centers by coaches who are affiliated to the German Tennis Federation (DTB). Players are selected within the same 12-month period (from January 1st to December 31st) based on a repetitive observation of the players' technical/tactical abilities as well as their competitive performance. While selection bias is likely to be mediated by several interacting mechanisms, maturational factors are often the focus of study (Musch and Grondin, 2001). This maturational hypothesis is based on the large inter-individual biological differences within the same chronological age groups during childhood and adolescence assuming that players born close to the selection date profit from their advanced physical and cognitive maturation (Baxter-Jones and Sherar, 2007). Even small age differences (i.e., months) within an annual age-group can provide substantial advantage in physical and cognitive maturity (Baxter-Jones et al., 1995). In a sport like tennis, in which height, strength, speed, and power are important performance factors (Fernandez-Fernandez et al., 2009), relatively older children are more likely to dominate youth tennis, be identified as "more talented" and be selected to be part of elite teams (Baxter-Jones et al., 1995; Edgar and O'Donoghue, 2005a). It is therefore possible that in order to remain competitive and have chances to be selected for the next levels of talent development, relatively younger players within their age group have to match either anthropometrics and/or physical fitness performances of the older players. However, an investigation into the link between anthropometrics, physical fitness, and RAEs in young tennis players has not yet been conducted.
Therefore, the aims of the present study were: 1) to test the existence of RAE in young German male tennis players, 2) to examine if the potential RAE was influenced by age and/or skill level and 3) to investigate whether players who were born later in the selection year and were still selected into the elite squads were likely to be similar across a range of anthropometric and fitness attributes compared with those born earlier in the year.
Between the years 2009 and 2011, a sample of the 348 best male young players in Germany (from the national and regional selection groups) was evaluated using a battery of standard anthropometric and physical performance tests implemented by the DTB at the national level. Players were recruited from their respective regional federations and all federations in the country were tested. For the purpose of the present study, the players were grouped on the basis of chronological age into 1-year age categories (i.e., from January 1st to December 31st). The cohort spanned 6 years and included U12 (n = 70), U13 (n = 96), U14 (n = 57), U15 (n = 57), U16 (n = 32), U17 (n = 36) male tennis players. The players and parents were informed of all experimental procedures and written informed consent was completed before participation. The study was approved by the institutional research ethics committee and conformed to the recommendations of the Declaration of Helsinki (World Medical, 2013).
To assess the prevalence of RAEs in tennis, a substantial data set had to be collected from different sources. The birth dates of all male players affiliated with the German Tennis Federation (DTB) born between 1992 and 2000 (11 to 17 years old) (n = 120,851) were analyzed and this group was labeled as "licensed players". Among all these players, various subgroups were subsequently made and retained for further analyses (Table 1). The first subgroup, defined as "ranked players", included all male players with official ranking in the German youth ranking list (players aged 11-17 years old, n = 7,165). The second subgroup, defined as the "regional squad", was made up of the most talented players in each region (up to 30 players per region, aged 11 to 17 years old), selected by the regional federations coaching staff based on their technical/tactical abilities and competitive performance (n = 381). A third subgroup, defined as the "national squad", was drawn from the best of the 381 regional players (previous group), selected by the national federation coaching staff based on their technical/tactical abilities and competitive performance (n = 57, from 11 to 17 years old). In addition, the birth dates of the first 50 senior players of the national ranking (i.e., including the Davis Cup squad) were collected from the DTB database. Moreover, the birth distribution of the whole male German population born between 1992 and 2000 was extracted from the Federal Statistical Office ("Statistische Bundesamt") (https://www.destatis.de/DE/Startseite.html).
All testing was completed in a three-week period, beginning at the end of September each year. Test sessions were undertaken between 14:00 and 20:30h and the players were assessed at their respective federation training centers. To ensure standardization of test administration across the entire study period, all tests were carried out in the same order and using the same testing devices and operators. All fitness tests were performed in an indoor tennis court (Rebound Ace surface). Testing began after a 15-min individual warm-up, which consisted of low-intensity forward, sideways, and backwards running, general dynamic mobility, multi-directional acceleration runs, skipping, and hopping exercises, and jumps of in creasing intensity. The following physical performance tests were conducted.
Anthropometry. Sessions started with the measurement of players' body dimensions, which included body height, body mass, and sitting height. Body height was measured with a fixed stadiometer ([+ or -] 0.1 cm, Holtain Ltd., Crosswell, UK), sitting height with a purpose-built table ([+ or -] 0.1 cm, Holtain Ltd., Crosswell, UK), body mass with a digital scale ([+ or -] 0.1 kg, ADE Electronic Column Scales, Hamburg, Germany). For the prediction of the age of peak linear growth according to Mirwald et al. (2002), leg length was estimated by subtracting sitting height from body height.
Maturity Status. Pubertal timing was estimated according to the biological age of maturity of each individual as described by Mirwald et al. (2002). The age of peak linear growth (age at peak height velocity) is an indicator of somatic maturity representing the time of maximum growth in stature during adolescence (Mirwald et al., 2002). Biological age of maturity (years) was...