The biological age of 14 year old boys and success

Research in Sports Medicine An International Journal

ISSN: 1543-8627 (Print) 1543-8635 (Online) Journal homepage: http://www.tandfonline.com/loi/gspm20

The Biological Age of 14-year-old Boys and Success in Adult Soccer: Do Early Maturers Predominate in the Top-level Game?

Sergej M. Ostojic, Carlo Castagna, Julio Calleja-Gonzรกlez, Igor Jukic, Kemal Idrizovic & Marko Stojanovic To cite this article: Sergej M. Ostojic, Carlo Castagna, Julio Calleja-Gonzรกlez, Igor Jukic, Kemal Idrizovic & Marko Stojanovic (2014) The Biological Age of 14-year-old Boys and Success in Adult Soccer: Do Early Maturers Predominate in the Top-level Game?, Research in Sports Medicine, 22:4, 398-407, DOI: 10.1080/15438627.2014.944303 To link to this article: http://dx.doi.org/10.1080/15438627.2014.944303

Published online: 08 Oct 2014.

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Date: 28 July 2016, At: 21:13

Research in Sports Medicine, 22:398–407, 2014 © 2014 Taylor & Francis ISSN: 1543-8627 print/1543-8635 online DOI: 10.1080/15438627.2014.944303

ORIGINAL RESEARCH

The Biological Age of 14-year-old Boys and Success in Adult Soccer: Do Early Maturers Predominate in the Top-level Game?

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SERGEJ M. OSTOJIC Center for Health, Exercise and Sport Sciences, Belgrade, Serbia Faculty of Sport and Physical Education, University of Novi Sad, Novi Sad, Serbia

CARLO CASTAGNA Football Training and Biomechanics Lab, Italian Football Association, Coverciano, Italy

JULIO CALLEJA-GONZÁLEZ Center for Health, Exercise and Sport Sciences, Belgrade, Serbia Faculty of Sport Sciences, University of the Basque Country, Vitoria-Gasteiz, Spain

IGOR JUKIC Center for Health, Exercise and Sport Sciences, Belgrade, Serbia Faculty of Kinesiology, University of Zagreb, Zagreb, Croatia

KEMAL IDRIZOVIC Center for Health, Exercise and Sport Sciences, Belgrade, Serbia Faculty of Sport and Physical Education, University of Montenegro, Niksic, Montenegro

MARKO STOJANOVIC Center for Health, Exercise and Sport Sciences, Belgrade, Serbia Faculty of Sport and Physical Education, University of Novi Sad, Novi Sad, Serbia

Received 20 February 2014; accepted 23 June 2014.

Supported by the Serbian Ministry of Science Grant No. 175037. We acknowledge the assistance of Dr Maja Marinkovic (University of California San Diego, CA, USA) in revising the language of the manuscript.

Address correspondence to Sergej M. Ostojic, Exercise Physiology Laboratory, Center for Health, Exercise and Sport Sciences, Stari DIF, Deligradska 27, Belgrade 11000, Serbia.

E-mail: sergej.ostojic@chess.edu.rs 398

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Talent identification and development in soccer is often biased by maturation-related differences of young athletes. However, there is no information available about success rates for youth maturing at different tempos to achieve success in elite adult soccer. The purposes of this study were to determine the prevalence of different maturational groups among boys playing soccer, and to track them for competence in adult performance. A prospective cohort study design was used to follow 55, 14-year-old boys playing in Serbian youth soccer Division I over eight years. At the age of 14, biological age using skeletal age rates was determined, and participants were categorized as early maturers (EaM), normal maturers (NoM), and late maturers (LaM). Game competence for adult soccer at age 22 was described as elite if an individual played for clubs competing in top-five international soccer leagues (La Liga, Premier League, Bundesliga, Serie A, and Ligue 1), and/or has become a member of an adult National team. Among boys in our study group, 43.8% were categorized as EaM, 35.4% as NoM, and 20.8% as LaM (P = 0.11). A significant difference in biological age was found among maturational groups at age 14, with EaM > NoM > LaM (P > 0.0001). When assessed for adult soccer competence, 33.3% of participants (16 out of 48 players) succeed in achieving elite level.

Elite soccer competence acquired 60.1% players from the group of LaM, 38.1% from NoM, and 11.8% from EaM (P > 0.0001). Our comparative analysis suggests that soccer excludes early maturing boys and favors late maturing boys as level of performance increases.

KEYWORDS skeletal age, youth, maturation, elite soccer

INTRODUCTION One of the confounding factors that critically interfere with the process of talent identification in soccer (association football) is maturity-related difference in performance among young athletes (Malina, Ribeiro, Aroso, & Cumming, 2007). Variation in size, function and skill associated with maturity status within the same age groups can be considerable (Figueiredo, Coelho E Silva, Cumming, & Malina, 2010). In particular, the advanced timing and tempo of maturation may contribute to the overall difference in soccer competence in youth soccer, which favors early maturing individuals (Malina, Eisenmann, Cumming, Ribeiro, & Aroso, 2004; Valente dos Santos et al., 2012). Early maturers tend to be physically superior and more proficient in childhood, and often outperform their less-mature counterparts of the same chronological age (Figueiredo et al., 2010), while their supremacy often

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decreases in later years when peers catch up with them (Malina et al., 2010).

This selection bias may result in late maturing and potentially talented players dropping out of the game at an early age, and not reaching the elite level of competition (Malina et al., 2000). Paradoxically, talented late maturers may have the potential to be better athletes later in life, if supported with high quality coaching and given the time to allow their physical maturity to catch up (Burgess & Naughton, 2010; Pearson, Naughton, & Torode, 2006). Several alternative protocols have been developed to determine maturational status of young athletes, and distinguish players of similar chronological age and contrasting maturity (Malina, 2011). It seems that assessment of biological age may help in long-term development of athletic children, and keep late maturers involved in sport programs (Ford et al., 2011). However, it remains open to investigation how many late maturing youth soccer players finally succeed in the top-level game. No studies so far have followed-up talented late and early maturers through their career, and compared their success in elite soccer as adults. Previous studies reported that adult players at top-level competition were early maturers at a young age (Deprez et al., 2013; Malina et al., 2000), suggesting possible discrimination related to maturational issues.

However, those studies used no sophisticated techniques for maturation evaluation (e.g. biological age assessment) to discriminate between late and early maturers, nor did they tracked down young individuals’ success toward adult soccer leagues. Therefore, the main aims of the present study were to find the prevalence of early and late maturers in top-level youth soccer using biological age evaluation, and to follow up these players longitudinally for adult competence in elite soccer. We hypothesized that late maturers are less prevalent among successful Serbian 14-year-old boys playing soccer, while they predominate in top international leagues and the National team as adult elite players at age 22.

METHODS Participants In 2006, 55 young soccer players (all born in 1992) competing at the youth Division I level were recruited for this study. All players were members of leading Serbian soccer clubs, with 12 boys also members of the National U14 team. All were apparently healthy (as assessed through routine medical examination), participating in soccer training (~ 9 hours per week), experienced in sport training (> 4 years), and had no medical history of congenital or developmental disorder. All participants and parents were fully informed verbally and in writing about the nature and demands of the study. All participants and parents gave their informed consent and volunteered to participate in the study, with the approval of the local IRB in accordance with the Helsinki Declaration.

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Experimental Procedures At initial assessment, plain radiographs of the left hand and wrist were obtained for biological age assessment using bone age rates according to TW3 method (RUS scores) (Tanner et al., 2001), with no orthopedic trauma suspected. RUS skeletal maturity estimation through TW3 method accurately rates skeletal age in children of age 14 (Ashizawa, Kumakura, Zhou, Jin, & Chao, 2005). All radiographs were assessed by a single observer, an experienced sports medicine specialist trained in radiology. To test the reproducibility of the assessments of the bone age, the same investigator re-evaluated randomly selected hand–wrist radiographs from 11 subjects two weeks after the initial evaluation.

The coefficient of reliability was 0.98, and intra-observer error was 0.16 years.

Participants were categorized according to biological age as early maturers (EaM, biological age above chronological age for > 0.51 years), normal maturers (NoM, biological age above or below chronological age for ≤ 0.50 years), and late maturers (LaM, biological age below chronological age for > 0.51 years). Using a band of ± 0.51 years allows for the error associated with the assessment of bone age. After the initial assessment, young soccer players from all three groups were followed for up to 8 years (through to December 2013) and categorized for adult soccer competence. Elite competence for successful adult player was described as: (1) one or more full seasons played for clubs competing in top-five soccer leagues (Spain’s La Liga, English Premier League, German Bundesliga, Italian Serie A, France’s Ligue 1), and/or (2) at least one official match played for an adult National team. Sub-elite competence was described as a full season played for teams from other soccer leagues (national and international), with no appearance with a National team.

Statistical Analysis We calculated that the enrolment of a minimum of 39 participants would be required to detect a 5% difference in prevalence between three maturity groups, at a 2-sided α level of 0.05, 80% power and effect size of 0.5 (G*Power version 3.1.7, G*Power, Dusseldorf, Germany). The percentage of EaM, NoM and LaM in the total sample was calculated at both young and adult age. Differences in prevalence of EaM, NoM and LaM were compared using the three independent samples Kruskal-Wallis test, with the Games-Howell posthoc test used to evaluate whether the differences between any two groups are significant. In addition, the comparison between biological and chronological age in each maturational group at age 14 was done using paired-samples T-test.

One-way analysis of variance was used to compare biological age among different maturational groups of 14-year-old soccer players, with the Tukey post-hoc test employed to identify the differences between individual sample pairs. The significance level was set at P ≤ 0.05. Data were analyzed using the SPSS program (version 21.0) (SPSS Inc., Chicago, IL, USA).

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RESULTS Seven participants from the initial group of fifty-five 14-year-old boys were excluded from the final analysis since no indicator of adult soccer competence was available at the follow-up. Forty-eight players (n = 48) were included in both initial categorization of biological age at age 14, and follow-up analysis for adult soccer competence. Average chronological age at baseline testing was 14.5 Âą 0.3 years. Among boys, 43.8% were categorized as EaM, 35.4% as NoM, and 20.8% are rated as LaM (P = 0.11). Biological and chronological ages of the three groups of soccer players at age 14 are presented in Figure 1.

Significant differences were found between chronological age and biological age in EaM (P > 0.0001), and LaM (P > 0.0001). Furthermore, results indicated significant differences in biological age among three maturational groups of young soccer players, with EaM > NoM > LaM (P > 0.0001) (Table 1). None of the participants attained the mature state (e.g. 1000 RUS score) at age 14.

When assessed for adult soccer competence, 33.3% of subjects (16 out of 48 players) played at elite level (e.g. National team, top five international soccer leagues), while 66.7% of subjects (32 out of 48 players) played at subelite level (national and/or other international leagues). Elite soccer competence at adult level was achieved by 60.1% of players from the group of LaM, 38.1% from NoM, and 11.8% from EaM (P > 0.0001) (Figure 2), with LaM > EaM (P > 0.0001) and NoM > EaM (P = 0.012).

FIGURE 1 Chronological and biological age among different maturational groups of 14-yearold soccer players. * Indicates significant difference (P < 0.05) between chronological versus biological age inside maturational group. â&#x20AC; Indicates significant difference (P < 0.05) in biological age between different maturational groups.

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FIGURE 2 Percentage of players from different maturational groups at age 14 who achieved elite level of adult soccer competence. â&#x20AC; Indicates significant difference (P < 0.05) between groups.

DISCUSSION This longitudinal study provided direct evidence that late maturers from the cohort of successful 14-year-old Serbian soccer players, with biological age retarded by more than 6 months, are more likely to achieve success in the toplevel game, compared with their early-maturing counterparts. Although early maturers prevail in youth soccer (43.8%), only 11.8% boys with advanced biological maturation at age 14 attain the elite level of competition as adults; it seems that early supremacy in youth soccer is not a predictor of later success.

Maturational Differences among Young Players In the past 50 years, different models of talent identification for team sports have been developed in attempts to recognize and cultivate potentially elite athletes (for a review see Burgees & Naughton, 2010). Research has highlighted a number of potentially important predictors of talent in soccer, yet no reliable evidence-based model has been proposed so far (Kannekens, Elferink-Gemser, & Visscher, 2011; Unnithan, White, Heorgiou, Iga, & Drust, 2012; Williams & Reilly, 2000). Due to the complex nature of talent, holistic approaches that consider physical, physiological, sociological and psychological characteristics of the young individual that plays soccer have been suggested in an effort to determine key predictors of performance (Ostojic,

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2013; Unnithan et al., 2012). However, performance assessment at a young age must account for the other aspects of the growing child, such as timing and tempo of maturation, which may not be related to talent per se. Previous studies reported that early maturing adolescents may have greater body mass, stature and muscular strength, which have been related to superior performance in youth soccer compared with late maturing boys of the same age (Figueiredo et al., 2010; Gastin & Bennett, 2014). Coaching staff may see this maturation-related superiority as an indicator of talent and favor early maturers, so this approach may prematurely exclude those late maturing athletes from soccer. Several studies over the past two decades evaluated the variation in biological maturation within single-age chronological age groups of young soccer players. Malina et al. (2010) evaluated 592 male players from Spain through regional and elite youth soccer programs. The authors reported over-representation of boys with advanced biological (skeletal) age among adolescent players, with early maturers being four times more prevalent than late maturers between the ages of 13 and 14 years.

Similar trends were found in cohorts of young players from Portugal (Figueiredo et al., 2010) and Belgium (Deprez et al., 2013). Our study confirmed results from previous studies as boys with advanced biological age were two times more predominant compared with LaM among 14-year-old Serbian soccer players (43.8% vs. 20.8%). On average, early maturers had a biological age 1.7 years greater than late maturing boys. We only partially assessed physical attributes of young players, yet higher prevalence of EaM in Serbian youth soccer may be attributed to the physical advantages of biologically older players of the same chronological age. Although we did not measure perceptions or attributions of the coaches, we could speculate that Serbian coaches in youth soccer are biased toward players of advanced maturation. If proven in subsequent studies, this may have a long-term impact on quality selection in youth soccer, since many of the qualities that distinguish top athletic performance in adults may not be apparent until late adolescence (Franks, Williams, Reilly, & Nevill, 1999).

Maturational Groups and Success in Adult Soccer During early adolescence, boys with inferior morphological growth during the selection process could be perceived by coaches as less talented, and often drop out of sport. On the other hand, if given time for their physical maturity to catch up and their skills to develop, late maturing athletes may have the potential to excel with age and be better athletes as adults (Pearson et al., 2006). Malina, Bouchard and Bar-Or (2004) reported that late maturers that continue with long-term skill development through late adolescence eventually catch up with their peers and perform better than early maturers, suggesting reduced significance of maturity-associated variation for adult

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performance in elite sport. After career tracking of the young soccer players from age 14 to age 22 in the current study, we found a significantly different proportion among players from different maturational status within single-age chronological age group for elite soccer performance. From our cohort of 14year-old top-level Serbian young soccer players, 60.1% of those initially categorized as LaM achieved elite level of adult competition, suggesting that they are more capable of succeeding in soccer with compared NoM and EaM. Late-maturing boys appear in top leagues 5.1 times more frequently than early maturers. Our study indicates that if late maturers have top-level soccer expertise at a young age and endure through their adolescent years, they more often become elite players. The fact that LaM regularly played soccer with both NoM and EaM at a young age might force late-maturing boys to overcome biological limits through development of other skills necessary for performance (e.g. agility, technical capabilities, tactical skills, and mental toughness). Those additionally-developed skills may flourish in later years when they catch up with early maturers, and may help them to achieve adult success. However, this assumption needs further investigation and long-term screening of performance outcomes in soccer skills during maturation. In accordance with the results of our study, it seems plausible that coaches in youth soccer should take special care of talented biologically inferior boys who successfully participate in youth competitions, for the long-term development of future elite players. Late maturers should be given enough time to develop through the experience of high quality coaching (Figueiredo, Gonçalves, Coelho E Silva, & Malina, 2009).

Limitations of the Study Although we used longitudinal design for this study, employed skeletal rates for biological age assessment, and recruited top-level young players at a critical age for talent identification in soccer, our study has several limitations.

First of all, the follow-up interval of 8 years could be considered somewhat short since soccer players may attain adult excellence later in their careers.

Long-term studies that follow individualsâ&#x20AC;&#x2122; entire careers in soccer are needed for future studies. Second, the sample size in our study could be considered relatively small to permit inferential analyses, and corresponds to the specific age group (14-year-old boys) for biological age assessment at a single time point. Upcoming studies should extend the sample and monitor other youth groups (including girls), accounting for individual maturational changes through adolescence. Third, biological age was obtained from an arbitrary criteria (discrepancy between chronological and bone age within a band of 0.51 years), while most of the previous studies used a 1-year band (Figueiredo et al., 2009; Malina et al., 2000). Although the band of Âą1 year provides a broad range of youths who are classified as on time in maturity status, the

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narrow range used for the current study (±6 months) is well outside the range of the error of bone age assessment (~2 months) so it might be relevant for the sample in this study. Finally, no inter-relationship of growth, maturity and changes in physical and/or physiological characteristics of youth soccer players has been evaluated in this study.

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CONCLUSION In conclusion, our study showed that 14-year-old soccer players with advanced biological age are over-represented in Serbian youth soccer, compared with their late-maturing counterparts. At the follow-up after 8 years, elite soccer competence seems to be achieved more often by the boys who were late maturers at age 14, while early-maturing boys less frequently attained top-level soccer. Our comparative longitudinal analysis suggests that soccer excludes early maturing boys and favors late maturing boys as the level of their performance increases.

CONFLICT OF INTEREST The authors declare that they have no conflict of interest.

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