Relative age effect and its relationship with morphological characteristics and performance in young soccer players

In soccer, the relative age effect (RAE) was observed in both adult and young players. The RAE appears to be more pronounced in elite sports, probably by the need to select the best players to compete internationally. This study review: (1) the prevalence of RAE in soccer players, (a) considering competitive level (b) and specific position and (2) association between RAE (a) and anthropometric characteristics, (b) physical fitness components and technical skills. A total of 12 studies met all inclusion criteria for this review. One trial (meta-analysis) was included after the eligibility process. Overall, 77675 young soccer players were analysed. In all studies, significance level of 0.05 was set for the type I error. There is a consensus about the presence of an RAE in men’s soccer, and the percentage of players born in the first quarter in the selection year for professionals is high, with peak values found for elite young athletes, and a large decrease is evident throughout the regional and school representation. The relationship between RAE and the specific position is controversial, according to few studies. It is likely that players born in the first quarter differ in a variety of anthropometric characteristics and physical fitness components compared with peers born in the last quarter. Researchers need to understand the mechanisms by which RAE increase and decrease in order, to reduce and eliminate this social inequality that influence the experiences of athletes, especially in periods of development. Organizational and practical intervention is required.


INTRODUCTION
In order to provide appropriate development, competition and equal opportunities for successful training, children are usually grouped by chronological age in many sports [1][2][3][4] .However, considerable complexities can arise due to the existence of significant interindividual variations related to growth and biological maturation especially evident in childhood and adolescence 5 .The Fédération Internationale de Football Association 6 (FIFA) established January 1st as the start date for the selection year for international competitions (specifically from 1st January to 31 December).This age difference among individuals in the same age group is referred to as relative age, and its consequence is known as the relative age effect 4

(RAE).
The majority of studies on this topic are focused on assessing the distribution of the different quartiles and their relationship with the sporting success.The strong linear relationship between month of birth and the proportion of players in the National Hockey League and Canadian leagues was demonstrated for the first time 7 .There are approximately four times more players born in the first quarter of the year compared to the last quarter 1,8 .Soccer, like ice hockey, is among the most popular sports around the world featuring a high prevalence of RAE 1 .
In soccer, RAE is observed in both adult [9][10][11][12][13] and young players [13][14][15][16][17] .RAE appears to be more pronounced in elite sports 18 , probably by the need to select the best players to compete internationally.Players belonging to high competitive level 19 teams tend to be slimmer and to show greater muscle mass compared to those of lower competitive level.
The influence of relative age is also observed in the functional and technical performance of athletes.Many classifications were created in an attempt to explain the specific psychomotor skills required by certain types of sports 8 .A key element of the selection process 20 is the analysis of the anthropometric characteristics and performance of participants.Considering the importance of these characteristics, different mechanisms have been proposed to explain the causes of RAE 21 .Variations in biological maturation and physical attributes (greater aerobic capacity, muscle strength and height) appear to be also important aspects.
The advantage of having been born at the beginning of the selection year may relate to physical precocity, in other words, players have the advantage of up to 12 months in physical maturation over their peers born at the end of the selection year 8 .This difference can lead to significant changes in anthropometric variables (height, body mass), development of physical fitness (strength, speed, endurance), cognitive skills (game analysis, insight, tactical ability) and psychological variables 5,8 .However, these athletes may not always emerge as the best athletes during adulthood 4 .Rather, these advantages may only contribute to the existence of RAE in soccer, especially among younger players 3,22 .
It is noteworthy that there are few studies that relate RAE, competitive level and specific position on anthropometric variables, physical fitness and technical skills of young soccer players.As a way to fill the gap in literature, this study aims to review RAE, competitive level (professional, amateur, elite, regional and local), specific position (defenders, midfielders and forwarders) and anthropometric variables, physical fitness and technical skills in young soccer players aiming to conclude what extent is the knowledge about this issue and what are the future research directions.
An extensive search of the online electronic database was held in Pub-Med without time limit.Text words, keywords and descriptors used in the research were: soccer AND (relative age effect).Inclusion criteria were studies published in English that included young soccer players in their sample.Repeated publications for the same studies were excluded.In the presence of any doubt about study inclusion, a final consensus decision was taken after the full text was jointly reviewed.The references of the selected studies were analysed to obtain other relevant studies.
Overall, 37 potentially relevant studies were identified by electronic database and the manual search of reference lists identified other four.A total of 12 studies met all inclusion criteria for this review.One trial (metaanalysis) was included after the eligibility process.Overall, 77,675 young soccer players from 11 different countries were analysed.In all studies, a significance level of .05 was set for the type I error.

RELATIvE AGE EFFECT AMONG PROFESSIONAL TEAMS
The demographic study 23 with professional teams affiliated in European Football Associations (UEFA) in thirty-one first-division championships stressed that players born in the first quarter (Q1) of each year are overrepresented (Q1 = 30% and Q4 = 19%) compared to those born in the last quarter (Q4).According to the above results 9 , professional soccer players born from 1935 to 2007 in Germany also showed significant differences among quartiles, with Q1 being overrepresented compared with the other quartiles.Only the group of players born from 1946 to 1950 showed similar distribution in the year of birth.
Professional soccer players of the second and third Belgian divisions 22 (1998/99 to 2002/03), were divided into 1A (old year selection, August) and 2A (new year selection, January).A strong association between quarter of birth and number of players in 1A group (p<.05) was demonstrated.Regardless of the selection criteria used, Q1 generated a greater number of players (1A, 28.8%; 2A, 31.5%), while the number of players born in Q4 was lower (1A, 19.9%; 2A, 19.5 %).
In the following decade, from 2000 to 2011, the distributions of professional players by date of birth were examined 12 in England, Portugal, Germany, Belgium, Netherlands, Spain, France, Italy, Denmark and Sweden.The comparative analysis of different ages revealed a significant increase in RAE from 2000-2001 to 2010-2011 (p<.05).While in the 2000-2001 season, 29.3% and 19.8% of athletes were born in Q1 and Q4, respectively, in 2010-2011, the relative frequencies were 31.9% and 18.4% respectively.In 2008 10 , all forty clubs that participated in the Brazilian Soccer Championship divisions A and B identified a greater number of players born in Q1.There was the preference of clubs of both divisions for recruiting athletes born in the 1 st half of the year (Q1 x Q4, p≤.001; Q2 x Q4, p=.001).

RELATIvE AGE EFFECT IN yOUNG SOCCER PLAyERS
The meta-analysis 1 of RAE in sport had its sample divided according to chronological age: <11 years, 11-14 years, 15-18 years and >18 years, and a progressive increase in RAE <11 years up to 15-18 years was observed.Comparison between Q1 and Q4 showed a small-moderate effect for individuals aged 15-18 years and decline in those >18 years.The same was observed in Japan 24 , when analysing young soccer players recruited between 1997 and 2000.At ages 10-15 years, 37.9-58.8% of players were born in Q1.Moreover, 3.2-13.5% were born in Q4.These results indicate a significant change in the distribution of month of birth among adolescent elite soccer players compared with the general population (p<.001).
Strengthening RAE in young Belgian 16 elite soccer players, the distribution among quartiles of the date of birth of players significantly differed from the "Flemish" population (U15, p<.001; U17, p<.001; U19, <.001).The date of birth of all French male players 11 affiliated to the French Football Federation during the 2006-2007 season from the database Federation revealed significant differences in age categories: U13 (p<.0001),U15 (p<.0001),U18 (p=.0001).A previous study 14 on German U17 soccer players belonging to forty teams in the premier league found a strong trend of selecting players born in Q1.Statistical tests were significant (p<.05) in twenty two of the forty teams (53.7%).
For the entire group of players 17 , about 40% were born in Q1.In contrast, only 16% of players were born in Q4.The same RAE was identified (p<.05) when analysing intra regions (Africa, Asia and Oceania, Europe, North America and Central and South America), in FIFA U17 World Cup competitions (1997 to 2007) 18 .

RELATIvE AGE EFFECT AND COMPETITIvE LEvEL IN yOUNG SOCCER PLAyERS
Box 1 shows the characteristic of eight studies that examined relative age effect and competitive level in young soccer players.Young elite soccer players 21 born in France (14-16 years old) were evaluated over eleven years (1994-2005).Players born in Q1 were the most represented, showing a decrease of the number of players born in subse-quent quarters (p<.01).Professional (p<.01) and non-professional (p<.01) players showed a greater number of soccer players born in Q1.Analyses in the upper levels 13 (U17 and U21) showed that young players from the first and second Spanish league have identical dates of birth in relation to soccer players from the U17 to U21 national team (p=.61), but are very different from those observed in professional players (p<.001).However, for young players, differences in distribution are not significant for this category (p=.64).
The meta-analysis 1 showed that RAE is more evident in the representative group (regional and national representation, Q1 vs Q4 = OR:2.77).Interestingly, results suggest that RAE is lower among elite players (senior, OR:1.42).The analysis of Spanish 3 soccer players showed that the percentage of players born in the first quarter was 43.9% in senior (p<.001),46.6% in youth elite (p<.001), 28.6% in youth regional representation (p<.001) and 27.1% in school players (p<.001).Players born in Q4 accounted for 12.2, 10, 21.2 and 22.9% respectively for the 1985/1986 to 2005/2006 seasons.On the other hand 26 , young soccer players from northern Portugal aged 11-17 years who participated in regional competitions showed similar distribution of birth date among quartiles (Q1 = 27%, Q2 = 25%, Q3 = 23%, Q4 = 25%).
The percentages in the distribution 24 for young elite players were: 13 years Q1 = 57.6% and Q4 = 3.2%; 14 years Q1 = 53.2% and Q4 = 6.4%; 15 years Q1 = 48.8% and Q4 = 7.3%.The distribution of players (U11 to U18) grouped into three groups was also analyzed 15 , showing considerable variation in each of the three groups studied by quartile (Elite 2005-2006, p<.001; Elite 2008-2009, p<.001, Amateurs, p<.001).The comparison of Elite groups with Amateur groups revealed that the distribution of births in 2005-2006 and 2008-2009 in Elite groups significantly differed from the distribution of births in Amateur groups.Similar results 6 were found for category U16 (Q1 = 48.96%,Q4 = 7.64%, p≤.01), when analyzing the distribution of birth dates of national teams U16, U18 and U21 participants in the 1999-2000 season of UEFA competitions.
In the Swiss 25 system, no significant differences were observed for the distribution of the Swiss population and 'J+S' (p≥.05).In all the selected teams (except for U20), Q1 for elite players were significantly overrepresented and Q4 underrepresented compared to 'J+S' (p<.01).Teams (national talent) showed a distribution of more than 35% in Q1, and less than 15% in Q4, which significantly differed from the distribution of 'J+S' (p<.001).

RELATIvE AGE EFFECT AND SPECIFIC POSITION IN yOUNG SOCCER PLAyERS
Box 2 shows the characteristic of the three studies that examined relative age effect and specific position in young soccer players.According to some authors 13,15 differences in the distribution of birth dates for specific position do not seem to be evident in young soccer players.However 25 , significant differences (U15 to U21) among goalkeepers, defenders, midfielders and forwarders were identified.Defenders, midfielders and forwarders were overrepresented at the beginning of the selection year and, in each case; there was a decrease in the number of players who were born in subsequent quarters.Defenders were significantly (p<.05) overrepresented in the first semester (79%) compared with forwards (57%).The other comparisons were not significant.

RELATIvE AGE EFFECT AND ANTHROPOMETRICS CHARACTERISTICS IN yOUNG SOCCER PLAyERS
Box 3 shows the characteristics of the five studies that examined relative age affect and anthropometric characteristics in young soccer players.
Analysing soccer players 13-14 years old belonging to Portuguese clubs 27 at regional level and local, it was found that regional soccer players, on average, were more experienced in soccer (p<.05), had advanced skeletal maturation (p<.01), were heavier (p<.01) and higher (p<.01) than those competing locally.It was also shown that soccer players for regional level and at specific positions were advanced in maturation [defenders: p<.01, midfielders: p<.01, forwarders: p<.05], were higher [defenders: p<.01; midfielders: p<.01, forwarders: p<.05] and heavier [defenders: p<.01; midfielders: p<.01, forwarders: p<.01] than players who compete at a local level.
Portuguese soccer players 28 belonging to eleven U19 teams were evaluated (5 elite, 6 non-elite).No significant interaction between competitive level and specific position on the field and anthropometric characteristics and training (p>.05) were observed.However, elite goalkeepers and cen- Carling et al. 21rench 14-16 Elite Significant differences in VO2, anaerobic power and concentric strength were observed among quartiles.
Coelho et al. 27 Portugal 13-14 Regional Local The regional group had better performance in explosive power and juggling than the local group.There were no differences between groups in competitive agility, YYIR1 and dribbling.
Deprez et al. 30 Belgian 10-16 Elite There was no significant difference in physical fitness (anaerobic power and muscular strength) among quartiles.
Rebelo et al. 28 Portugal U19 Elite Regional Players differed in physical fitness and technical skills in competitive level and specific position.

374
Relative age effect in soccer Pedretti & Seabra tral defenders tend to be larger and heavier than non-elite players in their respective positions.Elite midfielders showed moderate differences in body mass, but not when compared to non-elite players.Small differences in height and weight were observed between defenders and forwarders.A study carried out is Spain 19 with non-elite players aged 12-19 years showed that central defenders were taller and heavier in relation to external defenders, central midfielders, external midfielders and forwarders (p<.01).
There was a significant difference 24 between Q1 and Q4 for height only for players aged 12 years (p<.01) and 14 years (p<.05).The height of Q3 for players aged 12 years (p<.01), and Q4 for players aged 14 years (p<.01) were significantly lower than those of Q1 and Q2.In addition to these results, there was also a statistical difference in height for players aged 11 to 13 years when the maturational difference was statistically controlled, Q3 for players aged 13 years (p<.01),Q4 for players aged 11 years (p<.01) and 14 years (p<.05) were significantly lower than that those in Q1.A significant difference 21 was observed in height (Q4 vs Q1, p<.001) and weight (Q3 vs Q4, p=.049).In addition, the number of players born in Q1 was significantly higher (p<.05), indicating that the maturity offset in Q1 was lower in young soccer players in northern Spain 29 .On the other hand 16 , no significant effect on anthropometric variables was observed (height and weight) among quartiles within all age categories: U15, U17 and U19.

RELATIvE AGE EFFECT AND PHySICAL FITNESS AND TECHNICAL SkILLS IN yOUTH SOCCER PLAyERS
Box 4 shows the characteristic of the five studies that examined relative age effect and physical fitness components and technical skills in young soccer players.
In 30-m sprint, central defenders were faster than the other groups 19 .In squat jump (SJ) and countermovement jump (CMJ) tests, goalkeepers showed the best results, and the lowest values were found for external midfielders.Differences were not statistically significant.Better performance was observed 27 in the functional characteristics [SJ (p<.01), sprint (p<.01), sum of seven sprints (p<.01)], but in only one technical skill [juggling (p<.05)] at regional level.Local and regional participants did not differ in agility, aerobic endurance, speed dribbling, precision shooting and passing.It was also observed 27 that regional players showed better results on SJ [defenders: p<.01, forwards: p<.05] and 30-m sprint tests [defenders: p<.01, midfielders: p<.01] than players who compete at local level.In contrast, no significant differences were observed among positions in relation to agility trials, YYIR1, and technical skills (except for juggling, p<.05).Significant differences were found 21 in the year quarters for VO2max (Q4 vs Q1, p=.007), maximal anaerobic power (Q1 vs Q4, p=.039) and maximal concentric force (Q1 vs Q4, p<.001).
There was no 28 effect of the interaction between competitive level and specific position in any of the tests or functional ability (p<.05).Elite goalkeepers tend to perform better than non-elite goalkeepers on all tests.Differences in SJ, YYIR2, juggling and CMJ were observed.Moderate differences were also observed for the performance of 5 -and 30-m sprint and agility.Elite defenders tend to be different from non-elite defenders in SJ and juggling, while differences in agility and YYIR2 were moderate.Elite midfielders showed moderate differences in 5 -and 30-m sprint, agility, juggling and YYIR2 compared with non-elite.Elite forwarders showed moderate differences in agility and YIR2 compared with non-elite forwarders.
When young soccer players from northern Spain 29 were analyzed, older players showed better performance in 15 -(p<.01) and 30-m sprint (p<.01) and agility tests for 15 -(p<.01) and 30-m (p<.05).In general, players who were born in the first quartile showed significantly better performance than those who were born in the last quartile.Older players also showed better performance on YYIR1 and CMJ, although not statistically significant.On the other hand 30 , significant benefits for physical fitness components (muscular strength and anaerobic power) in soccer players (U14 to U17) born in the first quarter compared to players born in all other quarters were not evident.

FINAL CONSIDERATIONS
In soccer competitions, players are usually grouped according to their chronological age.This study review: (1) the prevalence of RAE in soccer players, (a) considering competitive level (b) and specific position and (2) association between RAE (a) and anthropometric characteristics, (b) physical fitness components and technical skills.
There is a consensus the presence of a RAE in men's soccer: Players born after the cut-off dates are overrepresented (professional, amateur, elite, regional or local level).The reason for this selection behaviour is justified by the thought of immediate success and avoids the long-term goals of promoting and developing talent.While the coaches' of the young soccer players judge according to the probable performance advantage of those born at the beginning of the year selection for your success present, this bias may worsen, with prejudice to the sport and the young soccer players.Some authors have shown that the percentage of players born in the first quarter of the selection year is high for professionals, but peak values were found for the youth elite and along the regional and school representation, a large decrease is evident.When compared to non-elite, the elite group (young players), it was observed that the distribution of births significantly differed.Given the relevance of the RAE and its potential to introduce a bias in the talent identification process, new studies correlating RAE and competitive level are needed for a better understanding of this issue, given that there are few studies focusing this theme.
The relationship between RAE and the specific position is controversial, according to few studies.While some authors show that there are no clear differences in the distribution of birth dates for specific position, others found that defenders, midfielders and forwarders were overrepresented at the beginning of the selection year.Some results have shown that young players with potential are neglected and that the subject is poorly documented, so further studies should be carried out in order to identify the possible spread and development of RAE.Thus, RAE and its relation to specific position is a gap to be filled.
It is likely that players born in the first quarter of the selection year differ in a variety of anthropometric characteristics and physical fitness components compared with peers born in the second half.Given this gap in literature and the lack of consistency in expected results on anthropometric variables, physical fitness components and technical skills, further studies should be carried out to better understand this selection bias, detection and development of 'talents'.

Box 4 .
Characteristic of studies that examined RAE and physical fitness components and technical skills

Box 1 .
Characteristic of studies that examined RAE and competitive level *Belgium; Denmark; England; France; Germany; Italy; Spain; Sweden; Netherlands; Portugal.

Box 3 .
Characteristics of studies that examined RAE and anthropometric characteristics Characteristic of studies that examined RAE and specific position.