In the present study, an historical analysis involving the most recent quarter-century (1993-2018) on RAEs among Japanese professional athletes was performed to clarify how the RAEs changed over time.
Relatively older players (i.e., Q1 players) may have greater opportunities for selection and experience in childhood because they are naturally heavier, taller, stronger, and faster, have greater endurance, and are more coordinated than relatively younger players during childhood [15, 16, 39], all of which translate into performance advantages in most sports. This has been often described as the ‘maturation-selection hypothesis’ [see a review, 15]. Judging from Table 1, our data in four professional sports including soccer, baseball, basketball, and volleyball, support these previous findings. That is, the percentage of Q1 among the four sports was the highest for 25 years, and that of Q4 was the lowest, although the effect size (Phi) gradually decreased over time. This was also supported by the OR data in Table 2. These data indicate that RAEs in fours sports existed for 25 years. Since the RAEs are influenced by player selection and training methods, it is necessary to promote the awareness of "adults (guardians, coaches, associations, etc.)". In other words, if adults have little knowledge about RAEs, relatively older players will be given priority and talented relatively younger players may be overlooked. Some previous studies reported cases to reduce RAEs [9, 40, 41]. For example, Mann & van Ginneken [41] reported a significant selection bias for the scouts of youth soccer in a no-age information group, and that bias remained when scouts knew the players’ dates-of-birth. The selection bias was eliminated when scouts watched the games knowing the shirt numbers corresponded to the relative ages of the players. However, with regard to the decrease in RAEs among professional players in Japan, it is unlikely that efforts to reduce the RAEs were made nationwide. Therefore, other factors need to be considered.
One of the possibilities is that the popularity of sports has decreased annually. Table 4 shows changes in the number of youth baseball teams, players, and players per team. Data on baseball were from official websites of the Nippon Junior High School Physical Culture Association [42] and Japan High School Baseball Foundation [43]. Since data from 1993 were not available, we used data from 1994. As a result, the number of youth players per team at junior high school clearly decreased in 2018. The numbers of teams at high school (hardball and rubber ball) were almost the same for 25 years. Therefore, we considered that the decrease in the OR for professional baseball players was partly related to the decrease in the popularity of baseball, especially in junior players. In contrast, youth baseball players who played at junior high school might tend to continue to play at high school. Katsumata et al. [3], focusing on Japanese youth baseball players in recreational levels at elementary school, junior high school, and high school, showed that significant RAEs were noted in recreational junior high school and high school players, but not in elementary school players, and the effect size became larger with increasing grade (0.063 in elementary school players, 0.151 in junior high school players, and 0.227 in high school players). Their data suggest that relatively younger players may gradually drop out from playing baseball even as recreational players. Delorme et al. [5] reported that the relative age was associated with sport dropout. They showed that relatively younger players in categories of 9-10 years old, 11-12 years old, and 13-14 years old tended to drop out from basketball, compared with relatively older players. Indeed, the present study did not directly elucidate the relationships between these backgrounds and effect sizes in professional baseball players; further studies are necessary to clarify the detailed mechanisms.
Table 4: Changes in the number of youth baseball teams, players, and players per team
|
Year
|
Team (n)
|
Player (n)
|
Number of players
|
|
per team (n)
|
Junior high school
|
1994
|
8702
|
-------
|
-------
|
(rubber ball)
|
2001
|
8391
|
321692
|
38.3
|
|
2010
|
8919
|
291015
|
32.6
|
|
2018
|
8384
|
166800
|
19.9
|
|
|
|
|
|
High school
|
1994
|
4104
|
142481
|
34.7
|
(hardball)
|
2001
|
4208
|
149622
|
35.6
|
|
2010
|
4115
|
168488
|
40.9
|
|
2018
|
3971
|
153184
|
38.6
|
|
|
|
|
|
High school
|
1994
|
627
|
14373
|
22.9
|
(rubber ball)
|
2001
|
549
|
12892
|
23.5
|
|
2010
|
482
|
11014
|
22.9
|
|
2018
|
428
|
8755
|
20.5
|
Next, we have to consider why the effect size of RAEs among soccer, baseball, and volleyball players decreased over time. As mentioned above, it is unlikely that efforts to reduce the RAEs were made nationwide. Therefore, other factors need to be considered. We proposed the acceleration of the age of peak height velocity (APHV) over time. Hermanussen et al. [44] reported that Japanese APHVs from 1948 to 2003 accelerated from 14.07 to 12.03 years old for boys and from 11.80 to 9.92 years old for girls. Yokoya & Higuchi [45] also reported that the Japanese APHV between 2006 and 2013 was 11.79 years old for boys and 9.55 years old for girls. Since these data cover up to 2013, the latest values of APHA should show even more acceleration. We hypothesized that if biological maturation through secondary sexual development is accelerated year by year, the timing for reducing RAEs might be more accelerated. In other words, the differences in physical performance, including, aerobic power, muscular strength, endurance, and speed, between relatively older and younger children might be reduced or masked in adolescents. Indeed, no significant RAEs in height were observed among baseball players in recreational high school [3]. In future studies, it will be necessary to examine the relationship between the acceleration of APHV and RAEs. On the other hand, the magnitude of the RAEs only among basketball players remained unchanged over time. The reason remains unclear, but we also considered that the popularity of basketball is associated with the unchanged RAEs. The Japan Professional Basketball League (B-league) only started in 2016, although there were semi-professional basketball leagues in Japan before 2016. The start of the new professional league may have affected RAEs of basketball players. The issue of RAEs in basketball players should be resolved in the future, and it would be needed to examine whether RAEs in basketball change with time. If basketball becomes more popular, RAEs might be observed in the future. To prevent this, it is necessary to promote the awareness and understanding of adults (guardians, coaches, associations, etc.) for RAEs involved in young basketball players.
As a limitation of the present study, our finding might be specific to Japan, because many activities, sports-related or academic, are based on a unique cutoff date (April 1st), which is not the case in other countries. Data on RAEs with an historical analysis should be examined in other countries.