PURPOSE The issue of the Relative Age Effect (RAE) has been studied in the theory of sports for more than 30 years. Most studies concentrate on team sports, while the area of some individual sports like swimming can be considered still underexplored. METHODS The aim of our study was to verify the RAE in young elite swimmers (n = 198) who participated in Czech Republic U14 Championship (1) in male and female samples (2) according to swimming disciplines and distances (3) and performance (times in individual disciplines) between individual quartiles / semesters of birth. The analysis was performed with the use of adequate statistical (chi-square test, Kruskal-Wallis H test, Mann-Whitney U test) and effect size (effect size w index, eta-square test, effect size r index) tests.RESULTS The results showed a different intensity of RAE sex-differences (male: w = 0.033; female: w = 0.006). In the division by the swimming disciplines and swimming distances, statistically significant values with large effect size were found in males in 50 m freestyle, 200 m individual medley, 100 m butterfly and 200 m butterfly. However, this did not apply for girls. Analysis of differences in performance showed a significant difference between the dependent variables (sex, distance, discipline) by different independent variables of quartile / semester of birth with large effect size only in cases of male 100 m breaststroke and female 200 m individual medley.CONCLUSIONS The issue of RAE should be circulated among the coaches working with youth, athletes, sports organizations, but also parents of athletes in order to avoid the termination of actively spent time or drop-outs.

The Relative Age Effect (RAE) is phenomenon that strongly influences youth sport development.The RAE refers to a disproportionately high percentage of athletes born early in the calendar selection year. Number of studies, especially in team sports, has proved this fact. However, there is still a lack of studies in individual sports, such as swimming, which makes it impossible to have a sufficient understanding of the RAE problem in a given sport. The aim of our study was to verify RAE in young elite swimmers (n = 202) who participated in Czech Republic U12 Championships (1) in male and female samples (2) according to swimming disciplines and distances (3) and performance (times in individual disciplines) between individual quartiles /semesters of birth. The analysis was performed with the use of adequate statistical (chi -square test, Mann-Whitney U test) and effect size (effect size index w and effect size index r ) tests. The analysis showed a significant RAE only in the whole sample (p = 0.015, w = 0.228, ES = medium), in sex distribution RAE was not present. A significant RAE, in the division according to swimming disciplines and swimming distances, was found in males in the breaststroke /100 m/ (p = 0.001, w = 0.763, ES = large), breaststroke /200 m/ (χ 2 (3) = 13.196, p = 0.004, w = 0.663, ES = large) and in the individual medley /200 m/ (χ 2 (3) = 12.214, p = 0.007, w = 0.638, ES = large). In females, there was a significant RAE in freestyle /50m/ (p < 0.001, w = 0.774, ES = large) and freestyle /100m/ (p = 0.001, w = 0.751, ES = large). Analysis of performance differences between research categories (Si ) in the form of time in disciplines, showed a statistically significant difference between the dependent variables (sex, distance, discipline) by different independent variables (Si ) only in case of female sample: freestyle /100m/ (p = 0.011), individual medley /100m/ (p = 0.028), butterfly /200m/ (p = 0.015) and individual medley /400m/ (p = 0.002).