Relative age effect on male futsal national leagues of Brazil and Spain: impact of foreign players.

Introduction: The Relative Age Effect (RAE) encompasses variations associated with the age of athletes within the same category and is present in various sports, including futsal. Objective: This study aimed to compare the RAE in male professional players from the Brazilian National Futsal League (Liga Nacional de Futsal [LNF]) and the Spanish National Futsal League (Liga Nacional de Fútbol Sala [LNFS]), considering differences between native and foreign players. Methodology: The sample consisted of 401 players registered in LNF and 307 in LNFS during the 2020/2021 season. Players were grouped according to their birth quartiles (Q1-Q4) and semesters (S1 and S2). The goodness-of-fit chi-square test (χ2) was used in the statistical analysis of RAE, comparing differences between observed and expected distributions. Cramer’s V was calculated as an effect size measurement. Odds ratios were also calculated for the likelihood of being born in Q1 vs. Q4 and for the first vs. second semester of the year. Results: The main results indicated that RAE was observed only in Brazilian players from the LNF, where players born in Q1 were more frequent than those born in Q3 and Q4. Discussion: Considering the natives and foreigners in the LNFS no deviation from the expected birthdate distribution was observed. Conclusions: Therefore, the conclusion is that RAE is a phenomenon in Brazil's LNF. However, no evidence exists in the LNFS of Spain, whether considering natives or foreigners.