RESUMEN
OBJECTIVE: This article aims to conduct a comparison of swimming performance during short course national championships (25-m) from 2019 and 2020 (before and during the pandemic). MATERIALS AND METHODS: The data points from the championships will be compared for 5 European countries: Poland, Spain, Russia, Turkey, and Denmark. Times achieved in the finals were calculated by the analysis of variance. Post hoc pairwise comparison analysis was performed using Tukey's test. The analysis plan included the assessment of the main effects and the effect of interactions in the groups of women and men. The strength of the effect was expressed by the partial eta-squared ratio. RESULTS: Two main trends were observed in the results. The first was a greater variation in the results in the group of men than in women (12-5 differences). The second was the development and improvement of sports performance in symmetrical strokes (69% improvement in recorded times measured during the pandemic, e.g., in Poland, women on 50-m breaststroke and butterfly Δ = 0.52, p <0.001 and Δ = 0.32, p = 0.034, men on 50-m and 100-m butterfly Δ = 0.34, p = 0.003 and Δ = 1.21, p <0.001). CONCLUSIONS: The main conclusion of our analysis is that the pandemic influenced the development of sports performance in symmetrical techniques, i.e., in breaststroke and butterfly.