INTRODUCTION: In swimming, Critical Speed (CS) is considered one of the key markers of aerobic capacity, which is important for monitoring training and competition. Recently, a 12x25m all out test was validated for CS determination in elite swimmers, a proxy of the 3 min all out test validated for other exercise modalities [1]. In young swimmers it may be pertinent to adjust the test to 10x25m so that the total duration approaches 3 min due to the lower maximal speed of the younger swimmers [2]. Our aim was to compare the CS calculated from the 10x25m with the traditional method using 200 and 400m efforts. METHODS: Twenty national level swimmers (12 male, 8 female, age 14.5 ± 0.7 years) performed three evaluations, with 48 h in between. Swimmers completed ten 25 m all out front crawl swims from a push start separated by 5 seconds rest, with a maximum of 2 underwater butterfly kicks, in a 25m pool. Subjects were told to swim as fast as they could on each 25 m repetition and hold nothing in reserve for the rest of the test. The CS10x25 was calculated as the mean speed of the slowest two 25 m efforts in the last four repetitions [1]. In the second and third session, swimmers performed, respectively, 200m and 400m maximal but constant efforts. CS400-200 was calculated from the slope of the regression analysis between the distances performed and the correspondent time [3]. T-test for paired samples was used to compare CS10x25 and CS400-200 and Bland Altman analysis and Concordance Corrrelation Coeficient (CCC) were used to determine the agreement between both methods. Significance was set at p<0.05. RESULTS: Despite a high correlation was observed (r=0.77; p<0.01), CS10x25 significantly overestimated CS400-200 (1.33±0.10 and 1.18±0.07 m·s-1, p<0.01, respectively). Bland Altman analysis showed that the mean difference between methods was 0.16 m·s-1 with wide limits of agreement (-0.03 to 0.29 m·s-1). CCC was 0.27, precision 0.77 and accuracy 0.36, which indicates a poor strength of agreement between the methods. CONCLUSION: Although the 10x25m is a test that mimic training sets and can be easily integrated in the swimming sessions, our results suggest that in young swimmers the 10x25 all out test overestimates the critical speed and can not be used interchangeably with the classical method based on 200 and 400m maximal efforts. Future studies may consider 5-6x50 to acknowledge the specific physiological responses of young swimmers.
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