Original ResearchEcological Validity and Reliability of an Age-Adapted Endurance Field Test in Young Male Soccer PlayersCastagna, Carlo1,2; Krustrup, Peter3; D'Ottavio, Stefano2,4; Pollastro, Carlo5; Bernardini, Andrea2; Araújo Póvoas, Susana C.6Author Information 1Fitness Training and Biomechanics Laboratory, Italian Football Federation (FIGC), Technical Department, Coverciano, Florence, Italy; 2Faculty of Medicine and Surgery, School of Sports and Exercise Sciences, University of Rome “Tor Vergata,” Rome, Italy; 3Department of Sports Science and Clinical Biomechanics, University of Southern Denmark, Odense, Denmark; 4Women's National Team, Italian Football Federation (FIGC), Rome, Italy; 5University of Pisa, Pisa, Italy; and 6Research Center in Sports Sciences, Health Sciences and Human Development, CIDESD, ISMAI, University Institute of Maia, Maia, Portugal Address correspondence to Dr. Carlo Castagna, email@example.com. Journal of Strength and Conditioning Research: December 2019 - Volume 33 - Issue 12 - p 3400-3405 doi: 10.1519/JSC.0000000000002255 Buy Metrics Abstract Castagna, C, Krustrup, P, D'Ottavio, S, Pollastro, C, Bernardini, A, and Araújo Póvoas, SC. Ecological validity and reliability of an age-adapted endurance field test in young male soccer players. J Strength Cond Res 33(12): 3400–3405, 2019—The purpose of this study was to examine the reliability and the association with relevant match activities (ecological validity) of an age-adapted field test for intermittent high-intensity endurance known as Yo-Yo intermittent recovery level 1 children's test (YYIR1C) in young male soccer players. Twenty-eight young male outfield soccer players (age 11.1 ± 0.9 years, height 142 ± 4.4 cm, body mass 37.0 ± 5.9 kg) with at least 2 years of experience in soccer competitions were tested twice using YYIR1C and an age-adapted competitive small-sided game (i.e., 9v9), 7 days apart in a random order. The YYIR1C performance showed an excellent relative (intraclass correlation coefficient = 0.94) and a good absolute reliability (typical error of measurement as percentage of the coefficient of variation = 5.1%). Very large and significant associations were found between YYIR1C performance and match high-intensity activity (r2 = 0.53). Large correlations were found between YYIR1C and match sprinting (r2 = 0.42) and high-intensity metabolic power (r2 = 0.46) distances. Match total distance was largely associated with YYIR1C (r2 = 0.30). The results of this study showed that YYIR1C may be considered a valid and reliable field test for assessing intermittent high-intensity endurance in young male soccer players. Because of the relevance of aerobic fitness in youth soccer, future studies testing the sensitiveness of YYIR1C are necessary. Copyright © 2019 by the National Strength & Conditioning Association.