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KRUSTRUP, P., M. MOHR, T. AMSTRUP, T. RYSGAARD, J. JOHANSEN, A. STEENSBERG, P. K. PEDERSEN, and J. BANGSBO. The Yo-Yo Intermittent Recovery Test: Physiological Response, Reliability, and Validity. Med. Sci. Sports Exerc., Vol. 35, No. 4, pp. 697–705, 2003.To examine the physiological response and reproducibility of the Yo-Yo intermittent recovery test and its application to elite soccer.Heart rate was measured, and metabolites were determined in blood and muscle biopsies obtained before, during, and after the Yo-Yo test in 17 males. Physiological measurements were also performed during a Yo-Yo retest and an exhaustive incremental treadmill test (ITT). Additionally, 37 male elite soccer players performed two to four seasonal tests, and the results were related to physical performance in matches.The test-retest CV for the Yo-Yo test was 4.9%. Peak heart rate was similar in ITT and Yo-Yo test (189 ± 2 vs 187 ± 2 bpm), whereas peak blood lactate was higher (P < 0.05) in the Yo-Yo test. During the Yo-Yo test, muscle lactate increased eightfold (P < 0.05) and muscle creatine phosphate (CP) and glycogen decreased (P < 0.05) by 51% and 23%, respectively. No significant differences were observed in muscle CP, lactate, pH, or glycogen between 90 and 100% of exhaustion time. During the precompetition period, elite soccer players improved (P < 0.05) Yo-Yo test performance and maximum oxygen uptake (V̇O2max) by 25 ± 6 and 7 ± 1%, respectively. High-intensity running covered by the players during games was correlated to Yo-Yo test performance (r = 0.71, P < 0.05) but not to V̇O2max and ITT performance.The test had a high reproducibility and sensitivity, allowing for detailed analysis of the physical capacity of athletes in intermittent sports. Specifically, the Yo-Yo intermittent recovery test was a valid measure of fitness performance in soccer. During the test, the aerobic loading approached maximal values, and the anaerobic energy system was highly taxed. Additionally, the study suggests that fatigue during intense intermittent short-term exercise was unrelated to muscle CP, lactate, pH, and glycogen.