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RICO-SANZ, J., M. ZEHNDER, R. BUCHLI, M. DAMBACH, and U. BOUTELLIER. Muscle glycogen degradation during simulation of a fatiguing soccer match in elite soccer players examined noninvasively by 13C-MRS. Med. Sci. Sports Exerc., Vol. 31, No. 11, pp. 1587–1593, 1999.The purpose of this research project was to noninvasively determine individual muscle glycogen [Gly] degradation during a test intended to predict individual fatigue in intense soccer matches.The [Gly] of the calf muscles of 17 elite soccer players [age = 17.4 ± 0.8 (SD)] were measured with 13C-MRS before and after an alternating velocity test to exhaustion. Blood samples were taken before and 3 min after the test for determination of blood metabolites.Average muscle [Gly] was 135 ± 53 mmol·(kg wet weight)−1 before and 87 ± 27 mmol·(kg wet weight)−1 (P < 0.001) after exhaustion (42 ± 25 min). There was a high correlation (r = 0.87, P < 0.0001) between muscle [Gly] at rest and net muscle [Gly] utilized. There was also a more moderate correlation (r = 0.62, P < 0.01) between net muscle [Gly] used and time to exhaustion during the soccer-specific test. There was some evidence of correlation (r = 0.42, P = 0.09) between resting [Gly] and time to exhaustion. Plasma lactate increased (P < 0.001) from 0.8 ± 0.4 before the test to 2.5 ± 1.0 mmol·L−1 at exhaustion, whereas ammonia was raised (P < 0.0001) from 44.1 ± 10.3 to 89.7 ± 14.9 μmol·L−1. Similarly, plasma free fatty acids were elevated (P < 0.0001) from 148 ± 106 to 797 ± 401 μmol·L−1, and glycerol was increased (P < 0.0001) from 48.3 ± 17.7 to 182.2 ± 61.8 μmol·L−1. Insulin levels (11.9 ± 3.7 vs 11.7 ± 4.8 μU·mL−1) remained the same. Creatine kinase levels increased (P < 0.0001) from 486 ± 501 to 640 ± 548 μmol·L−1 after the test.We conclude that exhaustion during soccer-specific performance is related to the capacity to utilize muscle [Gly]. The results underline the importance of dietary counseling (glycogen loading and resynthesis strategies) and proper training to enhance the glycogen levels and glycogenolytic capacity of the players.