Purpose: The study's purpose was to examine the effects of a short-term period with intensified training or training cessation of trained soccer players on VO2 kinetics at 75% maximal aerobic speed, oxidative enzymes, and performance in repeated high-intensity exercise. Methods: After the last match of the season, 18 elite soccer players were, for a 2-wk period, assigned to a high-intensity training group (n = 7) performing 10 training sessions mainly consisting of aerobic high-intensity training (8 × 2 min) and speed endurance training (10-12 × 30-s sprints) or a training cessation group (n = 11) that refrained from training. Results: For the training cessation group, VO2 kinetics became slower (P < 0.05) with a larger time constant (t = 21.5 ± 2.9 vs 23.8 ± 3.2 s (mean ± SD, before vs after)) and a larger mean response time (time delay + t = 45.0 ± 1.8 vs 46.8 ± 2.2 s). The amount of muscle pyruvate dehydrogenase (17%, P < 0.01) and maximal activity of citrate synthase (12%) and 3-hydroxyacyl-CoA (18%, P < 0.05) were lowered. In addition, the fraction of slow twitch fibers (56% ± 18% vs 47% ± 15%, P < 0.05), Yo-Yo intermittent recovery level 2 test (845 ± 160 vs 654 ± 99 m), and the repeated sprint performance (33.41 ± 0.96 vs 34.11 ± 0.92 s, P < 0.01) were reduced. For the high-intensity training group, running economy was improved (P < 0.05), and the amount of pyruvate dehydrogenase (17%) and repeated sprint performance (33.44 ± 1.17 vs 32.81 ± 1.01 s) were enhanced (P < 0.05). Conclusions: Inactivity slows VO2 kinetics in association with a reduction of muscle oxidative capacity and repeated high-intensity running performance. In addition, intensified training of already well-trained athletes can improve mechanical efficiency and repeated sprint performance.
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