Unchanged content of oxidative enzymes in fast-twitch muscle fibers and [Image: see text] kinetics after intensified training in trained cyclists

The present study examined if high intensity training (HIT) could increase the expression of oxidative enzymes in fast-twitch muscle fibers causing a faster oxygen uptake ([Image: see text]) response during intense (INT), but not moderate (MOD), exercise and reduce the [Image: see text] slow component and muscle metabolic perturbation during INT. Pulmonary [Image: see text] kinetics was determined in eight trained male cyclists ([Image: see text]-max: 59 ± 4 (means ± SD) mL min(−1) kg(−1)) during MOD (205 ± 12 W ∼65% [Image: see text]-max) and INT (286 ± 17 W ∼85% [Image: see text]-max) exercise before and after a 7-week HIT period (30-sec sprints and 4-min intervals) with a 50% reduction in volume. Both before and after HIT the content in fast-twitch fibers of CS (P < 0.05) and COX-4 (P < 0.01) was lower, whereas PFK was higher (P < 0.001) than in slow-twitch fibers. Content of CS, COX-4, and PFK in homogenate and fast-twitch fibers was unchanged with HIT. Maximal activity (μmol g DW(−1) min(−1)) of CS (56 ± 8 post-HIT vs. 59 ± 10 pre-HIT), HAD (27 ± 6 vs. 29 ± 3) and PFK (340 ± 69 vs. 318 ± 105) and the capillary to fiber ratio (2.30 ± 0.16 vs. 2.38 ± 0.20) was unaltered following HIT. [Image: see text] kinetics was unchanged with HIT and the speed of the primary response did not differ between MOD and INT. Muscle creatine phosphate was lower (42 ± 15 vs. 66 ± 17 mmol kg DW(−1)) and muscle lactate was higher (40 ± 18 vs. 14 ± 5 mmol kg DW(−1)) at 6 min of INT (P < 0.05) after compared to before HIT. A period of intensified training with a volume reduction did not increase the content of oxidative enzymes in fast-twitch fibers, and did not change [Image: see text] kinetics.