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 (math formula) response during intense (INT), but not moderate (MOD), exercise and reduce the math formula slow component and muscle metabolic perturbation during INT. Pulmonary math formula kinetics was determined in eight trained male cyclists (math formula-max: 59 ± 4 (means ± SD) mL min-1 kg-1) during MOD (205 ± 12 W ~65% math formula-max) and INT (286 ± 17 W ~85% math formula-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. math formula 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 math formula kinetics.
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