Unchanged content of oxidative enzymes in fast‐twitch muscle fibers and V ˙ O 2 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 ( V ˙ O 2 ) response during intense ( INT ), but not moderate ( MOD ), exercise and reduce theV ˙ O 2slow component and muscle metabolic perturbation during INT . PulmonaryV ˙ O 2kinetics was determined in eight trained male cyclists ( V ˙ O 2 ‐max: 59 ± 4 (means ±  SD ) mL min −1  kg −1 ) during MOD (205 ± 12 W ~65%V ˙ O 2 ‐max) and INT (286 ± 17 W ~85%V ˙ O 2 ‐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 ( μ mol 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 .V ˙ O 2kinetics 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 changeV ˙ O 2kinetics.