Skeletal Muscle Metabolic Adaptations in Response to 6 Weeks of High-Intensity Interval Training

Abstract

PURPOSE: There is sparse data in regards to skeletal muscle metabolic adaptations that occur in response to high intensity (80-100% VO2max) interval training (HIIT). Repeated short bouts of exercise at this intensity challenges both the aerobic and anaerobic ATP producing pathways and leads to an increased capacity of these pathways. We investigated the adaptive responses in the skeletal muscle's aerobic capacity and ability to distribute pyruvate to oxidative phosphorylation during high intensity exercise following 6 wks of HIIT, and hypothesized that there would be improved sensitivity in the regulation of PDH and reduced reliance on glycogenolysis and PCr. METHODS: 8 untrained but recreationally active subjects trained 3 d/wk for 6 wks on a cycle ergometer following a VO2max test and time trial to exhaustion (TTE) at 90% VO2max, which were repeated after training. Workouts consisted of ten 4 min bouts at 90% VO2max separated by 2 min of rest. This equated to 12 hours of training over 6 wks. Training power outputs (PO) were increased to maintain heart rate at ∼95 % of maximum. Muscle biopsies were taken during the TTE at rest, 5 min and exhaustion (EX). RESULTS: PO increased by 21% over 6 wk (209 ± 47 to 252 ± 51 W). Following training, VO2max increased by 9% (3.29 ± 0.24 to 3.58 ± 0.26 l/min) and TTE by 111% (7.1 ± 0.2 to 15.0 ± 1.1 min). The maximal activity of citrate synthase increased by 26% (18.4 ± 1.4 to 23.1 ± 1.2 mmol/min/kg wet muscle) and β-hydroxyacyl-CoA dehydrogenase increased by 29% (15.8 ± 1.1 to 20.3 ± 0.9 mmol/min/kg). PDH total increased by 23% (2.54 ± 0.21 to 3.09 ± 0.29 mmol/min/kg). Activation of PDH (PDHa) at 5 min in the TTE did not change with training (Pre, 2.32 ± 0.24; Post, 2.39 ± 0.18 mmol/min/kg) but was higher at EX (Pre, 1.94 ± 0.20; Post, 2.55 ± 0.27 mmol/min/kg wet muscle). The rate of glycogenolysis in the first 5 mins of TTE decreased (34.1 ± 4.8 to 23.3 ± 3.5 mmol/min/kg dry muscle) as did PCr utilization (12.5 ± 0.7 to 10.5 ± 1.1 mmol/min/kg dry muscle) and lactate accumulation (22.4 ± 1.1 to 17.9 ± 1.6 mmol/min/kg dry muscle) following training. Free ADP (ADPf) was reduced at 5 min after training (366.1 ± 42.5 to 265.4 ± 34.1 mmol/kg wet muscle). CONCLUSIONS: Only 12 hours of HIIT over 6 wks resulted in impressive increases in the aerobic capacity of the skeletal muscle. The reduced levels of ADPf but similar PDHa post-training suggest an improved sensitivity in the regulation of PDH. This, coupled with attenuated glycogenolysis, resulted in a greater proportion of pyruvate directed to oxidative phosphorylation. A decrease in PCr utilization supports these findings of HIIT-induced enhancements in aerobic ATP production and endurance performance during very intense exercise.