Abstract
The current study involved the completion of two distinct experiments. Experiment 1 compared fibre specific and whole muscle responses to acute bouts of either low-volume high-intensity interval training (LV-HIT) or moderate-intensity continuous endurance exercise (END) in a randomized crossover design. Experiment 2 examined the impact of a six-week training intervention (END or LV-HIT; 4 days/week), on whole body and skeletal muscle fibre specific markers of aerobic and anaerobic capacity. Six recreationally active men (Age: 20.7±3.8 yrs; VO2peak: 51.9±5.1 mL/kg/min) reported to the lab on two separate occasions for experiment 1. Following a muscle biopsy taken in a fasted state, participants completed an acute bout of each exercise protocol (LV-HIT: 8, 20-second intervals at ∼170% of VO2peak separated by 10 seconds of rest; END: 30 minutes at ∼65% of VO2peak), immediately followed by a muscle biopsy. Glycogen content of type I and IIA fibres was significantly (p<0.05) reduced, while p-ACC was significantly increased (p<0.05) following both protocols. Nineteen recreationally active males (n = 16) and females (n = 3) were VO2peak-matched and assigned to either the LV-HIT (n = 10; 21±2 yrs) or END (n = 9; 20.7±3.8 yrs) group for experiment 2. After 6 weeks, both training protocols induced comparable increases in aerobic capacity (END: Pre: 48.3±6.0, Mid: 51.8±6.0, Post: 55.0±6.3 mL/kg/min LV-HIT: Pre: 47.9±8.1, Mid: 50.4±7.4, Post: 54.7±7.6 mL/kg/min), fibre-type specific oxidative and glycolytic capacity, glycogen and IMTG stores, and whole-muscle capillary density. Interestingly, only LV-HIT induced greater improvements in anaerobic performance and estimated whole-muscle glycolytic capacity. These results suggest that 30 minutes of END exercise at ∼65% VO2peak or 4 minutes of LV-HIT at ∼170% VO2peak induce comparable changes in the intra-myocellular environment (glycogen content and signaling activation); correspondingly, training-induced adaptations resulting for these protocols, and other HIT and END protocols are strikingly similar.
Highlights
Endurance (END) training and interval training, consisting of either sprint interval training (SIT; repeated bouts of ‘all out’ exercise [e.g. Repeated Wingates]) or high-intensity interval training (HIT; repeated bouts of exercise below an all out intensity) result in comparable adaptations in both performance and skeletal muscle metabolism
All participants completed each exercise protocol at a work rate corresponding to 65.763.8% and 183.269.5% VO2peak for the END and low-volume high-intensity interval training (LV-HIT) exercise bouts, respectively
While no interaction effect was observed for either type I or type IIA fibres, a significant (p, 0.05) main effect of exercise for muscle glycogen content was present for type I (END–42%, low volume- (LV)-HIT–55%; Fig. 1B) and IIA
Summary
Endurance (END) training and interval training, consisting of either sprint interval training (SIT; repeated bouts of ‘all out’ exercise [e.g. Repeated Wingates]) or high-intensity interval training (HIT; repeated bouts of exercise below an all out intensity) result in comparable adaptations in both performance and skeletal muscle metabolism These include: improvements in maximal oxygen uptake (VO2peak) [1,2,3,4,5,6,7] and aerobic exercise performance [7,8,9], increases in markers of mitochondrial content, oxidative capacity [1,2,3,8,10], intramuscular glycogen [8] and triglyceride stores [3], capillary density [4,6,11], lipid oxidation [1,3,10], and reductions in substrate level (phosphocreatine and glycogen) phosphorylation [1]. While there is an abundance of literature demonstrating the efficacy of HIT at inducing adaptations, the mechanisms by which substantially lower doses of high intensity exercise induce adaptations typically associated with high dose END are not completely understood, similarities/differences in fibre-type specific adaptations induced by HIT and END have not been completely described
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