Fibre-Specific Responses to Endurance and Low Volume High Intensity Interval Training: Striking Similarities in Acute and Chronic Adaptation

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

Effects of endurance (END) and low-volume high-intensity interval training (LV-HIT) on anaerobic exercise performance and glycolytic capacity.

<p>A: Representative slides of α-glycerophosphate dehydrogenase (GPD) activity Pre and Post training (see Fig. 3C for corresponding fibre-type slides). B: Total work performed during all 4 Wingate bouts. C: Fibre-type specific fold change in GPD activity post-training compared to pre-training in type I, IIA and IIAX/IIX fibres. D: Absolute change in estimated glycolytic capacity. Bars represent 100 µm. AU, arbitrary units; µm, micrometre. †Significant (p<0.05) group by time interaction, *Significant (p<0.05) main effect of exercise, Significant (p<0.05) within group difference (derived from paired t-test).</p

Experiment 1 participant characteristics (N = 6), mean (SD) unless otherwise indicated.

<p>Note: LV-HIT mean power represents the average power output during the 8 intervals not including rest periods. BPM, beats per minute; cm, centimeters; kg, kilograms; min, minute; RER, respiratory exchange ratio; s, seconds; W, watts; yrs, years.</p

Effects of acute exercise on fibre-type specific glycogen depletion and phosphorylation of AMPKα.

<p>A: Representative slides of immunofluorescent fibre-type analysis (blue fibres are type I, green are type IIA, red/green are type IIAX/IIX) with serial sections of glycogen (periodic acid-Schiff staining) and p-AMPKα Thr¹⁷² content before (Rest) and immediately after (Ex) an acute bout of END or LV-HIT. B–C: Fold change in glycogen content from Rest and Ex in type I and IIA fibres. D–E: Fold change in p-AMPKα content from Rest and Ex in type I and IIA fibres. Bars represent 100 µm. AU, arbitrary units; µm, micrometre. *Significant (p<0.05) main effect of exercise.</p

Effects of 6 weeks of endurance (END) and low-volume high-intensity interval training (LV-HIT) on cardiovascular capacity and capillary density.

<p>A: Representative slides of capillarisation Pre and Post training. B: Absolute change in capillary density (capillaries/mm²). C: Change in O₂ pulse (mL O₂/beat). Bars represent 100 µm. mL, milliliter; mm, millimeter; O₂, oxygen; µm, micrometre. *Significant (p<0.05) main effect of exercise.</p

Effects of an acute bout of endurance (END) and low-volume high-intensity interval training (LV-HIT) on whole-muscle signaling.

<p>A–D: Fold change in p-AMPKα Thr¹⁷² (A), p-ACC Ser⁷⁹ (B) and p38 MAPK Thr¹⁸⁰/Tyr¹⁸² (C) content from Rest and Ex. D: Representative western blots, including loading control are also shown. AU, arbitrary units. *Significant (p<0.05) main effect of exercise.</p

Effects of training (LV-HIT) on aerobic exercise performance, fibre-type distribution and fibre specific oxidative capacity.

<p>Relative VO₂peak (A) and time to 500 kcal (B) at Pre, Mid, and Post for both END and LV-HIT groups are shown. C: Representative slides of immunofluorescent fibre-type analysis (blue fibres are type I, green are type IIA, red/green are type IIAX/IIX) with serial sections of succinate dehydrogenase (SDH) activity Pre and Post END or LV-HIT. D: Percentage of total fibre distribution of type I, IIA and IIAX/IIX fibres Pre and Post END or LV-HIT. E: Fibre-type specific fold change in SDH activity post-training compared to pre-training in type I, IIA and IIAX/IIX fibres. F: Absolute change in estimated oxidative capacity. Bars represent 100 µm. AU, arbitrary units; µm, micrometre. *Significant (p<0.05) main effect of exercise.</p

Experiment 2 participant characteristics (N = 19) for low volume high-intensity interval (LV-HIT) training (n = 10) and endurance (END) training (n = 9) at baseline (pre), mid-training (mid) and following 6 weeks of training (post); mean (SD) unless otherwise indicated.

<p>Note: *Main effect of training, p<0.05; ^†significant interaction, p<0.05. BPM, beats per minute; cm, centimeters; kg, kilograms; min, minute; mL, milliliter; RER, respiratory exchange ratio; s, seconds; W, watts; yrs, years.</p

Quantitative RT-PCR primer sequences.

<p>Note: GAPDH, glyceraldehyde 3-phosphate dehydrogenase; GCN5, general control non-derepressible 5; PGC-1α, peroxisome proliferator-activated receptor gamma coactivator 1 alpha; RIP140, receptor-interacting protein 140; SIRT1, silent mating type information regulation 2 homolog 1.</p