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.
Alex’s Notes: Chronic cardio or interval training? Both have their place and whichever one you chose will depend on your goals. However, high intensity interval training (HIT) has a distinct time advantage over endurance (END) training. That is, it has been shown numerous times to induce physiological adaptations common to cardiovascular training without the time requirement END demands. And despite the abundance of studies showing the benefits of HIT, its mechanisms are not completely understood and the muscle-specific adaptations have yet to be fully illustrated. The study at hand utilized two experiments, so to keep things organized it is best we look at them one at a time.
The purpose here was to see the muscle-specific effects of an acute bout of HIT compared to END. The HIT protocol consisted of eight 20-second intervals at 170% of VO2 peak separated by 10 seconds of rest eight times, for a total of four-minutes. During rest periods participants cycled against no load at a cadence of their choice. The END exercise protocol consisted of 30 minutes of continuous cycling at 65% of VO2 peak. The participants were six recreationally active males that didn’t do any aerobic exercise more than three times per week.
Both HIT and END significantly reduced type-I and type-IIA muscle glycogen in a similar manner, but neither had an effect on AMPK in either muscle type. The glycogen depletion can be seen as an indicator of muscle fiber recruitment during exercise, and you would expect glycogen to be reduced more significantly in the HIT protocol than the END protocol because it is higher intensity. Yet no difference was observed, and I would bet money that is because cycling at 65% VO2 is anything but low intensity, especially for 30 minutes. Now, according to the size recruitment principle, you will progressively recruit more type-I fibers until some threshold is reached at which point you will recruit type-II fibers. Since fiber recruitment was similar between treatments, take-away number one is that this threshold is below 65% VO2 peak.
Moreover, AMPK wasn’t different between groups, but the power output for the HIT group was significantly more. At submaximal intensities, AMPK increases with increasing intensity, and sprints really get AMPK ramped up. Thus, the brief HIT used in this study compared to the longer duration sprint study bring us to take-away #2 – exercise duration is an important factor for AMPK and related pathway activation.
With that in mind, we turn to experiment two, which looked at the fiber-specific adaptations of the previous training protocols performed 4 days per week for six weeks in 19 recreationally active persons (16 were male). Also, these participants were basically beginners with no structure training prior.
Unsurprisingly, all participants regardless of HIT or END similarly improved various parameters of aerobic capacity and exercise performance, and fiber-specific adaptations such as oxidative/glycotic capacity, glycogen and intramuscular fat storage, and capillary density were also similar between groups. In fact, the only difference was that HIT improved anaerobic exercise performance and whole muscle glycotic capacity more than END. Given the difference in training modalities, this would normally be expected, but it stands in contrast with the similar glycogen depletion from experiment 1.
Interestingly, both groups increased the ratio of type-I to type-II fibers, which is in stark contrast to other studies showing sprints to increase type-II fiber amount. It has been suggested that fiber contraction frequency influences fiber-type synthesis, with greater frequency increasing type-II fiber synthesis, and the current study had HIT and END cycle at the same speed with resistance being the determinant of intensity. This could explain the difference between these results and sprinting, something that requires maximal cycling and contraction rates.
To sum up, the results of this study suggest that 30 minutes of END exercise at 65% VO2 peak or 4 minutes of HIT at 170% VO2 peak induce comparable muscle fiber recruitment, changes in intramuscular signaling, and physiological adaptations of the muscle. Now, if we assume that exercise resulting in the largest degree of intramuscular signaling activation in the greatest amount of muscle fibers would result in the greatest degree of adaptation, then HIT has a clear advantage because it only took four minutes compared to 30. However, because exercise duration may be a key player in the signaling cascade, don’t overlook your chronic cardio. Perhaps, combine them. Do some brief intervals followed by some light jogging and end it with some tempo runs.