Fibre-specific responses to endurance and low volume high intensity interval training: striking similarities in acute and chronic adaptation.
Scribbans et al. (2014)
In recent years High Intensity Interval Training (HIIT) has become a phenomenon in both the science and fitness community. Several studies and organizations have suggested HIIT provides numerous benefits, many of which can be achieved to a greater extent than regular continuous endurance exercise in a much shorter time period. These positive adaptions occur at both a cellular and whole body level, with adaptations including mitochondria biogenesis and fiber type alterations.
The present study had a unique design examining both the acute and chronic adaptions to HIIT and its counterpart, steady state cardiovascular (SS). Split into 2 studies, the acute experiment compared specific fiber and whole muscle alterations. In experiment 2, they measured specific fiber and whole body markers of anaerobic and aerobic capacity over a 6 week period.
In the acute trial they recruited 6 active and healthy participants in a cross-over design. Participants reported to the lab in a fasted state and performed either 8x 20 second intervals at 170% VO2 max with 10 seconds rest or 30 minutes continuous exercise at 60% VO2 max. Muscle biopsies were taken immediately pre and post exercise, showing a significantly decrease in muscle glycogen of type 1 & 2 muscle fibres and an increase in p-ACC.
In the 6 week experiment 19 active participants were matched for VO2 peak and assigned either HIIT or SS. Results showed a similar increase in aerobic capacity, fibre type specific oxidative and glycolytic capacity, glycogen and muscle triglyceride stores along with whole muscle capillary density. However, in the HIIT there were also positive adaptations for anaerobic performance and estimated muscle glycolytic capacity.
Interestingly, despite a much lower work load the glycogen content of both type 1 and 2a fibres were reduced to a similar extent. In contrast to other studies both groups found a reduction in type 2 a and type 2 x fibres. This is interesting as other data in HIIT has found an increase in the fast twitch fibres and a reduction is slow oxidative fibres. Although the reasons for this are less clear, it may be dependent on study design, as this used a very short rest period or other variables such as participants experience / training level.
In summary, although both groups improved many markers of performance, these were increased to a greater extent in the HIIT group. As with other studies, these findings are quite remarkable when considering they exercised for less than 10% of the time that SS trained for (2 minutes 40 seconds vs 30 minutes). Further, HIIT improved both anaerobic and aerobic capacity, this may be highly significant for many athletes and team sports which consist of a mixture of sprints and SS exercise during training / matches.
This paper adds further support to the efficacy and ecological validity of HIIT, being highly time efficient and providing numerous health and performance benefits. One limitation of this study is the use of only “active individuals” rather than highly trained athletes, although other research has supported the benefits of HIIT in a well trained population.
How To Perform HIIT For Fat Loss:
Adding in HIIT is simple. Most aerobic exercises can be performed as HIIT sessions, just perform the exercise at full intensity then have a complete rest or active rest such as walking / spinning a bike.
Experiment with these work to rest ratios:
- 10 second sprint, 30 second complete rest.
- 20 seconds sprint, 40 seconds complete rest.
- 30 seconds sprint, 60 seconds complete rest.
- 45 seconds sprint, 90 – 120 seconds complete rest.
The most common type of sessions include:
- Spin Bike
- Hill Sprints