It is already known and understood that endurance training induces a number of metabolic adaptations in the skeletal muscle. The question sport scientists are now asking is whether high-intensity interval training (HIIT) has similar benefits.
Endurance training is known to give rise to mitochondrial biogenesis and the enhanced capacity to oxidize fuels like glucose and fats, which leads to improved metabolic health as well as reduced risk for chronic disorders. Among others, Gibala et al. have shown that HIIT can generate the same physiological adaptations despite a far lower volume of training.
A study to test the potential of HIIT was conducted by Gibala et al. using a safe training method as an alternative to the majority of studies that have looked at the ability of HIIT at "all out" intensity, such as maximal cycling used in the Wingate tests, to induce mitochondrial biogenesis.
The study involved seven healthy male participants aged 20 to 22. Though active, exercising around two or three times a week, the subjects were not participating in a structured training program. Six exercise sessions were spread out over two weeks. During the first two training sessions, participants completed eight intervals. This was increased to 10 during the following two sessions and finally ended at 12 for the last two sessions.
After a three-minute warm-up, participants completed 60-second high-intensity intervals at a workload close to their VO2 max. These were interspersed with recovery periods of low-intensity cycling lasting for 75 seconds.
Researchers found that the two weeks of HIIT led to similar results to two weeks of endurance training in regard to performance as well as maximal activity and protein content of mitochondrial enzyme cytochrome c oxidase (COX). For example, maximal activity of COX increased by 29%, and regulators of mitochondrial biogenesis, such as the protein content of PGC-1a, elevated on average 24%; SIRT1, an activator of PGC-1a and mitochondrial biogenesis, increased on average 56%. Low volume HIIT was also found to induce improvements in markers of metabolic control and vascular endothelial function just as in endurance training.
The implications of the study are that it is possible to use HIIT as a time-efficient strategy to induce mitochondrial biogenesis, other skeletal muscle metabolic adaptations, and functional exercise capacity.
As one of the most common reasons for not exercising sufficiently is lack of time, these findings are very significant for anyone looking to improve their fitness to the same extent as through endurance training, but with a lower training volume.