High Intensity Interval Training or HIIT is an exercise protocol that is often touted as a time-efficient method of training. But, how effective is it and what does it involve? In an effort to answer these questions a search of the scientific literature was undertaken for evidence of its benefits. Most of them point to quite impressive performance and health improvements (Sloth et al 2013) and one study in particular by Gibala et al (2006) examined their benefits against more traditional endurance training. The headline from this study was that six sessions of HIIT completed over two weeks yielded the same or slightly better performance gains as two weeks of traditional endurance training. It’s a fascinating study and one that is worth delving into for a better understanding of the benefits of HIIT.
In the Gibala study, two groups of eight healthy individuals were assigned either a HIIT or traditional endurance exercise protocol. The HIIT protocol was a Sprint Interval Training (SIT) session which consisted of repeated 30-s “all-out” efforts on a cycle ergometer. Subjects were encouraged to pedal as fast as possible throughout the 30s test. This was followed by a 4-min recovery period cycling at low cadence and against light resistance. The sessions were performed three times a week on alternate days (i.e., Monday, Wednesday, Friday) over 14 days. The number of intervals for each session from one through to six were as follows: 4,4,5,5,6,7. In contrast the control group were assigned six sessions of 90-120 minutes of continuous endurance cycling (ET) at 65% VO2peak.
The results from the study with respect to changes in exercise performance following training were impressive:
- The time required to complete a 750 kJ time trial (approx 25 miles) decreased by 10.1% and 7.5% in the SIT and ET groups, respectively. This translates to an average reduction from 61 minutes to 55 minutes for the SIT group
- There was an increase in mean power output during the 750 kJ time trial from 212 ± 17 to 234 ± 16 watts in the SIT group and from 199 ± 13 to 212 ± 12 watts in the ET group
- The time required to complete the 50 kJ test decreased by 4.1% in the SIT group (Post: 113 ± 6 vs Pre: 117 ± 6 s) and 3.5% in the ET group (Post: 122 ± 10 vs Pre: 115 ± 9 s)
- The mean power output during 50 kJ time trial increased from 435 ± 23 to 453 ± 25 watts in the SIT group and 416 ± 39 to 433 ± 40 watts in the ET group
The results for both protocols indicate the benefits of exercise, however, and this is really important the benefits of SIT arose from a total training volume significantly lower (~90%) than traditional endurance training (∼630 versus ∼6500 kJ). These data demonstrate that SIT is a time-efficient strategy to induce rapid adaptations in skeletal muscle and exercise performance comparable to endurance training.
For further information please click on the references below, the second of which reviews emerging evidence from recent studies into HIIT.