High Intensity Interval Training is a mode of exercise popularised for its time efficiency and effectiveness at burning fat and increasing a number of other fitness markers, such as VO2 Max.
During a single HIIT workout, the trainee performs short bouts of all-out effort and intensity (between 80 per cent and 95 per cent of maximal effort), followed by periods of rest.
These rest periods can be the continuation of the exercise at a low intensity, or by passive recovery. And these periods of work and rest are repeated for a given period of time, often between 10 and 20 minutes.
HIIT is also popular because it can be performed on a number of different pieces of exercise equipment. These include treadmills and rowers, exercise bikes, elliptical trainers and stair climbers. It can also be designed to meet the needs of any trainee, irrespective of age, gender and fitness level.
Why is HIIT effective?
HIIT workouts are generally much shorter than steady state cardio in terms of workout time. The increased intensity however sees an elevated metabolism extend beyond the end of your workout, as your body repairs muscle tissue.
HIIT takes many different forms, with some debate over which protocols are best for particular fitness goals. But what are the specific features of a HIIT protocol that can be adjusted?
It’s common for trainees to vary the type of exercise, intensity of the work interval, duration of the work interval, intensity of the recovery interval, duration of the recovery interval, number of intervals and overall duration are just some of the adjustable elements.
What Does the Research Tell Us?
Researchers have analysed how the different work-to-rest ratio protocols and the intensity in these work periods influence physical responses. Recent research has sought to determine the impact of varying the work duration and also the ratio of work-to-rest periods during a HIIT workout.
Not surprisingly, work to rest ratios of 1:1 and 2:1 (work-to-rest), see the biggest increases in cardiovascular fitness gains, but many experts feel this is something that needs to be progressively worked towards, particularly if you are new to HIIT training.
Some have recommended commencing your HIIT training at 1:4 and building towards a 1:1, 2:1 and even 3:1 work-to-rest ratio over time.
A recent study sought to explore the differences of varying work intervals, while fixing the work-to-rest ratio at 1:1. Participants trained on the cycloergometer. They compared the outcomes of working in 30 second, 60 second and 90 second blocks, with equal rest periods across a 10-minute HIIT workout.
What they uncovered was that there was no significant difference in aerobic capacity improvements irrespective of the work duration. All participants increased aerobic capacity significantly across the test period.
This was a surprising finding. As stated by the researchers, you would expect exercises at these intensities, and the aerobic systems being under maximal stress for longer, would provide the most effective stimulus for increasing maximum oxygen uptake (VO2max).
However, the results suggest that even at the 30 second work intervals, the stimulus is sufficient for improved aerobic capacity.
What the researchers did find was that performance drop-off was most pronounced in the 90 second intervals. Your work rate across the workout is lower because of the drop in performance. So, if you’re looking to increase the volume of higher intensity effort, using shorter bouts of work will ensure this across the workout.
When shaping your HIIT workout, according to the research, a protocol of 2:1 and 1:1 is a good foundation for effective work rate and intensity. However, increasing the length of time of your work intervals won’t necessarily see you perform at higher intensities across the workout.
Shorter intervals of 30 seconds and even 60 seconds will see you performing maximally and maintaining higher intensities across the work periods, and therefore, across the workout.