We discussed in part 1 of our article on HIIT versus cardio some of the background to the debate as to whether interval training or cardio is better while laying out the pros and cons of both forms of training. While there is a great deal of opinion as to which is better it seems that few proponents of either form of training use science as a basis to validate their opinions. We will begin the discussion of which is better by looking at some of the commonly stated arguments in favour of both approaches.

Does interval training’s EPOC make it superior to cardio?

Perhaps the most commonly stated justification for intervals being superior to steady state cardio is the claim that HIIT bouts can elevate metabolic rate for between 24 and 48 hours after the exercise bout depending on what exercise program the person is selling. By elevating metabolic rate for a long time after training you get an “afterburn” effect with intervals which does not apply to steady state cardio training. Does this claim hold up? A number of research studies (1,2,3,4) have shown greater fat loss shown by subjects performing interval work and has led to many people claiming intervals are superior.

A recent review[5] of the interval studies conducted to date demonstrated that while there is a greater EPOC effect with interval training it is not particularly significant amounting to just 6-15% of the actual total oxygen cost of the exercise. To put another way if you burned 100 calories through training EPOC would only amount to an additional 6-15 calories burned. Not exactly anything to get excited about. To quote the authors: “Notwithstanding the aforementioned, the earlier research optimism regarding an important role for the EPOC in weight loss is generally unfounded. This is further reinforced by acknowledging that the exercise stimuli required to promote a prolonged EPOC are unlikely to be tolerated by non-athletic individuals. The role of exercise in the maintenance of body mass is therefore predominantly mediated via the cumulative effect of the energy expenditure during the actual exercise.” By comparison with interval work, Laforgia’s group determined that the average EPOC from steady state cardio training amounts to around 7%. Based on the work of Laforgia HIIT can indeed burn more calories after training than regular cardio. We will turn next to how important this is likely to be.

Does the greater EPOC make intervals likely to burn more fat?

Using the calculations derived by Laforgia we can use the more generous estimate of a 15% EPOC with interval training to determine the amount of calories burned off via both steady state cardio and interval training. Although interval training leads to a greater calorie burn during the performance of the exercise than during steady state cardio this has to be set against the much lower calorie burn during the resting phase of interval training. Based on feedback from extensive testing the active component of interval work averages around 15 calories with around 5 calories during the resting phase. Assuming a work to rest protocol of 1:1 that equals 10 calories per minute something easily achieved using steady state cardio by anyone with a modicum of aerobic fitness. So the actual exercising phase of both HIIT and cardio will be the same under this example.

What happens when we add EPOC to the equation?

Table 1. Combined energy expenditure of exercise plus EPOC

	10mins	20mins	30min	40min	50min	60min
HIIT + 14% EPOC	100 + 14	200 + 28	300 + 42	400 + 56	500 + 70	600 + 84
Cardio + 7% EPOC	100 +7	200 + 12	300 + 21	400 + 28	500 + 35	600 + 42

Table 1 above shows clearly that with increasing time spent exercising energy expenditure during and after HIIT will be greater than with steady state cardio and the gap between the two increases over time. This is evidence for the greater fat loss benefits of HIIT work right?

Let’s look at the data again... While we can just about accept between 10-30 minutes of intervals (although 30 minutes of intervals would be taxing for even highly conditioned endurance athletes) the chances of anyone performing intervals over 30 mins is remote without leading to a marked decline in power output which would lead to less calories being burned during and after the exercise. By contrast, the notion of performing over 30 minutes of steady state cardio is relatively easy for most and, in fact, many could easily perform over 60 minutes of steady state cardio. If we then compare the calories burned during and after HIIT over the maximum realistic time span of 30 minutes versus the calories burned during and after cardio over 60 minutes, we see that cardio will end up burning off 642 calories versus the 342 calories burned during interval training. If we add to this the fact that steady state cardio can be performed on a daily basis versus a maximum of 3-4 times a week for HIIT it is clear that if intervals are superior to cardio then the EPOC is not responsible for this. Read Part 3 of our series on HIIT vs Cardio

References

1. EG Trapp et.al (2008): The effects of high-intensity intermittent exercise training on fat loss and fasting insulin levels of young women
2. EG Trapp et.al (2006): Metabolic response of trained and untrained women during high-intensity intermittent cycle exercise
3. A Tremblay et.al (1994):Impact of exercise intensity on body fatness and skeletal muscle metabolism.
4. I Tabata et.al (1994): Effects of moderate-intensity endurance and high-intensity intermittent training on anaerobic capacity and VO2max.
5. LaForgia J et. al. Effects of exercise intensity and duration on the excess post-exercise oxygen consumption. J Sports Sci. 2006 Dec;24(12):1247-64.