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If haven’t read part 1, you can find it here

What about HIIT?

Clearly, with all-out efforts (as with high-intensity interval training [HIIT]) you could conceivably exercise at higher intensities yet for a reduced total time, since you will often be working anaerobically and your blood lactate levels will rise steadily if there are limited recovery periods. Remarkably, given the recent promotion and marketing of HIIT in the UK, very little published research exists using this class format, although some rather unscientific claims have been made by extrapolating data from laboratory studies on elite athletes training on a cycle ergometer in the laboratory.

Even if working at a high intensity for three times as long were possible, the average total energy expenditure would be 720-1,080kcal

However, two relevant studies have been carried out recently in the US. One (by Olsen) is the subject of an abstract (i.e., not in full peer-reviewed journal format), while the other (by Porcari) is described in a letter to the editor of the Journal of Sports Sciences and Medicine. Olsen arrives at the conclusion that the average energy expenditure in her group of 13 men and three women was 54kcal for the typically recommended four-minute Tabata class protocol. Porcari’s research as described provides a little more detail. He took 16 trained volunteers (eight men; eight women) and used a laboratory-based assessment to arrive at heart rate and oxygen consumption values for each subject, later using these data to predict oxygen uptake and energy expenditure during the Tabata class. The workout consisted of four four-minute segments, with each segment comprising typical exercises (e.g., high-knee run, burpees, box jumps, jumping jacks). Subjects completed as many repetitions of each exercise as possible, followed by 10 seconds of rest and, after each four-minute segment, there was one minute’s rest. Blood lactate was measured at the end of each four-minute segment, along with ratings of perceived exertion (RPE) using the 6-20 scale. Energy expenditure was calculated using the HR/oxygen consumption equations created for each subject in the lab, with energy expenditure being calculated (indirectly) assuming a constant 5kcal/litre of oxygen consumed.

In this study, energy expenditure averaged 14.5kcal per minute with total energy expenditure ranging from 240-360kcal for the 20-minute workout. Heart rates averaged 86% of HR max, predicted maximum oxygen uptake was 74%, RPE was 15.4 and blood lactates averaged 12.1. In short, these were hard workouts for the subjects, with evidence of a progressively accumulating lactate concentration. Even if working at this intensity for three times as long were possible (and the lactate data suggests not), the average total energy expenditure the group would be 720-1,080kcal. This is entirely consistent with their maximum oxygen uptake data as provided by Porcari.


Of course, calories expended during exercise are not the only consideration, since advocates of HIIT argue that the simplistic calculations carried out (as above) ignore the ‘afterburn effect’ – or what is more technically known as excess post-exercise oxygen consumption (EPOC). Exercise recovery has two stages: the most immediate initial phase (‘getting your breath back’) lasting a few minutes followed by a slower second phase lasting a number of hours, which appears to be determined by exercise intensity and duration as the body restores itself to its pre-exercise level and adapts to the specific training stimuli associated with the workout.

Again, somewhat extravagant claims have been made for the number of extra calories that may be attributed to EPOC. It is possible that claims for ‘doubling’ the energy expenditure come from a misreading of some of the early research literature that looked at the potential 100% increase in EPOC with increased intensity. A recent review of the literature by the National Strength and Conditioning Association (NSCA) notes that EPOC values expressed in terms of the percentage of total kcal expended in the workout are modest, accounting for less than 15% of total energy expenditure. The NSCA concludes, “Many popular infomercials and programmes base their weight-loss and fat-burning claims on increased EPOC, yet EPOC values are modest compared to the actual energy expenditure from the exercise session itself, accounting for only 6-15% of the total energy cost. There is no evidence that EPOC alone can significantly contribute to weight loss.” Based on the current evidence, a reasonable rule of thumb would be to argue for any extra kcal associated with EPOC to be considered as being 10% of workout kcal. In the case of Porcari’s Tabata research described earlier, this would be 10% of 240-360kcal = 24-36 extra kcal per session.

Activity monitors and energy expenditure

Advances in technology have led to the increased availability of heart rate monitors and activity trackers that purport to measure your energy expenditure. None of them ‘measure’ energy expenditure; rather they measure something else such as heart rate and/or use a sensor (accelerometer) to track motion disturbances. This information is then used in an equation – an algorithm – perhaps alongside other personal data (e.g., weight, age, ‘fitness level’) to estimate likely energy expenditure. As a consequence, such monitors can never be entirely accurate. A recent study that looked at the calorie-counting accuracy of eight activity monitors reported an error range of 9.3-23.5% 8. Note that errors associated with algorithms that place greater emphasis on heart rate will probably over-estimate energy expenditure in classes that involve resistance training, sustained isometric contractions and exercises above shoulder height, since the heart rate response in each of these situations is no longer directly related to the oxygen cost of such activities. In short, heart rate is only a good indicator of exercise intensity/oxygen cost during whole-body, rhythmical aerobic exercise.

The extra energy expenditure associated with EPOC is probably around 10% of the total kcal expended in the workout


Can some people exercise at sufficient intensity for an hour to expend 1,000kcal? Yes – but these will be people who have a big, aerobic ‘engine’ and are motivated to work hard. For most people, it is likely that typical energy expenditure during class (or physical activity generally) will be more modest and probably around half of this value. The available evidence indicates that extra energy expenditure associated with EPOC is probably around 10% of the total kcal expended in the workout. Activity and heart rate monitors vary in their accuracy and may overestimate energy expenditure considerably if used during activities that include resistance training or sustained small muscle group or isometric work above shoulder level.