Finally! An exercise routine for the truly time poor – a fitness routine (that works) that involves working out for just 6 minutes a week! That’s the sensational headline to a segment on ABC’s Catalyst which looked into the powerful benefits of High Intensity Interval Training (watch it here: Fit in 6 minutes a week – note: I wrote this article some time ago, as you’ll be able to see from the date of the segment, but it never made it up onto the blog).

At the time, I was working as a personal trainer, and I watched the segment with horror, struggling with the concept of a new era in personal training delivery, involving 6 minute time slots per client and the need to multiply my client base by 5 or 6 times to compensate!

I am exaggerating of course but the show does raise some interesting questions, like: “why am I stretching myself with hours of exercise per week in amongst a busy lifestyle when there is a formula that gets the job done so much more efficiently?” After all, the most productive and successful people often owe their results to asking this question about everything in their lives – how can I automate, get more efficient, and game the system to get the same or better result? It’s only naturally we apply this reasoning to exercise as well.

Well, naturally, the devil is in the detail when it comes to HIIT. To start with, I’ll quote the program’s voiceover, which summarises the gist of the whole segment:

“When you do low-intensity exercise, even though it might feel like it, you are not actually using all of your muscle. You’re only using the amount of muscle that you need to do that particular type of exercise. Whereas when we do very high-intensity exercise, especially sprinting, then you need to recruit all of your muscle fibres at once.”

This is one of the key quotes from the program because it describes how we recruit the 3 types of muscle fibres present in the body (fast-twitch, intermediate and slow-twitch) during exercise.

The difference between HIIT and moderate exercise

Slower, steady state exercise which is commonly the prescription for endurance athletes and the ‘moderate’ type of exercise which almost all governments recommend we do for 45-60 minutes, 3 times a week, works primarily the slow twitch muscle fibres, with the rest not doing all that much.

As exercise intensity increases we use more and more of the other muscle fibres. At full sprint, everything is being recruited. It is this full muscle recruitment which makes HIIT so powerful, especially with regards to mitochondria, the organelles within muscle cells which use oxygen to convert fats and glucose into ATP, the substance which ultimately provides the energy to contract muscle.

As with any form of exercise, the stimulus of the exercise triggers adaptations. Slower, steady state exercise stimulates greater mitochondrial density within slow twitch muscle fibres but HIIT stimulates these changes in all 3 muscle fibre types. The implication is that we train the whole muscle with HIIT and the whole muscle responds with adaptations.

HIIT in Practice

The headline for the segment is based on a personal experiment conducted by the show’s host. She performed 4-6 x 30 second sprints, with full recovery in between, 3 times a week for 15 weeks. Each workout actually took about 20 minutes, with 2-3 minutes of that being full effort sprints and pretty damn uncomfortable (total ‘work’ time per week as 6 minutes).

Over the 15 weeks she managed to improve VO2 Max (the maximum amount of oxygen the body is able to consume during exercise), resting heart rate, waist circumference and body fat percentage by not insignificant amounts.

VO2 Max is quite an important improvement in relation to general health given studies which demonstrate that the largest lifestyle risk factor for preventable lifestyle disease is not body fatness, alcohol or even smoking (these do count, of course)… No, it’s cardio-respiratory fitness (a.k.a. your ability to climb a flight of stairs without losing your breath).


All very impressive but I do feel compelled to make some qualifying statements about HIIT before we throw away the baby with the bathwater to take up this magical exercise formula. Here we go:

From nothing to something: When a person goes from very little activity to some form of activity, as the host in Catalyst did, there will definitely be health benefits – enhanced cardiovascular functioning, lower body fat, lower resting heart rate.

The host had an above average baseline level of fitness but, by her admission, had not regularly exercised for some time. Any activity is better than none, so there will always be health benefits when we start doing more activity. That is to say, she would have improved her health with any form of exercise, given she hadn’t exercised for some time prior. You don’t have to do HIIT to benefit, but HIIT is effective for some quick health improvements, especially when the baseline is low.

Intensity matters: The original study on the benefits of interval training was led by Japanese researcher Dr Izumi In what has now become a famous and staple workout protocol he subjected highly trained athletes to a regime of 20 seconds of work at 170% of VO2 Max, followed by 10 seconds of rest, for 7-8 rounds (up to 4 mins), 5 times per week over 6 weeks. This has become a staple routine because the test subjects in this study, who were already very fit people, achieved some pretty impressive results.

So, what is 170% VO2 Max? It is, simply put, insanely intense. Essentially the concept of HIIT is that, for X health and fitness result, your time investment is inversely proportional to your effort. So, if you want to follow the regimen that the Catalyst host followed (amounting to roughly 6 minutes of actual hard work per week), you will need to work insanely hard during those 6 minutes. Anything less will require more intervals, or more time spent in slow, steady state exercise.

This is so relevant if you’re an exercise novice because a very common challenge for the untrained person is properly gauging effort level against maximum capability. Unfortunately, the novice might be dismayed to find that, while they feel like their intervals are being performed at an 11 out of 10 effort level, they are in fact operating nowhere near 170% VO2 Max.

In the Tabata study, and even in the Catalyst experiment, being able to push sufficiently hard to elicit the observed results was possible due to an existing degree of physical condition. The results you get from HIIT are proportional to the level of intensity you can achieve, and this does require some half-decent underlying aerobic and strength condition.

Injury is a concern: The Catalyst host was of above average fitness to begin with, however, she did sustain a minor injury during her training regime which took her out of action for a while. Pre-existing physical condition is not only necessary to elicit the full benefit of HIIT, but it is also necessary to avoid injury.

HIIT works on volatility. The high degree of stress stimulus delivered by maximal effort elicits a strong adaptation response, in the same way that slower, steady state exercise elicits adaptation in a slower, more manageable way. When it comes to HITT, we are sailing close to the wind, because at a certain point, which is different for everyone and influenced by pre-existing fitness, the relationship ‘breaks’ – too much stress in a single bout of exercise causes injury, something which takes much longer to recover from.

It is entirely possible to push the body past its physical limits during maximal effort workouts, like red lining a performance car. Our capacity to deal with that stress improves the fitter we get.

There is more than one way to skin a cat: A case in point – one of the studies quoted by Catalyst quoted showed how enhanced mitochondrial functioning can improve the youthfulness of skin by thickening the underlying collagen layer.

Another study Catalyst quoted showed how mice were able to reverse the degenerative effects of a genetically engineered early ageing disease through enhanced mitochondrial functioning.

But both of these studies enhanced mitochondrial functioning with a typically endurance style exercise prescription, such as moderate exercise for 45 mins, 3 times a week.

The point is that it wasn’t HIIT which enhanced mitochondrial function in these studies, it was simply the stimulus of some kind of exercise.

Horses for courses: Following on from the last point, if you’re looking for fitness which is specific to any particular sport, then it pays to train by doing that sport.

Endurance athletes need high VO2 Max because this means they can move faster while relying on fat as a primary fuel source, a near limitless fuel source even if an athlete is operating at 4% body fat (what is considered the minimum for health). The closer an endurance athlete goes to their VO2 Max during a race, the more rapidly they’ll use glucose for fuel, which depletes quickly, usually faster than it can be replaced. So, it pays to set that VO2 Max threshold as high as possible.

But if HIIT is so effective at stimulating mitochondrial density in all 3 muscle fibre types, and thereby enhancing VO2 Max, why is that endurance athletes consistently develop VO2 Max levels higher than any other type of athlete from endurance style training?

The answer lies in the fact that there is more to bodily adaptations to training than just mitochondria. For instance, endurance training actually changes the percentage make-up of muscle fibre types in the body. An endurance athlete’s muscle mass may be up to 90% slow twitch fibres, while a sprinter’s or power lifter’s muscle mass may only be 25% slow twitch fibre. This is relevant because the different fibres are capable of different levels of mitochondrial density, and slow twitch fibres will develop much (much) higher densities. The most aerobically fit people in the world are the ones with a high percentage of slow twitch muscle fibres.

An endurance athlete trains for higher VO2 Max by stimulating mitochondrial development as well as preferential development of slow twitch muscle fibre. This is achieved by a very small volume of HIIT training and a high volume of lower intently training, which is a typical endurance training regimen.

Mitochondria are not the be all and end all: Any kind of exercise provides a stimulus to the body to build upon (or at least maintain) a given bodily structure. Bones, blood vessels, the heart, lungs, hormonal regulation, you name it and chances are there is some kind of benefit to be derived from exercise.

One thing that HIIT (and endurance training) do not do well is to pull forcefully on bones to produce movement against resistance. But weightlifting does, and when we do this we strengthen bones and muscles, which means we better stand up to the rigours of everyday life and our functional movement patterns are better. It also helps reverse some of the degenerative effects of ageing which HIIT and endurance training do not always address, such as bone and muscle atrophy.

This is why resistance training is a general (meaning applicable to all people) exercise recommendation for people as they get older.

The skinny on HIIT

There most certainly is a place for HIIT training, especially for producing health benefits for time poor people. It can be done anywhere – in a park, gym or at home, so it is fantastically convenient. But there are a couple of caveats, like the need for some kind of pre-existing fitness in order to avoid injury and also push sufficiently hard to benefit fully.

Also, long term health in a general sense requires a broader scope of training, which means the inclusion of strength/resistance training.