In Defence of “Cardio”

The title of this article is somewhat misleading but, as I will attempt to illustrate, the word cardio has been bastardised to such an extent that people all over the world are being deterred from losing body fat and getting healthier using activities that are traditionally classed as “cardio”.

First things first, though – I need to make it crystal clear. I love HIIT! I love sprints and I love Tabata. All of these are a staple of my own training and my clients’ programmes. This post is not an attack on interval training, it’s a clarification of a few things that I think have become blurred.

As I sit here, some 50 minutes after I finished my 5k run, and 30 minutes since I slumped on the sofa to watch Dragon’s Den and now (as I write) Full Circle with Michael Palin, my heart rate remains elevated at 84 BPM. The reason I am monitoring my heart rate is to test a theory that is related to something that has been troubling me for a few months now – is “cardio” really counter-productive to fat burning?

For years, fat burning was thought to be most active whilst training in the “Fat Burning Zone” (60-70% max heart rate (MHR) ). More recently, however, “cardio”, as a means of body fat reduction, has become almost a swearword in the fitness world. Two of the main arguments against using cardio as a fat reduction method go something like this:

  • The body sees prolonged periods of cardio as a threat and therefore holds on to fat reserves.
  • Although, compared to HIIT (High Intensity Interval Training), a greater number of calories are burned during exercise, the EPOC (Excess Post-exercise Oxygen Consumption) effect generated by higher intensity exercise, and which can continue to burn calories for 12-48 hours after the cessation of exercise, leads to a greater total energy expenditure.

At this point it is, perhaps, useful to define exactly what “cardio” is.

Cardio is short for “cardio vascular exercise” which is synonymous with aerobic exercise. Aerobic means “with oxygen”, which means that the body primarily uses oxygen as an energy source. According to the Haskell and Fox formula, aerobic exercise takes place at 70-80% MHR. Note that this is higher than the Fat Burning Zone, which points to the first discrepancy in the argument against cardio as a method of fat reduction.

POINT 1: The fat burning zone is 60-70% MHR. The cardio zone is 70-80% MHR. They are different levels of intensity. If you’re going to attack anything, at least attack the right thing.


So, let’s come back to my run. I ran 5k in 22 minutes (15 seconds outside my personal best). My average heart rate for the run was 165 BPM, with a MHR of 186 BPM. Using the formula 220 – age (37) = MHR, my MHR is 183. This represents 90% of my MHR. From experience, however I’d say my true MHR is about 190 BPM, which means that 165 is 86% of my MHR. Therefore, by either calculation, my 22 minute run was done at 85-90% MHR (the anaerobic zone). So, let’s now look at the anaerobic zone.


Anaerobic means “without oxygen”, which means that glycogen stored in the muscle is the main source of fuel (as opposed to fat, which is used in the fat burning zone, and oxygen, which is used in the aerobic zone). According to the much lauded Poliquin Group, who are renowned experts on body composition and fat loss, anaerobic exercise is by far better for fat loss. And herein lies the problem, according to the Poliquin Group and a huge proportion of trainers and experts, anaerobic exercise means sprints, exclusively, and not any other kind running. So how come I’ve just spent the majority of 22 minutes in the anaerobic zone? Simple, as I eluded to in the opening sentence of this article, people have a tendency to take an extreme and often incorrect view of what something means.

POINT 2: Anaerobic doesn’t just mean sprints; it is any exercise that falls within the 80-90% MHR zone.

The question then arises: Why has the incorrectly termed “cardio” exercise and longer activity that doesn’t fall into the sprints or HIIT category become so maligned? I think the answer lies in the number of people who tread the pavement at a pedestrian pace or spend 60 minutes on the cross trainer, in the hope of getting a six pack or a body like a cover model without approaching anything like the anaerobic zone. The fact that they are doing what most people think of as “cardio” has led to a blanket view that any form of running, swimming, biking, or whatever, that lasts for longer than 45 seconds is ineffective at burning fat or generating the EPOC effect. The fact that my heart rate monitor says 73 BPM when I am lying on my sofa chilling and writing this, even though it’s 2 hours after I finished my (highly intense) run and my resting heart rate is about 48 means one of three things – either:

A, I have a heart problem (of which I show no symptoms).

B. I’m classed as unfit because my heart rate still hasn’t returned to normal (I’m actually in pretty decent shape – my resting heart rate is around 48 BPM and I’m fit enough to run a 22 minute 5k and 44 minute 10k without really running very much at all as I’m not a “runner”).

C, My 22 minute run was at sufficient intensity that it went into the anaerobic zone for long enough to generate a decent EPOC effect, of which my elevated heart rate is a sign.

POINT 3: It’s not the activity (such as running) or the duration (such as 10-30 seconds) that classifies an exercise as anaerobic, it’s the intensity at which the exercise is performed.

The point above raises a further question: Is it better to do 10 x 10 second sprints at 100% MHR than it is to do 22 minutes running at 85% MHR? In answering this it is, perhaps, useful to look towards the doyen of interval training – Professor Izumi Tabata.

The Tabata Protocol is a form of HIIT that was devised by the eponymous Professor Izumi Tabata. It involves working at maximum intensity for 20 seconds, with a 10 second rest period – repeated 8 times to give a total of four minutes. Professor Tabata’s study compared two sets of athletes – one group training at 170% VO2max and another group training at 70% VO2max. According to Professor Tabata “Tabata not only burns the same calories in four minutes as an hour of steady-state exercise (biking or jogging), but there’s also a significant “after burn” effect, where an additional 150 calories are being burned up to 12 hours after you leave the gym”. But here’s the rub, 70% VO2max for me would be around 145-155 BPM (the aerobic zone). Moreover, Tabata’s research compared “Effects of moderate-intensity endurance and high-intensity intermittent training on anaerobic capacity and VO2max“, so not only was the study not primarily concerned with body fat reduction (the main argument for using Tabata as a training method put forward by personal trainers), the study didn’t actually compare the exercise intensity level that is the most efficient at burning fat – exercise undertaken in the Fat Burning Zone.

So, the answer to the question: Is it better to do 10 x 10 second sprints at 100% MHR than it is to do 22 minutes running at 85% MHR? is probably no. I emphasise probably, but using Tabata’s research to justify a “yes”, doesn’t really hold water. It might, however, be better for fat loss and increasing VO2max to do 10 x 10 second sprints at 100% MHR than it is to do 60 minutes of running at 60-70% MHR. Having said that, longer runs can be beneficial for increasing fitness sub-maximally as it can be hard, especially for unconditioned exercisers, to push themselves sufficiently to get their heart rate up high enough due to lack of strength, as opposed to heart rate, becoming the trigger for failure.

Does this mean that I’m anti-sprints and pro-long distance running? Not at all! I agree that mindless pavement pounding and jogging for the sake of weight loss is probably counter-productive after the first 6-12 weeks or so. I also agree that high intensity training is key to both fitness and fat loss (the vast majority of my training with clients would fall into the high intensity category). In conclusion, what I’m arguing is that, as a profession, we need to clarify what we mean by “cardio”. 25 minutes of all out running at 85% MHR is a totally different proposition to 60 minutes of jogging / cross trainer at 60% MHR. With blanket statements such as “cardio is bad” (notwithstanding the fact that “cardio” is a level of intensity NOT AN EXERCISE) we are sending clients and the public down the wrong path. I have seen countless overweight people who think they can do 8 hill sprints three times a week and get a cover model body when they would make more progress with three 15-20 minute threshold runs. But this is not a “this vs that” article, it’s an article about balance and about defining what we mean so that we can be clear in both our own thinking and in what we espouse to our clients and the public.

It’s not steady state that’s the issue, it’s steady state at low intensity that’s the issue. So, let’s champion anaerobic exercise, not sprints. Let’s champion intensity, not duration. If we must bash something, let’s bash “The Fat Burning Zone”, not running. Let’s be clear, it’s not activities that are in question, it is the intensity of those activities!

N.B. My heart rate is now 65 BPM, 3 hours after my run (my resting heart rate is about 48 and my sitting and relaxing heart rate is 52ish)

Danny Sroda is owner and Lead Trainer at Reach Corporate Fitness for Business.