The Greatest Cardio Myths Exposed

Author: Lee Bell

February 6th 2019

In this article we break down some of the more popular cardio myths.

Whether your goal is fat loss or fitness, cardio is a great tool.

There’s a clear link between cardio levels and health, and a well-functioning aerobic system helps to reduce fatigue and even reduce your risk of life-threatening disease.

The problem is though, understanding cardio can be confusing.

With so much conflicting information out there it can be difficult to know exactly how to use cardio to get the best results.

Some say cardio is king whereas others think it should be avoided at all costs.

In this article we look at some of our reader questions and break down the science just for you. Want to know the biggest cardio myths and how to keep away from them?

We’ve got you covered…

#1. Is the fat burning zone best for cardio weight loss?

Many years ago, physiologists realized that the amount of fat you use for fuel during cardio exercise changed dependent on intensity. They found that while training above 80% of maximum heart rate used a higher proportion of stored carbohydrate for energy, lower intensities used more fat.

This is how the fat burning zone was created.

The theory was simple.

If working at 60-70% of maximum heart rate burned more fat as a percentage than any other heart rate, training at that intensity would lead to the greatest fat loss.

Makes sense, right?

All you’d need to do was exercise at a low intensity and the fat would fall off your hips, thighs and belly.

Well unfortunately there’s a lot more to it than that.

Working at higher intensities actually burns more fat

The problem with the attractive sounding ‘fat burning zone’ is that it forces you to maintain a low heart rate during exercise. Not only does this limit any fitness gains, it also leads to a low total calorie burn too.

The big issue is that low-intensity cardio just doesn’t burn many calories.

You’d be lucky if you topped 150 kcal in a 60-minute fat zone session.

We know that the ONLY way your body burns long-term fat stores is by achieving a calorie deficit – burning off more energy than you put in your body from food.

So, while working out at higher intensities might burn less fat as a proportion, the fact that your burning more total calories will lead to more noticeable fat loss.

Using more challenging workouts such as high-intensity interval training (HIIT) pushes calorie burning through the roof.

Studies show that regular HIIT training leads to [1]:

  • Decreased body fat
  • Lower body weight
  • Healthier hip-to-waist ratio
  • Greater fitness levels

HIIT workouts are also much shorter and efficient compared to hours and hours of low-intensity activity.

Great if you’re a busy person trying to cram everything into a jam-packed day.

Not only that, higher-intensity cardio also burns calories afterwards too. This so called ‘after burn’ effect is only seen when you train at intensities above 70% of your maximum heart rate.

How do you calculate your heart rate zones?

Although it might sound complicated, working out your training zones is quite simple.

All you need to do is use this formula:

220 – age = maximum heart rate (beats per minute)

Here’s an example for a 35-year old:

220 – 35 = 185 beats per minute

Once you’ve got this figure you can calculate the different percentages.

To calculate your training zones, multiply your maximum heart rate by 0.6 for 60% and 0.75 for 75% for example.


Cardio myths

Bottom Line: Fat burning zone is one of the more popular cardio myths. If you’re fit enough to up the intensity, do so. The harder you work, the more calories you’ll burn both during and after cardio.

#2. Does fasted cardio burn more fat?

Fasted cardio is a fat loss approach that got popular a few years back.

Bodybuilders who are on a weight cut are most likely to use it. Probably due to its popularity in hardcore gyms since the 1970s.

The idea behind fasted cardio is that training on an empty stomach forces your body to use stored fat for energy, rather than the calories from the food you just ate.

Much like the fat burning zone myth, it makes sense at face value.

If there’s lower carbohydrate in your body, the next energy source for your cells to turn to would be fat.

The reality is this – your body is a dynamic, complex organism that doesn’t work as simple as fasted cardio might have you believe. Your cells have an amazing ability to adapt and prioritize fuel use – even when carb stores are low.

Fasted cardio might burn more fat… but fewer calories overall

Research shows that there’s not really any difference between performing fasted or ‘fed’ cardio.

Schoenfeld and colleagues studies the effects of either condition using 20 female volunteers [2]. 10 of them were given a meal replacement shake immediately prior to training, and the other 10 were asked to fast overnight.

Both groups performed the same cardio workouts – 60-minutes of steady state cardio at 50% of their maximum heart rate, 3 times per week.

The results showed no significant differences between either group. Regardless of whether they were fed or fasted made little difference to body composition at all.

“Both groups showed a significant loss of weight (P = 0.0005) and fat mass (P = 0.02) from baseline, but no significant between-group differences were noted in any outcome measure”

Ultimately, fasted cardio is a choice of preference.

It might suit someone who just doesn’t like to eat before cardio. If that’s the case, don’t force down a meal and suffer in the gym. After all, fasted cardio probably makes little difference in the first place.

Fasted cardio myths

Bottom Line: Whether you eat first or train on an empty stomach doesn’t really matter. Fasted cardio is another of these cardio myths that you just don’t need to follow.

#3. Does cardio ruins your gains if your goal is to get strong?

This is probably one of the more recent cardio myths.

The idea that cardio makes you weaker has led to bodybuilders and athletes skipping endurance training for fear of muscle loss and strength reductions.

So, should you worry about going for a run or hitting the bike for a long ride?

The short answer is, not really.

The ‘interference’ effect won’t steal your gains

When you exercise, your muscles adapt to the specific stimulus presented.  

You lift weights and your muscle get bigger.

The damage caused to muscle fibers during resistance training triggers a series of cellular changes that lead to the growth of muscle cells. This process is led by an adaptation called the ‘MToR pathway’.

When triggered, MToR leads a molecular response that helps muscle fibers grow, boosts strength and increases force output. The more you can trigger MToR the bigger and stronger you’ll be.

In contrast, cardio training triggers a different adaptation. It harnesses the ‘AMPK pathway’. When activated, this helps muscle fibers build more oxidative capacity so you can train for longer without fatigue… but it also suppresses MToR.

So, the more cardio you perform, the less muscle building potential you have.

A study published in the European Journal of Applied Physiology [3] found that when cardio was added to a program of lower body power training, strength gains were lower. This had the bodybuilding and powerlifting community up in arms – fearing for their strength and mass.


Many of the studies into interference have only looked at very short term suppression of MToR. We currently don’t know enough about the long-term impact to talk about ‘ruined gains’ just yet.

Some studies have even shown that combining cardio and strength training leads to greater increases in muscle size [4]. This research used active individuals like you – not quite top level athletes that live and breathe training, but real people with real training programs.

Unless you’re a top level athlete, concurrent training is actually beneficial. Remember, cardio burns calories, improves overall fitness and decreases your risk of disease.

While it’s true that excessive amounts of cardio can lead to muscle loss for a number of reasons

  • More time doing cardio means less time lifting weights
  • Excessive cardio leads to an increase in the hormone cortisol which is catabolic
  • Lots of cardio can lead to type strength muscle fibers shifting to smaller endurance ones.

Again, this is when you expose yourself to lots and lots of cardio – not a 30-minute run a couple of times per week.


Bottom Line: The idea that regular cardio will lead to muscle loss is one of those cardio myths that’s been taken out of proportion. Okay, in pro athletes performing hours and hours of cardio each week might potentially be an issue… but in active individuals who want to get leaner and fitter it’s nothing to worry about.


Cardio is great tool for fat loss. It helps to boost fitness levels and decrease risk of illness – a well rounded program should include it.

But best of all it gets you lean, athletic and slim too.

Stop believing the myths around cardio and start to implement this fantastic training method into your weekly program. Your body will thank you for it.

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