What’s Better, Easy Or Hard Cardio?

This is part three in my blog post series on the importance of cardio exercise for your health. In part 1 I made the justification as to why cardio elicits unique benefits for our health. In part 2, I explain the physiological changes that cause the beneficial changes people tend to see from cardio exercise. In this post I will discuss the role of intensity in your cardio exercise and how exercising at the proper intensity helps produce the positive adaptations from cardiovascular exercise.

As with all exercise, there is a wide range of intensity you can work at. I tend to look at intensity from a minimalist point of view. I always ask myself what is the least amount of intensity I can apply to elicit the changes I am looking to achieve? The reason is because the more intense exercise is, the more stress we put on our bodies. Since there is no shortage of stress on us today, we need to make sure exercise does not overflow our stress bucket.

The hard part about finding the right intensity for cardio exercise is that the same exercise can range from not intense at all, to extremely intense depending on the person and their environment.

For example, for an endurance athlete going for a walk is very easy. For a bodybuilder who avoids all cardio for fear of it ruining their gainz, going for a walk can be near maximal intensity. In addition, going for a walk on flat terrain is very different from walking uphill at a 20% incline. We can even throw another level of complexity on top of this and talk about external factors like heat, cold, humidity, and elevation. A prime example is going for a walk at 9,000 ft elevation is much harder than going for a walk at sea level because there is much less oxygen available at 9,000 ft of elevation.

In order to better understand why the proper intensity is important for cardio exercise, it’s good to look at an example of what is happening in our body at various intensities.

A Peek Inside

I think a good way to look at what’s happening at various intensities during cardio exercise is to look at what happens during a VO2Max test.

A VO2Max test is a test meant to measure the maximal amount of oxygen you can intake. This measure is one of the best tests to assess health. In fact VO2Max has been correlated with all cause mortality. For every 1 ml/min*kg increase in VO2Max there was a 9% reduction in all cause mortality.[1]


As I outlined in my previous post, all the adaptations that produce the positive health benefits we are after occur from cardio exercise have to do with efficiently delivering oxygen to, and removing carbon dioxide from, the working muscle. So if we have a good VO2Max for our age, that is a pretty decent indicator that these adaptations are in place and we are getting all the health benefits from cardio.

The protocol for a VO2Max test is pretty straightforward, you start at a very low intensity and the intensity gradually increases as the test progresses until you get to the point where you cannot keep going.

While doing the test you wear a heart rate monitor, and a facemask connected to a device called a metabolic cart which collects and measures the amount of oxygen you breathe in and carbon dioxide you exhale during all parts of the test. It isn’t exactly the most fun test in the world, especially at the end, but you get a world of data from it that you can use to gain insight into the physiological processes that are occurring in your body.

Here is a short video of me doing a VO2Max test on a rower.

The cool thing is if you know how to interpret this data you can use it to better understand which intensities will help produce the greatest health benefits from your cardio exercise!

Measures Of Intensity

One way to look at intensity during any kind of cardio exercise is using heart rate. The reason why I like heart rate is because it is pretty accessible. Heart rate monitors have become relatively cheap, even the best heart rate monitor out there[2] cost about $80. To give you an idea of what your heart rate looks like during a VO2Max test, here is an example of my heart rate during a running test I did on a treadmill.

The light green line is my heart rate. The dark green line that goes up in a stepwise fashion is the treadmill speed. The light blue section of the graph is the warmup where I am walking. The active part of the test where the speed is increasing every minute is the red section. The dark blue section is the cooldown, where I was standing still.

As you can see, once the test begins my heart rate continuously increases pretty much linearly along with the speed.

As we found out in the previous post, breathing plays an important role in cardio exercise. In the below graph the purple line is my breathing frequency. As you can see this also increases as you would expect with the intensity of the test. It’s less linear than heart rate though, you can see much sharper increases as the last ⅓ of the test.

In addition to how many breaths I am taking we can see how much oxygen I am inhaling and how much carbon dioxide I am exhaling for each breath I take. In the graph below the blue line is the oxygen I am inhaling and the red line is the carbon dioxide I am exhaling. Notice that I am taking in more oxygen than carbon dioxide until the last ⅓ of the test when things switch.

It is one thing to look at how much oxygen is being taken into the body but ultimately that only is beneficial if that oxygen can then be utilized at the muscle to fuel the work. Luckily we can get a rough idea of if that is happening by using a sensor placed on the muscle. This type of sensor is called a near infrared spectroscopy sensor or NIRS sensor.

The way it works is by shining an infrared light onto the muscle and based on the light that gets reflected back to the sensor it can tell how much oxygen is being carried on the hemoglobin in the red blood cells flowing by the sensor.

During the VO2Max test I had one of these sensors placed on the quad of my leg. The amount of oxygen in my blood is represented in the light green line on the graph. As you can see, as the intensity of the test goes up, the amount of oxygen carried by my red blood cells goes down, to the point where the muscle pretty much can’t pull any more oxygen from my blood by the end of the test.

Perhaps one of the coolest things we can look at is the amount of fat vs carbs I am burning during the test. The colors on this graph make it kind of difficult to see what is what, but the dark green line is fat and the light green line is carbs (again the other dark green line increasing stepwise is the treadmill speed). As the intensity of the test increases we see the fat I am utilizing to fuel my muscles decreasing while the amount of carbs increases. Again as I approach my max intensity, fat utilization goes to near 0 and most of my energy is coming from carbs.

I know that can be a lot of information to understand, so let me summarize what a VO2Max test can measure in relation to changes in exercise intensity.

  1. Heart rate
  2. Breathing frequency
  3. Amount of oxygen consumed per breath
  4. Amount of carbon dioxide exhaled per breath
  5. Amount of oxygen in the blood at the muscle
  6. Amount of fat burned
  7. Amount of carbs burned

You might be thinking at this point why not work at the higher intensities? Harder is better, so we can get the health adaptations faster if we work harder. To better understand why that is not the case, it’s useful to understand what is happening inside our cells when we work easier and harder during cardio exercise

Our Body’s Powerhouse

Remember those cells we talked about in part 2 called mitochondria?

What happens in these mitochondria is important to the discussion of intensity when it comes to cardio exercise. The image below is useful for understanding why.


Don’t get overwhelmed by all the arrows and foreign language, for our needs we really only need to understand a couple of things.

First look all the way to the right. We see two terms, anaerobic energy and aerobic energy, these correlate to intensity. When you see the term aerobic, think easy exercise. When you see the term anaerobic, think very hard exercise.

Now look all the way to the left.

Here we see one of those capillaries we were talking about in the previous post with red blood cells flowing through it.

The red blood cells deliver oxygen and fat to the mitochondria and remove carbon dioxide as they flow by the muscle.

The mitochondria use the oxygen and fat to produce ATP. ATP is what our muscles use to contract. This process of using oxygen and fat to produce energy in the mitochondria results in 32 ATP molecules. It also results in carbon dioxide (CO2) being generated which is removed from the cell and taken away by the red blood cells to be returned to the lungs to be exhaled. This is what is referred to as aerobic (with oxygen) metabolism.

The cell can also produce ATP in another way. It can take glucose (carbs) from the bloodstream (or glycogen stored within the muscle itself) and turn that into ATP. This process only produces 2 ATP molecules, far less than aerobic metabolism. This is what we call anaerobic metabolism because it doesn’t require oxygen.

The upside to anaerobic metabolism is that it can happen much quicker than aerobic metabolism. This is why as intensity increases we see more and more carbs being used. We are working very hard and need ATP at a faster rate than aerobic energy production can run so our body ends up using the anaerobic pathway more and more.

Most people think that one or the other of these processes is happening at a time and you are either aerobic or anaerobic.

This is actually not the case.

How can we tell?

Remember that graph above showing how much oxygen my leg muscle was using during the test? At my maximal intensity the amount of oxygen that was in my blood was near 0 because the muscle was using it all. If my body was completely anaerobic why would it be taking the oxygen from my blood?

It must be that the aerobic process is still running.

Need more evidence?


Take a look at carbon dioxide production at the end of my test above, it continues to rise higher and higher, meaning aerobic metabolism is continuing to run.

It’s not that our body switches from aerobic to anaerobic, and the aerobic pathway is no longer used. We switch to anaerobic energy production when our aerobic pathway can’t keep up with the energy demands. Think of it as an additional energy source that kicks in when our bodies request more energy than our mitochondria can produce.

So what’s the big deal?

Why does it matter if we are using aerobic or anaerobic metabolism?

To understand why this matters, let’s understand the cost of running anaerobic metabolism.

Paying The Price Of Intensity

It’s great that we have the ability to produce energy for our muscles without oxygen, but it comes at a cost.

  1. We don’t get as many ATP from anaerobic metabolism.
  2. While the anaerobic pathway does not generate carbon dioxide like the aerobic pathway does, it does generate lactate and hydrogen ions.

The common thought is that lactate is “bad” and is what leads to that burning sensation in your muscle.

This is a myth!

Lactate can actually be used by the mitochondria as a fuel!

As you can see in the diagram below, lactate can be used within the muscle or it can go into the blood along with carbon dioxide and be delivered to other muscles throughout the body as a fuel that can be used by mitochondria.


So what causes the burning sensation in our muscles when we exercise at a high intensity?

It is actually the production of the hydrogen ions from anaerobic metabolism.

The body does the best it can to reduce the build up of these hydrogen ions in the muscle by binding them to other molecules within the muscle or transporting them out of the muscle. But if these mechanisms of reducing the buildup of hydrogen ions in the muscle can’t keep up with their generation from anaerobic metabolism it changes the acidity inside the muscle and that results in the burning sensation we often feel.


In addition to the buildup of hydrogen ions, our supply of glycogen (carbs) is fairly small compared to fat. We have about 500 grams of glycogen, at most, stored in our body, and when the brain senses that the supply is getting too low it will start setting off alarm bells.

If we run anaerobic metabolism too much, for too long without somehow replenishing the glycogen, the brain will eventually make us stop exercising. This is commonly referred to by athletes as “bonking”.

It’s All About Delivering Oxygen!

Remember the health benefits we see from cardio exercise are due to the body needing to deliver oxygen to the working muscles.

But as we discussed above even when our bodies use predominantly anaerobic metabolism, our aerobic metabolism is still running, so even at the highest intensities we can possibly reach oxygen is still needed.

Again, why not use high intensity exercise to elicit these changes?

It is all about stress!

Remember, my philosophy is to work at the lowest intensity possible to achieve the benefits we are after.

When it comes to cardio, all the benefits we are after happens when we primarily leverage our aerobic metabolism. This means we can work at a much lower intensity and still get all the benefits we are after.

Plus since it is less stressful we are more likely to not overflow our stress bucket and experience burnout and injury from pushing too hard too often.

How do we know if we are primarily using our aerobic metabolism while doing our cardio exercise?

There are a number of proxies we can use to make sure that we are using aerobic metabolism.

  1. We could look at oxygen consumption, both at the lung and at the muscle.
  2. We can look at lactate generation, as the more lactate present in the blood the more we are relying on anaerobic metabolism
  3. We can look at fat vs carb utilization. As long as we are burning more fat than carbs we are likely using aerobic metabolism primarily.
  4. We can look at our heart rate and find out at which heart rate we start to rely on anaerobic metabolism.

Now that we know we need to be focusing primarily on our aerobic metabolism and we have some options to figure out whether we are or not we are leveraging our aerobic or anaerobic metabolism we now need to talk about how to put this into practice in the exercise we are doing. In part 4 of this series we will do exactly that!

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  1. (n.d.). Long-term Change in Cardiorespiratory Fitness and All-Cause …. Retrieved March 29, 2022, from https://pubmed.ncbi.nlm.nih.gov/27444976/
  2. (n.d.). Accuracy of wearable heart rate monitors in cardiac rehabilitation. Retrieved April 1, 2022, from https://cdt.amegroups.com/article/view/25572/24196

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