Oftentimes I get asked the following question (or something very similar to it). This blog will help clear up some misconceptions when it comes to heart rate zones and exercise intensity.
"For cardio exercise there are various zones of heart rate, and I know higher heart rate burns more calories overall, but would I be better off dropping my intensity down to more of a fat burning heart rate?"
I think the problem people run into, is when they see the type of "Target Heart Rate Zone" chart like the one on the right. There isn't necessarily anything wrong with the chart, however the titles for each "zone" can be a bit misleading, especially for someone who may not fully understand what each zone means.
This can be especially confusing for newcomers to the health and fitness world, those who are interested in burning body fat, so they think that keeping their heart rate at a lower rate or intensity will help them burn body fat (adipose) faster, or more effectively.
To discuss this concept, it is actually better to think of this as a discussion on your body's energy systems rather than how high your heart rate is. Your body runs on (basically) 2 main sources of fuel: fat and sugar. Fat comes in the form of adipose tissue (body fat). Sugar comes from, well, sugar which is stored in the body in the form of long glucose chains known as glycogen which is mainly stored in muscle cells for times of fast, explosive bouts of strength/power. Fat, as many of you probably know, is stored as adipose tissue at various places on our bodies.
Fat metabolism (turning body fat into energy) requires oxygen to be present for the chemical reactions to take place. This is also known as oxidation or aerobic metabolism.
Fast, short bursts of energy come from burning glycogen (muscle sugar). This chemical reaction does not need oxygen to be present in order to happen. This is known as anaerobic metabolism.
When we are doing daily activities, such as walking, climbing a few stairs, folding laundry, making dinner, etc. our heart rate stays relatively low and our breathing rate also remains quite constant and in control. This is because our bodies are primarily using aerobic metabolism to meet its energy demands. I say primarily because you may have short bursts of effort throughout your day such as picking up your child, or quickly running to avoid a rainstorm. In those quick bouts of energy usage, our bodies quickly use some of the stored glycogen.
We are able to keep up with the energy demands placed upon us by breathing and keeping a steady heart rate. As oxygen enters your lungs, your heart pumps the blood to the working cells, as well as removing respiration waste products (Carbon Dioxide) back to the lungs to be breathed out. As long as we can keep a quick enough heart rate and breathe fast enough, our bodies continue to keep up with energy demands through aerobic means.
As your heart rate climbs, however, your body begins to transition from aerobic metabolism to anaerobic metabolism. As you work harder and push yourself, your body physically can't breathe in enough oxygen to keep up with the demands, so that is when you begin to transition to more of an anaerobic energy source. Keep in mind that this is never a "one or the other" scenario. Our bodies are constantly producing a bit of energy from both systems, it's just that at lower heart rates, our primary energy system is aerobic, and as our heart rate soars, we transition to an anaerobic system.
Even when you are in the anaerobic zone (high heart rate), however, and you are primarily burning sugar (glycogen) as a fuel source, oxygen is still present and fat is being burned. Not only that, but as we "spend" some of our anaerobic energy credits, our bodies go into oxygen debt. Oxygen debt has to be paid back! That is why, when you physically push yourself, you then have to catch your breath afterwards. This is known as excess post-exercise oxygen consumption (EPOC).
So, are high heart rates potentially better? Well, let's say you start walking on a treadmill and walk at a brisk pace for 30 minutes. During that 30 minutes of moderate exercise, your body is burning a few calories, your heart rate is slightly elevated, and your breathing has increased a bit. This is beneficial for your cardiovascular system, don't get me wrong, but there are times when you certainly have to push yourself harder than that, especially if you wish to burn more calories. Since you are able to keep up with the energy demands by slightly elevated respiration, your slightly elevated calorie burn stops once that 30 minutes is up.
On the other hand, if you are doing highly intense exercise, perhaps in intervals (HIIT), for 30 minutes, your heart rate is going up and down. Your breathing is laboured. You have to keep fighting to catch your breath. Although the exercise session may end after 30 minutes, your heart keeps pumping at an elevated rate and your breathing stays high for a short time afterwards while your body tries to re-pay that oxygen debt. Not only that, but since you used up some of those glycogen stores, your body will now spend the next few hours trying to replenish those stores, including snatching up any dietary sugar you will be ingesting.
I think a better heart rate chart is one that explains the various zones such as the one on the right. There is definitely something to be said about various heart rate zones, especially if you are training for an aerobic event such as a 10k or half marathon. You can't sustain a high heart rate in your anaerobic zone for too long, so you need to train your aerobic zone to be more efficient so that you can not only run an entire race, but so that you can improve your pace and time as well.
Again, various heart rates are beneficial for various reasons, and elevating your heart rate for extended periods of time is great for overall health and cardiovascular fitness, but in order to burn more calories and have a quick, efficient workout, I say go hard and then go home!
Tyler Robbins B.Sc. CSCS
Director of Fitness
Head of CrossFit