Why Heart Rate Zones Matter

If you want your body to perform optimally, you need to understand how your body metabolizes energy.

People who are truly fit have optimal metabolic flexibility, or simply put, they have the ability to switch from one energy system to another easily.
Think of your body as a hybrid vehicle.

How do you know what your body is using for fuel? Your heart will tell you.

Think of your body as a hybrid vehicle.

In the lower zones, you can run on your battery or the stored energy that your body has packed away as fat.

You need more power in higher zones, so you have to draw on fuel – the carbohydrates you’ve put in your tank.

This analogy is a straightforward way to think about burning energy. If you keep your output low but steady, you’ll primarily run off battery power from fat in the form of ketones. If you ramp up your output through resistance or speed, you’ll need more energy, faster, and you’ll get it from carbs in the form of blood sugar.

As your heart beats faster to keep up with the work you’re performing, your metabolism changes to keep up with your energy demand.

When your heart is beating slower, you can metabolize fats for energy. It’s a slow process, but it can keep you going all day.

When your heart starts beating faster, you metabolize carbohydrates (stored as sugar in your blood and muscle). This process will give you a lot of energy really quickly, but it won’t last long.

The higher your heart rate, the sooner you’ll run out of energy. Metabolizing fats for energy is preferential for most people.

If you’ve been doing CrossFit or HIIT for a while, you may recognize just how much time you spend in those higher zones. Training in the higher zones that require us to burn carbs for fuel isn’t bad, but it has its limitations and good fitness requires us to perform maximal work across all metabolic demands.

We must be able to go long and slow most of the time – consider this your BASE. That requires the ability to use fat for fuel efficiently. Most people don’t spend nearly enough time training in this zone.

But sometimes, we must be able to PUSH: to work harder for shorter periods. That requires the metabolic flexibility to burn fat and carbohydrates at their appropriate levels. 

And occasionally, we must go ALL OUT: to work extremely hard for very short periods. That requires the metabolic capacity to give our maximal effort when it’s needed.

In my last post, I talked about five heart rate zones, but today I’m going to simplify it to the three you’ll find yourself in the most at Arsenal.


BASE (Zone 2)

60-70% of Max Heart Rate 

Benefits: Builds Endurance and Improves Body Composition. 

Feels Like: Light breathing. No muscular fatigue. Can have a full conversation.

PUSH (Zone 3)

70-80% of Max Heart Rate 

Benefits: Improves Circulation, Metabolism, & Aerobic Capacity 

Feels Like: Light muscular fatigue and moderate breathing. Can speak a sentence or two.

ALL OUT (Zone 4)

80-90% of Max Heart Rate

Benefits: Improves Heart Rate Variability, Increases Lactate Threshold and VO2 Max 

Feels Like: Unsustainable. Muscular fatigue and heavy breathing. Can’t Speak.

Why leave out Zones 1 & 5?

In heart rate Zone 1, you’re moving your body but not struggling. This is shopping, walking, or mowing the lawn. Most people get plenty of Zone 1 work in their everyday lives. Think walking the dog, mowing the grass, carrying your groceries to the car, etc. Zone 1 is important, but it’s not where you need to be spending your time in the gym.

This is a good heart rate zone for active recovery, because increased blood flow will help with healing or moving waste out of your body, but it won’t make you fitter.

This is the “emergency” zone, and it carries a higher risk than any other zones—athletes who have experienced pushing to Zone 5 report cognition impairment (it’s like feeling drunk) and a decrease in movement quality – neither of those things is good when you’re working out.

Zone 5 is very challenging for most people to get to without some type of external motivation/factor. Zone 5 is a very short max effort lasting a few seconds and we can prepare to push ourselves there when called upon by training the other zones.