The cost of work performed is the energy used to perform it. We measure energy in kilocalories, and your body produces energy in different ways. The body has different systems for producing the energy required to do work.

Longer, slower, lighter: you can burn fat to create energy.

Shorter, faster, heavier: you can burn sugar to create energy.

This is called metabolism: breaking down chemicals in your body to create energy.

Think of your body like a hybrid vehicle.

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

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

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

Your batteries will power you at a low level for a very long time – a healthy adult will have over 130,000 kilocalories stored as fat, ready for use in low training activities. But when you start burning gas, your body switches over to that system and you stop running on battery power alone. And the higher your heart rate climbs, the less fat you burn.

How do you know which energy source you’re using?

Your heart will tell you. 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. This process is called lipolysis, and it’s slow, but it can keep you going all day.

When your heart starts beating faster, you start to metabolize carbohydrates (stored as sugar in your blood and muscle). This process is called the Y cycle, and it 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.

But you need both high-intensity exercise to improve your fitness, and lower-intensity exercise to change your body fat.

How much of each? It depends on the person. A No Excuses CrossFit coach will tell you! Click here to book a free consultation.