• FF Team

An Explanation of Heart Rate Zones 1-5 - Which do you use Most in Your Training?

Have you ever wondered which mode of training is most suited to burning fat? If you‘re anything like our clients then the answer is definitely YES! The amount of times clients ask us this question makes it an extremely pertinent and current topic to write about. Below I will run through the science behind the heart rate zones and the reasons why our bodies need to use all of them to produce the right amount of energy for the desired activity at the time of asking.





Here is a simple, clear explanation of the science behind HR zones.


When we raise our heart rate (HR) during any activity or workout we start to burn specific energy sources from within our liver and muscles to give us the extra energy our bodies need to perform the chosen activity at a specific Intensity. There are 5 different zones that our HR’s go through and depending on the intensity of the activity or the training session and your fitness level, will depend on which zone your body uses most.


Zone 1 - Is the lowest HR zone and is used by the body when at rest and during very light activity.

This zone does not require the body to produce large amounts of energy quickly therefore it has the time to aerobically produce the energy we need. Within this zone the body prefers to use fat as the main energy source as it produces large amounts of energy when broken down although it is a slower process so it suits lower intensity activities and periods of resting.


Zone 2 - This zone is also a fat burning zone and is used when we are moving and exercising at a light to moderate intensity level.

This zone is also used to help with recovery between higher intense efforts to allow the body to metabolise its energy from fat during active recovery periods to help save glycogen stores (sugar) within the muscle and liver cells. Zone 2 is a very productive way of burning the majority of your energy through fat stores and working within this zone will allow you to sustain a work load for a long period of time >60min.


Zone 3 - This zone is best known as the aerobic cardio zone and it uses its energy from a combination of fat and sugar throughout the activity.

At the lower end of the zone the body will be able to metabolise fat stores for energy as the intensity is still low enough to burn energy aerobically, I.e in the presence of oxygen. As you head up toward the higher end of zone 3 the body will gradually shit from fat burning to burning sugar for energy. It does this to match the demand of the exercise as the intensity increases and the need for more immediate energy is required. Within zone 3 you can expect to work for long periods of time although your fitness level and vo2max will have a huge impact on how long you can sit in this zone for - the fitter you are, the longer you can sustain zone 3 HR’s keeping lactic acid production down to a minimum.


Zone 4 - This is the first zone that is classed as anaerobic where your body burns energy without the presence of oxygen.

This doesn’t mean there is no oxygen in your body but it does mean that your body can no longer keep up with the intensity of exercise that you are asking from it whilst burning fat as the main energy. This means it needs to burn sugar instead at a much accelerated rate compared to a slower fat burn, therefore this sugar burn better suits and matches more effectively the higher intense type activities. Due to this burning of sugar without the presence of oxygen, a waste product called lactic acid is produced within our muscles and builds up in the bloodstream causing the PH level of our blood to drop to more acidic levels. If this build up keeps on progressing throughout the activity, your muscles will get to the point of exhaustion and will no longer be able to function well enough to match the high demands of the exercise. It is this lactic acid that we recognise as that deep burning sensation in our muscles and that funny taste of iron in our mouths when we are working hard.


Zone 5 - This is the last HR zone that our bodies can go into to and it’s this zone that is called the maximal intensity zone.

It’s the zone that is reached once all the other zones have been entered and used accordingly. It is saved for when the intensity of the exercise far outweighs the amount of energy your body can produce if it were to go for a long period of time toward an exhaustive state. Your fitness level will have a huge effect on when you enter this zone and how long you can sustain this level of intensity for, as the fitter your are the later you tend to enter this zone and the longer you can sustain this maximal HR for when most needed. This zone is again an anaerobic metabolising zone where it uses phosphates and sugars to produce the ultra-fast energy it requires to match the maximal intensity efforts. A high amount of lactic acid is produced within zone 5 and you generally would not be able to sustain this HR intensity for much longer than 90sec-3min again fully dependant on your fitness level and ability to produce energy at an intense anaerobic level.


If you found this article interesting and useful then please let us know with any questions around the topic you have.


Until next time,


Good health,

FF Team

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