Aerobic and Anaerobic Energy Explained by Marc Laithwaite


Endurance athletes are familiar with the terms “aerobic” and “anaerobic,” but what do they really mean? Marc Laithwaite provides a very simple explanation to one of the most misunderstood subjects in endurance sports.

The Aerobic and Anaerobic Engines.

We need energy for every minute of our lives. When we are resting, we don’t need much and when we are exercising we need considerably more. The energy is created by ‘burning’ fuels such as carbohydrates and fats and we have 2 engines within our body which are responsible for making this happen; Aerobic and Anaerobic.


1. The Aerobic Engine

The aerobic engine can burn both fat and carbohydrates and needs oxygen to make this happen. When we are at rest, we don’t need much energy, so we take in a small amount of oxygen and our aerobic system breaks down carbohydrates and fats to provide the small amount of energy required.

As we become more active (light exercise), our energy demand goes up. Generally this isn’t an issue for most of us. We take in more oxygen and break down more carbohydrates and fats to generate the extra energy required.

If we start to exercise at harder intensities, then at this point the aerobic energy system may start to struggle a little. The aerobic system has the choice of burning fat or carbohydrate and as the intensity of your exercise increases, it will favour the carbohydrate. The reason for this is that fat requires a lot more oxygen to break down, so it’s not the most efficient fuel. It’s fine when you are exercising at an easy intensity as your energy demand is small, but when you’re asking for higher amounts of energy, your aerobic system just goes for the easier option and prefers to burn carbohydrates. This is why we see a shift in fuels used as exercise intensity gets harder, from higher fat to higher carbohydrate.

If you continue to increase the intensity of exercise, your aerobic system may get to the point where it is finding it hard to match the energy requirement. At this point it asks for help from your ‘second engine’.

2. The Anaerobic Engine

When you reach that point where your aerobic system is struggling to generate the amount of energy required, it will call upon the anaerobic system to help out. Engine number 2 will ‘fire up’ and give you the extra energy required. At this point you will have 2 engines working together to supply the energy required.

IMPORTANT: We often hear the term ‘going anaerobic’ and it implies that we switch from aerobic to anaerobic energy. That isn’t the case, when the anaerobic engine starts to ‘help out’, the aerobic engine continues to work alongside. In essence, both engines are now working together rather than switching from one to the other.

The anaerobic engine can only use carbohydrate as fuel, so at this point your carbohydrate use will be pretty high. CRITICALLY, the anaerobic system also can’t use oxygen to break down the carbohydrate, so as a result it produces LACTIC ACID and CARBON DIOXIDE as waste products. At this point you’ll notice a significant rise in your breathing rate, this is due to the build up of carbon dioxide and as a result, you breathe faster to try and exhale as much and as quickly as possible!

If you continue to increase towards maximal intensity exercise, both engines are working together and both are close to their maximal capacity. At this point you’ll reach VO2 maximum, which is the maximum capacity of your aerobic system. IMPORTANT: As we said earlier, your aerobic system is still working, we don’t switch from aerobic to anaerobic, hence your VO2 maximum, a measurement of aerobic capacity, is only reached at maximal intensity.

As you approach maximal intensity, the anaerobic system is producing so much waste product (carbon dioxide) that no matter how fast you breathe, you can’t get enough carbon dioxide out. At this point, it’s like bailing out a boat which is filling quicker than you can bail! The carbon dioxide levels continue to rise , despite you nearly hyperventiliating and at that point, your brain will say stop!


How does fitness change the above?

As your fitness improves, the aerobic engine becomes more capable. At some point as your exercise intensity increases, your aerobic engine will start to struggle and will ask your anaerobic engine to ‘fire up’ and help out. Improvements in aerobic fitness mean that you can run and cycle at higher speeds and your aerobic engine can manage on it’s own. You’ll be able to reach much faster speeds and higher power outputs before it calls on the anaerobic engine to help out.


About Marc:

Sports Science lecturer for 10 years at St Helens HE College.

2004 established The Endurance Coach LTD sports science and coaching business. Worked with British Cycling as physiology support 2008-2008. Previous Triathlon England Regional Academy Head Coach, North West.

In 2006 established Epic Events Management LTD. Now one of the largest event companies in the NW, organising a range of triathlon, swimming and cycling events. EPIC EVENTS also encompasses Montane Trail 26 and Petzl Night Runner events.

In 2010 established Montane Lakeland 50 & 100 LTD. This has now become the UKs leading ultra distance trail running event.

In 2010 established The Endurance Store triathlon, trail running and open water swimming store. Based in Appley Bridge, Wigan, we are the North West’s community store, organising and supporting local athletes and local events.

Check out the endurance store HERE

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3 thoughts on “Aerobic and Anaerobic Energy Explained by Marc Laithwaite

  1. You could add a section called “How does your nutrition change all this?” since those whose bodies (cellular mitochondria) are well-adapted to burning fat for energy (i.e. those who eat a lower carbohydrate and higher fat diet) will be more easily able to tap into their fat reserves for energy than those who eat a daily diet of cereal, sandwiches for pasta and heavily rely on energy gels during runs.

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