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Aerobic Fitness: The Base to Performance and Health

If you’ve seen any of my (Tom) endurance training lately, you would have seen and heard me harp on about ‘aerobic fitness’/‘aerobic base work’. 

You may have also heard me say – ‘The bigger the base, the taller the pyramid’

So, what is aerobic fitness and what is aerobic base work?

Aerobic fitness is a measure of your body’s ability to take oxygen from the atmosphere and use it to produce energy for your muscle cells. Many factors influence aerobic fitness, including your lung capacity, cardiac function, gender, age and genetic makeup.

Put simply, aerobic base work is targeted exercise that is prescribed to improve an individual’s aerobic fitness. 

Aerobic base work‌ should be at the bottom of any fitness pyramid. A greater aerobic fitness level allows for many things, including an increase in performance, a greater potential to get stronger (through an increased ability to recover and perform more total work) and improvements to cardiovascular health markers (lower resting heart and lower blood pressure).

An athlete’s aerobic capacity ultimately boils down to their ability to turn food (chemical energy) into mechanical/usable energy (muscle contraction).

Let’s start with some basic physiology to gain a greater understanding of how the body produces energy and how aerobic base work can enhance your fitness potential.

ATP (adenosine triphosphate) is synthesized by skeletal muscle and then used as energy. ATP is responsible for energy in all human cells and is constantly being synthesised during exercise. 

There are 2 main mechanisms in which this is done – aerobically and anaerobically.


  • Denotes that oxygen is being used in a process of energy production
  • Primary fuel source = Fats 
  • Primary muscle fibre recruitment – Type 1


  • Denotes that oxygen is NOT being used in the process of energy production
  • Primary fuel source = Carbohydrates
  • Primary muscle fibre recruitment – Type 2a and 2b

For both aerobic and anaerobic exercise, fats and carbohydrates (at differing rates) are used to produce energy. The intensity and duration of exercise (ie the metabolic/physiological stresses placed on the body), as well as the muscle fibre types recruited to complete said exercise, will dictate which energy system and substrate will be used to produce energy.

Generally, working at around 55 – 75% of max HR, predominantly use fats for energy production. This process is called oxidative phosphorylation and occurs in the mitochondria.

Intensities @ or above 75% of max HR, carbohydrates will begin to become the primary source of fuel. 

The reason for this switch in energy production – fats cannot synthesize/replenished fast enough at intensities above 75% of max HR. In simple terms, working at lower intensities allows your body to utilise fats as fuel.

Now, before I explain why this is so important, let’s circle back to our muscle fibre types – Type 1 and Type 2a/2b

Type 1 fibres

  • Are referred to as slow-twitch
  • Are the first muscle fibre to be recruited 
  • Have a high mitochondrial density (Where fats are broken down into ATP)
  • Have a low force output but do not fatigue quickly

Type 2 fibres (There are multiple types but let’s keep it simple for today)

  • Are referred to as fast-twitch fibres
  • Are recruited as intensity increases and more force or speed is needed
  • Have a lower mitochondrial density 
  • Fatigue quickly

When training at lower intensities (below 75% of max HR as mentioned above) we recruit type 1 fibres and therefore stimulate mitochondrial growth, mitochondrial function and ultimately improve the body’s ability to use fat as energy. This is one of the keys to long term athletic success. 


Think about a longer endurance event, say, a marathon. If you can burn fat for fuel throughout the race, saving your carbohydrate stores for the final kick at the end (when your opponents are SUFFERING), you are a more efficient athlete and stand a better chance of winning the race.

What about if you’re not interested in doing an endurance race? Having a higher aerobic base allows you to increase fat utilisation at lower intensities (think sitting around at work). Put simply, you burn fat easier which makes losing body fat easier while dieting (Who wouldn’t want that).

So, now we know the importance of having a strong aerobic base, how do we train that aerobic base? Enter the 80/20 principle…

The 80/20 principle is a method in which 80% of an individual’s endurance training is done at zone 2 or an aerobic pace. This would denote that the other 20% of training is done at a higher intensity. 

For simplicity, ease of application and in my experience – The Maffetone HR is a simple and effective benchmark to work from.

You can establish your Mafatone HR using this equation:

  • Maffetone HR @ 180 – (age) = Top-end aerobic HR

Once you have your Mafatone HR, you can use the below equation to calculate your Zone 2 HR zone:

  • Maffetone HR – (10 – 15%) = Zone 2 range 

80% of endurance training is to be done in this Zone 2 range. Training in this zone 2 range is quite simply the single BEST way to build your aerobic base. Consistent and targeted training in this HR zone will provide you with benefits such as:

  • Increased mitochondrial density. The more mitochondria you have, the greater your aerobic fitness base, the more fat you can utilise at lower HR levels and the faster you can recover between sets/after workouts.

  • Increased metabolic flexibility. This will allow trained athletes to utilise both fats and carbohydrates efficiently as energy sources.
  • A lower resting HR and blood pressure. A stronger heart muscle particularly the left ventricle = a more efficient heart. This means the heart has to work less to pump blood around the body. It also means there is less pressure on your arterial walls which can reduce your risk of a heart attack.
  • Reduces the risk of injury. Training at more manageable paces ensures less impact on the body, faster recovery times and more consistent training over the long term.

By building your base you’ll be increasing mitochondrial density, improving your metabolic flexibility, lowering your resting HR and blood pressure. Aerobic base work will help you ‌increase your performance and improve health for the future.

Again, The bigger the base, the taller the pyramid! Now go get after it.

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