How To Improve Aerobic Fitness

Over the last few years it’s been popular to bash the importance of aerobic fitness. It seems like you’re told to avoid long slow distance work. You’re told that the best way to get ‘fit’ is through high-intensity intervals (HIIT). Or something like a CrossFit metcon. People think going for a long-distance run is going to kill your strength gains, and make you as skinny as a rake.

In reality, the aerobic system is one of the most efficient, and important energy systems. It’s the one with the most room for improvement.

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The aerobic system has the potential to increase your aerobic capacity and aerobic power. But it also helps the other energy systems in the body recover between efforts. Team and individual sports lasting over 2 minutes rely on the aerobic system, for sustained energy production.

The recent hype of high-intensity training has led most of the population to believe that incorporating HIIT, with reduced rest periods, is the best way to train the aerobic system. While there is a carryover to developing the aerobic system this way. It’s important not to neglect the other benefits that come from long-slow-distance work (LSD).

Only working on your aerobic system in one way (LSD, HIIT, fartlek etc.) won’t allow you to reach your true aerobic potential. So, it’s important to mix up the stimulus.

Before we get into the nitty gritty of all the science. It’s important to understand what I’m talking about.

There are three energy systems within the body that help provide energy allowing us to move.

  1. The aerobic system.
  2. The anaerobic lactic system.
  3. The anaerobic alactic system.

All 3 energy systems are always working. Only one is dominant at any time.

Both anaerobic systems require no oxygen and last a maximum of around 2 minutes. They cause negative by-products to accumulate which lead to fatigue or burn out. These two systems are important in short duration sports. And they both rely on the aerobic system to refuel and clear out the by-products that accumulate (when these systems fatigue).

I’ll go into more depth on these systems at a later date. But this week we’re focusing on the development of the aerobic system.

Improve Aerobic Fitness
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All the energy systems create energy through breakdown down of a molecule called ATP to ADP. The aerobic system uses oxygen to do this. It is a much slower process than the anaerobic systems due to the increased number of chemicals and processes.

But as I’ve already hinted at, the aerobic system provides energy for the longest amount of time. This contributes the most amount of energy in any activity over 90 seconds or so.

It can also produce energy with very little negative by-products that cause fatigue. Making it a very clean and efficient system. This makes it possible for ultra-marathoners and Ironman athletes to compete for hours and even days on end.

This means that the aerobic energy system is only limited by oxygen supply. Our muscles ability to utilise that oxygen. And enzyme availability (which we call substrate availability).

The aerobic system is also the only system that can breakdown fats. Fats provide the most energy (per gram). But they take the longest to break down. This makes them useful for endurance events and general activities of daily living.

Improve Aerobic Fitness
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There are two main areas in which to think about the efficiency of your aerobic system:

  1. Aerobic power: how FAST you can produce energy using the aerobic system
  2. 2. Aerobic capacity: how MUCH energy you can produce using the aerobic system

Both areas have their benefits when it comes to sports performance. Depending on your sport, one may have more impact than the other.

For example, a marathon runner competing for 2+ hours is going to rely a lot more on aerobic capacity. They don’t need to produce energy as fast.

While an MMA fighter (fighting 3×5 minute rounds), will need to produce that energy at a much faster rate. It will still rely mostly on aerobic energy.

And as I said earlier there are three main areas we are looking to improve when training the aerobic system:

1. Oxygen Supply:

How capable your heart, lungs, and blood vessels are at delivering oxygenated blood to the muscles. As well as carrying de-oxygenated blood away from the muscles. Here we are looking to increase our cardiac output, by increasing the size of the hearts chambers, or how hard they can contract.

2. Oxygen Utilisation:

How well your muscles use the oxygen that’s delivered. I’m sure most people are aware that the muscles in your body are made up of fast and slow twitch fibres. Slow twitch fibres rely on oxygen to fuel them and create energy at a slower rate. But for a longer period of time. While the fast twitch fibres are bigger and more powerful, they fatigue faster.

They can create energy with or without oxygen present. So, while people may use the excuse ‘I’m fast twitch dominant and can’t run for very long’. It’s important to understand that you can absolutely increase the endurance capabilities of fast twitch fibres. As well as making them bigger, stronger and more powerful.

Regardless of the type of fibre, we want to increase the number of mitochondria (energy powerhouse of the cell). And enzymes (help in the chemical breakdown of ATP, fats, sugars) in the cell.

3. Substrate Availability:

The amount of fuel (in the form of stored sugars and fats) and enzymes available to breakdown ATP with oxygen. Running out of the stored fuel will cause you to ‘hit the wall’. This is more important in endurance events but can still be important for longer duration team sports. The better your endurance training, the better you are at storing and using these fuels.

Developing one or all these pathways will go a long way in optimising your aerobic energy system. Therefore, it’s important to train the aerobic system in a combination of different ways (which I’ll go through soon).

Avoid choosing purely high-intensity intervals or long-slow-distance. If you only train your aerobic system in one way, there’s a good chance that you’re missing something. And there’s a very good chance you’re not reaching your potential.

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So how do we improve our aerobic system through training?

Here’s 5 different training strategies you could incorporate into your program. This will ensure you’re ticking off each of the 3 factors mentioned above. Each method will lead to a more efficient aerobic system.

  • Oxygen supply.
  • Oxygen utilization.
  • Substrate availability.

1. Long-slow distance work

This method works on increasing stroke volume.

Stroke volume = how much blood the heart pumps per beat.

It does this is by increasing the size of the left ventricle which increases the total volume of the heart. More volume in the left ventricle = more blood that can fill the heart between beats. Therefore, you can pump out more blood with each beat.

If you can pump out more blood each beat then your heart doesn’t have to beat as many times to get the same amount of blood around the body. The result is a decrease in resting heart rate.

Reduced heart rate means you can do MORE work for LESS effort (increase in cardiac efficiency).

Note that if you train too hard, and increase your heart rate too high, you will start to get a different adaptation (discussed in the next point). So ensure that you keep heart rate to the recommended range below. Here’s some guidelines to follow:

  • Aim to train with a constant heart rate between 120–150 (I’d stay below 140bpm)
  • Do this for 45–90+ mins
  • Increase volume at or less than 10% each week. For example 50 mins in week 1. 55 minutes in week 2.
  • Perform this with running, cycling, swimming, any other type of cardio equipment. You need to be able to maintain this heart rate for extended periods.

2. Long high-intensity intervals

Rather than forcing your heart to stretch out and increase in volume like the LSD work. This method works to increase the thickness of the cavity wall and its ability to contract more forcefully.

Much like strength training increases the size of your muscles. These high-intensity methods work to increase the size of the muscles in your heart. The result is similar. In that the result is an increase in stroke volume (how much blood can be pumped per beat).

At higher heart rates, the strength of the heart is more important. As the time between each beat reduces, the potential for the heart to fill decreases.

A stronger, thicker heart also increases the number of mitochondria in the heart’s tissue. This helps your endurance at high heart rates.

So how do we train for this?

  • Intervals of 1–2mins x 4–12 sets depending on duration of intervals and training age
  • Rest for 2–5mins
  • Heart rate should be pushing maximum above 150bpm
  • Perform this with running, cycling, swimming, any other type of cardio equipment. You need to be able to maintain this heart rate for extended periods.

3. Anaerobic threshold work

This method works to increase maximum rate of aerobic energy production. You do this by training at your anaerobic threshold a.k.a. the absolute limits of aerobic energy production without sliding into the anaerobic system.

Through this kind of training the contractile properties of your slow-twitch fibres are enhanced. And the total number of aerobic enzymes increases.

It’s quite difficult to figure out your anaerobic threshold without lab testing. For this training to be most effective, you want to remain within 5 beats per minute, either side of that number. However, you can determine an approximate anaerobic threshold through a few different ways. One way to do it is to run as far as you can in 30 minutes. Use the average heart rate and / or pace for the last 20 minutes to find your anaerobic threshold. It’s not perfect but it’s better than nothing.

You could also find other methods on the internet and different calculators for 5k race times and the like. But this one seems simple enough.

Training for this involves:

  • 2–5 sets of 3–10 minute intervals depending on duration of intervals and training age
  • Keep heart rate within +/- 5bpm of anerobic threshold.
  • Rest 1–5 minutes between sets
  • Perform this with running, cycling, swimming, any other type of cardio equipment. You need to be able to maintain this heart rate for extended periods.
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4. High resistance intervals

These intervals work on increasing the endurance of your fast twitch fibres. This method will increase how effective your muscles are at utilising the oxygen.

In this form of training you will be completing intervals focused on low speeds. But high resistance.

This way we recruit the fast twitch fibres (due to the high-resistance). And force them into increasing their mitochondrial density (number of mitochondria). Therefore, their ability to maintain high-power outputs for extended periods improves.

We train this system by:

  • Performing 15–20 reps (10–12 seconds per rep) at high-intensities
  • Rest until your heart rate is back down to 130–140bpm
  • This is best performed sprinting up a steep incline. With sled pushes / drags. Or on a spin bike at a very high resistance.

5. Strength training for slow-twitch fibres

This method also works to improve how our muscles utilise the oxygen in the working muscle. This method is different to the previous. As it targets the development of slow twitch fibres. Our aim is to stimulate a hypoxic environment (low oxygen). Which causes our fibres to grow.

By increasing their size (cross-sectional area or CSA) we increase their oxidative capabilities. This allows us to create more energy using the aerobic system. For endurance athletes the bigger your slow-twitch fibres the better.

And for team sports or sports completed over a longer time domain, it’s also important to increase the CSA of slow-twitch fibres.

In doing so, they indirectly improve the endurance of fast-twitch fibres by helping prevent fatigue. As well as delaying how early we start producing energy ANAEROBICALLY.

How do we train this?

  • Strength training exercises focused on the major compound lifts. Such as squats, deadlifts, RDL’s, push-ups, pull-ups and overhead presses.
  • Control the tempo for 2–3 seconds on the way up and the way down
  • Do 3–5 sets of 8–10 reps on 3–5 exercises

As mentioned earlier, these methods are not the only ways of increasing our aerobic fitness. We haven’t even touched on nasal breathing, altitude, heat and other strategies. Joel goes through more strategies in his book. But these are a good starting point.


The current craze in strength and conditioning is to bash aerobic training. The focus seems to be on strength and power training.

While the strength and power are important, we shouldn’t brush aerobic training to the side. Aerobic fitness helps set the foundation for the other energy systems in the body. As well as helping you recover faster between your strength, speed, and power training days.

If there’s one thing to take away from this article it’s this.

The aerobic energy system is extremely important for energy production and your ability to recover between explosive efforts. Training it is more complex than doing HIIT or LSD work alone. Aim to train the aerobic system across multiple intensities and time domains for the best results.

If you feel like you’re lacking aerobic fitness. Or you’d like some specific guidance on how you can incorporate it into your training. Please don’t hesitate to contact me.

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