Six things you need to know about… anaerobic capacity

Why anaerobic capacity is important, what it tells us and how to improve it

Most riders nowadays are familiar with the concept of threshold heart rate or FTP power – the heart rate or power you can sustainability hold for an hour. However, your threshold isn’t the only metric you should be worrying about if you want to produce your best performance on the bike.

Anaerobic energy has a big bearing on the amount of power you can put out. Put simply, the greater your anaerobic energy, the higher your top-end power output. Sounds good, right?

So in this article we are going to take a closer look at anaerobic power. What is it? How do we use it? And how we can train to increase it?

If you remember my article on training zones, you’ll have seen that the sixth of the seven zones I outlined was called the ‘anaerobic zone’ – when you’re riding in short, sharp bursts at between 121% or 150% of FTP.

As a result, when people hear the term ‘anaerobic’ their first thought is often sprinting. This is because we have always been told anaerobic means ‘without oxygen’ and, when we sprint, we release energy so quickly it has to come from anaerobic rather than aerobic sources. It’s because the aerobic pathways simply can’t keep up and why these types of efforts are short and unsustainable.

This, of course, is 100 per cent true – however, what is often forgotten is that we can use both anaerobic and aerobic energy pathways together to produce power on the bike.

As covered in my piece on how to perform an FTP test, your FTP power is 95 per cent of the average power your produce in a 20-minute test. Why only 95 per cent? Because in an FTP test, five per cent of your power output also comes from anaerobic energy pathways.

What is the significance of that? Well, it just shows how even in a relatively long effort we are still using some anaerobic energy alongside aerobic output. The more anaerobic energy available, therefore, the greater amount of power we can put out on the bike.

Regardless of whether you’re producing a short, sharp hill climb effort, sprinting for the finish line, closing the gap in a race, or attacking from the bunch, increasing your anaerobic capacity will help you become a stronger cyclist.

It’s also worth mentioning that anaerobic capacity is called various things dependent upon which training platform you choose to use. Golden Cheetah for example will refer to it as W’ (pronounced ‘w prime’) whereas TrainingPeaks WKO will refer to it as FRC (functional reserve capacity).

Other articles may, as we have here, refer to it as anaerobic capacity or even anaerobic work capacity. It all means the same thing, however, and you can use the models we’ll cover here in the same way to predict both time to exhaustion and the maximal sustainable power output for a given period of time.

Anaerobic + aerobic = power output

To understand how both your aerobic and anaerobic systems work together to produce power, we need to think of your anaerobic system as a big barrel of water.

Now consider that there is a hose-pipe going into the top of the barrel. This represents how much energy you can produce through aerobic pathways – or, in other words, your FTP power.

On the bottom of the barrel there is a tap, which represents how much power you are putting out – this is your power output

The water level in the barrel represents your anaerobic capacity.

Bear with me, as this model will demonstrate how anaerobic energy and capacity works as we delve deeper into the subject. First up, however, we can use this model to explain a number of things relating to how much power you can put out on the bike.

Your maximal sustainable power (how much water you can allow out of the tap without emptying the barrel) has to equal your FTP power (the hose-pipe or water coming into the barrel). That power is only sustainable if the amount of water in the barrel (anaerobic capacity) is not decreasing; therefore the maximal sustainable power output equals your FTP power (hose-pipe).

Power outputs above FTP power are produced from a combination of FTP power (hose-pipe) and your anaerobic capacity (water in the barrel). These power outputs are only sustainable while there is still water in the barrel (your anaerobic capacity isn’t empty).

Once the barrel (anaerobic capacity) is empty, a bigger FTP (hose-pipe) will refill the barrel (anaerobic capacity) quicker than a smaller FTP (hose-pipe). We call this recharging your anaerobic capacity.

To understand how this process works in the real world we need to look at your power profile curve.

Power profile curve

You may have come across a power profile curve on Strava or TrainingPeaks (you can read more about six of the key training metrics found on Strava in this article). This graph plots the maximal amount of power you can sustain for any given length of time and looks something like this.

We can take this graph and add a line representing where FTP power would fall on the graph. This is the point at which the graph flattens out – or the highest sustainable power output.

We know that for short periods of time it is possible to sustain a higher power output than FTP power, represented by anything above the dotted FTP line.

Returning to the barrel model, we also know that in order to put out more than FTP power, you need to use some of the water contained in the barrel – your anaerobic capacity.

To achieve your best power output at the end of an effort the barrel needs to be empty. If the barrel isn’t empty you had some more water in reserve that you could have used.

So it follows that if we are looking at maximal ten-minute power we would need to let out the water in the barrel slower than for a maximal three-minute effort.

Let’s also consider the shape of the barrel. We know that to hold the same amount of water the barrel can take various different shapes as long as the volume of the barrel remains the same. So the barrel could be tall and thin or wide and flat, as long as it holds the same amount of water.

We can see this process in action on the power profile graph. Below I have drawn onto the chart two barrels. You can see that both barrels, when placed along the FTP line and the y axis, touch the power profile line.

Because both barrels perfectly fill the gap between the power profile line and FTP power, we can see that anaerobic capacity is always constant for this particular rider.

Essentially, anaerobic energy can be used for a short burst at a significantly higher power output than FTP (tall, thin barrel) or for a longer burst closer to your FTP power (short, wide barrel). The total amount of anaerobic energy available is constant, and so the only way to improve your ability to sustain/produce a higher power, is to improve your anaerobic capacity (or the volume of your barrel).

If you know your anaerobic capacity, therefore, you can calculate how long you would expect to be able to sustain a certain power above FTP.

Predicting power outputs

So far we have seen that anaerobic capacity can be used in conjunction with aerobic energy (FTP power) to produce short-term power outputs.

We have also seen that the amount of anaerobic energy (water in the barrel) is constant but limited. Finally, in the last graph we saw that the amount of anaerobic energy available is the same across all power outputs above FTP power.

Therefore, if we know a rider’s FTP power and we know how large their anaerobic capacity is, in theory we can predict how long they can sustain a certain power output or, to flip that on its head, what power output they can sustain for a specific period of time.

Anaerobic capacity is measured in joules – this is important as one watt of power = one joule per second.

Our example rider has an aerobic capacity of 20Kj (20,000j) and an FTP of 300w.

We can now calculate how long they can sustain 400w:

400w (flow rate out of tap) minus FTP power (300w) (flow rate into barrel) = 100w (this needs to come from the water in the barrel)

100w = 100 joules per second (the amount of extra water that needs to come from the water in the barrel)

20,000j (water in barrel) divided by 100 joules per second (flow rate needed from barrel) = 200s (how long before the barrel is empty)

200s = 3 minutes 20 seconds, so this is how long this rider would be able to produce 400w.

Likewise lets say a rider has set out to complete a maximal ten-minute effort during a ride, we can easily calculate the power output they can sustain for ten-minutes.

10 minutes = 600s

20,000j divided by 600s = 33.3w

300w (FTP) + 33.3w = 333.3w – this is what this rider can sustain for ten minutes

Testing your anaerobic capacity

So far, so good, but everything we’ve talked about ultimately relies on you knowing the size of your anaerobic capacity. Luckily, there are a number of tests to determine this.

The quickest (read: most painful) is known as the CP3 test: a three-minute max effort where you start off by sprinting as hard as possible and then try to continue and make it to the three-minute mark. The idea is to empty the barrel as quick as possible – this takes approximately two minutes – and then continue riding for another minute. Once you have used all your anaerobic energy (i.e. the barrel is empty), then the average power you can sustain for the final minute is approximately equal to your FTP power (the flow rate of the hose-pipe).

Once you have a score for your FTP you can calculate the size of your anaerobic capacity based on the average power for the full three minutes. So if your FTP is 300 watts and your average power for the three minutes was 400w, we know your anaerobic capacity provided an additional 100 watts over your FTP for the test. Now it’s time for the calculation: 100w = 100j/second and three minutes = 180 seconds, therefore your anaerobic capacity provided 18,000j or 18Kj.

This is not a test I would recommend doing on your own though – only do this under laboratory supervision as it is incredibly intense.

Therefore, an easier (but longer) way to predict your anaerobic capacity is over a number of individual time trials ranging from three minutes to 15 minutes. I normally ask clients to do a three, five, eight, ten,12, and 15-minute TT over a few days. Once you plot the power from each of these time trials on a graph you should get a power curve like the ones earlier in this article.

With a bit of clever maths you can then find the line of best fit for this power curve and where the curve flattens is equal to your FTP. Luckily for those of us who aren’t Excel experts, TrainingPeaks WKO and Golden Cheetah are just two of the programs that will calculate this for you.

Once you have the curve and know your FTP you can work backwards from any of your short time trials to calculate your anaerobic capacity. This second method is actually a much more reliable way to calculate both your FTP and anaerobic capacity, so is what I would recommend – the downside is that it takes a few days to do.

How to improve your anaerobic capacity

In order to improve your anaerobic capacity you need to give your body a stimulus that indicates your barrel isn’t big enough.

Here are two training sessions to do just that.

Training session one – 30-second hill sprints

Sprinting as hard as you can for 30 seconds is a great way to quickly empty your anaerobic capacity.

If you feel like you are dying in the last few seconds of the effort then you know that you have emptied the tank and given your body the stimulus it needs to improve.

Allow sufficient time between sprints to recover and recharge your anaerobic capacity. Five to six minutes of recovery should allow you to complete multiple sprints – aim for five sprints in total.

Training session two – 3×3 minutes

In this session you need to pace the effort so that at the end of the three minutes you are completely empty, rather than running out of steam too early.

Aim to maintain a consistent power output throughout. What you should notice is that the first 30 seconds feel easy but after that the effort gets harder and harder. Again allow five or six minutes between efforts to get three good efforts in.

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