Thursday 29 December 2022

Estimating my Anaerobic Work Capacity

My previous blog post showed how my critical power, how it compares to those of professional riders, and also to those of other amateur and recreational riders.

It showed that I'm relatively good at short duration efforts, less than a minute or so.  For those sort of efforts, my power numbers are in the top 2-3% of the 40-49 year-old amateur rider data stored on  Furthermore, I'm actually better than the bottom 10% of professional riders (which is a nice little ego boost at this time of the year, post-Xmas, when I'm feeling fat and slow!).  I speculated that my good short duration power numbers might be a result of having a good anaerobic work capacity (AWC).  I've never analysed my AWC before, and this blog post describes a quick analysis I did to estimate my AWC.

What is Anaerobic Work Capacity?

Most serious cyclists will be familiar with functional threshold power (FTP), which is the maximum power that a rider can sustain for 60 minutes.  Riding at or just below FTP is an aerobic activity in which the build of lactate and other metabolic by-products reaches an uncomfortable but tolerable steady-state level, whereby the body is able to process lactate as quickly as it is generated by the muscles.

People will intuitively know that it's possible to cycle at powers above their FTP, but only for short periods of time.  At powers above FTP, the body is working anaerobically, because it is producing more lactate than it's able to process.  As a result, lactate and other by-products build up until they are painfully high and the rider must stop or slow down. 

The concept of Anaerobic Work Capacity (AWC), which is also called W prime or W', is that a certain amount of energy, a certain number of Joules, is available to allow a cyclist's power to exceed their person's FTP for a limited time.  AWC can be thought of as a 'battery' that has a limited number of Joules, that can be used in a number of ways.  High powers, significantly exceeding FTP (see the red rectangle in the figure above) will 'drain' the battery quickly and so can only be maintained for short periods.  Alternatively, if the power only slightly exceeds FTP (shown by the blue rectangle above), that anaerobic battery can maintain those powers for longer.  Note that the area of both red and blue rectangles is similar, because the area represents the AWC, which is calculated from the duration multiplied the the power above FTP (because power multiplied by time equals energy).

Anybody that cycles a bike will implicitly understand how these things work. You can push hard, but only for a short time, and the harder you push the shorter the time you can do that.  This AWC concept also explains the shape of a person's critical power curve, which tends to be hyperbolic, asymptoting towards their FTP value at higher durations.  At very short durations, the AWC model breaks down because a person's power will be limited by their sprint strength and their associated neuromuscular capabilities, instead of their anaerobic work capacity. This AWC concept is therefore a somewhat simplistic model of reality, but like all models it has some value.

Calculating my AWC

I calculated my AWC from my critical power curve that is shown by the black dashed line in the plot at the top of this post.

At each duration, I calculated the excess amount of energy, having made an assumption about my FTP.  My FTP is quite stable throughout the season and is always in the region of 275-295W.  I calculated my AWC for three FTP assumptions of 275, 285 and 295W, to get a feeling for how sensitive the results would be to that FTP assumption.

The plot below shows the results for durations ranging from 1 second to 1200 seconds (1200s is 20 minutes).  For reasons explained previously, the AWC values calculated for the very short durations, less than 30 seconds, are not valid due to the neuromuscular component of the effort that limits the power.

At the other end, the 20-minute value (1200 seconds) is not a good representation of an anaerobic effort either, because of the large aerobic contribution for that duration. However, the shape of the curves at the 10-20 minute duration gives a clue about my FTP.  The very high AWC calculated with the 275W FTP assumption suggests that 275W is too low as a value for my FTP.  It's very unlikely that I could achieve an average power of 303W for 20 minutes if my FTP was only 275W.  Similarly, the FTP of 295W seems too high when looking at the AWC values for 10 and 20 minutes when compared to the short durations.  I conclude from this that 285W is the best FTP assumption.

The AWC values for the durations of 30s, 60s and 300s are, I feel, the most reliable values.  For those durations, assuming an FTP of 285W, my AWC is around 18-20kJ.

Is my AWC any good?

It's difficult to find good data about what a 'good' AWC value is, but I thought it would be interesting to calculate the AWC for 2018 Tour De France winner Geraint Thomas.  During a 2022 podcast, Geraint mentioned that he set his best ever 1-minute power of 730W during a stage of the 2022 Tour De France.

This impressive 730W number exceeds my best 1-minute power by 111 Watts.  However, when you consider that Geraint's FTP is significantly higher than mine, probably 130-150W better, the amount of power in excess of his FTP is actually lower than I achieved.  Calculating Thomas's AWC, assuming his FTP is 420W, gives an AWC of 18.6KJ.  This is actually slightly lower than my AWC.

Before I pat myself on the back, it's worth remembering that his effort was set up a climb in the middle of a stage in the middle of a grand tour, when he wouldn't have been fresh, whereas my power records have generally been set fully rested when going for Strava KOMs.  Furthermore, we shouldn't forget that AWC in itself doesn't achieve anything, and instead it's the power that can be achieve for a certain duration that's important.


It's also worth bearing in mind that anybody's critical power curve will be a bit lumpy, because it's created from a number of discrete efforts of different durations.  Therefore, a better estimate of your AWC is achieved by selecting the durations at which the peaks in the power curve are seen.  For me, these are the durations of my efforts up certain hills and segments when I've been going for Strava KOMs.  If I calculate my AWC values for these best durations, instead of the 'standard' 60 seconds, 300 seconds etc, my AWC numbers are even better, as shown below. 

My best AWC values, 22.9kJ @75s and 25.1kJ @146s were achieved on two memorable occasions when I went especially deep, trying to get a good time up a couple of short hill segments.  I was also very fresh when I attempted those Strava KOMs.


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