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 intervals.icu.  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.

Finally...

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.

Critical power curves of professional riders (and how mine compares)

I recently came across an interesting set of data, summarised on Twitter, which contains the critical power data from 188 professional cyclists between the years of 2013 and 2021.

I'm fascinated by this kind of data.  It's interesting and humbling to see the kind of numbers that the pros produce.  Until now, the best source of data of this type that I knew of is the chart in Table 4.10 of Hunter Allen & Andrew Coggon 2010 book 'Training and Racing with a Power Meter 2nd Edition' (Ref. 1).  That chart is also readily available on the Training Peaks power profiling blog page.  However, it has always been unclear to me, at least from reading the book, what data Allen & Coggon used to create that chart.

With this new data, from Pedro L. Valenzuela et al (Ref. 2), data has been analysed from 188 cyclists (144 of them male), from 7 teams.  It contained both Pro Tour and World Tour level athletes.  Furthermore, the 144 male athletes were a mixture of all-rounders, climbers, sprinters, time triallists and GC contenders.  The data, collected from ~130,000 race and training data files, was analysed to identify the best power values at standard durations.

I don't have access to the full journal article, but the key information was already published on Twitter and also analysed on the website wattkg.com.

I have further analysed the data to compare this new data with the older chart from Allen & Coggon. I also looked at my own critical power curve to see how I compare.

My Analysis

In the chart below you can see in the three red and pink lines the new data from Pedro L. Valenzuela et al, for the 10th, 75th and 90th percentiles of the 144 male riders.  The blue symbols and the error bars show the range of W/kg values from Allen & Coggon for 'World Class' and 'Domestic Pro' categories.

It can be seen from the plot that the two sources of critical power data agree very well for 1-minute, 5-minute and 60-minute durations.  For the 5-second duration, the Allen & Coggon data shows higher W/kg values than the data from Pedro L. Valenzuela.  However, the author commented on Twitter, in reply to one of my tweets, highlighting that they had only 11 sprinters in their database as a possible explanation why his data for the 5-second critical power might seem to be relatively low.

Nevertheless, I would say that the two sources of data agree rather well.

My Power Curve versus the Pros

The dashed black curve in the chart above shows my own critical power curve, for comparison against the pros.

At the lower durations, less than a minute or so, my critical powers aren't too bad. In some cases, they actually exceed the worst pro riders (the 10th percentile pro riders).  However, at the longer durations, my lack of aerobic fitness is clearly visible, with the pro riders having critical powers approximately 30-60% better than me.  This is further illustrated in the graph below, which shows how much better, as a percentage, the pros are compared to me.

What I conclude from this is that my power over short durations is pretty good considering that I am, at best, a very mediocre amateur racer.  However, the longer durations reveal my lack of aerobic capabilities.

This probably explains why I've been able to get and hold many local Strava KOMs during the last 5-10 years, over one hundred of them, mainly on segments lasting <2 minutes, whereas I've never won a bike race of any kind!  All this suggests that I have a relatively good anaerobic work capacity (AWC).  I will analyse my AWC as a next step - something I've never done before - and write a blog post to explain my findings.

My Power Curve versus other amatuers

Finally, I want to show quickly how my power curve compares to other amateurs, because I think this shows a broadly similar picture of my strengths and weaknesses.

The website intervals.icu provides an excellent and free set of analytical tools for your cycling data files that are stored on Strava.  Furthermore, the power curve analysis page allows you to see how your critical power numbers compare to other athletes in the same demographic, showing you graphically where you sit on the 'bell-shaped curve'.  For my 40-49 age range, that's a good-sized population of around 12,000 cyclists in that age range.  The people using intervals.icu are likely to be fairly serious recreational riders and amateur riders, so that's also worth keeping in mind.

The pIot below shows that I'm at about the 80th percentile mark for my best 5-minute and 60-minute powers, but my critical powers for 1-minute and 5-second durations are much better, where I'm in the ~95th and ~98th percentiles respectively.

Again, this goes to show that my strengths are with the relatively short duration efforts. 

References

1) Hunter Allen & Andrew Coggon 2010. Training and Racing with a Power Meter 2nd Edition

2) Valenzuela PL, Muriel X, van Erp T, Mateo-March M, Gandia-Soriano A, Zabala M, Lamberts RP, Lucia A, Barranco-Gil D, Pallarés JG. The Record Power Profile of Male Professional Cyclists: Normative Values Obtained From a Large Database. Int J Sports Physiol Perform. 2022 May 1;17(5):701-710. doi: 10.1123/ijspp.2021-0263. Epub 2022 Feb 21. PMID: 35193109.