Calculating Ironman CTL Targets

Now that I’ve set my target times for Ironman NZ, I want to know how fit do I need to be to hit those times? More to the point, how fit for the swim, how fit for the bike and how fit for the run? These questions sound like they will get a pretty vague answer, but with all the data I’m capturing during every training session, and the advent of the Performance Management Chart, I should be able to come up with three magic numbers. If my fitness exceeds these magic numbers, then my Ironman goals are achievable.

Oh, and just so you know, this post is a…


The magic numbers I want for swim, bike and run are the Chronic Training Load (CTL). The CTL is a metric that can be used to track your fitness. Rather than just accumulating your hours logged, CTL accumulates the duration of workouts and the intensity of the workouts; CTL also decays over time (as your fitness does). Russ Cox has a good primer on CTL and how it works. Up until now I’ve been treating CTL as just another number (like heart rate and power) which I watch going up and down. When I train more and train harder it goes up, and when I train less and train easier it goes down. It’s time for CTL to show its worth!

For the past year I have been tracking my fitness (CTL) for swim, bike and run. I’ve also imported the previous years stats (I had to make some estimates) giving me two years worth of CTL values. Now that I have this data, what I really REALLY want are some target CTL numbers that I can aim for in my IMNZ training plan. Specifically, what CTL values do I need to do a 13 hour Ironman? Even more specifically, what CTL values do I need for a 90min swim, 7 hour bike and 4 hour run?

There seem to be two key ingredients to predicting your Ironman time (barring nutrition issues, hydration problems, injuries, mechanicals, bike setup, aerodynamics, weather, course profile and surface etc). These ingredients are: (i) the speed at which you can swim/bike/run, and (ii) your ability to maintain that speed over the distance.

WARNING. I’m making this theory up as I go along!

Speed = FTP. In the Performance Management Chart, speed is measured by FTP (Functional Threshold Power). By doing some tests (which you need to do to track your CTL) you get your FTP values for swim, bike and run. Basically, this is the maximum speed you can maintain for one hour.

Ability to maintain speed = CTL. Your ability to maintain speed over time is measured by your CTL (fitness). The higher your CTL, the longer you can maintain a given speed and the less your speed will drop-off over time.

So I have these two numbers (FTP and CTL) available from my training data. Using my target race times I can calculate how fast I need to do each leg of the race, and therefore what percentage of my FTP I need to sustain for that leg. The percentage of FTP that is maintained during a session is called Intensity Factor (IF). The table below show for each discipline, what my target Intensity Factor is:

target if.jpg

*Bike power to speed conversions are estimates based on data from my rides. 150 watts is the estimated conversion from 25.7 kph; I’ve attempted to take the difficulty of the Taupo course into account.

Now the question is, how fit do I need to be to hit these Intensity Factors? In fact, what is the optimum speed to race at for a given CTL and FTP? Obviously I can’t race at my FTP because, by definition, that is the maximum speed I can sustain for an hour. The last 6 hours of my 7 hour bike would be very ugly indeed!

Googling around revealed a range of suggested IF values for Ironman pacing on the bike: eg. 0.65-0.75 from BreakThrough MultiSport, 0.70-0.76 by Allen and Coggan, 0.60-0.68 from Alan Couzens at EnduranceCorner, 0.67-0.80 from Friel and Byrn in the book “Going Long”, 0.70-0.71 is mentioned on a lot of the forums at Endurance Corner (a 0.70 bike IF is about 30.5 kph for me which is a 5:55 bike leg). I could find no references to IF targets for swim and run.

I can’t believe that everybody should target the same IF regardless of their fitness (CTL). It seems logical that the fitter you are, the higher the IF you should be able to sustain. However, FTP and CTL are not independent – FTP will increase as CTL increases (you tend to get faster as you get fitter) – so it is possible that there is a single Intensity Factor that everyone should use. But I still reckon that a fitter athlete would be able to hold a higher percentage of their FTP for longer. I’d love to see some numbers to prove/disprove this.

Now that we know our target pace and intensity, the typical approach is for the athlete to perform long training sessions at the target pace/intensity and check for heart rate decoupling – if your heart rate increases more than 5% percent over the duration then you aren’t fit enough and you need to train more. But I don’t want to wait until I can pass a decoupling test to know what my CTL will be – I want to know the CTL right now, before I create my training plan, so I can use it to help create my training plan!

What I really want is a little table (per discipline) that will tell me what percentage of my FTP I can race at based on my CTL. Something like:

if possible for given ctl.jpg

These are just totally made up figures. I don’t know if a table like this exists. “Going Long” comes close with a table showing IF by target bike times. The most likely location is “Training and Racing with a Power Meter” which I don’t have yet. And since this is my first Ironman, I have no data to build my own table with (although I will be able to get a fairly good idea during my long training sessions). But if this table did exist, it would be a piece of cake to work out what CTL I need to reach to hit my race targets!

Problems with this Theory.

There are a few problems with this theory:

1. FTP is not constant. I’m basing my target CTL on my current FTP. But my FTP will increase as my CTL increases, which will reduce my target IF, which will reduce my required CTL. The target CTL is a moving target because I can’t know what my FTP will be come race day. Unless we can perhaps estimate how much FTP will increase per CTL increase…? I can mitigate this problem by revisiting these figures as my FTP changes and modifying my target CTL accordingly.

2. You need an accurate FTP. I’ve been performing my FTP tests using a 20min hill climb. I know that I cannot maintain the same power on the flat in the aerobars as I do on a hill. I also suspect that 20 mins is not long enough for me to estimate my 60 min power – I doubt that I could manage anywhere near the same power on a 2nd repeat of this same hill. (Note to self: I should try this). If I am going to use Table 2, which is based on other riders FTP/CTL values, I need to ensure that my FTP/CTL values are accurate (see the “Seven Deadly Sins” for ways to set your FTP).

Problems aside, if I can get a rough estimate of my target CTLs, it’s then a matter of tweaking the training plan to ensure that I accumulate enough CTL.

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1 Response to Calculating Ironman CTL Targets

  1. jontsnz says:

    On 24 October I did perform my FTP test twice, back to back. My power dropped almost 5% over the 2nd attempt of the 25min FTP hill. Less than I expected. I used the average Normalised Power for the two attempts as my new FTP. This equated to 97% of my CP20 during the first attempt.

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