Critical Power vs. FTP: Unraveling the Key Distinctions

Critical Power versus FTP

Ok, i’m going to preface this blog post by saying it’s could well end up a little geeky, but hopefully it’s an interesting chat that’ll provide some food for thought.

Many cyclists - especially if you have a power meter - will have heard of FTP (functional threshold power) which is thought of as the highest power you can sustain for 60-mins (except that’s incorrect and i’ll come back to that) and some will have also heard of CP (critical power) which is also the power that you can sustain for a long time (also about 60-mins, except it’s not).

Ok, so what are the definitions of each? FTP is defined as the highest power output that can be sustained in a quasi steady state for about an hour (with a time range of about 35 to 70-mins depending on the athlete’s fitness, genetic ability, etc).

Critical power on the other hand is the power that you can sustain at high intensity indefinitely. Obviously, the indefinitely is hypothetical and is really a mathematical construct. For most people the duration that they can sustain CP for is about 30 to 40-mins.

What happens if you ride at an intensity that is less than CP or FTP? Quite obviously, you can ride longer at a lower intensity and the duration that you can ride for depends on how much lower than CP/FTP you’re riding at, the nutrition you take on, and your mental tenacity, etc.

What happens if you ride at an intensity that is above your CP or FTP? Again, quite obviously, this will have to be for a shorter time period than that which you can sustain FTP or CP for. If it’s just 5 W above our CP or FTP then that time frame will be a whole lot longer than if it’s 200 W above your CP or FTP.

Interestingly, the work that you can do above CP or FTP has a metric associated with it. If you use CP the work that you can do above it is called W’ (W prime). If you use FTP the work you can do above FTP is called FRC (Functional Reserve Capacity). These numbers are calculated differently, and in the case of FRC the calculation is proprietary to Training Peaks and are slightly different (in magnitude). However, in both cases they’re measured in KJ (kilojoules). For example looking at my FRC it’s about 12 KJ.

Why is W’/FRC measured in KJ? This is because you can think of these metrics as a spare reserve battery, which you can tap into whenever your power goes above CP/FTP. The rate at which W’/FRC depletes is proportional to the power above CP/FTP that you’re riding at. Let’s pretend your CP/FTP is 200 W, and your W’/FRC is 10 KJ. You come to a hill and start riding up it at 300 W.

You use your aerobic power at up to 200 W (in this example). So, 300 W - 200 W = 100 W

You have available 10KJ = 10000 Joules

1 W = 1 Joule/second

Therefore, (in this example) you can ride at 300 W for
Time = W’ or FRC / (power you’re riding at - FTP or CP)
= 10000 / (300 - 200)

=10000 / 100
=100 seconds or 1.67 minutes

If i then plug in my data (CP/FTP = 270 W and W’/FRC = 12.8 KJ)
i should be able to ride at 300 W for 426.67 seconds (which is just over 7-minutes)

So, I then (as a way of checking the calculation) looked at what my best power over ~7-minutes was. I see that my best power over 6-mins and 45-secs is 294 W. For clarity, I’ve neither done an all-out max test over 7-mins or seen how long I can maintain 300 W for. The above (294 W) was from a set of intervals I was doing up a hill (that happened to take me about 6:45) and with an effort I could do 3 intervals as hard as possible. That’s close enough for me to feel that one 300 W effort would be doable for just over 7-mins.

So, which are better metrics? CP/W’ or FTP/FRC
Some coaches and athletes use these terms interchangeably, and, indeed in the above examples I’ve suggested that they’re actually the same (that was just to minimise calculations though). Some coaches and sports scientists suggest that the CP/W’ terms are ‘better’. Certainly, these are generally thought of as the correct terminology in physiological circles. However, most coaching platforms tend to use FTP/FRC terminology.

Scientists and coaches who use CP/W’ terminology frequently cite that CP/W’ is better because it requires more testing and you therefore have to maximally calculate CP and W’. That is true (you need 2 or 3 tests over durations of a few minutes such as 3 through to about 15 to calculate these data). On the other hand, many people who use FTP only think of the 20-minute test and the former scientists and coaches who use CP/W’ often (but not always) suggest that for FTP you just need to know the 20-minute test. However, Andy Coggan (the inventor of FTP etc) has said, since the initial development of FTP that you should also do other testing over other durations (5-secs, 60-secs and 5-minutes) so that you can calculate additional data and see how an athlete performs over these differing durations.

Furthermore, just prior to FTP being developed I came up with the idea of ramp testing in conjunction with sprint testing and TT testing to develop an overall picture of the athlete. At CycleCoach for nearly all the athletes we work with we have riders completing a 5-sec, ramp/MAP test and 20-mins TT test to ascertain a complete picture. For some athletes we do additional testing. Looking at the 20-min test in relation to the athlete’s MAP provides actionable data on how they may need to train (depending on their goals and time availability)

All of these systems are useable. In fact at CycleCoach we use our 3 tests but also have all the athletes data in WKO5 (which is the ‘desktop’ version of Training Peaks) and thus it calculates FTP/FRC. If the coach wants to they can also calculate CP and W’. We tend to find that all 3 ideas match up within a few watts of each other…