At Working Triathlete, we have shifted away from the more common methods of testing for functional threshold power (FTP) and have moved towards a more insightful method involving calculating critical power (CP) via two+ max power tests—typically a 3-min and 12-min max power test. CP is close to FTP, though CP is a superior measurement because it considers your unique mean max power curve to establish more individualized intensity zones and to get a sense of what “type” of rider you are, or what your current strengths/weaknesses are (e.g., is your current fitness more like a top crit rider, climber, 40K time trialist, or Ironman triathlete?). It can also shed some light on how you should be training.
What is Functional Threshold Power (FTP)?
At the risk of oversimplifying, FTP is the maximum power you can put out for 60 minutes. Most athletes/coaches use it to calculate intensity zones. In the traditional five-zone intensity model, FTP falls in the middle of Zone 4, and your other intensity zones are calculated in relation to it. Because riding all-out for an hour is brutal, FTP is commonly calculated via tests in training wherein you ride as hard as you can for a shorter period and then calculate a percentage of that average power output to estimate your FTP. The most common protocol is doing a 5-min blast and then riding as hard as you can for 20 minutes, multiplying your average power over the 20-min effort by 95%. Ramp tests are another way of estimating your FTP.
FTP testing in this manner is a solid way of extrapolating your anaerobic threshold, or LT2, or VT2. This is the power output or pace wherein the lactate concentration in your blood spikes upward dramatically. However, FTP, by most definitions, is your max 60-min power output. Your actual anaerobic threshold, or LT2, or VT2, could be closer to your 35-min max power, 45-min max power, 65-min max power, etc. In this way, testing FTP isn’t directly considering your unique power curve or empirically establishing where your lactate spikes (you need to do an actual blood lactate or ventilatory threshold test for that). It gets close, but we can do better. This is where CP testing comes in.
What is Critical Power (CP)?
Critical Power represents the power at the actual physiological point where things start to go awry in your body (read about it in depth here). Below CP, markers like lactate concentration, oxygen uptake, hydrogen ion concentration, etc. are relatively stable. Above it, these markers rise, and you’re on borrowed time. You can only maintain an intensity above CP for a short period. The amount of energy you have available to burn above CP is unique to you and is called W’(pronounced “W Prime”). By executing multiple max power tests, we can calculate your CP more accurately and establish your W’ (Note, the W’ Balance Model was refined/studied by Dr. Philip Friere Skiba. Check out some of his work here).
Is CP really better than FTP?
In reality, CP is very close to FTP (in our testing, around 4% higher than 60-min max power), though it is generally a better metric. For many athletes, especially those whose power curves follow a typical distribution, using CP or FTP when calculating intensity zones won’t yield a meaningful difference when training. However, if one has a very high, or very low, W’, or an atypical power curve, the difference can be meaningful. We account for this potential difference in “high-end” vs. “steady-state” power output ability, and the ability to put out power above CP, by doing multiple peak power tests at different durations. We then take the results and consider how short-duration max power outputs relate to longer-duration max power outputs to arrive at an accurate CP.
How do you calculate CP?
The formula to calculate CP with two tests is:
((duration of longer test * avg power)-(duration of shorter test *average power))/(duration of longer test-duration of shorter test)
For example, if an athlete put out 350 watts for a 12-min test and 435 watts for a 3-min test, the formula would be:
((720 seconds * 350 watts)-(180 seconds * 435))/(720-180)= 322 watts. Meaning: the athlete’s CP is 322 watts.
Perhaps counterintuitively, if this athlete put out MORE power during the 3-min test, their CP would be LOWER, as it would represent a steeper decline from 3-min to 12-min power, and therefore the assumption would be that this relative decline would continue down the power curve to CP (you can confirm this directly with tests of varying duration). An athlete with a high 3-min vs. 12-min power output would have a higher W’, which is useful for crits or any race that requires time spent above CP. It could also be the case that this athlete needs to develop “aerobically”/build endurance (especially if they are a new athlete). Alternatively, if an athlete has a low 3-min vs. 12-min power output, it may be worthwhile focusing on developing higher-end fitness or tweaking high-intensity workouts to accommodate the athlete’s lower max power or W’.
What is the Deal with W’?
As mentioned above, W’ is the amount of energy (measured in kilojoules) you have to burn above CP before you’re toast. The athlete who put out 350 watts for a 12-min test and 435 watts for a 3-min test would have a W’of around 20, meaning they have 20 kilojoules (or 20,000 joules) before their battery is depleted. In this way, if you know your W’, you know exactly how hard you can go in training or racing before exhaustion. I talk about W’ more in-depth in this podcast where Miguel Mattox and I discuss the results of his CP test. Again, if you want to really dive in, check out the work done by Dr. Philip Skiba (linked further above), who did his Ph.D. on the concept.
Racing/Training Intensity Zones
Once you know your critical power, you can use it like you’ve used FTP. Because CP is typically slightly higher than FTP, feel free to drop each intensity zone by a percentage point or two. In reality, most people would be fine replacing it directly (especially if you’ve done shorter tests to establish your FTP, which we’ve found tend to overestimate 1-hour max power), but you need to confirm this in training. Especially in the race-specific training phase, you should dial in power targets by experimenting and confirming what your fitness allows you to do.
Race power targets as a percentage of Critical Power
If you’re a beginner or slower athlete, err toward the lower end of the range. If you’re elite, target the higher end:
Sprint Triathlon: 90-100%+ of CP
Olympic Distance Triathlon: 85-95% of CP
70.3 Triathlons: 75-90% of CP (mid-pack athletes should target around 80-83%)
Ironman Triathlons: 65-80% of CP (mid-pack athletes should target around 70-72%)
Conrad Goeringer is an Ironman Certified Coach based out of Nashville, TN. He is the founder of Working Triathlete and author of the book The Working Triathlete. His passion is helping athletes of all levels and with all schedules achieve their endurance goals. Reach out to learn more about Working Triathlete coaching packages and for a free consultation with a WT coach.