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Can I Validate My Functional Threshold Power?

It’s all about power.

Power Tap indicator

Power Tap indicator (yellow ).

Of the two main variables in a training program, volume and intensity, volume has been easily measured for decades. On the other hand, intensity has suffered from a lack of accuracy until recently. Power meters can quantify intensity like no other device outside of a laboratory. The power meter revolutionized training and racing.

An Intensity Benchmark

Power Tap wheel hub

Power Tap hub on rear bike wheel

Power is certainly accurate, and relatively easy to measure. But absolute power values are meaningless from one workout to the next. A benchmark had to be developed for the sake of comparisons and that benchmark is called functional threshold power (FTP) [1]Ballinger, A. 2020. What is FTP in Cycling and How Do I Test and Improve It? Cycling Weekly.[2]Anon. . What Is Functional Threshold Power. TrainingPeaks. It is most commonly defined as the maximum effort one can sustain for 60 minutes on the bike (Allen et. al, 2019)[3]Allen, H., Coggan, A. and McGreggor, S. 2019. Training and Racing with a Power Meter, 3rd ed. Velo Press. Boulder CO.

Functional Threshold Power is a point estimate of a theoretical construct so it is not going to solve all your training problems. But it can be very useful.

Testing FTP

There are different ways to estimate FTP. My coach, Sami, and I use a method similar to that of Friel (2016)[4]Friel, J. 2016. The Triathlete’s Training Bible 4th Ed. VeloPress, Boulder, CO. p.47 which involves a 20 minute maximum effort on the bike. For this discussion, let’s call that Max20. The resulting average watts of the 20 minute effort can be multiplied by 0.95 to approximate the actual 60 minute functional threshold power (FTP). In our implementation of the test, there is an extra five minute all-out effort before the 20 minute effort. It gives us a little extra data to play with and as long as each subsequent test has the same protocol, I can compare results among tests to see how my fitness is improving.

Why pedal for 20 minutes to approximate a 60 minute effort?

Sixty minutes of pedaling at your highest constant effort is difficult. In fact, it’s grueling. It is very hard to maintain the power you believe should be your FTP for that amount of time. And in order to perform the test correctly you must have a pretty good idea of what your FTP is going to be before you even start the test so you can pace it correctly.

All of this has lead me to the question:

Can I validate the results of my 20 minute FTP test? or How accurate is FTP20 for predicting a 60 minute effort (FTP)?

Validation vs. Confirmation

I know from the get go that I won’t be able to validate my FTP estimate from a 20 minute test (FTP20).

Statistics. That’s why I can’t validate my FTP20. There is one experimental unit (myself) and, at this point, there is one replicate (one recent measure of FTP20). That’s what they mean when they say, “n equals one.” Therefore, this is not so much a validation, but a confirmation of sorts.

So, to confirm my FTP20, I need to generate a 60 minute all out effort (FTP60) and see if

FTP60 = FTP20 = 0.95 x Max20

The 20 Minute Test (FTP20)

This workout was held on 11/14/2020 and it went something like this:

  1. 10′ warm up: easy spin

  2. 3 x [60″ @ z3, 60″ @ z1] (z3 can be based on RPE or previous FTP estimate)

  3. 5′ @ z1

  4. 5′ all out

  5. 10′ @ z1 recovery

  6. 20′ all out

  7. 5′ cool down

‘ = minutes, ” = seconds

Power (watts) generated over the course of the 20 minute FTP test workout

Summary statistics for the 20 minute all-out effort

Average power for the 20 minute effort was 220w. Multiply that by 0.95 and we get FTP = 209w. Now, let’s get a little rest and then confirm that estimate with an “hour of power.”

The 60 Minute Test (FTP60)

After a week of mostly low intensity and low volume training it was time to complete the “hour of power” on 11/21/2020.

Low intensity and low volume preceded the second test in order to allow sufficient recovery from the first test.

This workout was a bit more simple than the previous FTP test.

  1. 15′ warm up.

  2. 60′ all-out

  3. 5′ cool down

Power (watts) generated over the course of the 60 minute FTP test workout

Summary statistics for the 60 minute all-out effort

In the first test of FTP, we determined FTP20 to be 209w. From the table on the right, we can see that the average power for the 60 minute effort (FTP60) was 209w.

I think high power statistics are not necessary to conclude that my FTP60 confirms my FTP20.

Like I said above, I do not consider this a validation. It’s possible that the two estimates are identical by chance. After all, “n equals one.” There are many sources of variation and I accounted for only a few. Also, notice how the minimum power is zero in both tests. During the 20 minute and 60 minute intervals I never stopped pedaling which means either the measuring device in the wheel hub is dropping watts or the head unit is dropping watts. The effect may be inconsequential, but I don’t know for sure.

Another factor that may have affected the outcome is bias. I may not have pedaled as hard as possible on the second test, because I knew what my initial test results were. It’s possible that I subconsciously tried to make the results match. I truly doubt that’s the case because I can not pedal smoothly enough to hold the power output constant so the power meter displays a very large range of numbers. Aiming for one particular number would require a higher level of averaging or smoothing than what I was monitoring.

For now, however, I have a high degree of confidence in my current estimate of FTP and the 20 minute testing protocol.

I’ll Take 20 Over 60 Any Day

The 20 minute test is hard enough. Putting out a maximal sustained effort requires pacing and focus. The 60 minute maximal effort is brutal. If you plan on trying it (and I suggest you do if only because my misery loves your company), here are some of my notes and observations:

  1. Both tests were performed on a training stand with hydrostatic resistance.

  2. The power meter is a PowerTap® in the hub of the rear wheel.

  3. I used my FTP20 as a guide for pacing the 60 minute effort.

  4. In the first 10 minutes, I aimed for an average watts of 200 to 205.

  5. After that I just tried to increase the power slowly.

  6. The effort felt easy for the first 15 minutes.

  7. From minute 15 to 30, the effort steadily got harder and harder to maintain the same power. I considered quitting the test.

  8. From minute 30 to 45, I was in hell. Too late to quit.

  9. In the last 15 minutes hell rejected me so I had no one to blame but myself if I did quit. Something inside kept pushing, but at one point, I begged for the watch to move faster.

  10. About 50% of my time was in the aero bars compared to about 60% for the 20 minute test.

  11. No video games like Zwift®, Rouvy®, etc.; no music, no videos.

So, in conclusion, it seems that my FTP20 is an acceptable estimator of my FTP. Now on to the next question,

How can I be confident that my FTP is an accurate estimator of my anaerobic threshold?

Until next time…


References ↑1 Ballinger, A. 2020. What is FTP in Cycling and How Do I Test and Improve It? Cycling Weekly. ↑2 Anon. . What Is Functional Threshold Power. TrainingPeaks. ↑3 Allen, H., Coggan, A. and McGreggor, S. 2019. Training and Racing with a Power Meter, 3rd ed. Velo Press. Boulder CO. ↑4 Friel, J. 2016. The Triathlete’s Training Bible 4th Ed. VeloPress, Boulder, CO. p.47

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