There is a lot of chatter online about this, which makes it confusing. It’s probably best to go back to the original invention of FTP, over 20 years ago now.
This text contains the rational and thought process for the invention of FTP as a tool to guide power-based training. So buckle up and pretend it’s the early 2000s and you’ve never heard anything before these paragraphs about using power in training:
Power at LT is the most important physiological determinant of endurance cycling performance… As such, it is (most) logical to define training levels relative to an athlete’s threshold power… At least in theory, the most precise way of determining an athlete’s power at LT would be to rely on laboratory-based testing with invasive blood sampling. Very few individuals, however, have access to such measurements on a routine basis. Furthermore, while LT is often defined by sports scientists as the initial non-linear increase in lactate with increasing exercise intensity (Fig. 2), this intensity tends to be significantly below that which coaches and athletes tend to associate, on the basis of practical experience, with the concept of a ‘threshold’ exercise intensity. The latter corresponds more closely to what the sports science community has termed OBLA (onset of blood lactate accumulation, defined as a blood lactate concentration of 4 mmol/L), but is really conceptually closest to MLSS (maximal lactate steady state)… In terms of understanding the physiology of exercise, it actually makes little difference which of these various definitions is used, since they are all highly interrelated…
Basically Cogan is saying, “there’s really no agreed upon definition of what threshold even is in the first place, but in my opinion it makes the most sense to use the concept of lactate threshold as the anchor for power zones. In practice this is a bit higher than what athletes and coaches associated with “threshold” intensity (whatever threshold meant in the pre-power meter era), which is closer to the concept of OBLA or MLSS. But it doesn’t really make a significant difference IRL as these are all closely related anyways. “
Given the limitations of laboratory testing as discussed above, probably the easiest and most direct way of estimating a rider’s functional threshold power (first time this term was introduced) is therefore to simply measure their average power during a ~40 km (50-70 min) TT. This highly pragmatic approach is justified by laboratory research showing that the power a cyclist can generate for 60 min correlates very highly with, but is slightly greater than, their power at LT (defined as a 1 mmol/L increase in blood lactate over exercise baseline) (2)…
Here, Cogan is saying that your average power for a 40k TT is a reasonable real-life way to estimate what your “threshold” power is.
An even easier way of estimating an athlete’s threshold power is to just measure the power that they can routinely produce in training during long intervals or repeats aimed at raising LT (e.g., 2 x 20 min at level 4).
Providing another potential way to estimate threshold power.
And that’s it.
“FTP” in its literal and original sense is just a rough but pragmatic way to estimate what your power in the LT/OBLA/MLSS region is. It is not a precise concept, but a pragmatic, back-of-the-napkin sort of idea.
Notice how he’s describing what FTP is - it’s the power you can hold in real life threshold efforts, where “threshold” in this era is in practice based off things like RPE (because PMs don’t exist yet).
And the only purpose of this estimate was the idea that power in the lactate threshold region was a reasonable and logical way to predict appropriate training zones for you.
If you wanna use some other point on the PDC curve to guide power targets, more power to you! As long as it gives you appropriate workouts.
If you have some other idea to test for what this rough power is, great! As long as it gives you appropriate workouts.
If you wanna forego power-based training altogether, and use HR and RPE, great! As long as it gives you appropriate workouts.
And the markers of “appropriate” workouts are if they feel right at the time, and if they are producing the results you’re looking for.