Every triathlete has done it. You warm up, hammer a 20-minute effort, multiply by 0.95, and call it your FTP. You plug that number into your training software, and it spits out five colour-coded zones. Job done.
Except it is not done. Not even close. That single number is telling you far less about your physiology than you think, and it is probably steering your training in the wrong direction.
What an FTP Test Actually Tells You
A 20-minute FTP test tells you how well you can sustain a 20-minute effort. That is it. It provides one data point at one duration, and then applies a coefficient to estimate a threshold value. Your fitness is a curve, not a number, and a single point on that curve cannot capture the shape. The problem is that this number represents a performance metric, not a physiological one. It tells you what you did. It does not tell you how your body produced the energy to do it.
Think of it this way. Two athletes could test at identical FTP values and have completely different physiological profiles underneath. One might have a massive aerobic base with a low ceiling. The other might be glycolytically dominant with a narrow aerobic zone. Prescribing the same training zones to both athletes based on the same FTP number would be a mistake, because they need fundamentally different training to improve.
The Problem With One Number
The core issue with any single-threshold approach is that it is impossible to build a truly individualised program from one number. You cannot determine where an athlete's aerobic threshold sits. You cannot see the gap between their first and second thresholds. You cannot classify whether they need to build their aerobic base or raise their ceiling. All of those insights require at least two anchor points.
This is why the Tremayne Performance system uses two thresholds, not one. Your aerobic threshold and your anaerobic threshold, both identified from the shape of your power and pace curves. Two thresholds. Three zones. A genuine physiological map.
Why Two Thresholds Change Everything
Once you have two threshold values, the gap between them becomes a profiling tool. That gap tells you which energy systems dominate, where the athlete has capacity to develop, and what kind of training will produce the biggest return on investment.
An athlete with a narrow gap between the aerobic and anaerobic thresholds and a low first threshold has a compressed aerobic zone. They are glycolytically dominant. They need extensive sub-threshold work to build their aerobic engine. Prescribing them lots of VO2max intervals based on an FTP number would be the worst possible approach.
An athlete with a wide gap and a high first threshold has the opposite profile. Their aerobic system is well developed, but their ceiling is low. They need targeted high-intensity work to expand their capacity above the second threshold.
You simply cannot see any of this from a single FTP test. The number might be identical for both athletes, but the training they need is completely different.
The Shape Over The Number
Some coaches have moved past FTP and use critical power testing instead. That is a step in the right direction. Critical power at least uses multiple data points across different durations to build a curve, which gives you far more than a single twenty-minute number. But two problems remain.
First, critical power testing is still a battery of maximal efforts. It has a recovery cost, it is noisy, and it only updates when you rebuild the protocol. Second, raw power alone cannot separate a good-day result from a bad-day one. The same 250W on a hot afternoon at hour three of a ride is a completely different effort to 250W on a cool morning at the start of a block.
How the Curve Identifies Your Thresholds
Every ride and every run contributes power and pace data across different durations, with the heart rate cost of producing each output recorded alongside. Over a short period of normal training, the curve fills in. Where the curve bends reveals where both thresholds sit. No lab. No ramp test. No blood lactate or gas exchange mask. Just your training, a power meter, a GPS watch, and the analysis that sits on top of the data.
The heart rate normalisation is the part that matters. Every output is anchored to the heart rate cost of producing it, so day-to-day noise from heat, fatigue, sleep, and hydration gets stripped out. Real physiological change shows up in the numbers instead of being buried by them.
The result is a genuine physiological profile that tells you not just how fit you are, but what kind of fit you are, and exactly how your training should be structured to keep improving. If you are new to physiological data, heart rate training is a practical place to start. And if you suspect your current zones are off, here is how to tell whether your training zones are actually right, and why your creeping heart rate on long sessions is usually a zone problem, not a fitness problem.
Two athletes with the same FTP can need completely different training. You will never know which one you are from a 20-minute test.