Everyone gets handed FTP first. It’s a fine number and it’s easy to talk about, which is most of why it won. But it’s a single point on a curve, and you don’t race at a single point. You race at whatever the road asks for, for however long it asks.
The curve itself is old news, and better news. In 1965 Monod and Scherrer noticed that if you plot how much power a muscle group can hold against how long it can hold it, you get a hyperbola, and a hyperbola has exactly two parameters. That’s still the model. Two numbers, and between them they describe most of what you can do.
The engine
The first is critical power. It’s the asymptote, the level the curve flattens toward and never quite touches, and it turns out to be a real physiological border rather than a curve-fitting artifact. Poole, Burnley, Vanhatalo and Jones laid this out in 2016. Below CP, your body can find a balance: oxygen uptake settles, blood lactate settles, muscle phosphocreatine and pH settle. High, uncomfortable, but stable. You could hold it a long time and the limit becomes fuel, heat, saddle, boredom.
A watt above CP and none of that stabilizes. Lactate climbs without a ceiling, phosphocreatine drains, oxygen uptake drifts up toward maximum whether you asked it to or not. There’s no steady state to find, and the clock starts.
The battery
The second number is W’, pronounced “W prime.” It’s the fixed amount of work you can do above CP before you stop, measured in kilojoules. Think of it as a battery: it doesn’t care much how fast you spend it, only that it’s finite. Spend it slowly at 20 watts over CP and you’ll last a long time. Spend it in a 30-second attack and it’s gone.
And it recharges, which is the part that matters tactically. Drop below CP and W’ refills, slowly, in a way that depends on how far below you go. That’s the physiology under every hard race you’ve ever done. Attack, recover, attack again, and the last attack is small because the battery never got fully topped up.
Why two numbers beat one
Take two riders with the same 300 W critical power. One has a W’ of 12 kJ, the other 25 kJ. On a long steady climb they finish together. In a crit, the second rider wins every sprint and every gap, and the first one has to make the race hard early or lose it in the last 200 meters. Same FTP, completely different athletes, and a single threshold number can’t see the difference.
There’s a wrinkle worth knowing. The two-parameter model says you can produce infinite power for zero time, which is obviously false. Morton’s 1996 three-parameter version adds a maximum instantaneous power so the short end of the curve behaves. That’s the model Allure fits to your own best efforts, which is also where FTP comes from in the app rather than the other way around.
Getting an honest fit
The model is only as good as what you feed it. It wants genuinely maximal efforts across a spread of durations, roughly two to fifteen minutes, done reasonably fresh and reasonably close together in time. Feed it three efforts that were all paced, or all the same length, or one from March and one from August, and it will confidently return a curve that describes nobody.
So if your numbers look strange, the usual cause isn’t the math. It’s that you haven’t actually gone all in on a five minute effort since last season. Go find out. The curve gets honest fast when you do.
Sources
- Monod H, Scherrer J. The work capacity of a synergic muscular group. Ergonomics. 1965;8(3):329-338.
- Poole DC, Burnley M, Vanhatalo A, Rossiter HB, Jones AM. Critical power: an important fatigue threshold in exercise physiology. Med Sci Sports Exerc. 2016;48(11):2320-2334.
- Morton RH. A 3-parameter critical power model. Ergonomics. 1996;39(4):611-619.
- Jones AM, Vanhatalo A. The “critical power” concept: applications to sports performance with a focus on intermittent high-intensity exercise. Sports Med. 2017;47(Suppl 1):65-78.