Your FTP is a fresh-legs number
Durability — how long your power actually holds as the work piles up — is personal, trainable, and almost no app measures yours
2026-06-24 · 4 min read · Brevet · AI-assisted
Drafted with AI assistance and reviewed by a human. The science is sourced and real; the article isn't medical advice.
The short version: Your FTP is measured on fresh legs, but races and big rides aren't ridden on fresh legs. How much power you keep three hours and 2,000 kilojoules deep — your durability — is a separate, personal trait. Most apps quietly assume you hold your FTP all day. You don't, nobody does, and the gap is trainable once you actually measure it.
The climb that should have been easy
Hour three of the big ride. There's a climb you've done a hundred times, the one you take at 280 watts without thinking. Today you settle in, look down, and the legs are giving you 250 for the same burn.
You didn't get unfit between breakfast and now. You spent something — two and a half hours, a couple of thousand kilojoules of work — and the power that comes easy on fresh legs quietly walked out the door. That's not a bad day. That's durability, and it's one of the least-measured, most-decisive numbers in endurance sport.
The number everyone quotes is the one ridden least
Here's the thing about your FTP: you test it fresh. Twenty minutes, legs you've rested for it, a number you're proud of. Fair enough — it's a real ceiling.
But you almost never ride fresh. The decisive moments — the late climb, the closing hour of a gran fondo, the third lap of the crit — all happen after the tank's been drawn down. And the lazy assumption baked into most training apps is that your hour-three watts equal your hour-one watts. They build the plan, set the targets, and grade the ride against a number that only exists before you've done any work.
The ride does not care about your fresh-legs number.
What durability actually looks like
Picture your sustainable power not as a single value but as a line that holds flat, then bends. Early on, accumulated work barely touches it. Then you hit a point — call it your fade point — where the line starts sliding down, and from there every extra kilojoule costs you a few more watts.
The shape of that bend is yours. Two riders with the identical FTP can fade completely differently: one holds 97% of fresh out to four hours, the other is down to 88% by two. Same test result, different rider, and it's the fade that decides who's there at the finish.
That gap, the red wedge, is the whole story. It's the work you can't do when it counts, hiding behind a fresh-legs number that looks fine on paper.
Why almost nobody measures yours
Durability is harder to pin down than FTP, which is exactly why most tools skip it. A 20-minute test gives you one clean number. Your fade only shows up in long, hard rides, read deep into accumulated work — the watts you can still hold at 1,500 kilojoules, at 2,500, compared to where you started the day. You need real rides, enough of them, and a model that's looking for the bend.
How we read it — and how we say "not yet"
We estimate your fade from your own rides: how your power behaves as the kilojoules stack up, ride after ride, until a personal shape emerges. Once it does, it stops being trivia. Your daily brief can tell you when a session's hardest demands land past your fade point — the difference between a workout you'll finish strong and one that falls apart in the last interval — and we can prescribe the long, fuelled, deep-work sessions that actually push your fade point later. Durability isn't just a readout. It's trainable, and that's the point of measuring it.
And here's the honest part, because it matters most on this one: a fade curve drawn from three rides is a guess wearing a lab coat. Until we've watched you go deep enough, often enough, we use a population-shaped estimate and label it as exactly that — "population, not you" — rather than dress up a default as your personal physiology. You earn your own curve by riding long. We won't pretend you have one before you do.
Train the fade, fuel the fade, and the late climb stops being where your day quietly ends.
Your FTP is the first chapter. Durability is the rest of the ride — and it's the part you can still write.
A note on what this is
Brevet is a training and coaching tool, not a medical device. Durability here is a performance trait — how your power holds across a ride — described relative to your own history, never a health assessment. The platform recommends; you decide every session.
Our methods are written up in plain language on the Science page, with the limits stated honestly — including where a population estimate ends and your own data begins.