The WattPlan Podcast · Episode 36 · 9 min
Durability: what your FTP can't tell you about hour five
Next episodes: What is cardiac decoupling telling you? · Why is my power lower indoors? · Has my FTP gone up? How WattPlan decides, and why it asks
Read the transcript
James. Hello, and welcome to the WattPlan Podcast. I'm James. I've got a family, a day job, a few podiums I like to mention, and WattPlan to build in whatever's left, plus a sports science habit I really should kick. This week it's the question every long ride asks eventually. Why do some riders still have legs at hour five, and can a number actually measure that? Lyra's with me, our AI research voice, and every study is linked in the show notes.
Lyra. It can estimate it, which is a slightly different thing. Durability is how much of your fresh power is still there after hours of riding. Your FTP and your power curve are measured on fresh legs, so they're silent on it. Two riders with the same FTP can be miles apart by the fourth hour.
James. Miles apart how? Give me a feel for it.
Lyra. Picture one rider still holding about 85 percent of their FTP at hour four, and the other down to 70. On paper they're identical. On the road, the first one is long gone. Nothing in a twenty minute test tells you which of those you are.
James. Is this a proper research idea, though, or a word someone made up to sell a feature?
Lyra. Proper, and fairly young as a named idea. The key paper is a 2021 review in Sports Medicine by Ed Maunder and colleagues. Their point was that labs test riders well rested, but the numbers they measure don't hold still during long exercise. They defined durability as when, and by how much, those numbers deteriorate over time. [1]
James. So my FTP is basically my fresh legs number.
Lyra. Exactly. Fine for setting zones, but nobody rides a sportive on fresh legs. And the Maunder review went further. It argued that the models people use to profile endurance competitors don't account for that drift at all, and called for ones that do.
James. What's actually drifting, though? What's going on inside me at hour three?
Lyra. Several things. Glycogen runs down, you get hotter, muscles take damage. The one you can see on a head unit is heart rate. A 2001 review in Exercise and Sport Sciences Reviews, by Coyle and González-Alonso, looked at what they called cardiovascular drift. After ten to twenty minutes of steady work, heart rate starts creeping up and the blood your heart pushes per beat falls. Their new perspective was that the rising heart rate comes first, and the smaller beats follow from it. [2]
James. That's the creep I see on a long steady ride. Same watts, heart rate edging up all afternoon.
Lyra. You've seen it in the wild, then. And the rider version has a name, cardiac decoupling. Compare power per heartbeat in the first half of a steady ride with the second. If it falls, you've decoupled. Under five percent is generally read as a well developed aerobic base.
James. So low decoupling, fit rider. High decoupling, unfit rider. Simple.
Lyra. Not quite that simple. A hot day, a hard session the day before, or just riding the long ride too hard will all push it up. One bad number is a ride, not a verdict on you. What matters is the pattern across lots of steady rides, which is exactly how a durability model should treat it.
James. Right. So how does WattPlan get from my heart rate creeping to a score?
Lyra. It extends your power curve past where you've measured it. Beyond an hour, power fades with time from your FTP, and after three hours it takes off a bit more each hour for glycogen. How steep that fade is gets personalised. It reads your predicted power at five hours as a share of your FTP. Sixty percent or less scores zero, 85 or more would score a hundred.
James. Hang about. If glycogen's the problem, can't I just eat my way out of the fade?
Lyra. You can eat your way out of some of it, which is why fuelling long rides belongs in durability training. But stores still run down over hours even when you're eating well, so the model takes that extra bit off after three hours whatever you do.
James. And what sets the steepness? Please don't say my age.
Lyra. Not your age. Two things from your riding, plus a small nudge if your efficiency has been improving. The decoupling on your latest endurance ride, where lower means a gentler fade. And your own long rides of two hours or more, comparing average power in the second half with the first. Once it has three of those, they carry 60 percent of the weight and decoupling 40.
James. But surely the second half of my long ride is slower because I stopped for cake and rolled home with a tailwind? Isn't that measuring my café, not my legs?
Lyra. Partly, and that's a fair hit. Stops don't count, because it only averages the time you were pedalling. A tailwind home, or a big climb early, will still colour a single ride. That's why it waits for three, and why it's blended with decoupling rather than trusted alone. The more steady long rides it sees, the more those quirks wash out.
James. Okay, I'll stop blaming the café. What would a typical score look like, then? Am I chasing a hundred?
Lyra. You're not, and nobody is. The model never assumes anyone fades slower than a set minimum, so in practice the score tops out in the low eighties. With no long rides or decoupling at all, it starts from a baseline that lands in the high thirties. Elite begins at 80, Strong at 55.
James. So if I'm just starting out and I see a thirty-something, that's not a judgement on me.
Lyra. Not at all. If you're new to structured training, it probably hasn't met you yet. The card says whether it's working from a baseline estimate or from your rides, and how confident it is. The way to move it is the same as building any aerobic fitness. Ride long, ride it easy enough to talk, eat from the first hour, and keep doing it for months.
James. And if you train with power and a heart rate strap? What does it need from me?
Lyra. Steady long rides with both recording. The strap gives it decoupling. The long rides only count if the first half averaged at least 60 percent of your FTP, so a very gentle cruise with friends won't teach it anything. That's slightly annoying for pure Zone 2 days, but it means it's measuring fade under real work.
James. And for anyone racing long events, or riding a big sportive?
Lyra. That's where it earns its keep, because pacing a long day off your FTP is how people blow up at hour four. Route analysis uses your same fade rate. On a course it expects to take over an hour, it sets targets from the power you can hold for the whole predicted time, not the first hour. Durability and Route analysis are both Premium.
James. Where does this run out, though? What should I not believe about my score?
Lyra. Treat the five hour figure as a model, not a measurement. The shape of the fade and the extra loss after three hours are WattPlan's calibration, not numbers lifted from a trial. The Maunder review asked for durability models precisely because the field didn't have good ones. And we haven't seen a study testing how well a score like this predicts a real five hour ride for amateurs.
James. Fair enough. So what do I do with all this on Saturday?
Lyra. Three things. Open Progress and read the Durability card, and check whether it says baseline estimate or long ride history. Then ride your plan's long ride steady, with power and a heart rate strap, eating from the first hour. After three or four of those, look again and see whether the confidence has gone up.
James. So, check whether it actually knows me yet, wear the strap on the long steady stuff, eat early, and let it learn. And stop blaming the café.
Lyra. And keep this one in mind on the bike. Your FTP is what you've got on fresh legs. The ride is usually decided by how much of it you've still got when your legs aren't fresh any more.
Lyra. The full article and every study we mentioned are on the WattPlan blog, linked in the show notes. And if you've enjoyed this, follow the WattPlan Podcast wherever you're listening, so you don't miss the next one.
James. And if you want a plan built around the time you've actually got, get WattPlan on the App Store or Google Play. Thanks for listening.
Voices produced with ElevenLabs. James's voice is a clone of his own, Lyra is an AI voice, and James reviews every episode.
Your power-duration curve tells you what you can do fresh. It shows your 5-second sprint, your 5-minute VO2max, your 20-minute threshold power. But it says almost nothing about what happens at hour three, hour five or hour seven. That's where durability comes in.
Durability Score predicts how your power fades over multi-hour efforts. It extends your power-duration curve beyond measured data to estimate what you can realistically sustain for the full duration of an endurance event. For cyclists targeting gran fondos, sportives or ultra events, this is the metric that matters most.
Why Short-Duration Power Doesn't Predict Long-Ride Performance
Two cyclists can have identical FTPs and very different long-ride capabilities. Cyclist A holds 85% of FTP at hour four. Cyclist B is down to 70%. On paper they're equal. On the road, Cyclist A finishes 30 minutes ahead.
The difference is durability, the ability to resist power fade as duration increases. It's influenced by
- Aerobic development, mitochondrial density, capillarisation, fat oxidation
- Fuelling efficiency, glycogen sparing and exogenous carb absorption
- Cardiac stability, resistance to heart rate drift at steady power
- Training history, accumulated volume of long rides
None of these show up in a 20-minute FTP test. They only reveal themselves when you ride for hours.
The Power-Duration Curve and Its Limitations
The classic power-duration model (Monod, 1965) describes power output using two parameters, Critical Power (CP) and W' (anaerobic work capacity). It works best for efforts of roughly 2 to 20 minutes. Beyond that, it assumes you can hold CP indefinitely.
That assumption is wrong. Nobody holds CP for five hours. Glycogen depletes. Heart rate drifts upward. Core temperature rises. Muscles accumulate damage. The sustainable power at hour five is significantly lower than the sustainable power at hour one.
The Monod model stops at the door where endurance events begin. Durability Score picks up where it leaves off.
How Durability Extends the Curve
WattPlan's durability model uses a three-zone approach to predict power across all durations.
Short durations, under 2 minutes. The standard Monod model applies. WattPlan uses CP and W' here, which is a rougher guide for very short sprints than for efforts of a few minutes.
Transition, 2 to 60 minutes. A smooth transition from the Monod model to the extended model. This covers threshold efforts and longer intervals.
Extended durations, over 1 hour. A logarithmic fade model with glycogen depletion. This is where durability matters.
The extended model has two components.
Logarithmic power fade captures the natural decline in sustainable power as duration increases. The rate of fade is personalised to you.
Glycogen depletion adds an additional power reduction after approximately 3 hours. Even with perfect fuelling, muscle glycogen stores deplete over time, reducing the power your muscles can produce. This factor increases progressively from the 3-hour mark onward.
Personalisation via Cardiac Decoupling
The fade rate, how quickly your power drops with duration, isn't the same for everyone. WattPlan personalises it using your cardiac decoupling data.
Cardiac decoupling measures how much your heart rate drifts upward (or power drifts downward) during a steady ride. Low decoupling (under 5%) means your cardiovascular system holds steady under load. High decoupling (over 10%) means your heart has to work progressively harder to maintain the same output.
The relationship is direct. Lower decoupling equals lower fade rate equals higher durability.
This is how decoupling adjusts the model.
| Latest Endurance-Ride Decoupling | Fade Rate Multiplier | Interpretation |
|---|---|---|
| Under 3% | 0.80x | Exceptional durability |
| 3-5% | 0.90x | Strong aerobic base |
| 5-8% | 1.00x | Average (baseline) |
| 8-12% | 1.15x | Room for improvement |
| 12% or more | 1.30x | Significant fade expected |
If you don't have enough decoupling data yet, the model uses a baseline fade rate and improves its predictions as you log more rides.
What Is a Good Durability Score?
Your Durability Score is a single number from 0 to 100 based on your predicted power at 5 hours as a percentage of FTP.
| 5h Power as % of FTP | Durability Score |
|---|---|
| 60% or below | 0 |
| 65% | 20 |
| 70% | 40 |
| 75% | 60 |
| 80% | 80 |
| 85% or above | 100 |
The scale is linear between these points. A score of 50 means about 72% of FTP for 5 hours. A score of 100 means 85% or more, which is exceptional for any cyclist. WattPlan labels scores Developing below 35, Moderate from 35, Strong from 55 and Elite from 80.
For most amateur cyclists targeting events of 3-5 hours, a score above 50 is solid. Scores of 80 and above are rated Elite.
How Do I Improve My Durability?
Durability improves through the same training that builds your aerobic engine.
Ride long. There's no substitute for time in the saddle. Weekly rides of 3+ hours teach your body to spare glycogen, oxidise fat and maintain cardiac output under sustained load.
Ride easy enough. Long rides should be in Zone 2, at a conversational pace. If your cardiac decoupling is above 5% on endurance rides, you're probably riding too hard. Back off and let your aerobic system do the work.
Fuel properly. Glycogen depletion is a major driver of power fade. Practising race fuelling on long training rides improves gut absorption and teaches your body to use exogenous carbs efficiently.
Build volume gradually. Durability is a product of accumulated training load over months, not weeks. Consistency trumps heroic one-off efforts.
Monitor your Efficiency Factor. Rising EF at endurance intensities is a leading indicator that your durability is improving.
How Route Analysis Uses Durability
Route analysis uses your durability profile to set realistic power targets for a long event. The durability model gives the power you can sustain for the whole length of the ride, and Route analysis spends that budget where it buys the most speed, harder on the climbs and easier on the flat. A longer day means a smaller budget, so every target comes down with it.
Viewing Your Durability Score
Your Durability Score appears on the Progress page alongside your power-duration curve. Tap the button that shows the 1 to 7 hour prediction to see
- Measured power curve from your actual ride data (solid line)
- Predicted extended curve projected by the durability model (dashed amber line)
- Predicted sustainable power at 3h, 5h and 7h durations
- Overall Durability Score (0-100)
The extended curve shows where your measured data ends and the model's predictions begin. As you log more steady endurance rides with power and heart rate, the model has more to go on. The card shows its confidence and which data it used.
Durability Score is a Premium feature. Get started with WattPlan to build your fitness profile and track your endurance development.
Related Reading
- What Is Cardiac Decoupling in Cycling and What Is a Good Number
- What Is Efficiency Factor in Cycling and How to Improve It
- How to Read Your Power Curve
- Fitness, Fatigue and Form Explained for Cyclists
Sources
- The Importance of ‘Durability’ in the Physiological Profiling of Endurance Athletes, Sports Medicine
- Cardiovascular Drift During Prolonged Exercise: New Perspectives, Exercise and Sport Sciences Reviews