How to Test Your Durability (the Fatigue Power Reserve).
Written by Damian Ruse
Founder of SEMIPRO Cycling. Fourteen years coaching cyclists from beginners to the WorldTour.
The short version.
A normal FTP test tells you what you can do rested. It says nothing about what you keep once you are tired, which is the thing that decides long rides and races. The Fatigue Power Reserve closes that gap. You compare a fresh effort to the same effort after accumulated work, and the gap between them is a number you can track and improve. Measure it, train it, retest it.
Why a fresh test only tells you half the story.
Every standard test is done fresh. Rested legs, maybe a rest day before, sometimes a taper. That number is real, but it only describes your best case. Your races and your hardest rides happen after hours of work, when your ceiling has already come down. So the one number most riders own describes the version of them that rarely shows up when it counts. To train durability, you first have to see it, and that means testing the fatigued version of you, not just the fresh one.
The Fatigue Power Reserve test.
You can run this on any ride where you can do two hard efforts with an hour between them.
- Warm up properly, then do a maximal five minute effort while fresh. Record your average power. Call this your fresh number.
- Ride 60 minutes at a steady endurance pace. Not easy soft pedalling, not racing, just honest steady work that puts some fatigue in the legs.
- Repeat the same maximal five minute effort. Same road or same trainer setting if you can. Record your average power. Call this your fatigued number.
- Work out the drop. Fresh power minus fatigued power, divided by fresh power, times 100. That percentage is your Fatigue Power Reserve.
- Retest in the same conditions every couple of months so you are comparing like with like and can watch the number move.
Lower is better. The most durable pros lose under five percent. Most riders lose more than that, and the good news is that it responds to training. If you want the maths done for you, run your two efforts through the Durability Score calculator.
What fatigue actually changes.
Here is something useful about how power is lost late in a ride. In one study, when a rider repeated a hard effort after heavy accumulated work, the result looked like this.
| The same hard effort | Fresh | After heavy accumulated work |
|---|---|---|
| Power | 402 W | 390 W |
| Cadence | 92 rpm | 89 rpm |
| Torque per pedal stroke | 42 Nm | 42 Nm |
The torque, the force on each pedal stroke, barely changed. The power dropped almost entirely because cadence dropped. The legs were still pushing just as hard, they were simply turning over more slowly. That matters, because cadence is one of the earliest and easiest signs of fatigue to track in your own files. When your cadence starts sliding late in a ride at the same effort, your durability is eroding underneath you.
How to train the number down.
Testing is the start, not the point. Once you have a Fatigue Power Reserve, you bring it down the same way durability is always built: a strong aerobic base from regular long rides, quality efforts placed late in those rides rather than only when fresh, and fuelling that actually supports the work, in the range of 90 to 120 grams of carbohydrate an hour on long days. Test, train, retest. The number moving is the proof.
The verdict.
You cannot train what you do not measure. A fresh FTP tells you your ceiling. The Fatigue Power Reserve tells you how much of that ceiling survives once it counts. Run the test, get your number, then go and make it smaller. That is durability you can actually see.
Common questions.
How do I test my durability at home?
Do a hard five minute effort fresh and note the average power. Ride an hour at a steady pace. Repeat the same five minute effort and note the power again. The percentage drop between the two is your Fatigue Power Reserve. Lower is better, and you can track it over time.
What is a good Fatigue Power Reserve?
Lower is better, because it means you keep more of your power when tired. The most durable pros lose under five percent. Most riders lose more than that, and the number responds well to training, so a high score today is simply a starting point, not a verdict.
Why does my cadence drop when I get tired, even at the same effort?
Because cadence tends to fade before raw force does. Late in a ride your legs often still push nearly as hard per stroke, but they turn over more slowly, and that slower cadence is where most of the lost power comes from. It is one of the earliest durability signals in your data.
Can I actually improve my durability score?
Yes. Durability responds to training independently of your fresh fitness. Build the aerobic base, put quality work late in long rides, fuel them properly, and retest in the same conditions. Watching the number come down is how you know it is working.
Want the full picture this comes from? Read about the durability framework, or see how a structured plan applies it.