Every number in your profile was collected by someone who felt fine. Rested legs, good sleep, coffee, a warm-up, a lab or a quiet road. That’s the right way to test, and it’s also the reason the numbers keep lying to you at the point in a race where they matter.
Nobody loses a gravel race in the first hour.
What happens to the curve
In 2018, Clark and colleagues had cyclists ride two hours of heavy-intensity work and then retested their power-duration relationship. Critical power came back roughly 10% lower. Not tired legs, not lower motivation. The boundary itself had moved down. W’, the finite work available above that boundary, started dropping even earlier, by around the 80 minute mark.
Sit with what that means. If your critical power is 300 W fresh, the version of you at hour three might be a 270 W athlete, and every plan built on the fresh number is describing somebody who left the race a while ago.
Maunder and colleagues gave the idea a name in 2021 and a definition worth quoting carefully: durability is the timing and the size of the deterioration in your physiological profile over the course of prolonged exercise. It’s a second dimension. Not how good you are, but how well the good part survives.
It’s a real separator
Van Erp, Sanders and Lamberts went through professional race files and found that the ability to hold power as accumulated work piles up tracks with success in the peloton. Not peak power. The retention.
Spragg, Leo and Swart followed with a series of studies on the same question. One useful finding from their 2024 work: what shifts your curve down isn’t simply the total kilojoules you’ve burned, it’s the intensity of the work you did to burn them. Four hours of steady endurance and four hours of surging cost you very different amounts, even at the same job number on the screen. Anyone who has done a fast group ride versus a solo long ride already knew this in their body. It’s satisfying to see it in the data.
The spread between athletes is enormous. In the pooled studies, the drop in critical power after prolonged work ranges from about 1% in some people to 31% in others. That is not a minor individual difference. That’s the difference between a rider who is still dangerous at kilometer 160 and one who has quietly become a passenger.
Heat counts as accumulated work
Kilojoules are the usual bookkeeping, but they aren’t the whole bill. A hot day drives core temperature and skin blood flow up, cardiac output gets split between working muscle and cooling, and the same power costs more heartbeats and more glycogen. Anyone who has ridden the same route in May and again in August has felt the second version arrive early.
Dehydration compounds it. Lose enough body mass in fluid over a long day and plasma volume drops, heart rate climbs at a fixed power, and the curve slides down faster than the work alone would predict. That’s why two rides with identical files, one in the cold and one at 35 degrees, are not the same day of training and shouldn’t be read as one.
It’s also why late-race power in a hot event is a fitness question and a logistics question at the same time. You can train durability all winter and still hand it back by starting a July race with one bottle.
Some of it is just fuel
Clark’s follow-up in 2019 found that carbohydrate intake during the ride blunts the decline. Not eliminates, blunts. So a meaningful share of what people call “fading” is a fueling problem in a costume, and it’s the cheapest part of durability to fix. If you’re taking 40 grams an hour on a five hour day, you don’t have a durability problem yet, you have an empty tank.
Training it, and seeing it
The practical version isn’t complicated, it’s just unpopular. Put the quality late. A four hour ride with the intervals at hour three teaches something a four hour ride with the intervals at hour one does not. Do some long days deliberately underfueled if you know what you’re doing, and do most of them properly fueled, because the goal is to be fast at the end, not to be a martyr in the middle.
There’s an ordering to it that’s easy to get backwards. Durability work is expensive and slow to recover from, so it belongs in the specific block, close to the event, not scattered through the base period because it feels productive. Two or three genuinely long sessions with late quality per month does more than a weekly attempt that leaves you too cooked to hit the actual interval days.
And it’s specific to what you’re training for. A four hour road race and an eight hour gravel event fail in different ways at different times, so the long day should rehearse the right failure. If the decisive moment in your event comes at hour five, a well-executed three hour ride is not preparation for it, however hard the three hours were.
Then measure it honestly. A best five minute power means one thing off the front of a fresh ride and something very different after 2,500 kJ of work. OSO keeps the context attached to the effort, so you can see your own curve at hour one and at hour four instead of pretending they’re the same athlete.
They aren’t. The second one is the one that gets the result.
Sources
- Clark IE, Vanhatalo A, Bailey SJ, et al. Effects of two hours of heavy-intensity exercise on the power-duration relationship. Med Sci Sports Exerc. 2018;50(8):1658-1668.
- Clark IE, Vanhatalo A, Thompson C, et al. Dynamics of the power-duration relationship during prolonged endurance exercise and influence of carbohydrate ingestion. J Appl Physiol. 2019;127(3):726-736.
- Maunder E, Seiler S, Mildenhall MJ, Kilding AE, Plews DJ. The importance of “durability” in the physiological profiling of endurance athletes. Sports Med. 2021;51(8):1619-1628.
- Van Erp T, Sanders D, Lamberts RP. Maintaining power output with accumulating levels of work done is a key determinant for success in professional cycling. Med Sci Sports Exerc. 2021;53(9):1903-1910.
- Spragg J, Leo P, et al. The intensity rather than the quantity of prior work determines the subsequent downward shift in the power duration relationship in professional cyclists. Eur J Sport Sci. 2024;24(4):449-457.