Durability: why your power fades late in long sessions

At a glance

What is durability?

Durability is the gap between what you can do fresh and what you can do after hours of exercise. Sport scientists Ed Maunder, Stephen Seiler and colleagues made the case for measuring it in 2021. Their point was that lab tests are almost always done on rested athletes, while long races are decided by how an athlete performs once they're tired.

The classic model of endurance performance rests on three things: VO2max, how much of it you can sustain, and how economically you move. In 2024, physiologist Andrew Jones argued for a fourth. All three get worse during prolonged exercise, and by very different amounts from one person to the next. He called resistance to that decline physiological resilience, which is durability by another name.

Durability isn't the same as stamina. Stamina is how long you can hold an effort from fresh. Durability is how much of your fitness is still there three hours in.

Why it matters

The fresher your numbers, the less they tell you about the end of a long race. In a small field study of 12 professional cyclists, riders did a 20-minute time trial fresh, and again after about four hours of riding (around 40 kJ of work per kilogram of body weight). Average power fell by about 3%. The spread was the interesting part: one rider lost 8.5%, while another went slightly faster. None of the usual fresh measures, such as VO2max, threshold or peak power, predicted who would fade.

Recreational athletes see it too. A study of more than 82,000 marathon runners compared heart rate with running speed through each race. In about a third of runners the two stayed closely matched to the finish. In another third, heart rate rose well out of step with pace, from about 19 km on average. The runners who held together finished roughly 20 minutes faster on average, though they also ran at a higher share of their critical speed, so pacing and fitness play a part.

Add a swim before the bike and a run after it, and durability matters even more in long-course triathlon.

How to see durability in your own data

You don't need a lab. Most of what you need is already in your long-session files.

• Fatigued power curve. Compare your best 5-minute and 20-minute power from the last hour of long rides with your fresh bests. Some platforms let you filter best efforts by how much work came first, often in kilojoules per kilogram. The smaller the drop, the more durable you are.
• Heart rate drift. On a steady long ride or run, compare the ratio of power or pace to heart rate in the first half with the second. If heart rate climbs while output holds, or output falls while heart rate holds, you're drifting. Many coaches treat a drift under about 5% on a steady aerobic session as a sign the effort was well within your aerobic range. That's a coaching convention rather than a research threshold.
• Late-run pace. On long runs, and on runs off the bike, check whether your pace in the last third holds at the same heart rate and effort as the first third.

Compare like with like. Heat, wind, hills, fuelling and how well you slept all move these numbers, so look for patterns across several sessions rather than reading too much into one.

A simple durability test

Do the same effort twice: once fresh, and once at the end of a long session.

1. Pick an effort you can repeat accurately, such as a 5-minute or 20-minute all-out effort on the same climb or the same indoor set-up.
2. Do it fresh, after a standard warm-up.
3. Within a week, do a long steady session of two and a half to four hours, fuelled the way you'd race, and finish with the same effort.
4. Work out the drop as a percentage of your fresh result.
5. Repeat the pair every six to eight weeks under the same conditions.

Results vary from day to day even when nothing has changed, so treat a difference of a percent or two as noise until you've seen it more than once.

What changes durability on the day

Fuelling is one of the biggest levers you control on race day. In a 2026 study, 16 trained cyclists and triathletes rode for three hours at a moderate intensity, then tested their critical power. Fresh, it averaged 277 W. After three hours on water only it was 236 W, with 60 g of carbohydrate an hour 257 W, and with 120 g an hour 266 W. An earlier study of 12 cyclists and triathletes found a similar pattern at the first threshold: carbohydrate during a long ride roughly halved the drop. High intakes take practice, so build them up in training rather than trying them for the first time in a race. Many athletes race on less, and how much your gut can handle is individual.

Heat makes the same work harder. Hot conditions push heart rate up and power or pace down for the same effort, so a hot long session will look less durable than a cool one.

Pacing early decides how much you have late. Going out a little too hard spends the resource durability depends on. Many coaches advise an even or slightly negative split to leave more for the end.

Can you train durability?

Yes, though the research is young. In a ten-week study of 35 previously inactive and recreationally active adults, both low-intensity and high-intensity training reduced the strain of a three-hour ride. The changes in drift itself were small, so ten weeks may be too short to see the full effect.

Coaches commonly use three approaches:

• Long steady sessions that build the volume your body needs to keep working late.
• Quality late in long sessions. A block of tempo or threshold in the final hour of a long ride, rather than at the start, trains you to produce power when you're already tired.
• Race-style fuelling in training, so your gut can handle the intake you'll need on race day.

Retest with the same protocol to see whether it's working. See Treat each block as a test in Why FTP isn't the whole story. Your own record of how you respond is a better guide than any average.

Related

• Internal and external loadThe work you do and what it costs you, why the same session costs different amounts, and how to measure both with session RPE.
• Why FTP isn't the whole storyWhat FTP tells you, what it doesn't, why two riders with the same FTP can need different training, and what else to look at.
• Fitness, Fatigue and FormYour long-term load, your short-term load and the gap between them, and why the chart is so often over-read.
• HRV for trainingWhat heart rate variability measures, what a good HRV is, how to read your trend, and how far to let it guide your training.

Sources

1. Maunder E, Seiler S, Mildenhall MJ, Kilding AE, Plews DJ. The importance of "durability" in the physiological profiling of endurance athletes. Sports Medicine. 2021;51(8):1619–1628.
2. Jones AM. The fourth dimension: physiological resilience as an independent determinant of endurance exercise performance. The Journal of Physiology. 2024;602(17):4113–4128.
3. Valenzuela PL, Alejo LB, Ozcoidi LM, Lucia A, Santalla A, Barranco-Gil D. Durability in professional cyclists: a field study. International Journal of Sports Physiology and Performance. 2023;18(1):99–103.
4. Smyth B, Maunder E, Meyler S, Hunter B, Muniz-Pumares D. Decoupling of internal and external workload during a marathon: an analysis of durability in 82,303 recreational runners. Sports Medicine. 2022;52(9):2283–2295.
5. Norte B, Slinn M, Johnson KO, Mahon E, Shepherd S, Strauss JA, Louis J. Graded carbohydrate ingestion up to 120 g·h⁻¹ attenuates the reduction in critical power following 3 h of moderate-intensity exercise in a dose-dependent manner. Scandinavian Journal of Medicine & Science in Sports. 2026;36(6):e70326.
6. Dudley-Rode H, Zinn C, Plews DJ, Charoensap T, Maunder E. Carbohydrate ingestion during prolonged exercise blunts the reduction in power output at the moderate-to-heavy intensity transition. European Journal of Applied Physiology. 2025;125(5):1349–1359.
7. Matomäki P, Heinonen OJ, Nummela A, Laukkanen J, Auvinen E-P, Pirkola L, Kyröläinen H. Durability is improved by both low and high intensity endurance training. Frontiers in Physiology. 2023;14:1128111.
8. Hopkins WG. Measures of reliability in sports medicine and science. Sports Medicine. 2000;30(1):1–15.