Durability: The metric that separates amateurs from pros
Hour 5 decides the race, not minute 5. Why Durability is the most exciting development in modern training diagnostics.
One of the most exciting developments in endurance sports science over the past few years has an unassuming name: Durability. Roughly: capacity to perform under accumulated fatigue. It's the reason why some riders in the WorldTour pro peloton can still attack in the fifth hour of a race, while others who post similar 5-minute power in a fresh test are long out the back by then.
What does Durability measure?
Classic performance testing looks at your best values in a fresh state: best 5 minutes, best 20 minutes, CP. Durability asks something different:
What do your best values look like after you've already done 1,500 or 2,000 kJ of work?
A 5-minute best at kJ 0 is a fresh value. The same effort at kJ 2,000 carries an honest fatigue component, exactly what counts in a race.
The Durability Index
In the research (Leo et al. 2022, Mateo-March et al. 2022) the following index has become standard:
Durability Index = Fatigued Power / Fresh Power × 100 %
Rough ranges:
- > 95 %: excellent, typical for well-trained pros.
- 85 to 95 %: solid, where many ambitious amateurs sit.
- < 85 %: clear room to improve the aerobic base.
Important: these thresholds apply in combination with the matching pre-load. Pros work with 25 to 30 kJ/kg of pre-load, amateurs with 10 to 20 kJ/kg. trace uses 20 kJ/kg by default, a good middle ground for ambitious athletes.
How does trace show your Durability?
In the CP curve you can switch on the „Fatigued Overlay". Suddenly you see two curves:
- Blue: your best efforts from a fresh state.
- Orange: your best efforts after 20+ kJ/kg of pre-load.
- The red difference band between them: your Durability gap.
A narrow gap means: you hold your form under load. A wide gap means: you're strong when fresh but drop noticeably under fatigue.
How do you train Durability?
This is where the real value of the metric lies. Durability is trainable, contrary to what many believe. The main levers:
- High volume in Zone 2: mitochondrial density and fat oxidation are the foundation. 12 to 18 hours per week at low intensity build the base that supports hard training later.
- Hard intervals after long pre-load: classic example: 4 h endurance, then 4 × 6 min at VO2max intensity. It's unpleasant, but it's exactly what improves durability specifically.
- Carbohydrate periodization: occasional training in a slightly glycogen-depleted state boosts fat oxidation. Don't overdo it, the effect is small and the injury risk climbs fast.
Why this metric emerged
For a long time Critical Power was treated as the most honest performance number. Until studies showed that two riders with identical CP can have dramatically different residual power after 2,000 kJ of work. Suddenly it was clear: the classic testing model underestimates a decisive aspect, namely the ability to hold form under fatigue.
Who should pay attention?
If you race longer than three hours, ride granfondos, or compete in middle- to long-distance triathlon: Durability is one of the most relevant numbers you can track. If you primarily race crits or short tests, the classic CP curve stays your main focus.
Either way: the first time you turn on the Fatigued Overlay, take a moment. It's the most honest diagnosis many athletes will ever get.