Load & readiness
Durability
How well your efficiency resists fatigue late in long runs.
Built on the aerobic decoupling heuristic, so it inherits its caveats (heat and fuelling inflate it). The underlying quality is a real, emerging research concept; the 5% and 10% lines are conventions.
Durability asks how well your efficiency holds up deep into a long run, when fatigue would normally start to pull it apart. A durable runner covers the last few kilometres of a long effort nearly as efficiently as the first few. A less durable runner fades: the same pace starts costing more and more heart rate as the run wears on.
It is an emerging idea in endurance research, the ability to resist fatigue-induced loss of efficiency late in prolonged exercise. We turn that idea into a single number by leaning on a measurement we already trust: aerobic decoupling.
The idea in one line
We take your aerobic decoupling on each of your recent long steady runs (runs of about 45 minutes or more) and average it over the last 42 days. A low average drift means your efficiency held steady across many long efforts, which is exactly what durability describes.
Durability = average decoupling across long steady runs (last 42 days)Each run's decoupling compares first-half efficiency against second-half efficiency, exactly as described on the aerobic decoupling page. Durability just rolls those per-run numbers up into one figure.
Reading the number
Because it is an average of decoupling percentages, a smaller number is better: it means your efficiency barely moved across your long runs.
- Under about 5% average drift rates as Solid.
- A middle band sits between the two lines.
- Over about 10% average drift rates as Weak.
Worked example
Worked example
Three recent long runs with aerobic decoupling of 3%, 5%, and 4%:
- Add the three drifts: 3 + 5 + 4 = 12
- Divide by the number of runs: 12 รท 3 = 4%
- Compare to the line: 4% is under 5%, so durability rates as Solid
The average sits below the 5% threshold, which means this runner held their efficiency well across their recent long efforts.
Honest caveats
Durability inherits every caveat of aerobic decoupling, because it is built entirely from it. Heat, dehydration, and caffeine all inflate decoupling, so a stretch of hot or under-fuelled long runs can push your durability toward Weak with no real change in fitness. It only reads cleanly when the underlying runs were steady, single-intensity aerobic efforts, so intervals and hilly routes do not contribute a meaningful signal.
The thresholds are conventions, not validated cutoffs. The underlying quality, resistance to fatigue-induced loss of efficiency, is genuine and actively researched, but the specific 5% and 10% lines are rules of thumb carried over from the aerobic decoupling heuristic, not measured boundaries. In the research this quality is assessed after much longer or more depleting efforts (often 1.5 to 3 hours), so our 45-minute floor is a pragmatic proxy that only loosely approximates the full construct. Read durability as a soft, directional signal that trends over weeks, not as a precise score. Evidence tier: coaching heuristic, built on the aerobic decoupling heuristic, measuring a real and emerging research concept.