Durability: The Fourth Pillar Your Zone Test Can't See

1. Durability was proposed as a fourth parameter in 2021 — and it's stuck.

Maunder, Seiler and colleagues published the paper that named the concept in Sports Medicine in 2021 ^[1]. Their argument was simple: the physiological attributes we measure in routine profiling are not static; they drift during prolonged exercise, and if you don't account for the drift, you're regulating intensity off numbers that no longer apply. They proposed durability — resilience to the deterioration of those attributes over time — as the fourth parameter of endurance, alongside VO₂max, fractional utilisation and economy.

Five years later, the concept has its own methods papers ^[7], systematic reviews ^[6], and a growing set of studies that quantify it. That's unusual speed for sport science. It's because the data keep agreeing.

2. All three classic determinants deteriorate — measurably — inside two hours.

Zanini, Folland and Blagrove took well-trained male marathon runners and re-measured everything after 90 and then 120 minutes of running ^[2]. The results are the cleanest picture we have of what happens to "your fitness" during a long effort:

Read the bottom row twice. Speed at threshold — the number most tightly linked to marathon pace — was 3% slower at 90 minutes and 6.6% slower at two hours. Notice, too, that the decline isn't linear: VO₂peak fell subtly at 90 minutes and then more than doubled its loss by 120. And notice that fractional utilisation went up — you're working at a higher percentage of a smaller engine. That's exactly what a rising heart rate at a fixed pace on your long run is telling you.

3. The size of the fade predicts race performance — in ordinary athletes, not just pros.

Hunter and colleagues tested eighteen 2024 London Marathon finishers — average finish time 3:17, average age 41, in other words you — fresh, and again after a 90-minute run at threshold speed ^[3]. VO₂peak dropped (56.7 → 53.4 mL·kg⁻¹·min⁻¹) and threshold speed dropped (12.8 → 12.1 km/h). Economy and fractional utilisation didn't change in this group.

The key result: the percentage drop in threshold speed was significantly associated with marathon time. Smaller deterioration, faster marathon. The authors' conclusion is worth quoting in spirit — physiological markers deteriorate during prolonged exercise, and the ability to resist that decline is now itself a marker of marathon performance.

4. Durability is largely independent of VO₂max and threshold.

This is the part that should change how you train. Barsumyan and colleagues had twenty well-trained amateur cyclists do a 5-minute and a 20-minute test fresh, then repeat both after riding at 80% of their 20-minute power until they'd accumulated 1,000 kJ of work ^[4]. Twenty-minute power fell by 10.1% on average (±6.5%) and 5-minute power by 10.8% (±7.8%). Look at those standard deviations: some riders lost 4%, others lost 17%.

And the size of the loss had no significant correlation with VO₂max or with relative FTP. The fittest riders on paper were not the most durable. The authors' conclusion: durability is a parameter independent of the traditional physiological measures of cycling performance. A fresh test cannot tell you who will still be riding well at hour four.

The professional data point the same way. Spragg, Leo and Swart measured how the entire power-duration relationship — critical power and W′ — shifts downward in U23 professional cyclists after prolonged work, and asked whether standard lab measures could predict the size of that shift ^[5]. Their follow-up found that it's the intensity of the prior work, more than the total quantity, that determines how far CP drops ^[8]. A 2025 systematic review pulled it together: power output declines of 10–20% after high-intensity prior work, smaller declines after similar or greater volumes at lower intensity, experienced athletes more resilient — and carbohydrate intake during the effort directly modulating the size of the drop ^[6].

5. It's trainable — and not only with more miles.

The same Loughborough group ran a randomised controlled trial: ten weeks of supplementary strength training in well-trained runners, then 90 minutes of heavy-intensity running followed by a high-intensity performance test ^[9]. Strength training improved the durability of running economy through the 90 minutes and markedly improved performance in the fatigued high-intensity test — the bit that looks like the last 5 km of a race. Separately, their observational work found that runners who did regular long runs and carried higher weekly volume had better economy durability than performance-matched runners who didn't.

WHAT THIS MEANS