Everybody knows one explanation for the marathon wall: you ran out of glycogen. It is correct, but over recent years it has become clear that it is incomplete. Race fuelling has moved a long way forward — 90, 100, 120 grams of carbohydrate an hour are no longer exotic — and runners still fall apart at kilometre 32.

In 2026 the Journal of Functional Morphology and Kinesiology published a review that proposed reframing the question. A year earlier, a study had already supplied the numbers behind it.

What was measured

European Journal of Sport Science, 2025, Hunter and Muniz-Pumares. The logic of the experiment is simple and surprisingly rare: marathon runners were given standard laboratory tests before a long run and the same tests again afterwards.

The load between the two rounds of testing was 90 minutes of running at a heavy intensity. The retest results:

VO₂peak: from 56.7 to 53.4 ml/kg/min — a drop of about 6%
Speed at lactate threshold: from 12.8 to 12.1 km/h
Running economy: unchanged
Fraction of maximum that can be sustained: unchanged
Blood lactate, heart rate and perceived exertion: all significantly higher

It is worth spelling out what those lines mean. After an hour and a half of running you are, physiologically, a different athlete. Your maximum is 6% smaller and your threshold has moved down by almost a kilometre an hour. Every zone you calculated from fresh testing is, by that point, no longer yours.

And this is not even a marathon yet. For a 3:30 runner, ninety minutes is roughly half the distance.

What durability is

The term is still settling into everyday coaching vocabulary, although the idea behind it is old enough: resistance to wear over a prolonged effort. The authors' definition is the extent to which endurance markers deteriorate over the course of prolonged work.

The key point is that this is a separate quality. Two runners with identical VO₂max, threshold and economy in a fresh state can end up in completely different places after 90 minutes of work. The standard testing protocol does not see that difference at all — it only ever captures a fresh athlete.

The 2026 review, “Hitting the Wall in Marathon Running,” takes the idea further: it proposes treating the wall not as a metabolic event but as a psychophysiological collapse, the product of an interaction between physiological strain, perceived exertion, pace regulation and the ability to preserve function under prolonged load. In its formulation, the deterioration runs along several lines at once: metabolic, thermoregulatory, gastrointestinal, neuromuscular and biomechanical. Durability, rather than fitness alone, explains why runners with similar aerobic potential finish marathons so differently.

What this changes in practice

Your zones in the fourth hour are not your zones. If the threshold has shifted from 12.8 to 12.1 km/h, then a pace that was comfortably sub-threshold at the start has become supra-threshold by kilometre 30. Your sensations are not lying to you — the calculator built on a fresh test is.

Heart rate stops being an anchor. For the same work, heart rate climbs (cardiovascular drift), while the physiological cost of that work climbs faster still. Holding a target heart rate in the second half of a marathon means either slowing down imperceptibly or, conversely, overreaching, depending on which way your particular physiology drifts.

Running economy did not change in this study — and that is interesting. Economy is considered one of the main determinants of performance, and its deterioration is usually the prime suspect. Here the ceiling and the threshold moved while the cost of a kilometre stayed put. Different studies disagree on this, and the scatter most likely reflects differences in the duration of the load and in the calibre of the subjects.

Nobody has cancelled carbohydrate. The review does not say glycogen is irrelevant; it says glycogen is not the only cause. Fuelling remains the first lever — just not the last one.

How to train it

The logic is straightforward: if durability is a separate quality, it has to be loaded separately. The work has to happen on top of fatigue, not only on fresh legs.

Quality work at the end of the long run. The final 20–30 minutes of a long run at marathon pace is the most direct method there is.
Long runs on accumulated load. Schedule the long run the day after a hard session rather than after a rest day.
Strength work. In a separate study, ten weeks of strength and plyometric training barely changed fresh running economy but preserved it through to the end of a 90-minute run.
Test yourself fatigued. If you can, measure lactate or threshold pace not only fresh but also after an hour or an hour and a half of running. The gap between the two numbers is your metric.

How to apply this

Plan your pace around the second half, not the first. Your target pace should be one you can hold with a threshold 5–6% below where it started.
Do not chase a heart rate zone in the second half of a marathon. Go by pace and by feel; by that stage heart rate is telling you about drift, not about intensity.
Assume your ceiling at kilometre 30 is about 6% below the laboratory figure. A race plan calculated from a fresh VO₂max is optimistic by construction.
Do not cancel your fuelling. 60–90 g of carbohydrate an hour remains the foundation; durability is about what you still stand to lose even with perfect fuelling.
Long runs are not only about kilometres, but about the quality of the finish. Simply grinding out 30 km easy trains the wrong thing.
Put your key sessions in conditions that resemble race day: the same time of day, the same food, the same accumulated fatigue.

Key points

Hunter and Muniz-Pumares, EJSS 2025: marathon runners completed laboratory tests before and after 90 minutes of hard running.
VO₂peak fell from 56.7 to 53.4 ml/kg/min (about 6%), and speed at lactate threshold from 12.8 to 12.1 km/h.
Running economy and the sustainable fraction of maximum did not change; lactate, heart rate and perceived exertion rose.
Durability — the degree to which endurance markers deteriorate during prolonged work — is a separate quality that standard testing does not measure.
The 2026 review proposes treating the wall as a psychophysiological collapse rather than glycogen depletion alone.
Zones calculated from a fresh test are already inaccurate by kilometre 30; heart rate stops being a reliable guide.
Durability is trained by working on top of fatigue and by strength training.

Sources: Hunter B., Muniz-Pumares D. «Durability of Parameters Associated With Endurance Running in Marathoners», European Journal of Sport Science, 2025. https://doi.org/10.1002/ejsc.70073 · «Hitting the Wall in Marathon Running: An Integrative Psychophysiological and Durability-Based Review», Journal of Functional Morphology and Kinesiology, 2026. https://doi.org/10.3390/jfmk11030291