Two persistent ideas circulate about carbon-plated shoes, both plausible and both until now untested.

The first: the magic works while the legs are fresh, and at the thirty-fifth kilometre, when the calves are shot, the plate gives you nothing any more — because the mechanism is built on elastic return at the ankle, and a tired muscle cannot spring.

The second: hence the variation between people. Some gain 6% in economy, others gain nothing, and the explanation is supposedly the strength of the plantar flexors: strong calves, big pay-off; weak calves, none at all.

European Journal of Applied Physiology has published a study that tested both hypotheses in a single protocol. Neither was confirmed.

How it was tested

A New Zealand–Belgian group (Bidois et al., 2026) recruited 64 runners — deliberately mixed in level rather than elite only. The design:

first session — a VO₂peak test;
second — two running economy tests, in a conventional shoe and in a carbon one, before fatigue;
then two rounds of a plantar-flexor fatigue protocol;
then a repeat economy test in one of the two shoes (the order counterbalanced across participants);
plantar flexion power was measured at baseline and before/after each fatigue round;
biomechanics were filmed in every test.

The fatigue was real: plantar flexion power fell by 6.5%.

What came out

Carbon works. Economy in the super shoe was 35.3 ± 5.0 ml/kg/min versus 36.8 ± 5.2 in the control. A difference of about 1.5 ml/kg/min, that is roughly 4% — exactly the order of magnitude claimed by the first Vaporfly studies.

Fatigue gets in the way. Before fatigue — 35.7 ± 5.1 ml/kg/min, after — 36.4 ± 5.2. Running tired really is about 2% more expensive.

But the two effects do not interact with each other. No interaction was found, p ≥ 0.476. Which means the carbon advantage after fatigue is the same as before it. It simply adds to the deterioration from fatigue rather than being swallowed by it.

And the strong-calves theory was not confirmed either. Neither baseline plantar flexion power nor the size of its drop correlated with how much carbon helped a given person (p ≥ 0.566). That directly contradicts the popular explanation of individual variation.

Biomechanics behaved predictably: in carbon, ground contact time was shorter, the foot strike angle smaller, cycle time and duty factor larger; fatigue also increased cycle time and duty factor. But the authors honestly write that the magnitude of these differences was mostly within the resolution of the equipment — so there is nothing to celebrate here.

What this means in practice

The first conclusion is simple and pleasant: shoes do not “switch off” at the marathon finish. If a model gives you 4% on fresh legs, it will give you roughly the same 4% when your legs are falling off. What is more, in absolute terms those percentages are worth more over the final kilometres: you are running more expensively, and any reduction in the cost of a kilometre works off a larger base.

The second conclusion is less pleasant: strengthening your calves in order to “unlock the potential of carbon” is pointless. Plantar-flexor work is an excellent thing in itself: it protects the Achilles, improves economy regardless of footwear and helps on climbs. But the idea that “I will build up my lower legs and the shoes will start working better” is not supported by this study.

The third conclusion: if carbon does not help you, the reason is something else. The variation is real — for some runners the gain is close to zero. The candidate explanations that remain: body mass, running speed (the plate works worse at slow paces), foot strike type, how well the geometry of the last fits. But not calf strength.

Honest limitations

They induced the fatigue in isolation, not with a marathon. The protocol hit the plantar flexors specifically. A real marathon fatigues everything at once — quadriceps, glutes, the central nervous system — and adds dehydration, hyperthermia and glycogen depletion. Transferring this straight across to the 35th kilometre is not valid, and the authors do not claim it.

Economy is not race time. A treadmill at a fixed speed does not reproduce tactics, terrain or weather.

There is a conflict of interest, and it is disclosed. One of the authors is an employee of Salomon SAS, and the company provided all the footwear for the study. That said, according to the authors it took no part in collecting or analysing the data, and two other authors are lecturers for The Running Clinic. There was no external funding. Note also that the study's main result is negative on both of the hypotheses tested, which is uncharacteristic of a “sponsor” study.

Two specific models were compared, not carbon in general. Different manufacturers use different plate stiffness, different foam and different geometry.

What to do

Do not save carbon “for the first half”. Rotating super shoes “for fresh legs” makes no physiological sense.
Test shoes on tired legs. Not to check whether the plate still works, but to catch chafing, instability and foot overload — those are exactly what show up under fatigue.
Train the calves, but with the right motivation: Achilles health, economy as such, climbs. Not to “activate” carbon.
If you feel no advantage, change the model rather than your strength programme. Check the speed you are running at and the fit of the last: those are the more likely causes.

The bottom line

The study (EJAP, 2026) involved 64 runners of varying level; fatigue reduced plantar flexion power by 6.5%.
The carbon shoes gave 35.3 versus 36.8 ml/kg/min — about 4% of economy.
Fatigue worsened economy from 35.7 to 36.4 ml/kg/min — about 2%.
There is no interaction between shoe and fatigue (p ≥ 0.476): the carbon advantage is retained in full on tired legs.
Calf strength does not explain the individual response to super shoes (p ≥ 0.566).
The biomechanical changes were statistically significant, but in magnitude mostly at the level of the equipment's resolution.
Salomon supplied the footwear for the study and one of the authors is its employee; the result is nonetheless negative on both hypotheses.

Source: Bidois B., Cumming C., Giandolini M., Nguyen A.P., Hébert-Losier K. “Economy benefits of running in advanced footwear technology shoes remain with plantarflexion fatigue”. European Journal of Applied Physiology, 2026;126(7):3843–3857. DOI: 10.1007/s00421-026-06190-0. Registration: ACTRN12624000753550