Durability (fatigue resistance)

Evidence: moderate

A newer “fourth determinant”: how well VO₂max, threshold and economy hold up late in a long effort. The direction is robust, the numbers still immature.

Durability is the ability to resist the deterioration of physiological characteristics during prolonged exercise. Proposed as a fourth determinant of endurance performance, it captures something the classical three, measured fresh, miss: a runner’s VO₂max, threshold and economy all worsen over the course of a long effort, and the rate at which they worsen varies between people (Maunder et al. 2021). For the marathon and beyond, the decisive physiology is what remains in the final hour, not what could be measured at the start.

This is a genuinely new line of work, with most key papers from 2021 onward. The direction of the findings is robust; the precise numbers are still immature.

How it is assessed

Two approaches, one rigorous and one practical.

Pre- and post-fatigue laboratory testing. Measure threshold, economy or critical speed when fresh, impose a prolonged bout of exercise, then remeasure. The shift is the durability deficit. This is the rigorous method, and it shows clear deterioration:

• Speed at the first lactate threshold (LT1, the aerobic threshold) fell by around 5 to 6% after 90 minutes of easy running (Eur J Appl Physiol 2025). This is the lower of the two thresholds, so the figure should not be read as the drop in the higher threshold that sets race pace.
• Critical power can decay by roughly 10% after fatiguing exercise, enough to push a pace that was sustainable early into the unsustainable domain late on (Maunder et al. 2021).

The catch is that the result depends heavily on the dose of prior exercise used to fatigue the athlete, so protocols are not yet standardised and reproducibility is a live concern.

Decoupling, the field metric. In normal training, durability shows up as cardiovascular drift: heart rate climbing through a steady-paced run even as pace holds constant, driven by rising core temperature, fluid loss and glycogen depletion. In one protocol heart rate drifted up by around 14 to 15 beats per minute over three hours (Front Physiol 2023). It is quantified as aerobic decoupling: the pace-to-heart-rate ratio in the first half of a steady effort compared with the second half (TrainingPeaks decoupling). A commonly used scheme reads under 5% as good aerobic durability, 5 to 10% as moderate and over 10% as a sign of insufficient base, though those cut-offs are a practitioner convention, not a physiological law. The metric has real-world validity: marathon runners with the least heart-rate-to-pace drift tend to hold pace better and finish faster.

What trains it

Durability appears trainable, and partly separable from aerobic capacity. In one 10-week study it improved with both low- and high-intensity training, while VO₂max rose only in the high-intensity group (Front Physiol 2023). The practical levers are the obvious ones: accumulated easy volume and the long run, which rehearse holding output as fatigue builds. Fuelling interacts strongly, since much late-race deterioration is glycogen depletion, so in-race carbohydrate and loading directly defend durability, and resistance to muscle damage may also contribute.

The body of evidence is still limited and developing, but the practical use is already clear: track decoupling on long steady runs as a cheap durability check, and treat a falling drift over a training block as a sign the aerobic base is deepening even if VO₂max has not moved.

Related

• VO₂max
• Lactate threshold
• Muscle tone (Bakken) — a distinct, practitioner concept often confused with durability
• The long run
• In-race carbohydrate
• GPS watches and metrics