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Thresholds, VO2max and durability

Two thresholds, a dozen names between them, and how far VO2max can realistically be moved. Holding your numbers late in a race is a separate quality again.

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Two thresholds, a dozen names

There are two intensities worth naming. The first is where lactate first rises above resting levels and breathing starts to change. That is the top of genuinely easy work. The second is the highest intensity at which lactate production and clearance still balance, above which nothing stabilises.

The naming is a mess for a concrete reason: dozens of threshold concepts have been published, defined by different criteria and different test protocols, and reviews of them find the various definitions do not identify the same intensity in the same athlete ¹. Aerobic threshold, LT1, VT1, first turn point, and aerobic-anaerobic transition are all pointing near the first. MLSS, anaerobic threshold, LT2, VT2 and OBLA are all aiming at the second, with real disagreement about where it lands.

Critical power targets the same upper boundary from a different direction, performance rather than blood sampling, and marks the point above which oxygen uptake drifts to maximum and exhaustion becomes a matter of time ². Functional threshold power is a field estimate that behaves usefully but does not reliably coincide with that boundary; agreement between FTP and laboratory measures of maximal metabolic steady state has been poor, in both directions ³.

VO2max and what moves it

VO2max is the ceiling on how much oxygen you can take in and use per minute. It sets the roof on sustained aerobic power, but it does not by itself say where your thresholds sit under that roof, which is why two athletes with the same VO2max can race very differently.

It is trainable, within limits. Pooled across high-intensity interval training studies, average absolute improvement lands on the order of half a litre per minute, with the largest gains coming from protocols that actually spent time near maximal intensity rather than merely calling themselves hard ⁴. Individual response to the same programme varies widely.

Expect the biggest movement early in a training history and progressively less as you approach your own ceiling. Athletes who have trained for years typically improve race performance through threshold, economy and fatigue resistance rather than through further VO2max gains.

Durability: a separate axis

Every threshold and VO2max number above is measured fresh. Durability is how much of that profile survives after hours of work: whether your second threshold at hour four is close to your second threshold at minute ten ⁵.

It varies between athletes independently of their fresh numbers, which is what makes it worth measuring separately rather than treating late-race fade as noise or as a fuelling problem. Physiological resilience has been argued as a distinct determinant of endurance performance alongside VO2max, threshold and economy ⁶.

What this changes about testing

A fresh ramp test measures your best case, and events longer than a couple of hours are decided somewhere else. Assessing durability means looking at what happens to the same effort late: heart rate drifting up at a fixed power, power falling at a fixed heart rate, or a repeat of a submaximal effort placed deliberately after several hours of work ⁵.

Accumulated work is the thing being tested, so the assessment has to include it. Training that never goes long enough to expose the fade will also never tell you whether it is improving ⁶.

References

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