Running · 5 min read
Marathon performance is governed by three physiological qualities and how they interact. Knowing which of the three is currently limiting you is the difference between training more and training the part that is holding you back.
By Chris Gkoufas, DVOTION Labs ·
Endurance performance rests on VO₂ max, the fraction of it you can sustain, and how much oxygen it costs you to run at a given speed. None of the three is sufficient on its own, and the interaction between them is what produces a finishing time.
That is why VO₂ max alone is a poor predictor. Two runners with an identical maximum can run very different marathons if one holds a higher fraction of it for longer, or runs more economically at race pace. It is also why chasing the maximum is often the wrong project: for a race lasting three or four hours, the sustainable fraction and the cost of each stride do more work than the ceiling does.
Measurement tells you which of the three is yours. Without it, the default is to train whichever one your programme happens to emphasise and hope it was the right one.
Marathon pace for a trained runner sits close to, and usually a little below, the second lactate threshold. That makes the threshold the ceiling the race is run against — raise it and sustainable pace rises with it.
Blood lactate sampling identifies where that threshold actually sits, stage by stage, from your own blood rather than from a percentage of maximum heart rate or an estimate produced by a watch. The two thresholds bound the training intensities: below the first is genuinely easy work, between them is the tempo region, above the second is where fatigue accumulates progressively.
The practical value is in the specificity. A threshold pace inferred from a recent race carries every condition of that race with it — the weather, the course, how the taper went. A measured threshold does not.
Research on training intensity distribution in middle- and long-distance runners consistently points towards most training volume sitting at genuinely low intensity, with a smaller proportion at high intensity. Polarised distributions have been shown to produce greater improvement in key endurance variables than threshold-dominant ones.
The difficulty is that "genuinely low intensity" is defined physiologically, not by feel. In practice a large number of runners spend their easy days above their first threshold — fast enough to accumulate fatigue, slow enough to feel comfortable. The result is a programme where the easy days are not easy and the hard days are therefore not hard enough.
A tested first threshold puts a number on the ceiling of easy. For many runners that number is slower than the pace they have been running, and applying it is the single change that most often makes the rest of the programme work.
Test early enough that the result changes the training. A measurement taken in the final fortnight before a race tells you what you brought to the start line; one taken twelve to sixteen weeks out sets the zones the block is built on.
For a marathon the useful combination is a blood lactate profile and a maximal respiratory test in the same appointment, run separately so neither interrupts the other. The lactate profile gives LT1 and LT2 with the speeds and heart rates at each. The maximal test gives your aerobic capacity, maximum heart rate and the ventilatory thresholds. Where the two threshold systems agree you can hold the zones with confidence. Where they diverge, that divergence is informative and worth discussing rather than averaging away.
Retesting at twelve to sixteen weeks is long enough for adaptation to have occurred. Sooner than that and you are largely measuring day-to-day variation.
A marathon is long enough that fuelling and hydration become performance factors rather than details. Carbohydrate availability late in the race and fluid and sodium losses across three or more hours both influence the last hour more than the first.
Sweat rate and sweat sodium concentration vary enormously between runners, and neither can be inferred from the other or from how much you appear to sweat. For a spring marathon in London the conditions are usually temperate, which reduces the fluid problem without removing it — and a warm year changes the arithmetic considerably.
Running economy is the third piece and the one least often addressed. It is influenced by biomechanics, neuromuscular factors and training history, which is why movement and force data are worth having alongside the physiology for a runner whose economy is the limiting factor rather than their engine.
There is no single value that produces a given time. Performance depends on the interaction of VO₂ max, the fraction of it you sustain and your running economy, and runners with the same maximum record substantially different times. A measurement tells you where you sit on all three rather than against a target figure that does not exist.
Wearable estimates are derived from heart rate and pace data against population models. They are useful for tracking change over time and they are not a measurement of your threshold. Where a tested threshold and an estimated one disagree, it is the estimate that is doing the guessing.
Twelve to sixteen weeks before race day, so the zones shape the block rather than describe it. A second test late in the block is optional and is better used to confirm that adaptation has occurred than to change the plan.
No. The protocol adapts to the intensity you can produce, and the zones are individual either way. What matters is whether you train to a structure you intend to follow — the measurements are equally accurate for a five-hour marathon and equally useless if nothing is built on them.
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