Triathlon · 5 min read
Long-course triathlon is decided by the intensity you can hold for eight to seventeen hours, and by whether your fuelling and hydration hold up over that time. Both are measurable, and neither is well estimated by feel.
By Chris Gkoufas, DVOTION Labs ·
An Ironman is raced a long way below maximum. That makes the aerobic ceiling less decisive than it is over shorter distances, and makes the sustainable fraction of it — the intensity you can hold without progressive fatigue — the quality the race is actually run on.
The first lactate threshold matters more here than in almost any other event. Long-course racing sits around or below it for most of the day, so where that threshold falls determines your realistic race intensity. Riding or running above it early is the most common way a long day comes apart in its final third.
The second threshold still matters, because it bounds what you can do in the hard sessions that raise the first. But for pacing decisions on race day, the lower of the two is the one to know.
Thresholds are mode-specific. A cyclist-turned-triathlete and a runner-turned-triathlete can have quite different threshold heart rates on the bike and on foot, and applying one set of zones to both disciplines produces a programme that is too hard in one and too easy in the other.
Heart rate at a given metabolic intensity also differs between cycling and running for the same person, which means a single heart-rate zone table used across both is wrong for at least one of them.
Where testing budget is limited, the bike is usually the more valuable of the two for long-course racing: it is the longest segment, it is where pacing errors are most punishing, and power gives a directly controllable variable to pace against.
Over eight or more hours, carbohydrate availability and hydration stop being support acts. The intensity you can hold late in the day depends on what you have absorbed across it, and that has a ceiling set by your gut rather than by your legs.
Sweat rate and sweat sodium concentration vary substantially between athletes and cannot be inferred from one another. An athlete losing a litre an hour of concentrated sweat and one losing two litres an hour of dilute sweat can lose similar total sodium and need entirely different fluid plans.
What measurement gives you is a starting strategy to rehearse in training. What it does not give you is a plan to debut on race day — gut tolerance is trained, and a fuelling rate that works on paper and has never been practised is a race-day experiment.
At the intensities long-course racing is held at, a meaningful proportion of energy comes from fat. The intensity at which fat oxidation peaks — FatMax — is measurable from gas exchange during a graded test, and it sits low enough to be directly relevant to Ironman pacing in a way it is not for a 10 km.
One caveat is worth stating plainly, because it determines whether the measurement means anything: FatMax is heavily affected by what you ate beforehand. A carbohydrate meal before the test substantially reduces fat oxidation and shifts the measured intensity, which is why the assessment requires a fast and why a result taken fed should not be treated as your FatMax.
What it changes is how the long sessions are structured, and how much of the race can realistically be fuelled by carbohydrate you carry versus fat you already have.
Early in the build, so the zones shape the volume. A test in the final weeks describes what you are bringing rather than directing what you do.
Retesting mid-block is more useful in long-course than in shorter racing, because the blocks are longer and the adaptation window is wider. Twelve to sixteen weeks between tests is a reasonable interval; closer together and you are mostly measuring variation.
The hydration work is best done separately and closer to the event, at an intensity and duration representing race day, and ideally in conditions resembling the ones you expect. Conditions change sweat rate substantially, so a figure measured indoors in winter is a poor guide to a hot race.
Both, if the budget allows, because thresholds are mode-specific and a single set of zones applied to both disciplines will be wrong for one. If only one is possible, the bike is usually the higher-value test for long-course racing.
It is a useful field measure and it is an estimate of one point on the curve. It does not identify the first threshold, which is the one long-course pacing sits around, and it does not tell you anything about fat oxidation or substrate use across intensities.
Separately from the physiology testing, and closer to the event. It needs an intensity and duration representing race day, and ideally conditions resembling what you expect, because sweat rate moves substantially with heat, humidity and airflow.
For the gas exchange work, yes — six hours minimum and overnight preferred. VO₂ max and the thresholds hold up whether or not you have eaten, but FatMax does not, and it is the one measurement in the test that depends entirely on the fast.
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