In a hot final the cooling budget decides the pacing, not the legs

Championship distance races in heat are won by whoever spends least on staying cool, which is a decision made before the gun.

A major championship schedules a distance final at seven in the evening in a coastal city in August, humidity high, air still. Every athlete in that field is fit. Within twenty five minutes a third of them are running with the deliberate, upright, faintly stunned action of someone whose body has quietly changed its priority from moving forward to not overheating.

That is a competition between cooling systems, not engines.

The mechanism is simple and it is unforgiving. Working muscle produces far more heat than mechanical work, and the only large scale way to shed it is by moving blood to the skin and sweating. Blood at the skin is blood not at the muscle. In still, humid air, sweat does not evaporate efficiently, so the cooling costs more circulation for less return, and cardiac output has to rise just to hold the same pace. The athlete slows down or their core temperature climbs. There is no third option and no amount of will alters the arithmetic.

Which is why pacing in heat has to be decided in advance and defended stubbornly. Going out at goal pace because that is what the training said is how people end up walking at thirty two kilometres in a marathon or dropping out with three laps to go on the track. The correct opening pace in those conditions is slower than feels right, and it feels wrong precisely because the athlete is not yet hot. The cost arrives later.

The preparation that pays is the part that happens weeks earlier. Heat acclimation is one of the few interventions in endurance sport that reliably produces a large effect in a short time, and it is boring, uncomfortable and mostly a matter of turning up to sit in the heat repeatedly. Plasma volume rises, sweat starts earlier and contains less salt, and the same pace runs at a lower core temperature. Athletes who skip it because it interferes with quality sessions are protecting the training at the expense of the race.

Pre-cooling before the start helps too, though not as much as the ice vest photographs suggest.

The uncomfortable conclusion is that in these finals, the best runner in the field is often not the one who wins, and the result tells you more about who prepared for the weather than about who was in the best shape.