In the heat, the finishing time is decided in the first two laps

Core temperature rises on a trajectory set early, and a distance runner who starts too fast in warm conditions has already lost minutes.

Twenty eight degrees, high humidity, no wind, and a ten thousand metre field goes through the first kilometre on personal best schedule. Everything after that is arithmetic.

The body sheds heat mainly by evaporating sweat, and when the air is already close to saturated that route narrows sharply. Meanwhile heat production scales with running speed. So a runner in those conditions is filling a bath with the drain half blocked, and the rate at which the water rises was set by the tap setting in the first few minutes. Core temperature does not respond quickly to a change of mind at six kilometres. The trajectory has already been established, and the runner will be slowing down whether or not she chooses to.

What follows is not really willpower. As core temperature climbs, the body reduces the drive to the working muscles, partly through central mechanisms and partly through the competition for blood flow between the skin, which needs it for cooling, and the legs, which need it for everything else. The runner feels it as a loss of belief, and describes it afterwards as a bad day, and it was mostly a pacing decision made twenty five minutes earlier.

The practical version is unglamorous. In heat, target an effort rather than a time, accept from the start that the split will be slower than your fitness deserves, and understand that the fastest runner on the day is likely to be the one who slowed least rather than the one who started best. Championship distance races in hot cities are almost always won from behind, and the video looks like a heroic late surge when it is really everyone else paying a bill.

Precooling helps and is underused outside well funded teams. Ice slurry before the start, cold towels, a cooler vest during the warm up, all of it buys a small amount of headroom at the top end and delays the point where the trajectory bites.

Heat acclimation helps more, and it needs ten to fourteen days of genuine exposure, not a sauna the week before.

But the largest lever is free and nobody pulls it, because pulling it means letting the field go in the first kilometre while a stadium watches and wonders whether you have lost your nerve.