You do not run out of fuel in a long sprint, you run out of relaxation

The tightening arms and climbing shoulders of a dying one-lap runner are not a symptom of the problem; they are most of it.

Three hundred and twenty metres into a 400, something happens that every spectator can see and almost every commentator describes wrongly. The runner's shoulders come up. The hands open and start slapping rather than driving. The face goes tight, the head rolls slightly, and the whole action becomes noisier while producing less. This is usually explained as the athlete running out of energy.

Look again at what is actually costing them.

Tension is expensive. A sprinter who is holding the trapezius, the jaw and the forearms is doing muscular work that puts nothing into the track, and worse, co-contraction around the hip and knee means the agonist is fighting a partly braced antagonist on every stride. The mechanical efficiency of the running drops precisely at the moment there is least capacity to spare, so the same speed now costs more than it did on the back straight. It is a spiral, and it feeds itself.

Which is why the training answer of just doing more grim repetition work is at best half right. Yes, you have to be able to keep producing under acidosis, and there is no gentle route to that. But an athlete who tightens at three hundred metres will tighten faster and earlier with more of that work, because you have been rehearsing the exact thing you want to eliminate. Every rep run in a locked, straining shape is a rep teaching the nervous system that this speed requires straining.

The coaches I trust run speed endurance with a hard rule about form. The moment the shape breaks, the rep is over. Not finish it anyway for character. Over. What the athlete is being taught is that this pace and a loose jaw belong together, and that lesson only lands if the pairing is never broken.

Relaxation is trainable and it is barely trained, mostly because it does not photograph as hard work and it cannot be written in a log as a number.

I would take an athlete who can hold a smooth action at ninety per cent over one who can grind out a faster time in training while falling apart. The first one has room left. The second one is already at their ceiling, and they will meet it again in the final, in front of everybody.