Fatigue is a decision-making problem before it is a muscle one

Late-game errors get blamed on legs, and the thing that degrades first is the judgement that puts the legs in the right place.

Eighty-eight minutes. A defender steps out to press, arrives half a second late, and gets turned. The commentary says he is tired. He probably is, and the tiredness that beat him was not in his quadriceps.

Physical output in the final minutes of a match falls less than most people assume. Top speed holds up reasonably well; total high-speed distance declines, though a good deal of that is teams changing how they play rather than being unable to run. What deteriorates faster and more reliably is the quality of choices. Scanning drops off. Players check their shoulders less often, so they hold a poorer picture of what is around them, and a poorer picture produces a slower decision even when the body is perfectly capable of executing it.

The mechanism is that attention is expensive. Maintaining a mental model of twenty-one moving people while tracking a ball and rehearsing your own next action is demanding work, and it degrades under both physical and cognitive load. When it degrades, players fall back on defaults. They mark the man in front rather than the space behind. They pass to the obvious option. They stop making the run that is only worthwhile if somebody else sees it.

Which makes a lot of standard conditioning slightly beside the point. Running players until they are exhausted trains the legs to tolerate a state in which the brain has already stopped working properly, and it trains it with no decisions attached, because a shuttle run requires none.

The better design puts the decision inside the fatigue. Small-sided games at the end of a session, with a scanning constraint, or a rule that changes mid-drill so players have to update a plan while breathing hard. It is harder to organise and harder to measure than a set of runs, which is exactly why it loses the argument on a busy week.

Substitutions look different in this light too. A fresh player is not primarily fresh legs. He is a functioning decision-maker dropped into a game where several people on both sides have stopped being one.

That is the advantage. Most coaches spend it on running.