By the time the load data reaches the bench it is history

Live tracking promises in-game injury prevention and mostly delivers a very precise record of a decision that was already made.

The vest data is on a screen behind the dugout. It updates constantly. Distance, high speed running, accelerations, all of it live, and somewhere in the seventieth minute a number crosses a threshold that suggests a left back is in the zone where soft tissue injuries become likelier.

By then he has already been in that zone for four minutes, the manager is watching a corner being taken and not a laptop, the substitution he wants to make is the attacking one because the game is level, and the fourth official's board goes up two minutes later with somebody else's number on it.

That is the honest picture of in game load monitoring at most clubs, and I say it as somebody who thinks the technology is genuinely useful. It is useful between matches. It is close to decorative during them, because the loop from measurement to decision to action is longer than the window in which the decision would have mattered.

The bottleneck is not the sensor. Sensors are fine. The bottleneck is that a substitution is a competitive decision with a tactical cost, made by one person who is being asked to weigh an abstract elevated risk against a very concrete match situation he can see with his own eyes. Nobody takes off a full back who is playing well, at nil nil, because a trace suggests fatigue. Nobody.

What would actually help is quieter and less impressive. Pre agreed rules made on Friday, when nothing is at stake and nobody is emotional. This player comes off on seventy minutes if he started midweek, and the decision is not reopened during the game. That is a policy, not a monitor, and it requires the manager to give up authority in advance, which is the part that fails.

There is a counterargument worth respecting. Removing judgement from a live situation is how you take off the player who was about to win you the match. Fine. Then say so, and stop pretending the screen is what protects the squad.

Precision is not the same thing as usefulness, and a very exact record of an injury you did not prevent is still just a record.