Limb tracking tells you about the hamstring long after the hamstring knew

Wearable data is very good at describing what already happened and much worse at warning anybody in time.

The pitch is covered in sensors now. Vests between the shoulder blades, optical tracking overhead, sometimes markers on the thigh, and a sports scientist with a laptop who can tell you what a player's stride length did in the eighty-third minute compared with his own baseline.

That is a real capability and I do not want to be sniffy about it. The question is whether it prevents anything.

Here is my worry. The signals that reliably precede a muscle injury are small, they vary enormously between individuals, and they show up in the same window in which a perfectly healthy player is simply tired. Stride length shortens. Ground contact time goes up. Asymmetry between left and right widens a few percent. All of that also describes a fit midfielder at the end of a hard game, which means any threshold sensitive enough to catch the injury will also flag half the squad every week.

So the department faces a choice nobody envies. Set the threshold tight and become the people who cry wolf until the manager stops listening. Set it loose and catch the injury on the replay, where it is a description rather than a warning.

What the data is genuinely good at is the boring middle of the week. Chronic load against acute load, making sure a player returning from three weeks out does not go from nothing to a full session, controlling the ramp. That is where wearables have earned their money, quietly, and it is why hamstring reinjury rates are the number worth watching rather than first injury rates.

Where it gets oversold is the in-game promise. The idea that a screen will go amber at minute sixty and a substitution will save a hamstring assumes that the useful signal exists, arrives early enough to act on, and is distinguishable from ordinary fatigue in real time. Two of those three are shaky.

There is also a human problem. Players are not stupid, and a player who knows his numbers are being watched can run in a way that keeps the numbers happy.

The most reliable early warning system in football is still a physio who has known the player for four years watching him warm up and saying, quietly, he is not right today.