The vest cannot tell a sprint from a panic

Wearable tracking counts metres and accelerations honestly, and then coaches read intention into numbers that contain none.

A GPS unit between the shoulder blades knows three things. Where the torso was, how fast it got there, and how hard it changed direction. It does not know why. That gap is where most of the bad decisions in sports science live.

Two players finish a match with almost identical high speed distance. One of them made eight timed runs in behind, arriving at pace because he read the pass early. The other spent the game chasing after the ball had gone, late and stretched, braking hard at the end of every effort. On the report they look like twins. In the legs they are not remotely alike, because the eccentric load of a hard stop is a different injury conversation from the load of a smooth acceleration into space.

I would go further. The most dangerous efforts a footballer makes are the ones that end badly, and the vest is worst at exactly those. It captures the sprint. It does not capture the stretch, the reach, the moment the hip goes past its comfortable range while the foot is planted. Hamstrings do not tear because of total distance. They tear at the end of a lengthening contraction that the athlete did not choose.

None of this means throw the data out. Trends are real. If a player's high intensity output falls for three weeks he is telling you something and you should listen. Aggregate load over a block is a decent guide to whether you have asked too much of a squad.

But I have sat in rooms where a number on a screen overruled the eyes of a coach who had watched the player for ten years, and it is a strange inversion. The eye sees a player labouring. The vest says he covered his usual eleven kilometres. Somebody decides the vest wins.

The fix is not more sensors. It is context on top of the sensors, which mostly means video synced to the peaks, so that when the trace spikes you can ask what he was doing. Half the time it turns out he was jogging back after a corner and the ball hit him.

Measure what you can. Then go and watch it. The number is a question, not an answer, and treating it as an answer is how a fit player gets rested and a tired one gets picked.