The vest data that matters is the braking, not the running

Total distance covered is the number that gets on television, and it is close to the least useful thing the tracking system records.

Twelve point one kilometres. It goes up on screen next to a midfielder's face and everybody nods, because it sounds like effort and it is easy to understand.

It tells you almost nothing about what he did or what it cost him.

A player can accumulate a large distance figure by jogging endlessly between positions in a possession-heavy side, and that is a low-cost afternoon for his body. Another can cover two kilometres less while producing forty hard decelerations, twenty changes of direction and a dozen maximal sprints, and walk off with legs full of micro-damage. The second one is the one at risk. The tracking vest knows this perfectly well. It records accelerations and decelerations at a resolution of tenths of a second. The broadcast graphic ignores all of it in favour of a single cumulative number that would have made sense in about 1994.

Deceleration is where the actual load sits. Slowing from a sprint puts eccentric strain through the hamstring and the quadriceps in a way that steady running never does, and the positions that do it most are the ones with the worst injury records. A full-back in a side that presses high is doing this constantly, sprinting up to close down, checking, turning, recovering. His distance total may be unremarkable. His body is taking a beating.

Which leads to a conclusion that sounds backwards. A lot of soft-tissue injuries happen not because players sprint too much but because they sprint too little in training, then get asked to hit maximum velocity in a match without having been exposed to it during the week. The tissue has not been prepared for the speed it is asked to produce. Wrapping a fragile player in cotton wool through a training week and then starting him on Saturday is roughly the worst available option, and it is chosen constantly for entirely understandable reasons.

There is also the human problem with all this. Players know they are being measured. Some of them, entirely rationally, manage their numbers. If the coach reads the distance report, you get distance. It is the same failure mode as any metric attached to a consequence.

So when a fitness graphic appears at half-time, treat it as entertainment. The useful version exists, it is on a laptop in the medical room, and it is mostly about how hard people are stopping.