The fourth over is where a quick tells the truth

Length drift under fatigue is the most reliable early warning of a breaking down fast bowler and almost nobody tracks it.

Bowlers do not get slower first. That is the intuition and it is wrong often enough to be dangerous. What tends to go first, as a fast bowler accumulates fatigue across a spell or across a season, is the consistency of where the ball lands, and specifically a creep in length, because the front leg is not blocking as firmly and the release point drifts a few centimetres.

The speed gun will show something eventually. By then he has been compensating for weeks, and compensation is where injuries come from. A bowler whose front leg is tired will unconsciously find a way to keep the pace, and the way is usually more trunk flexion or more counter rotation of the shoulders, which is exactly the pattern that stress fractures grow out of.

So the number I would want, and which almost no team reports publicly, is the standard deviation of pitch length across a spell. Not the average. The spread. A bowler landing everything within a tight band in his first over and scattering it a metre either side in his fourth is telling you something the radar cannot.

There is an obvious objection: variation is deliberate. Slower balls, bouncers, yorkers, all of them move the landing point on purpose. Fine. Strip those out. Look only at the stock ball, which is easy to identify by grip and speed, and measure how much its landing point wanders as the spell goes on.

I would use that reading for two decisions and neither is medical. First, whether to give him the eighteenth over, because a bowler whose stock ball has already begun scattering will not hit a yorker under pressure and everybody in the dugout knows it by feel and pretends not to. Second, whether he has an extra spell in him next week.

The reason it has not become standard is that ball tracking of this granularity is expensive to keep across training as well as matches, and the useful comparison is against his own baseline rather than a league average. Franchises that only measure in games are seeing a small fraction of his bowling.

My blunt version is this. Watching a fast bowler's fourth over will tell an experienced coach more about his condition than a fitness test on Tuesday, and we have built entire medical departments around the Tuesday test.