The fourth spell is where fast bowlers break

Nobody breaks down in the first hour; they break down at ten past four, with a captain who has nobody else to turn to.

Bowling fast is a controlled collapse. The front foot lands with several times bodyweight going through it, the trunk hyperextends and then snaps forward, and the whole thing is held together by timing. Take the timing away and the forces do not go where they are supposed to.

Fatigue takes the timing away. That is the entire injury story and everything else is detail.

Watch a quick bowler's fourth spell of a long day and the changes are visible if you know where to look. The run-up shortens by half a stride. The front arm drops earlier so it is no longer pulling the body through. The hips stop rotating fully and the shoulder takes over, which is how a bowler ends up delivering with the trunk bent sideways rather than braced. That sideways bend, repeated, is where lumbar stress fractures come from, and it is not something that happens in one ball. It accumulates across a season of afternoons.

Which brings me to the thing that irritates me about workload management as it is currently practised. It is counted in overs. Overs bowled per week, per month, and there are perfectly sensible guidelines about not increasing them too sharply. But an over in the first spell and an over in the fourth are not the same unit, and anyone who has bowled will tell you so.

I would weight them. Overs in a spell beyond the third, on a hot day, in the second innings, on a flat pitch where nothing is happening and the bowler is trying harder for less, should count for more than a fresh burst with a new ball. Track that number rather than the raw total and I suspect the picture of who is at risk would change substantially.

There is also a selection consequence. A four-man attack where one seamer is not trusted is really a three-man attack, and the two who are trusted will bowl fourth and fifth spells routinely. Everybody talks about the balance of the side in terms of runs. Very few teams talk about it in terms of whose back is going to pay for the imbalance in August.

The captain who takes a bowler off while he is still going well is doing the most useful thing available to him. He rarely gets any credit for it.