Compactness is really a time measurement in disguise

Distance between the lines is the wrong way to describe a defensive block, because what an attacker experiences is delay, not metres.

Twenty five metres from the first line to the last. That is the number teams quote and analysts reproduce, and it is a reasonable shorthand that describes the wrong thing. An attacker on the ball does not perceive metres. He perceives how long he has before somebody reaches him, and those two quantities come apart constantly.

A block spread over thirty metres, defended by quick players facing forwards, can be less penetrable than a twenty metre block full of tired legs with their weight on their heels. Same picture on a tactical camera, entirely different experience for the man trying to pass through it. What matters is the time from the moment the ball is played to the moment pressure arrives, and that is a function of reaction, orientation and acceleration, not of the gaps you can measure on a still frame.

This is why fatigue destroys defensive blocks in a way that does not appear in the shape. Watch a compact side in the eightieth minute. The lines are still where the coach wants them. The distances have not changed. But every arrival at the ball is now a third of a second late, and a third of a second is enough for a good midfielder to turn, which means the block has failed while still looking correct.

It also explains a thing that confuses people, which is why some deep defences are extremely hard to play against and others get carved open despite defending the same territory. The difference is almost always the first step of the man nearest the ball, not the arrangement of the ten behind him.

If I were building a measure of it, I would time pressure instead of drawing it. Ball played into the block, stopwatch running, stop when a defender is within a yard. Average that over a match and compare it across the halves. That single number would tell you more about a defensive performance than any heat map, and it would expose exactly which teams are surviving on organisation and which are surviving on legs they will not have in April.

Organisation is what you can coach. Legs are what you buy. Most clubs confuse the two and end up short of both.