Rehab percentages compare you to a number nobody wrote down

Ninety per cent of what, exactly, when the healthy baseline was never recorded before the injury happened.

A player is cleared at ninety per cent. Ninety per cent of what?

In most cases the honest answer is: of the other leg, today. Which is a substitute for the number anyone actually wants, namely what that specific limb could do before it was hurt, and that number does not exist because nobody tested him in March when he was fine. So the uninjured side stands in for the missing baseline, and everyone agrees to treat the substitution as free.

It is not free. The uninjured side has been doing more work than usual for eight weeks. It has taken the landings, the push-offs, the single-leg everything while the injured side was protected. If it has gained five per cent over that period, and there is every mechanical reason to think it might, the ratio you are reading understates the recovery. Push the same logic the other way: in a long lay-off with reduced total training, the healthy side detrains too, and now the ratio flatters the athlete because the comparison has fallen towards him.

The denominator is moving. That is the problem in one line. A percentage whose denominator drifts in an unknown direction across the exact window you are measuring is not a measurement, it is a mood.

Squads that do preseason testing think they have solved this, and partly they have, but only if the preseason test is the same test on the same device with the same protocol, and only if it was recent enough to still describe the athlete. A baseline from eighteen months and one growth spurt ago is a historical document. For a young player it can be worse than nothing, because it sets a target the limb has outgrown and encourages an early return.

What I want on the form is both comparisons side by side, plus the date of the baseline in the same size type as everything else. Ninety per cent of the other leg, seventy-eight per cent of his own March number, baseline taken fourteen months ago. Three facts and a caveat, and it takes one line.

One number cannot carry that. The reason the single percentage survives is not that it is better. It is that it fits into a sentence a manager will listen to.