The Achilles Problem That Warms Up and Then Gets Worse Tomorrow
There is a pattern so characteristic that experienced clinicians recognize it from the description alone. You are stiff and sore for the first few minutes of a run. Then it eases. By a mile or two in, you barely notice it. You finish the run thinking it is improving. Then you get up the next morning and the first few steps to the bathroom are considerably worse than the day before.
That sequence is the signature of Achilles tendinopathy, and the reason it catches so many runners out is that the warm up effect gets interpreted as evidence that running is helping. It is not. It is just what tendons do.