A tired server hurts the shoulder with the legs

Serving injuries usually get attributed to volume, but the mechanism is a leg drive that quits before the arm does.

The shoulder is the last link in the serve, and last links only fail when the earlier ones stop contributing. That is the part of serving injury that gets skipped in favour of counting serves, which is easy to do and tells you almost nothing about whether any given serve was dangerous.

Here is the chain. The legs push, the hips rotate, the trunk uncoils, the shoulder externally rotates to a position that looks anatomically alarming, and the arm whips through. When the legs are fresh the shoulder is being carried by everything beneath it and the extreme position is passed through quickly under momentum that was generated elsewhere. When the legs are tired, the push shortens, the hips rotate less, and the arm has to make up the difference with its own muscular effort, at the same speed, in the same extreme position, without the momentum. Same serve on the scorecard. Completely different load on the joint.

Which is why the serve that injures you is usually not the two hundredth of a session. It is the fortieth, hit at the end of a long rally, on the second point after a twenty three shot exchange, when the legs have nothing and the player still wants the same number on the radar. Volume counting misses this entirely because it treats every serve as equivalent.

If I were monitoring a young server I would care about jump height on the serve far more than about the count. It is easy to film, it degrades visibly as the legs go, and it is a direct proxy for how much of the work is being done by the parts that should be doing it. When the contact point starts dropping, stop. Not because the shoulder hurts, but because it is about to start.

The coaching implication annoys people, because it means the fix for a sore shoulder is often squats and conditioning rather than a technical rebuild of the arm action. Nobody wants that answer. It is slower and it does not feel like tennis.

There is also a scheduling version of this argument, but that is a different piece and other people make it better.

The arm is the fuse. Fuses blow because of what is upstream.