The radar gun is measuring the wrong part of your serve

Racquet speed is produced by the trunk and the legs, so the scoreboard number says almost nothing about where the load went.

The number goes up on the screen and everybody looks at the arm. It is the wrong place to look. By the time the racquet is moving, the arm is mostly along for the ride, and the speed on the board was produced somewhere lower down.

A serve is a sequence. The legs drive up, the hips rotate, the trunk rotates a fraction later, and the separation between those two rotations stores energy the way twisting a towel does. The shoulder then delivers it, the forearm rotates violently through contact, and the racquet arrives fast because everything before it timed correctly. The arm is the last link. It contributes least and it absorbs most.

Which is why serve speed is such a poor proxy for anything a medical team cares about. Two players can hit the same number, one by using the ground and the trunk properly, the other by hauling with the shoulder because his legs are tired or his separation is small. Same reading, completely different bill. The second player is the one who develops shoulder trouble in his mid twenties, and no scoreboard in the sport will flag him.

Fatigue makes it worse in a specific and predictable way. Legs go first. When the drive weakens, the kinetic chain shortens, and a player defending his speed compensates upstream with the shoulder and the trunk. So the most dangerous serves a player hits are the ones in the fourth hour that still read the same as the ones in the first.

The number I would rather see, if we are going to televise a metric, is the drop in average first serve speed across a match. That decline tracks the legs going, it predicts a drop in first serve percentage before it happens, and it tells you something honest about who is in trouble.

There is a related myth about the kick serve being safer because it is slower. It is not obviously safer. The heavy internal rotation and the trunk side bend involved in a big kick serve load the shoulder and the lower back in their own way, and back injuries in servers with extreme arch are not rare.

Look at the legs. If they have stopped driving, the number on the screen is being paid for by a shoulder.