The kick serve is bought with the lower back

Arching and twisting to brush up the back of the ball puts repeated load on a small area of bone that does not forgive volume.

Every second serve that jumps up over a returner's shoulder was produced by bending the spine backwards and twisting it at the same time, at speed, several hundred times a week.

The mechanics are unavoidable rather than optional. To get topspin on a serve you must contact the ball while the racket is travelling upward and across, which means the body has to lean back and to the side of the tossing arm so the racket can drop below the ball. That posture puts the lumbar spine into extension and rotation together, and the small joints at the back of each vertebra are the structures that carry the load in that position. The bony bridge between them is thin. Load it hard, repeatedly, with a rotation on top, and it develops a stress reaction the same way a runner's shin does.

What makes this a serving problem specifically rather than a general tennis problem is asymmetry. Groundstrokes rotate the trunk in both directions across a match. The serve only ever goes one way. So the load piles onto one side of the spine, and the imaging, when a young player finally gets some, tends to show it on the non-dominant side, which surprises people who assume the racket arm side takes everything.

Young bodies are where this bites hardest, because growing bone has weaker points than mature bone and junior players with a good kick serve get told, correctly, that it is their weapon. Then they hit five hundred of them a week for three years at the age when the structure is least tolerant. It is not the technique that fails. It is the arithmetic.

The interesting part is what does not fix it. Core strengthening helps stability but does not remove the geometry. Stretching does nothing about a bone that is being bent. The only real lever is how many, how often, and whether the flat serve gets used enough on second balls to spread the pattern. That last one is a genuine argument, because a flatter second serve is a riskier second serve and every coach knows it.

I would take the double faults. A player who serves flatter and loses two points a match is a player whose spine is still there in ten years, and I have watched enough gifted juniors disappear at nineteen with back pain to think that trade is not close.