A heavy ball is a wrist problem before it is a shoulder one

Damp conditions and worn balls load the forearm in a way that gets misdiagnosed for weeks as a technique fault.

A ball that has absorbed moisture, or one that has been in play long enough for the felt to mat down, is heavier and slower and it hits the strings with more momentum for the amount of pace it carries. That combination is unpleasant for the forearm in a specific way. The impact shock rises while the racket head speed you can generate falls, so you end up gripping harder to control a ball that is fighting you, on a shot that is producing less reward than usual.

Grip pressure is the mechanism. A relaxed hand lets the frame rotate a little and the shock disperses through the arm and trunk. A tight hand transmits it straight to the wrist and the extensor tendons at the elbow. Players tighten their grip when they feel the ball is heavy, which is exactly the moment they should be doing the opposite, and the discomfort that follows tends to get labelled a technique problem or an overuse injury rather than an equipment and conditions problem.

You see this most in autumn and winter club tennis, on damp outdoor courts with a tin of balls that should have been retired a month ago. The complaint rate goes up and everybody talks about grip size and racket weight. Some of that matters. Most of it is that the ball has changed and the player has responded by squeezing.

The counterintuitive fix is a heavier racket, not a lighter one, because more mass at the head resists the twisting and reduces the amount the hand has to do. Light rackets feel easier on the arm and are frequently worse for it, which is one of the least helpful facts in the equipment business, since light rackets are easier to sell to exactly the people they will hurt.

The second fix is retiring balls sooner and nobody does it because balls cost money and the difference is gradual enough to argue with.

I would rather buy new balls than new physiotherapy.

That is not a sophisticated position. It is just cheaper.