A heavier frame is only stable while you are fresh

Swing weight is bought with shoulder work, and the interest on that loan is paid late in long matches.

Everyone tells the beginner to use a stable racket, and stability comes from mass distributed away from the hand. That advice is right for the ball and incomplete for the arm.

The number that matters is not how much a racket weighs on a scale but how much effort it takes to swing, which depends on where the mass sits. Move a few grams to the head and the resistance to being swung climbs steeply, because it scales with the distance from the hand squared. So a modest change in the tape at twelve o'clock produces a large change in what the shoulder has to do on every stroke.

Fresh, that is a good trade. A head-heavy frame plows through the ball, twists less on off-centre contact, and gives a player free depth. The muscles paying for it are the ones around the shoulder blade and the rotator cuff, which have to both accelerate the racket and, more expensively, stop it afterwards.

Fatigue changes the accounting. The stabilising muscles around the shoulder blade are small, they work continuously rather than in bursts, and they tire earlier than the big movers. When they fade, the blade stops positioning itself correctly during the swing and the arm ends up working from an unstable base. Now the extra swing weight is being decelerated by a joint that is no longer being held where it should be, and the tissue in the narrow space at the top of the shoulder gets pinched on every follow-through.

That is why the third hour is where equipment choices show up as injuries. Nothing about the racket changed. The support around the joint did, and the load stayed constant, which is the same as the load rising.

I would put it this way. A player should choose a frame for the last set he intends to play, not the first. Almost nobody does, because rackets get chosen in a twenty minute hit on a good day with fresh legs, which is the single least representative condition available. Then the shoulder spends a season telling him about it in a language he reads as tiredness.