The open stance forehand is a hip movement pretending to be an arm movement

Loading and unloading the back hip is what produces modern spin, and the joint has a limited tolerance for that twist.

Take away the arm entirely and you can still tell a heavy topspin forehand from a flat one, because the difference is visible in what the back hip does before the racket has even started forward.

In an open stance the player does not step into the ball. He turns and loads onto the outside leg, letting that hip take the whole body weight while it rotates inward, then drives back the other way to launch the trunk. The hip joint is the hinge for all of it. That inward rotation under load is what stores the energy, and the stroke is essentially the hip giving it back.

The joint is well built for this and not infinitely so. The ball of the hip sits in a socket with a rim of cartilage around it, and the position of deepest inward rotation with the thigh bent is exactly the position in which the neck of the bone comes closest to that rim. Load it there, hard, repeatedly across an adolescence and a career, and the tissue at the front edge of the socket takes contact it does not enjoy. Players describe it as a pinch in the front of the groin at the end of the load, not as pain in the swing.

Grass court players of an earlier generation did not accumulate this in the same way, because a step-in stance passes the load through both legs and reduces the demand on any single hip rotation. Modern hard court tennis, played wide of the sidelines with recovery steps in both directions, has effectively made the outside hip the engine of the sport.

There is an argument that this is simply the cost of playing well, and I would half accept it. What I do not accept is the frequency. A junior hitting eight hundred forehands a day is producing eight hundred maximal hip rotations, all in one direction, on one leg, on an unforgiving surface. Nobody would program eight hundred single leg rotational lifts and call it developmentally sound.

The tell that something has gone wrong is not usually a limp. It is a player who starts stepping in more, hitting flatter, and losing depth, and everybody assumes his technique has drifted. It has not drifted. It has been renegotiated by a joint that will no longer sell him the rotation at the old price.