The shoulder trades stability for reach and the muscles pay the rent

A joint with almost no bony containment stays located only while the surrounding muscle keeps working, and muscle tires.

Put a golf ball on a coin and you have roughly the geometry of the shoulder. The humeral head is large, the socket it sits against is shallow and small, and only a fraction of the head is in contact at any moment. Every other joint in the body gets some containment from its own architecture. This one gets almost none, and buys the most range of motion in the human body with the difference.

What holds it together is active. The rotator cuff muscles compress the head into the socket and steer it, adjusting continuously as the arm moves, so that the centre of rotation stays roughly where it should be. The labrum deepens the socket a little. The capsule and ligaments matter mostly at end range, when they are being stretched, and they are the last line rather than the first.

Fatigue therefore does something to the shoulder that it cannot do to the hip. As the cuff tires, its ability to keep the head centred declines, and the head migrates upward under the pull of the larger muscles above it. The space between the head and the bone above narrows. Tendon that was passing freely through that space starts being compressed on every overhead repetition, which is why throwing and swimming injuries so often present as pain that arrives at a specific point in a session and not before it.

Then the scapula, which is the part people forget. The socket is on a bone that floats on the ribcage, held by muscle alone, and it must rotate upward as the arm rises to keep the socket underneath the head. When the muscles controlling that rotation fatigue, the socket stops arriving on time. The arm goes overhead against a platform that has not finished moving.

So a throwing shoulder is not a joint that fails when loaded too heavily. It is a joint that fails when its stabilisers run out of endurance, and endurance is a function of pitch count, laps, sets and the fourth hour of a tournament day.

Which makes volume the thing that hurts shoulders, not intensity.

Strong through six innings and unprotected in the seventh is the same shoulder, and only one version of it is the one that gets injured.