https://www.sciencedirect.com/science/article/abs/pii/S0021929022001130
The shoulder blade makes up the socket of the "ball and socket joint" of the shoulder (the humerus making up the ball). The shoulder blade is attached to the collar bone at the shoulder (Acromio-clavicular joint) and then loosely to the rib cage in the back (scapulo-thoracic). This study looked at how much the scapulo-thoracic motion contributes to external rotation of the shoulder. They found that with the arm by your side (0° abduction) it causes 10° of external rotation and with the arm by your side (90° abduction) it causes 15° of external rotation. This shows how the shoulder blade contributes to external rotation and should be looked at in shoulder issues.
