Abduction - Upward lateral movement of humerus cute to the side, away from the body
Adduction - Downward movement of humerus medially toward body from adbuction
When the arms are in their neutral position (0 degrees of adduction) the result will be a neutral pose of the scapula which is vertical.
First 30 degrees (fixing phase of the scapula), scapula will not move as the arm/humerus abducts upward.
Beyond 30 and up to 180 degrees, there is a 2:1 relationship between the abduction of the arm and the rotation of the medial boarder of the scapula.
45 degrees of arm rotation:
At a 45 degree rotation of the arm, the medial border of the scapula does not rotate for the first 30 degrees which leaves 15 degrees of rotation remaining. At a ratio of 2:1, this results in a 7.5 degree rotation of the medial border of the scapula.
90 degrees of arm rotation:
At a 90 degree rotation of the arm, the medial board of the scapula does not rotate for the first 30 degrees which leaves 60 degrees of rotation remaining. At a ratio of 2:1, this results in a 30 degree rotation of the medial border of the scapula.