Final answer:
Superior rotation of the scapula is essential for full shoulder abduction, as it prevents impingement between the humerus and the acromion. Without it, abduction would be limited to roughly 90 degrees.
Step-by-step explanation:
Without superior rotation of the scapula, full shoulder abduction would be significantly limited. During shoulder abduction, superior rotation allows the glenoid cavity of the scapula to move upwards, avoiding any impingement of the greater tubercle of the humerus on the acromion process of the scapula. If this rotation did not occur, full abduction of the arm above shoulder height would be prevented due to this interference.
The deltoid muscle is the primary abductor of the arm; however, without the assistance of scapular rotation, its action would be restricted. The scapulohumeral rhythm is a term used to describe the coordinated movement of the scapula and humerus which allows for full abduction of the shoulder. Hence, without the superior rotation of the scapula, shoulder abduction would generally be limited to around 90 degrees, not achieving the full range of motion.