Which shoulder muscle is primarily involved in initiating arm abduction and is commonly injured in rotator cuff disease?

• A. Infraspinatus
• B. Subscapularis
• C. Supraspinatus
• D. Teres minor

Answer: (C)

Initiating arm abduction relies on the supraspinatus. This muscle sits in the supraspinous fossa and its tendon passes under the acromion, where it can be compressed in rotator cuff disease. Its job is to start the lift of the arm, accounting for the first roughly 15 degrees of abduction, while simultaneously stabilizing the humeral head in the glenoid so the deltoid can elevate the limb smoothly. After that initial phase, the deltoid becomes the main mover for further elevation. Because the supraspinatus tendon is the most commonly affected in rotator cuff pathology—due to its location and vascular supply—it’s frequently injured. The other muscles contribute to other shoulder actions (infraspinatus and teres minor are mainly external rotators; subscapularis is an internal rotator and stabilizer) and do not principally initiate abduction.