During isotonic external rotation of the glenohumeral joint in shoulder rehabilitation, which muscle is primarily involved?

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

Answer: (A)

External rotation of the shoulder is produced mainly by the rotator cuff muscles that act to rotate the humeral head outward and stabilize the joint. The primary muscle driving external rotation is the infraspinatus. It attaches from the infraspinous fossa to the greater tubercle of the humerus and generates the largest external-rotation torque, while also helping center the humeral head in the glenoid during movement. Teres minor also contributes to external rotation but to a lesser degree. Subscapularis and teres major are internal rotators (and adductor in the case of teres major), while supraspinatus mainly initiates abduction and stabilizes the joint rather than acting as a primary external rotator. So, during isotonic external rotation, the infraspinatus performs the principal work.