Comprehensive Osteopathic Medical Licensing Examination (COMLEX USA) Practice

What motion is primarily performed by the infraspinatus muscle?

1. Abduction

2. Internal rotation

3. External rotation

4. Flexion

The correct answer is: External rotation

The infraspinatus muscle is primarily responsible for external rotation of the shoulder joint. As a rotator cuff muscle, it plays a crucial role in stabilizing the shoulder while allowing for this specific motion. The infraspinatus originates on the infraspinous fossa of the scapula and inserts onto the greater tubercle of the humerus. When it contracts, it pulls the humerus outward, rotating it away from the body, which is characteristic of external rotation. Additionally, the muscle helps to stabilize the glenohumeral joint during arm movements, contributing to overall shoulder function. In contrast, abduction involves moving the arm away from the body and is primarily performed by the deltoid and supraspinatus muscles. Internal rotation, on the other hand, is primarily carried out by muscles such as the subscapularis and the pectoralis major. Flexion of the shoulder is primarily influenced by the anterior deltoid and pectoralis major. Thus, the infraspinatus is uniquely positioned and functionally specialized for external rotation.