Which muscle is known as the primary external rotator of the shoulder?

The infraspinatus is recognized as the primary external rotator of the shoulder due to its anatomical position and functional role. This muscle originates from the infraspinous fossa of the scapula and inserts onto the greater tubercle of the humerus. When activated, the infraspinatus primarily facilitates the rotation of the humeral head away from the body, a movement termed external rotation.

In the context of shoulder mechanics, external rotation is vital for various upper extremity movements, including reaching overhead or behind the back. The strength and functionality of the infraspinatus are also essential for shoulder stability, as it helps to align the humeral head properly within the shallow glenoid cavity during arm movements.

The other muscles mentioned have their distinct roles: the subscapularis primarily acts as an internal rotator, the supraspinatus is essential for initiating arm abduction, and the teres major assists with internal rotation and adduction. However, it is the infraspinatus that is most critically involved in external rotation.