Which muscle's primary function includes assisting in external rotation of the shoulder?

The infraspinatus is a key muscle in the rotator cuff group that plays a primary role in external rotation of the shoulder. It is located on the posterior aspect of the scapula and attaches to the greater tubercle of the humerus. When the infraspinatus contracts, it pulls the humerus backward while simultaneously rotating it outward, allowing for the external rotation movement. This function is crucial for various activities that involve throwing or reaching overhead.

Other muscles mentioned have different primary functions: the supraspinatus primarily assists in shoulder abduction, the subscapularis is mainly responsible for internal rotation, and the teres major is involved in internal rotation and adduction of the shoulder. Therefore, the infraspinatus stands out as the muscle specifically associated with external rotation of the shoulder.