Question

what rotator cuff structure is primarily responsible for the internal rotation of the shoulder?
supraspinatus
subscapularis
infraspinatus
deltoid
teres minor

Explanation:

The subscapularis is part of the rotator cuff. It originates from the subscapular fossa of the scapula and inserts onto the lesser tubercle of the humerus. Its anatomical position and muscle - fiber orientation make it the primary muscle among the rotator cuff muscles for internal rotation of the shoulder. The supraspinatus is mainly for abduction, the infraspinatus and teres minor are for external rotation, and the deltoid (though not a rotator cuff muscle in the strictest sense as the rotator cuff is usually defined as the SITS muscles - supraspinatus, infraspinatus, teres minor, subscapularis) has functions like abduction, flexion, and extension but not primary internal rotation of the shoulder.

Answer:

Subscapularis