The teres minor is one of the four rotator cuff muscles. It serves three functions - external rotation, adduction (moving the humerus closer to the scapula), and holding the head of the humerus firmly against the glenoid fossa (approximation).
The origin of the teres minor is on the lateral (axial/outside) border of the scapula for two-thirds of its length. The remainder of the origin is shared as an aponeurosis by the infraspinatus and the teres major.
It inserts into two parts of the humerus. The upper fibers of the muscle insert into the greater tubercle near the head of the humerus, like the supraspinatus and the infraspinatus. The lower fibers insert directly into the humerus just below the greater tubercle.
During a normal follow-through, this muscle contracts to stop internal rotation and to keep the humeral head in place. When a follow-through has upward motion (arm moving above the shoulder and away from the scapula), this muscle faces the added stress of also trying to decelerate this upward movement. This eccentric contraction is powerful, and in one documented case led to a stress fracture of the scapula ("Scapular Stress Fracture in a Professional Baseball Player: A Case Report and Review of the Literature", American Journal of Sports Medicine).
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