In Plain English
The posterior delts are the rear portion of your shoulder muscles, running from the shoulder blade to the upper arm. They pull your arms backward and outward, helping create a thicker, rounder shoulder while balancing the muscles at the front of the shoulder.
Technical Definition
The posterior deltoid is the rear portion of the deltoid muscle, originating from the spine of the scapula and inserting onto the deltoid tuberosity of the humerus. Its primary actions include shoulder horizontal abduction, shoulder extension, and shoulder external rotation. Its posterior fiber orientation makes it a major contributor during rowing, reverse fly, and other pulling movements where the arm moves behind the body.



















