The shoulder complex serves as the anchor for the upper extremity much like the hip does for the lower extremity, however there are a few key differences. Both joints are ball and socket joints, however the ratio of the ball and socket sizes are very different. The hip is composed of a relatively large socket and smaller ball, whereas the the shoulder has a relatively smaller socket and larger ball. This anatomical difference makes the shoulder inherently less structurally stable than the hip. The socket of the shoulder is located on the scapula (aka shoulder blade) and the scapula and the arm must move in a 2:1 ratio during overhead reaching tasks in order to maintain the normal mechanics of the shoulder and avoid impingement of important structures like the rotator cuff and biceps tendon. Therefore, scapular strength and movement coordination is essential to maintaining a healthy shoulder https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3445067/.
Understanding the Human Body as an Integrated Kinetic Chain - Part 3 The Shoulder