This wide range of motion comes at the expense of joint stability. Because there is little contribution from ligaments, the rotator cuff muscles are responsible for both movement and stabilization of the joint. The arm may be abducted away from the body till vertical. Abduction of the arm occurs in two parts. The first 120 degrees is done through the shoulder with the remaining 60 degrees accomplished by rotation of the scapula.
Adduction brings the arm back toward the body.
The shoulder joint allows 60 degrees lateral rotation which is done by rolling the arm out so that the humerus is moved about its own axis and the palm of the hand turns away from the body. Thirty degree medial rotation may be seen by rolling the arm inward and turning the palm toward the body. All rights reserved.