The shoulder is different from most other joints in the body because it is designed to provide a great deal of movement. For example, the architecture of the shoulder joint enables us to reach up overhead, back behind the body, across the chest, and we can even rotate or arm internally and externally. When you compare the shoulder with other joints – such as the ankle, knee, or elbow, which basically move only forward and backward – it can be seen that the shoulder is indeed a joint with a lot of mobility.
The shoulder is capable of allowing this wide range of movements as a result of the way it is formed. Basically, the shoulder joint consists of the round surface of the upper arm, called the humerus, connected to the flat surface of the shoulder blade, or scapula. This “round-on-flat” relationship means the arm does not fit tightly onto the shoulder blade, and it is this loose fit that allows for a large amount of motion. Unfortunately, in providing greater motion, this loose fit fails to provide bony protection and stability for the shoulder joint, which makes it more susceptible to injury.
Due to a lack of joint stability at the shoulder, proper motion requires a complex set of muscles to help control and stabilize shoulder movement. The primary muscles that provide this control are the Rotator Cuff muscles. The rotator cuff is a group of four muscles that cross the shoulder joint and hold the arm tightly onto the shoulder blade. When the arm is moved in any direction these muscles have to contract to hold the round surface of the humerus in place against the flat surface of the shoulder blade. If these rotator cuff muscles fail to contract properly the upper arm is not held tightly onto the shoulder blade and the shoulder joint becomes unstable. When this happens it places a tremendous amount of strain on the rotator cuff muscles as well as the ligaments and other tissue of the shoulder joint, leading to shoulder pain and injury.Social tagging: rotator cuff > shoulder