The shoulder is capable of allowing the wide range of movements as a result of the way it is formed. Basically, the shoulder joint consist of the round surface of the upper arm, called the humerus, connected to the flat surface of the shoulder blade, or scapula. This allows for a large amount of motion, but also requires tremendous stability from the surrounding soft tissues.
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. 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.
Every time you lift, push, pull, or carry anything with your arm the shoulder muscles must contract to stabilize the shoulder and protect it from injury. A small but constant contraction of the shoulder muscles is even required with something as simple as sitting in front of a computer, in order to hold the arm and shoulder in a proper position.
In addition to high levels of muscle activity that is inherent in normal daily activities, many factors also place additional strain and work load on the shoulder muscles. For example, repetitive use with certain sports or occupants, poor posture, muscle imbalances, or previous injuries that may not have been fully treated or rehabilitated can further strain the muscles of the shoulder girdle.Social tagging: rotator cuff > shoulder