The shoulder is one of the most used parts of the body. As the principle of the human body goes, structure determines function. The movement of the shoulder is versatile and efficient because of the structures that form it. One of the important components of the shoulder which is mostly responsible for its wide range of action is the glenohumeral joint.
Components of the Glenohumeral jointCredit: http://www.maitrise-orthop.com/viewPage_us.do?id=797
The glenohumeral joint is the major joint of the shoulder. Also known as the scapulohumeral joint, it is mainly comprised of the glenoid fossa and the head of the humerus to which it articulates to. The glenoid fossa is a shallow space which is deepened by a strip of fibrocartilage called the glenoid labrum.
Extent of MovementCredit: http://www.stjohn-clarkptc.com/FrozenShoulderTreatment.htm
There are different kinds of joints, and these joints have different degrees of freedom which determine how they move and how they influence the movements of muscles that are attached to them. The glenohumeral joint is special because it is a ball-and-socket joint, which gives it three degrees of freedom. Because structure determines function, this allows the glenohumeral joint to do a number of movements.
1) Flexion or extension in the transverse axis
2) Abduction or adduction at the antero-posterior axis
3) Internal and external rotation at the vertical axis
4) Circumduction, which is a combination of two or three axes.
This is the reason for the variety of movements your upper extremities can make. This is why you can raise, stretch or rotate your arms. The shoulder joint also enables you to lift, push and pull. These are the movements which allow you to do the butterfly stroke or the freestyle, swing a baseball bat or throw a ball.
Muscle StabilityCredit: http://www.begin2dig.com/2010/06/amazing-shoulder-part-2-glenohumeral.html
The shoulder joint is not stable on its own. Certain muscles reinforce the joint. These are the rotator cuff muscles, which act as a dynamic ligament pressing the head of the humerus into the glenoid fossa keeping the joint intact. The rotator cuff muscles are made up of the supraspinatus, infraspinatus, subscapularis and teres minor muscles. These muscles are important in stabilizing the glenohumeral joint. Tears in the rotator cuff muscles cause a great deal of shoulder immobility. The other muscle that stabilizes the glenohumeral joint is the tendon of the long head of the biceps brachii. This also forces the humeral head medially on the joint.
In summary, the glenohumeral joint is one of the body structures that make our extremities mobile. The glenohumeral joint is a ball-and-socket joint with three degrees of freedom that allows the arms to be flexed or extended, abducted or adducted and rotated. This causes a wide range of motion for the arms and hands, which allows us to do a lot of actions. On its own, the glenohumeral joint will not be stable, but it is supported by muscles such as the rotator cuff muscles and the tendon of the biceps brachii. These muscles reinforce the head of the humerus into the glenoid fossa which maintains its position and keeps the joint intact.