The shoulder is known as a ball and socket joint. This type of joint allows for a great amount of motion in the shoulder, 360° worth. This motion does come with a price. Because of the increased motion, the shoulder is inherently unstable. The knee on the other hand has much less motion due to being what is known as a hinge joint, but is much more stable. One way to understand the anatomy of the shoulder, and this instability, is to realize that the shoulder is much like a golf ball on a golf tee if it were to be turned on its side.
In an effort to provide the shoulder with more stability, around the glenoid is tissue called the labrum. The labrum along with the ligaments that hold the humeral head to the glenoid, create stability, helping to prevent the shoulder from either subluxating or dislocating. It needs to be noted that the majority of shoulder dislocations are anterior dislocations. This means that the ball slides out of the front of the glenoid.
To help move the arm to allow for overhead activities, the rotator cuff muscles are involves. These 4 small muscles are the ones that help to center the humeral head in the glenoid during activities. These muscles are the: Supraspinatus, Infraspinatus, Teres Minor and Subscapularis.
The rotator cuff muscles are very important for overhead activities. Not so much in generating arm speed but in actually slowing the arm down after the overhead activity. The rotator cuff also keeps the humeral head centered in the glenoid. Some examples would be pitching / throwing and serving.
Overhead athletes place a tremendous amount of stress onto the labrum, ligaments (joint capsule) and the rotator cuff. Theses repetitive stresses as well as traumatic incidents, may damage these structures. Many times this requires surgery to repair and allow the athlete to enjoy their sports of choice. Different types of shoulder injuries will be discussed in this site.