Osteology: (the bones involved)
The two main bones of the shoulder are the humerus (the upper arm) and the scapula (shoulder blade).
The point where they meet (aka the joint cavity) is cushioned by articular cartilage covering the head of the humerus and face of the glenoid. This cartilage helps guide movement and protects the joint surfaces. As in any joint, it is suseptible to injury and general “wear and tear.”
The scapula is located on the back of the thorax (along the mid back) and extends to the top and the front of the shoulder via a projection known as the acromium. This bone can be thought of as the “roof” over the glenohumeral (aka shoulder) joint.
Important to note is that the scapula (and thus the arm) are only attached to the bony skeleton by ONE connection – the clavicle. All the rest of the arm and shoulder’s stability comes from muscle, ligament, and fascia (connective collagen tissue). For all of you gymnasts out there, now how much more impressive is an iron cross or a handstand?
The joint articulation:
The end of the scapula, called the glenoid, meets the head of the humerus to form a glenohumeral cavity that acts as a flexible ball-and-socket joint. Because the humeral head is so much larger than the glenoid, the arm has a tremendous ability to move.
The mobility thus requires secondary stability, so the joint is stabilized by a ring of fibrous cartilage surrounding the glenoid called the labrum. This makes the glenoid seem larger and increases stability, while being flexible and elastic enough to stretch and allow the shoulder to maintain almost 360 degrees of mobility (after all it is the most mobile joint in the body).
Ligaments and the joint capsule:
Ligaments connect the bones of the shoulder (giving it stability), and are mixed with and encased in a joint capsule that seals of the inside of the joint from the outside of the joint. Think of the joint capsule as a saran-wrap like covering that holds joint fluid (synovial fluid) in and seals out the rest. There are also tendons around the shoulder that join the bones to surrounding muscles. One of the most visible and important in this area is the biceps tendon which attaches the biceps muscle to the shoulder and helps to stabilize the front (anterior) portion of the shoulder joint.
The rotator cuff:
The rotator cuff is composed of four small, short muscles that originate on the scapula and pass around the shoulder where their tendons fuse together and attach on the humerus. Known as the “SITS” muscles, they are:
- supraspinatus
- subscapularis
- infraspinatus
- teres minor
These muscles have two main functions. First, they stabilize the joint and help center the humeral head in the glenoid fossa (important in preventing pain and injury), And second, they control rotation of the shoulder and allow for complex movements. Thus, they are very important to shoulder function, especially in gymnasts where the shoulder often becomes a weight-bearing joint (something most people don’t often require).
Other musculature:
The shoulder is also crossed and affected by many other muscles and each will be discussed in the appropriate injury section. Just know that it is one of the most complicated joints and is hard and lengthy joint to rehab – so be patient if you have shoulder problems!
Gymnastics Injuries 