1.1.4 The shoulder joint and its movements
Now let's look at the shoulder joint. To understand the shoulder joint, let's get acquainted with the upper half of the humerus.
This is the head of the humerus. The articular surface is half of a sphere. On the anterior aspect is a well marked groove known as the bicipital groove, because the tendon of the long head of the biceps runs in it. At the proximal end of the groove are the lesser tubercle, and the greater tubercle. Because it's between two tubercles, the bicipital groove is also known as the inter-tubercular groove. Down here on the lateral aspect of the humerus, almost halfway down the bone, is a rough spot, the deltoid tubersosity.
Here's the shoulder joint, also known as the gleno-humeral joint. This loose sleeve of tissue which encloses the joint is the joint capsule. The capsule doesn't hold the bones together, it's quite a weak structure. What it does is to permit movement. The structures which hold the two bones together are muscles, as we'll see. Here's the tendon of one of those muscles.
Let's look at the movements that can occur at the shoulder joint. Movement forward and upward is called flexion. Movement downward and backward is called extension. Movement away from the side of the body is ab-duction. The opposite movement is ad-duction. Rotation which moves the front of the arm towards the body is internal rotation. Rotation the other way is external rotation.