About the Mechanics of Shoulder Stability.

Figure 1 - Glenohumeral joint

Figure 2 - Deep socket

Figure 3 - Ligaments under tension

Introduction

Shoulder stability

One of the many remarkable features of the normal shoulder is that the glenohumeral joint (the ball and socket joint between the upper arm bone and shoulder blade) is very stable in spite of the vast range of motion available to the joint (see figure 1).

The glenohumeral joint does not have a deep socket or ligaments that are always under tension (see figures 2 and 3). Thus, the glenohumeral joint must be stabilized by mechanisms quite different than those which stabilize the hip or knee. The mechanisms of shoulder stability are different but effective: the humeral head, which is slightly smaller than a billiard ball, is held precisely centered on the glenoid, which is about the size of a tea spoon. It is amazing that such an arrangement can allow the shoulder to throw, pull up, lift, punch and do gymnastics without coming apart!

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