About the Mechanics of Shoulder Stability.

Last updated Thursday, February 10, 2005

Figure 4 - Socket

Figure 5 - Humeral head and glenoid

Requirements for shoulder stability

Sockets, capsules, ligaments

We have found that glenohumeral stability requires that there be a socket into which the ball can be pressed, and that the muscles around the joint work in a balanced way to press the humeral head into the glenoid (see figures 4 and 5).

Glenohumeral stability also requires that the capsule and ligaments check the motion of the joint so that it does not rotate to positions where the forces become unbalanced. You can see more about how the ligaments help control shoulder stability in activities such as the baseball pitch. However, it is interesting to note that in many important positions of the joint these ligaments do not stabilize the shoulder. The normal joint is, in fact, quite loose. In most positions, the ball is held in the socket primarily by muscle action as described above.

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