Glenohumeral instability is a common shoulder condition. The joint is very flexible, and the socket of the glenoid is shallow.
It is amazing that the shoulder can do everything we ask of it.
To understand, evaluate, and manage shoulder instability one must first understand how the joint normally is stabilized. Doug Harryman did a wonderful job of explaining shoulder stability in the videos Shoulder Mechanics 1 & Shoulder Mechanics 2 by our late colleague.
The principles of shoulder stability are detailed in Chapter 3 of the freely available Practical Evaluation and Management of the Shoulder (see this link.).
The shoulder does not have any isometric structures to limit its range. The soft tissues that surround the shoulder remain. The shoulder is lax when it comes to most of its functional positions, with the exception of those that are performed at extremes (such as the baseball pitch shown in the picture above).
(N.B.Laxity and instability are not the same. Laxity of the normal glenohumeral ioint: A quantitative in vivo assessment)
The hip…
