The large head of the humerus, which forms a joint with a small pelvis, allows for extreme range of motion at the expense of the stability characteristic of other joints. Similarly, there is a scapula on the chest wall. It is very mobile, which allows it to move in the wake of the humerus and sets the pelvis in the right way. It does not allow trapping of the humerus by the shoulder appendage.
Bone stability of the shoulder joint is enhanced by the fibrous-cartilaginous vertebra, which enlarges and deepens the pelvis, while strengthening the alignment of the articular surfaces. However, the stability of the shoulder is mainly a matter of the soft tissue structures that intersect with it.
The ligaments and articular bag are static stabilizers, and their function is to limit the displacement and rotation of the head of the humerus in the pelvis. The upper brachial ligament has been shown to be a very important lower stabiliser. The medial ligament of the shoulder joint provides stability with respect to the forward displacement of the shoulder in the position of external rotation and visitation not exceeding 90 degrees. The most important front stabilizer when visiting 90 degrees and external rotation of the shoulder (which is considered the least stable setting of the shoulder) is the lower shoulder ligament.
In turn, the muscles provide dynamic stabilization of the shoulder joint in many ways. During contraction, they increase the stiffness of the bag-ligament system, which improves the stability of the joint. They also act as dynamic ligaments when their non-working elements are stretched. What is very important, they are a component of the pair of forces that control the setting of the humerus and scapula. They thus help to give the right direction to the forces acting on the shoulder joint.