When athletes, or anyone for that matter, move their arms in an overhead movement too much, the result can be damage to the shoulder capsule, the lining of the shoulder joint. When the lining stretches, the shoulder joint no longer works like it should because the restraining element, the part that keeps the joint in place, is stretched out. The person ends up with a condition known as multi-directional instability where it can move too far in every direction.
Instability means that the shoulder is too loose and has a tendency to slip out of the socket.
If the shoulder slips completely out of the socket, it has become dislocated. Repeated dislocations are not only a nuisance, but can cause further injury to the shoulder and can lead to arthritis of the shoulder if not treated. This injury is common among people who throw a lot, like quarterbacks or pitchers, swimmers, or people who do a lot of aerobics. It can also be a problem for people who sustain injuries to their shoulders from a traumatic experience. Traditionally, patients would have to undergo a surgery that opens their entire shoulder area.