Your shoulder is more flexible than any other joint in your body: while this offers a great advantage in terms of day-to-day function, it brings challenges in terms of stability of the joint. To help keep the various muscles and bony attachments in position, a tough capsule, made of connective tissue, surrounds your shoulder joint.
This is a companion discussion topic for the original article at https://patient.info/news-and-features/how-to-cope-with-a-frozen-shoulder