This information is taken from Myofascial Pain and Dysfunction, The Trigger Point Manual, (Volume I pub. 1999, Volume II pub. 1993), by Travel and Simons.
There are three different situations that are called “frozen shoulder.”
1. The term “true frozen shoulder” refers to a syndrome due to adhesive capsulitis of the shoulder joint, which has been defined as tendonitis of the long head of the biceps, the supraspinatus, and the subscapularis tendons.
It will be difficult to hold the arm up, as when brushing teeth. There will be weakness and pain in reaching above the head. It will be hard to lift the arm to the side or reach back to throw a ball. If caused by trigger points, the symptoms include pain in the back, side, and front of the shoulder, the side and back of the arm, a band of pain around the wrist, and possibly pain at the elbow.
In the medical setting frozen shoulder is treated first with a steroid injection. Then, if steroid injection is not successful, it may be treated with surgery or forceful manipulation under anesthesia to release adhesions.
Working on the muscles can sometimes do as well or better than surgeries, since trigger points can cause the same symptoms.
2. Thoracic Outlet Syndrome, in which nerves are being compressed between the upper ribs and the collar bone, is also often diagnosed as “frozen shoulder.”
Pain is felt in the chest, along the shoulder blade in mid-back, down the front and back of the arm, all the way to the wrist and hand (mimicking carpal tunnel syndrome).
Turning the head side to side is restricted. Lifting the arm over the head is difficult.
To alleviate this syndrome, these muscles must be released: scalenes, subclavius, and pectoralis minor.
3. There is also Pseudo-Thoracic Outlet Syndrome, which produces the same pain symptoms and restricted movements as Thoracic Outlet Syndrome, but does not actually compress the nerves. It too is sometimes diagnosed as “frozen shoulder.”
Reaching behind the back, either to throw a ball, or to hook a bra, is difficult and painful, as is reaching across the body to the opposite shoulder. The wrist is often painful in a strap-like band.
The muscles involved in this syndrome are: pectoralis major, latissimus dorsi, teres major, and subscapularis.
(It seems this syndrome can be caused by events that affect the blood flow in the brain, although I haven’t found enough information to tell why that is.)
Other muscles that can be affected when subscapularis is tight: pectoralis major, teres major, latissimus dorsi, long head of triceps, anterior deltoid.
Deep tissue massage on the joint capsule and/or tendons, especially of supraspinatus and subscapularis, can be helpful.