The test for painful arc is easy to do. Simply abduct/raise the arm straight out to the side and toward the ceiling. Full range of motion would be 180 degrees, with 0 being fingers pointed toward the floor and 180 with their fingers pointed toward the ceiling. With painful arc, pain is felt in the range of roughly 45-60 degrees, give or take. The pain is caused by either the bursa in the joint or one of the rotator cuff tendons getting pinched between the head of the humerus and the lateral edge of the acromion process of the scapula. If it is the bursa, it is considered bursitis. If it is a tendon, it is considered tendonitis. Most doctors don't bother making a critical distinction between the two because they treat both conditions the same way - a shot of cortisone into the joint capsule. The reality, however, is that they are two distinctly different conditions and would be treated differently from a bodyworker's perspective. In either case, however, I would personally use lymphatic drainage to drop the inflammation, edema and pain.