This is a condition that is often very painful, reduces the motion your shoulders make and when left untreated can develop into loss of function in the arm (often called ‘Frozen Shoulder’).
It is due in part to the tendon and nerve being pressed upon by the tubercle of the humerus and pinched between the acromion process of the scapula. When the pressure or pinch becomes constant it is difficult to raise your arm above your head.
Normal shoulder motion allows you to lift your arms away from your sides all the way up to your head. When this motion is interfered with, you might be lucky to move your arms a few degrees about the horizontal plane.
A few simple tests are all that is needed to diagnose this problem, however x-ray of the shoulder may be needed to assess if there is any abnormal bone growth or other injury to the bones that are causing the symptoms.