Whenever you contract a muscle, referred to in that moment as the prime mover, the brain is sending a signal to your muscle to cause that muscle to contract; at the same time, a signal is being sent to the opposing muscle to cause it to relax so that the prime mover can move the associated body part.  For example, when you fire the biceps to move the forearm, the triceps must be relaxed to allow the forearm to flex at the elbow.  Because of the intense pain caused by the pinching of the supraspinatus tendon between the acromion of the scapula and the humeral head, the teres minor or latissimus dorsi muscles can tense up and refuse to relax entirely when the deltoid is pulling the arm straight up from the side of the body into abduction.  In addition to the pain, this is why a client with a frozen shoulder often has such a hard time raising the arm up into abduction; treatment of the frozen shoulder will require dealing with the cause of the intense pain first, and also require a retraining of the client to use the shoulder properly again and without fear of the intense pain returning.