The exercise should follow in optimal form as this. Retract the scapula and depress it while lifting arms up over head. As long as you keep scapular retracted, the glenohumeral joint will be stable and rotator cuff will be in safe position. You may feel discomfort and lack of range of motion which is fine as the muscles are now associating a stretch response and must be held in position to inhibit overactive stretching muscles.
However, if the tension then becomes somewhat painful, it may indicate that you are progressing into corrective exercise too soon. Building the association between the correct contractions (scapular retraction and depression) and stretching the inhibitors will also stretch the necessary muscles to override kyphosis. Keeping the navel drawn in and core activated will condition the brain to associate good core function with overhead movements.
By holding this optimal spinal position, you will deactivate the kyphosis muscles (pectorals, latissimus dorsi, and subscapularis) that act as the inhibitors to functionality in this exercise. Once the muscles at the anterior chain of the shoulder begin to relax, the cervical spine will align naturally since the kyphosis muscles are no longer compensating. This will now put the cervical protractors into a position of neutrality, bringing balance to that region of the spine.
Developing associations of functionality between your abdominal systems and scapulothoracic region is paramount to effective dynamics in reality. In ideal circumstances, the wrists, elbows, shoulders, neck and pelvis should be in relatively good alignment. This alignment will secure that the body is operating efficiently at all points, and that we will in turn get efficient stimulation in the region of the middle trapezius.
You should feel the main contraction between your scapulae. Your mid/lower trapezius, if in perfect position, will retract and depress your scapula without any type of anterior scapular tilt. This is one of the most key exercises to master if we want to have full functional capabilities in the shoulder and rotator cuff. It will be especially important in association with neutral spine activity.