The Apley Scratch Test (aka zipper test, or Back Scratch Test) involves trying to touch your middle fingers behind your back, with one arm reaching back over the shoulder and the other reaching up from behind. How can I increase shoulder mobility (most likely scapular mobility) to be able to do this? What specific shoulder function is it testing?
I can do it with my right arm, but not my left. Is it possibly a natural shoulder-blade alignment difference from being right-handed?
Edit: Here is a video of me attempting the test. Even through my back hair, you can see my left shoulder blade moves differently right from the start. I believe this is a muscle issue because a muscle activation therapist massaged various muscles and I was able to touch my fingers on the left just as well as on the right... but only during that one session. I have not been able to achieve it with self-massage. Which muscle could be responsible for the difference?