Originally Posted by AlphaBettor
Main movers in external rotation are the infraspinatus and teres minor, with some assistance from the posterior delt. Since this done while in abduction, supraspinatus should also be active, according to: https://www.ncbi.nlm.nih.gov/pubmed/21064160
Supraspinatus might be a good place to start (especially if you can do something like lying dumbbell external rotations pain-free) but diagnosing injuries is not so simple.
Cool, thanks. This sounds about right.
I forgot to mention that yes, I can do external rotations (not lying, I do them standing) with my arm in adduction completely pain free.