This was covered in a previous message that perhaps was deleted during the hacker attack. Anyway the anatomical length of the superior glenohumeral ligament is short (supportive) in external rotation and is lengthened (slack) in internal rotation.

Given that a patient does not exhibit any muscle activity about the glenohumeral joint post stroke (flaccid paralysis) then it would stand to reason that position would affect subluxation. I believe the laxity in the GH joint capsule alone permits about 2.5-3cm subluxation without any muscle activity. So if you add it all up - flaccid shoulder, internal rotation (i.e. lengthening of the superior glenohumeral ligament) = subluxation. Therefore maintaining an amount of external rotation (even a neutral GH joint) should assist minimise the rate and likelyhood of subluxation.

I have seen a few studies looking at muscle stimulation for the external rotators during flaccid paralysis to prevent subluxation but none have proven any direct benefit as yet. That said I believe that was more the fault of the study and that one should perhaps combine this approach with Upper Limb exercises focusing on lateral and behind reaching activities to facilitate practise and stimulate restoration of function.