Warning: post about shoulder biomechanics, please ignore if not interested.
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DF: What exercises are you doing to strengthen?
internal rotation: arms shoulder height in front of you, palms facing out
external rotation: arms shoulder height in front of you, palms facing up.
Almost every (overhead and other) movement we do strengthens the muscles used to internally rotate the shoulder. So even if you don't do a lot of shoulder work (training), most people already have an imbalanced ratio of internal vs external rotation strength, not to mention inflexible muscles in the front of the shoulder and chest. When you contract your deltoid, it pulls the humurus upwards to the roof of the shoulder while your infraspinatus and teres minor (muscles in the back of the shoulder that are usually weak) stabilize it by pulling it down. If the infraspinatus and teres minor are not strong enough, the humurus will rub against the roof of the shoulder (acrominion) and hence why you feel pain/friction and eventually inflammation and eventually fibrosis
So 3 things:
1)Strengthen the muscles that pull the humurus down (infraspinatus, teres minor, rhomboids helps too, all muscles in the BACK of your shoulder)
2)Stretch the front (internal rotators) so that when you need to externally rotate the shoulder, your already-weak external rotators don't have to work overtime and against your tight chest/shoulder muscles.
3)If you're doing something biomechanically un-sound (ie: swimming or rowing or any other shoulder intense training), fix your technique.