I do external rotation by holding the dumbbells at shoulder level, elbows out, and rotate up towards my head. That's my standard exercise, but I also will do cable pulls for external and internal rotation. Internal rotation on cable pulls I use 40 lbs on our machine. (I doubt machines are fungible.)
Do you think I'm going too heavy? My shoulders are fine now that I've gotten rid of that nagging supraspinatus/teres minor inflammation or whatever it was. The back of my shoulders have always been very strong, btw.
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