I figure your muscles tell you, I'm in a similar situation myself (rotator cuff & tendonitis in my left arm.) Fiddle is the worst to play for contorting yourself, and raising your left arm so high. I'm going to get a shoulder rest attachment (Shoulder pad on about 2-3" legs attached between bottom of fiddle) for my fiddle.

I find guitar much easier to play. Mandolin is somewhere in between the two--the position is O.K. but I have to tense up my left arm muscles to finger cleanly, it's a problem after awhile.

Good luck eh...

Bill