I can dislocate both of my shoulders using just the muscles and put them right back in place and it doesn't even hurt. From my research, this is something called positional non-traumatic shoulder dislocation. It was not caused by an injury, I was just born that way. That's how I escape from being tied up so easily when doing escape artist stuff. I've been doing this all my life and only recently figured out this is what I was doing with my shoulders. I never experience any shoulder problems either.
