This is from page 28 of Andrew Cutler's book, "Amalgam Illness diagnosis and treatment". I just read this last night and it kind of sounds like what you are experiencing.

"A feeling of weakness occurs in the upper arms. It occurs on that part of the upper arm and shoulder that is on top if arms are being held out to the sides. The feeling of weakness actually occurs on the part of the upper arm between the biceps and triceps- there is no muscle there to BE weak, but the feeling is very distinctly localized in that area. The actual muscle that causes the weak feeling movements is the top portion of the deltoid on the shoulder."