EMG is a Electromyography. It's uncomfortable, but not really painful generally. Here's a link that explains:

http://en.wikipedia.org/wiki/Electromyography

Through a repetitive nerve stimulation test, they can detect how quickly a muscle fatigues. It is measured as a decrement in response to the electrical stimulation of the muscle. They will stimulate the muscle, recording it's response, have you "exercise the muscle" then stimulate it again, then stimulate it a third time allowing a brief rest period between the sets. On me, the muscle at the wrist only shows a 6% decrement - not enough to dx MG, but the trap muscle in my shoulder and back show a 76% decrement, and the facial muscle showed a 23%. I believe anything over 10% is considered a high enough decrement to demonstrate MG. Generally, for the RNS they should test 3 areas, and choose both proximal (close to the body trunk, like shoulder or thigh) and distal (farther away from the trunk, like hand).

Interestingly, and as Annie pointed out, you don't necessarily have to be aware of the weakness in a muscle for it to show the decrement. I have a neighbor that went in due to ocular problems. When they performed the RNS (repetitive nerve stim) on him, they found weakness in his legs that he hadn't realized.

Mestinon has a very short half life, so if you are off it for at least 24 hours prior to an EMG it shouldn't interfere with the test. The test is normally performed by the neuros themselves and most have the equipment in office.

Hope this helps you to understand! There is so much to learn when you first get a diagnosis. You may want to hone up on your googling skills so you can check things out at will! I'm getting better at it, Annie is awesome at rooting stuff out, but you can find things too.

For instance, to find the link I posted here, I just typed EMG into my search bar, then chose a link with a description that looked like what I wanted. Of course, any of us are happy to give you the information we have, but I find it interesting to go search various elements, tests, etc. myself - I'm up in the middle of the night quite often and it's a great opportunity to root around.

Best of luck to you. Feel free to let those questions fly. We're always here for you!