I must say that I have never had any stiffness/rigidity. If you have a good MG doc, then go back to them and ask the questions.

Mestinon has helped me with getting a good breath (esp with activity or much talking), and reduced fatigue with or without any type of activity.

However any improvement that I have experienced has only lasted 4 or 5 months, before I have had to increase the dose. I am now on 90 x 6 and have pretty much the same problems as I had when this started 2 years ago.