I've just figured that out. Church, restaurants, movie theaters--I usually go in fine, but have a hard time getting out. I say that sitting "makes me weak," but it's probably more accurate to say it triggers an episode of severe weakness.

My muscles are also fatiguable in what I think is a typical myasthenia way, but this is not nearly as dramatic and variable, or as severe.

The weakness is not dizziness or light-headedness. When it's really extreme, I can feel the weakness coming over me. When it's milder, sometimes I don't notice it until I try to stand up. The weakness seems to affect my legs most of all, so that I can't support myself, and slowly collapse to the floor. But I feel it all over my body.

So, what's up with that? This is not typical of myasthenia. I thought of LEMS (tested negative once), but my understanding is that with LEMS, the muscles get stronger when you walk. For me, the episodes last from 45 minutes to two hours, and then slowly lift--but the weakness lifts quickly enough that I can feel it lifting. I say, "Oh, I'm coming out of it."

Strangely, I have never had an episode like this upon awakening in the morning.

Abby