hi, Lydia ... I'll jump in for a minute, although I'm sure that Wright will have the definitive answer.
This notion of "fatiguability" (spelling?). I don't recall reading anything about that on Forum and apologize if it has been addressed. Both my GP and neuro seemed really interested in how I feel as the day progresses, but that idea seems to have stalled. I feel mighty good in the morning
Probably the reason "fatiguability" isn't mentioned on this forum is because it is not a symptom of ALS ... but it's almost the definition of Myasthenia Gravis. People with MG generally become weaker and weaker (or more and more tired, whatever word works for you) as the day goes on.
Patients with MG cannot perform repetitive tasks ... like hammering a nail, or sawing wood, or kneading bread.. Usually, they can perform an action a few times, but it becomes harder and harder with each repetition. If they rest the muscles involved until they recover, they can again resume the action, but with the same increasing weakness, requiring rest again.
Hope this helps a little.
BethU