My neurologist has a patient with Parkinson's disease, who has symptoms of too much medications on one half of his body and too little on the other.

But, at least it is consistent.

With MG it can go from side to side.

A bout of exercise can lead to a dramatic change in the expression of various genes in the muscles. (the on-off switching you talk about).

mitochondrial dysfunction (which appears to be part of the problem, at least in some MG patients) can significant alter this normal process.

Different muscles have a different ultra-structure of their fibers. (there are fast fibers and slow fibers. there is a different pattern of innervation etc. ).

etc. etc. etc.