Neuropathy is probably one of the big reasons. Muscle deconditioning, and that depends on the individual, if they're bedridden or not, can walk at all. So many things have to be taken into consideration.
As we age, falls, other neurological illnesses apart from ME, (such as my arms are very weak...I look normal, but I can no longer lift even a saucepan),later stages of cancer, other illnesses along the way. There is no one reason, I think it depends upon how life plays out, what you have to deal with....there is no point in trying to predict the future. So many unknowns interfere and, believe it or not, some are good. Yours, Lenora.