Polio was both a myelitis (inflammation of the motor neurons in the spinal cord) and an encephalitis (inflammation of the brain). Actually in all cases of Polio an encephalitis exists as it affected the brain first. David Bodian was the one who discovered that there were three types of poliovirus and that it went from the intestines to the lymph node into the blood before it went to the motor neurons. That is what allowed a polio vaccine to work.
The most common and sever poliovirus damage was at the bottom of the brain, in the brain stem or "bulb". Bodain found that the brain stem was involved in even mild cases of polio. This is the part of the brain that controls our automatic muscles, the ones that control the diaphragm, blood pressure, heart rate, and the muscles in your throat. It was severe damage to the bulb that made polio a killer, ie the heart stopped working. Also what caused use of the iron lung as the diaphragm muscles are what make your lungs work.
It is important to remember that every polio survivor, with or without obvious swallowing or breathing problems, had "bulbar" polio, since the reticular formation was the one area of the brain Bodain found was always damaged by the polio virus.
If you don't already own a copy of The Polio Paradox, get one, or see if your library has it.
The damage done in the brain by polio affects our production of dopamine, other brain-activating neurons were killed by the poliovirus, including those in the Hypothalamus and the Thalamus. It also damaged the neurons that make endorphin and enkephalin, the bodys own morphine.
Any injury including falls can trigger the PPS symptoms.
P.S. it isn't the muscles that fail in PPS or in the original Polio .. it is the motor neurons that control the muscles. The posterior horn motor neurons, the anterior motor neurons are the one that allow you to "feel". This is why when Polio patients are paralyzed they can still feel the limb.