What causes "post-polio syndrome"?

Polio + aging: Post-polio syndrome results in spinal motor neurons (those that supply voluntary muscles) which were affected often decades earlier by polio virus to degenerate & stop functioning normally. Theory is that the neuron is weakened & now succumbing to continuing muscle use which overtaxes the neuron metabolism. Reduced activity is the only remedy but won't reverse damage already done.

Cause is not known: The exact cause of Post Polio Syndrome is not known.But the New weakness is related to degeneration of nerve terminals in the motor units that remains after the initial illness