The purpose of this protocol is to apply specific neuromuscular, electrophysiological, histological, virological, and immunological tests to answer pertinent questions regarding the pathogenesis of the post-polio syndrome. Studied patients will undergo a series of clinical neuromuscular evaluations, quantitative muscle testing, electromyography including single fiber EMG, immunogenetic, viral and immunochemical studies in the serum and spinal fluid, muscle biopsy and swallowing evaluation. Although this is not a therapeutic study, the information obtained would help us understand the pathogenetic mechanisms of the new weakness and could help us design possible therapies.