Clinical evaluation, quantitative analysis of the EMG, and motor unit fiber density were carried out on 34 selected patients that suffered paralytic poliomyelitis. 50 of the subjects developed a late and slowly progressive weakness. Automatic analysis of the electromyogram showed a great increase in mean amplitude in weak muscles but also in hypertrophic ones, and in other muscles that had normal strength. Increase in mean amplitude and in motor unit fiber density was greater in the weaker muscles. The increased amplitude and motor unit fiber density found in clinically unaffected muscles confirms that neurogenic atrophy is more widespread than is suspected clinically. Thus, the late deterioration of function developed in some of the patients always takes place in muscles which are previously damaged and partially depleted in motor units. Widespread neurogenic involvement of the muscles can play an important role in the late deterioration of these patients.
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