QUANTITATIVE ELECTROMYOGRAPHIC CHANGES ASSOCIATED WITH MUSCULAR WEAKNESS

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Abstract
Voltage/tension curves of the biceps or triceps measured by means of an electromyograph and a strain gauge dynamometer were studied in 12 healthy subjects and 26 patients with weakness of these muscles. In cases of weakness associated with motor neurone disease, peripheral neuritis and cerebral vascular disease the maximal muscle contractions were weaker but the curves were within the normal range. In cases of muscular dystrophy and polymyositis the curves indicated decreased neuromuscular efficiency. The same was true in polio patients and the occasional high voltages associated with maximal contractions were explained by the enlargement of motor units by collateral sprouting. Muscular weakness due to interference with the nerve or upper motor neurone produces no alteration in the curves because voltage and tension would be proportionately less in relation to the reduction of activated motor units. Weakness due to a structural change in the muscle, produces steeper curves because for a given tension a larger number of fibers are being activated thus producing a greater integrated voltage.