EMG dynamics in polymyositis. A quantitative single motor unit potential study.

A multidimensional quantitative EMG analysis method with averaged motor unit potentials (MUPs) was applied to a study of different stages of polymyositis. The duration parameters had a general tendency to increase in polymyositis. The only exception was the total MUP duration, which decreased in the onset stage of acute polymyositis. The amplitude tended to decrease in acute polymyositis but increase in chronic, progressive polymyositis. The number of fast components both in the whole MUP complex and in the main MUP tended to increase in all stages of polymyositis. The mean fast component duration in the main MUP tended to decrease in polymyositis. Satellites (small signals time-locked to the main component but part of it) were numerous in active stages of polymyositis. There were significant quantitative differences in most of the parameters in different stages of polymyositis.