Quantitative electromyography in polymyositis and dermatomyositis.

Twelve patients with polymyositis or dermatomyositis were examined repeatedly during the course of the illness by muscle strength tests, serum enzyme determinations, frequency analysis of the electromyographic signal, and estimations of the spontaneous activity. All methods can be used to follow the course of the disease. Increasing muscle strength was correlated to decreasing serum enzyme values, decreasing spontaneous activity, and decreasing proportion of high-frequency components in the EMG signal, except during the initial period of treatment, when a temporary increase in high-frequency components was recorded.