EMG power spectra obtained during a sustained isometric contraction were analyzed in a group of 10 children affected by Duchenne muscular dystrophy (DMD) and compared with those obtained in a control group of 5 normal children. In myopathic subjects the isometric contraction caused an increase of the total power, a progressive increase of power of the lower frequencies, a decrease of that of the higher frequencies, and a shift downward of the median frequency. In normal children an increase of the total power without a significant median frequency shift was noted. The modifications observed in DMD children were explained by a decrement of the firing rate of the more damaged fast twitch motor units. This decrease was probably induced by a relative predominance of activity of the slow twitch motor units, which are less damaged by the pathological process.
Child Diagnosis, Computer-Assisted