Measurement of rise time of averaged muscle action potential in normal subjects and patients with myopathy.

As a result of diminution of the number of muscle fibers in patients with myopathy, the rise time of the fast negative phase of the spike potentials in the EMG becomes shorter. By using a small special purpose computer (averager) a mean action potential is obtained from which the rise time of the "spike" is determined. In normal muscles the mean value is 509 microseconds (SD 118). Myopathic muscles show a mean value of 225 microseconds (SD 76). There is a slight overlap and uncertainty between 280 and 370 microseconds. Because of the short time needed to examine the muscle for this parameter many muscles may be examined during one routine EMG.