The purpose of this study was to detect the motor neuron losses in progressive neurologic diseases before the weakness or wasting of muscles became apparent using the motor unit number estimation technique. Method for estimating the number of motor units based on electrical stimulation of motor nerves was introduced first by McComas. The author describe the method which was applied to the median nerve innervated thenar muscles in healthy subjects. This study showed that the results of motor unit number estimation were 196.9⁑56.2 in male, 187.6⁑79.2 in female, and the most influencing factor for the number of motor unit was the size of amplitude or area of compound muscle action potential.