Neurophysiological Methods for Motor Unit Number Estimation in Human Muscles

The number of motor units is a perfect marker of the severity or the progression of neurogenic muscle weakness, such as, for instance, in amyotrophic lateral sclerosis. The number of motor units cannot be determined exactly, but various methods for its estimation have been described. Methods that impose upon the all or none principle of action potential generation were described first. They require both time and a specially trained examiner. More recently, another method was proposed, the motor unit number index (MUNIX). This method is far less demanding, but its accuracy needs to be determined. Finally, it should be noted that a reduced number of motor units can also be estimated with the standard concentric needle-EMG.