New methods of estimating the number of motor units in a muscle

Two electrophysiological methods have been described for estimating the number of motor units (MU) in a muscle. The methods are based on the original methods of Mc Comas and others, which involve dividing the maximum compound potential (MCP) of muscle evoked by supramaximal electrical stimulation of nerve by a mean motor unit potential (MMUP). The important modifications in the methods are the incorporation of the fluctuations in the response of MU to electrical excitation and a possible correction for the overlap in the firing levels of motor axons. The methods have been used in the estimation of the number of motor units in the first dorsal interosseous, thenar, hypothenar, and extensor digitorum brevis muscles of normal subjects and patients with various neuromuscular disorders. The results indicate that previous motor unit estimates were in general erroneously high.