A method for the longitudinal study of human thenar motor units

Methods currently available to study the electrophysiological and contractile properties of single human motor units (MUs) preclude the same MU from being studied on repeated occasions. We have developed a method which allows single thenar MUs to be studied longitudinally. Sites were located along the median nerve where, using percutaneous stimulation, it was possible to selectively activate a single thenar motor axon as determined by: (a) the presence of an "all-or-nothing" surface-detected motor unit action potential (S-MUAP); (b) the occasional presence of F-responses identical in size and shape to the direct S-MUAP; (c) excitation thresholds widely separated from the next highest threshold motor axon so as to allow high frequency stimulation. To ensure that the same motor axon had been located and stimulated in successive studies, additional criteria included: (a) the motor axon was found at approximately the same site along the nerve; (b) the threshold for excitation remained widely separated from the next highest threshold motor axon; (c) the shape of the S-MUAP was almost identical on repeated studies; (d) S-MUAP and F-response latencies were similar. This method provides, for the first time, the ability to track longitudinally the electrical and contractile properties of single MUs in health and during the course and treatment of disease.