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George E. Carvell, PhD, PT   University of Pittsburgh
PERIPHERAL NERVE CONDUCTION: Normal Motor Nerve Conduction
 Motor Nerve Conduction Studies record responses from a distal muscle innervated by the nerve to be examined. Stimuli are delivered at several locations along the course of the nerve. Brief (0.1 msec pulses at 1/sec) but strong (supramaximal strength) stimuli are used. Responses are recorded with surface electrodes placed over the muscle belly (active electrode) and distal muscle tendon (reference electrode). A ground electrode is also placed over non-contractile tissue (not shown). The example shown is motor nerve conduction for the Median Nerve. Recording electrodes are placed over the Abductor Pollicis Brevis Muscle. Proximal stimulation is located at the elbow adjacent to the brachial artery. Distal stimulation is located at the wrist lateral to the palmaris longus tendon & proximal to the carpal tunnel. Stimuli are monophasic DC pulses; cathode (-) located distal to the anode (+). Stimuli build from submaximal to supramaximal strength. Recordings are simulated digital oscilloscope traces (note calibration).
Normal motor nerve conduction waveforms are large. Motor units act as biological amplifiers since each motor axon innervates hundreds of muscle fibers. Summation of all motor unit muscle potentials produces amplitudes in the 8-12 mV range (compared to mV ranges for sensory potentials). Distal latencies for the median nerve should be less than 4 msec and conduction velocity should be > 50 M/sec for adults. Actual normal values vary according to age. Distal & proximal stimulation site waveforms should have similar shape and amplitude.
Normal Motor Nerve Conduction
GMOMM  2001