Long train (50) electrical pulses were applied to the ulnar, median, and deep peroneal nerves. The probability of firing, or firing index, of the first motor units (MU) recorded by surface and needle electrodes in the first dorsal interosseous, thenar, and extensor digitorum brevis muscles varied from 0-100% as the stimulus was increased from a minimum threshold voltage (V) to V + delta V (delta V = 2-4 volts). The voltage interval, delta V (firing level range), of even the first few MU recruited greatly overlapped in normal subjects, but may overlap less in some neuromuscular disorders such as amyotrophic lateral sclerosis. The mean size of the single MU isolated by (1) isometric voluntary contraction method, (2) F recurrent discharge method, and (3) stimulation at multiple locations along the nerve, were more than twice as large as the mean size of the incremental steps evoked by graded electrical stimulation. These results suggest that methods of estimating the number of MU in a muscle should incorporate a correction for the fluctuations in excitability and overlap in firing levels of MU, and in addition should include larger MU in the estimation of the mean MU potential.
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