Abstract

The discharge properties on maintained maximal voluntary effort, axonal conduction velocity (a.c.v.) and contraction time (c.t.)of single human motor units were studied. Electromyographic (EMG)techniques were used and sufficientselectivity was obtained after repeated lesions to the terminal nerve twigs and consequent collateral spouting, or by blocking the main muscle nerve in subjects with an accessory nerve supplying just one or a few units, or using high impedance wire electrodes. Most units with a.c.v. above 45 ms and c.t. below 50 ms had maximal voluntary firing rates of about 50 Hz. on prolonged maximal effort, however, their rates rapidly decreased and after some seconds to a minute they ceased to respond tonically. As long as their motoneurons fired their EMG potentials were mainly intact and their twitch tension was significant. Most units with a.c.v. below 40 ms and c.t. above 60 ms fired initially at 30 Hz, decreased slowly in firing rate and continued at 20 Hz forsome minutes. We conclude that there is a central fatigue of units with short c.t. and high a.c.v. which proctects their peripheral regions from severe exhaustion, but that there is no significant central fatigue of units with long c.t. and low a.c.v.We emphasize, however, that most units are interme- diate in their properties. Verf.-Referat