To test the hypothesis that the nerve fiber conduction velocity of a motor unit is correlated with the muscle fiber conduction velocity of the unit, the collision technique was used to discriminate the motor unit action potentials elicited by stimulation of a group of nerve fibers with known conduction velocity. Muscle fiber conduction velocity was determined from the propagating action potentials of the unit recorded by a surface electrode array. A weak but significantly positive correlation was found between nerve and muscle fiber conduction velocities in the thenar muscle of human subjects. The results indicate that the size principle that governs the relation between nerve and muscle fiber size is a fairly weak principle with occasional exceptions.
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