Correlation of motor nerve conduction velocity and number of innervated muscle fibers.

Distal and proximal motor (M) responses were recorded from the "Abductor Pollicis Brevis" (APB) muscle by using the collision method; median motor nerve was stimulated at distal (elbow) and proximal (wrist) regions in a concurrent manner. The delay between two stimuli (ISI: Inter-stimulus Interval), beginning at 9 ms, was decreased by 0.1 ms steps, until the proximal potential completely disappeared. Areas of M responses recorded for each ISI were calculated. Because the area difference between two subsequent ISIs is proportional to the number of muscle fibers innervated by the conduction velocity group at that interval, the relative numbers of muscle fibers for each velocity group were calculated. The results show that the motor nerve conduction velocities belonging to the innervating APB muscle vary between 38 m/s and 57 m/s; the conduction velocity of the group innervating the greatest number of muscle nerves was found to be 55-57 m/s, which comprised 10% of all fibers.