Conduction velocities of α‐motor fibers innervating human thenar muscles and order of recruitment upon voluntary contraction

Abstract

Conduction velocity (CV) of single α-motor fibers was measured using a combination of microneurogrphic and spike-triggered averaging techniques on 7 healthy subjects (aged 25 to 42). Two tungsten microelectrodes were inserted percutaneously into the median nerve at the wrist and 4 to 6 cm more proximally. Motor unit activities recorded from thenar muscles during voluntary isometric contraction with a third microelectrode, were isolated with a window discriminator, and used to trigger an averager. CVs were calculated by dividing the interelectrode distance by the conduction time between unitary activities in the proximally and distally averaged neurograms. The CVs of 100 units ranged between 34 and 66 m/s, with the peak at around 50 m/s. The relationship between CVs and recruitment orders was compared within sets of 2 or 3 units recorded simultaneously. Statistical analysis using a χ² test revealed that slower motor units tended to be recruited earlier than faster units (P < 0.001).