Age-related changes in fastest and slowest conducting axons of thenar motor units

Single thenar motor unit F waves (FMUPs) were collected from 23 healthy vol unteers (age range 21-91 years, mean 46 +/- 20 SD). In each subject, 10 dis tinct FMUPs were recorded, using surface stimulating and recording electrod es, and the conduction velocity (CV) of each motor unit was calculated. The distribution of CVs (overall range 42-66 m/s; individual FMUP CV dispersio n range 6-27% of the maximal FMUP CV) was close to those previously reporte d whatever the technique used. With age, a progressive CV reduction was obs erved, and maximal FMUP CV was significantly correlated with age (r = -0.58 , P < 0.01), whereas no statistically significant correlation was found bet ween minimal FMUP CV and age (r = -0.27, ns). Individual FMUP CV dispersion presented a statistically significant decrease with age (r = -0.46, P < 0. 05). Furthermore, thenar motor unit number (MUNE), estimated by the adapted multiple point stimulation method, decreased progressively with age and wa s statistically correlated with maximal FMUP CV (r = 0.59, P < 0.01), where as there was no correlation with minimal FMUP CV (r = 0.34, ns). Thus, we p ropose that motor unit loss is progressive with age throughout life, affect ing particularly the largest and fastest conducting motor units. Preferenti al involvement of these fibers could be responsible for the age-related cha nges in motor nerve CV. (C) 1999 John Wiley & Sons, Inc.