PHYSIOLOGICAL-PROPERTIES OF SINGLE THENAR MOTOR UNITS IN THE F-RESPONSE OF YOUNGER AND OLDER ADULTS

The purpose of this study was to characterize the properties of single thenar motor units in the F-response of healthy younger (n = 15; age 33 +/- 11 years) and older subjects (n = 15; age 68 +/- 3 years). Trai ns of 300 stimuli at intensities evoking M-potentials 10%, 20%, and 30 % of the peak-to-peak amplitude of the maximum M-potential, were deliv ered to the median nerve. In the young, observed firing probabilities of surface-detected motor unit action potentials (S-MUAPs) extracted f rom the F-response ranged from less than 1-10%, the S-MUAPs varied in size from 0.015% to 5.3% of the maximum M-potential negative peak area , and they were similar in size to the population of S-MUAPs collected by multiple point stimulation of the median nerve. The percentage dif ference between the slowest and fastest conducting fibers for individu al subjects ranged from 8% to 20%, which translated to conduction velo cities (CVs) of 48-68 m . s(-1) (mean 59 +/- 4). The preceding were ai l independent of stimulus intensity. The S-MUAP sizes were significant ly larger in older subjects (39%), and the range and distribution of m otor unit CVs (38-61 m . s(-1); mean 52 +/- 3) were markedly shifted t o reflect a slower population of motor fibers. These findings suggest that age-related axonal slowing may uniformly affect all median motor fibers. (C) 1994 John Wiley & Sons, Inc.