Physiological properties of single thenar motor units in the F‐response of younger and older adults

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Abstract
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). Trains of 300 stimuli at intensities evoking M-potentials 10%, 20%, and 30% of the peak-to-peak amplitude of the maximum M-potential, were delivered to the median nerve. In the young, observed firing probabilities of surfacedetected motor unit action potentials (S-MUAPs) extracted from 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 difference between the slowest and fastest conducting fibers for individual subjects ranged from 8% to 20%, which translated to conduction velocities (CVs) of 48–68 m · s−1 (mean 59 ± 4). The preceding were all independent of stimulus intensity. The S-MUAP sizes were significantly larger in older subjects (39%), and the range and distribution of motor unit CVs (38–61 m · s−1; mean 52 ± 3) were markedly shifted to reflect a slower population of motor fibers. These findings suggest that age-related axonal slowing may uniformly affect all median motor fibers. © 1994 John Wiley & Sons, Inc.