AbstractThe 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 moto
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