title: Physiological properties of single thenar motor units in the F-response of younger and older adults.
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 surface-detected 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.
authors.affiliation: Faculty of Kinesiology, University of Western Ontario, London, Canada.
authors.forename: T J
authors.initials: TJ
authors.lastname: Doherty
authors.forename: T
authors.initials: T
authors.lastname: Komori
authors.forename: D W
authors.initials: DW
authors.lastname: Stashuk
authors.forename: A
authors.initials: A
authors.lastname: Kassam
authors.forename: W F
authors.initials: WF
authors.lastname: Brown
mesh_terms: D000200:Action Potentials / Q000502:physiology*; D000328:Adult; D000368:Aged; D000375:Aging / Q000502:physiology*; D001369:Axons / Q000502:physiology; D006801:Humans; D008875:Middle Aged; D009046:Motor Neurons / Q000502:physiology*; D009132:Muscles / Q000502:physiology*; D011930:Reaction Time