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The Journal of Neurophysiology Vol. 82 No. 5 November 1999, pp. 2081-2091
Copyright ©1999 by the American Physiological Society
1NeuroMuscular Research Center and
2Department of Biomedical Engineering,
Erim, Zeynep,
M. Faisal Beg,
David T. Burke, and
Carlo J. de Luca.
Effects of Aging on Motor-Unit Control Properties. J. Neurophysiol. 82: 2081-2091, 1999. It was
hypothesized that the age-related alterations in the morphological
properties of a motor unit would be accompanied by modifications in the
control aspects of the motor unit, as either an adaptive or
compensatory mechanism to preserve smooth force production. In
specific, the objective of the study was to investigate the age-related
alterations in the concurrent firing behavior of multiple motor units
in the first dorsal interosseous (FDI) muscle in isometric contractions
at 20 and 50% of the subject's voluntary contraction level. Analysis
of the data collected from 10 young (24-37 yr of age) and 10 elderly
(65-88 yr of age) subjects led to three novel observations regarding
the firing behavior of aged motor units. 1) Among
elderly subjects, there is a decrease in the common fluctuations that
are observed among the firing rates of motor units in the young.
2) The relationship observed between the firing rate and
recruitment threshold of young subjects is disturbed in the elderly.
Although in young subjects, at any point in a given submaximal
contraction, earlier recruited motor units have higher firing rates
than later-recruited units; in aged subjects this dependency of firing
rate on recruitment rank is compromised. 3) The
progressive decrease observed in the firing rates of concurrently
active motor units in constant-force contractions in the young is not
seen in the aged. In addition to these original findings, this study
provided support for earlier reports of 1) decreased
average firing rates probably reflecting the slowing of the muscle,
2) a shift in recruitment thresholds toward lower force
levels in line with the shift toward type I fibers, and 3) multiphasic action potential shapes indicative of the
reinnervation process that takes place during aging. Taken as a whole,
these findings indicate significant age-related modifications in the control properties of human motor units.
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