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1 Faculty of Kinesiology, University of Western Ontario, London, Ontario, Canada N6A 3K7; and 2 Neuromuscular Unit, Department of Neurology, New England Medical Center and Tufts University, Boston, Massachusetts 02111
Received 10 July 1995; accepted in final form 21 August 1996.
Doherty, Timothy J., and William F. Brown. Age-related
changes in the twitch contractile properties of human thenar motor
units. J. Appl. Physiol. 82(1):
93-101, 1997.
The purpose of this study was to examine the
effects of aging on the contractile and electrophysiological properties
of human thenar motor units (MUs). Percutaneous electrical stimulation
of single motor axons within the median nerve was used to isolate and
examine the twitch tensions, contractile speeds, and surface-detected
MU action potential (S-MUAP) sizes of 48 thenar MUs in 17 younger
subjects (25-53 yr) and 44 thenar MUs in 9 older subjects
(64-77 yr). A wide range of twitch tensions, contractile speeds,
and S-MUAP sizes was observed in both age groups. However, older
subjects had significantly larger MU twitch tensions and slower MU
twitch contraction and half-relaxation times. These changes were
accompanied by increased S-MUAP sizes. These findings suggest that the
human thenar MU pool undergoes significant age-related increase in MU
size and slowing of contractile speed. Such adaptation may help to
overcome previously reported age-related losses of thenar MUs.
aging; electromyography; skeletal muscle; motor unit force
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