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Articles in PresS, published online ahead of print July 19, 2002
J Appl Physiol, 10.1152/jap.00414.2002
Submitted on May 14, 2002
Accepted on July 15, 2002
1 School of Kinesiology, University of Western Ontario, London, Ontario, Canada
2 School of Kinesiology, University of Western Ontario, London, Ontario, Canada; Department of Anatomy & Cell Biology, University of Western Ontario, London, Ontario, Canada
3 School of Physical Therapy, University of Western Ontario, London, Ontario, Canada
4 School of Physical Therapy, University of Western Ontario, London, Ontario, Canada; Department of Physiology, University of Western Ontario, London, Ontario, Canada
* To whom correspondence should be addressed. E-mail: csklein{at}uwo.ca.
This study examined, in nine old men (82±4 y), whether there is an association between the magnitude of change in motor unit discharge rate and the amount of twitch potentiation following a conditioning contraction (CC). The evoked twitch force and motor unit discharge rate during isometric ramp-and-hold contractions (10-18 s) of the triceps brachii muscle at 10 %, 20 % and 30 % of the maximal voluntary contraction (MVC) were determined before and 10s, 2 min, 6 min and 11 min after a 5 s CC at 75 % MVC. Following the CC, there was a potentiation of twitch force (~2 fold), and the discharge rate of the 47 sampled motor units declined (P < 0.05) an average of 1 Hz 10 s after the CC, compared to the control condition. The CC had no effect on the variability (coefficient of variation) of both force and discharge rate, as well as the electromyographic (EMG) activity recorded over the triceps brachii and biceps brachii muscles. In contrast to our earlier study of young men, the magnitude of the reduction in discharge rate after the CC was not associated (r = 0.06) with the amount of twitch potentiation. These findings suggest an age-related alteration in the neural strategy for adjusting motor output to a muscle after a CC.
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