Document Detail


Multiple features of motor-unit activity influence force fluctuations during isometric contractions.
MedLine Citation:
PMID:  12702706     Owner:  NLM     Status:  MEDLINE    
Abstract/OtherAbstract:
To identify the mechanisms responsible for the fluctuations in force that occur during voluntary contractions, experimental measurements were compared with simulated forces in the time and frequency domains at contraction intensities that ranged from 2 to 98% of the maximum voluntary contraction (MVC). The abduction force exerted by the index finger due to an isometric contraction of the first dorsal interosseus muscle was measured in 10 young adults. Force was simulated with computer models of motor-unit recruitment and rate coding for a population of 120 motor units. The models varied recruitment and rate-coding properties of the motor units and the activation pattern of the motor-unit population. The main finding was that the experimental observations of a minimum in the coefficient of variation (CV) for force (1.7%) at approximately 30% MVC and a plateau at higher forces could not be replicated by any of the models. The model that increased the level of short-term synchrony with excitatory drive provided the closest fit to the experimentally observed relation between the CV for force and the mean force. In addition, the results for the synchronization model extended previous modeling efforts to show that the effect of synchronization is independent from that of discharge-rate variability. Most of the power in the force power spectra for the models was contained in the frequency bins below 5 Hz. Only a model that included a low-frequency oscillation in excitation, however, could approximate the experimental finding of peak power at a frequency below 2 Hz: 38% of total power at 0.99 Hz and 43% at 1.37 Hz, respectively. In contrast to the experimental power spectra, all model spectra included a second peak at a higher frequency. The secondary peak was less prominent in the synchronization model because of greater variability in discharge rate. These results indicate that the variation in force fluctuations across the entire operating range of the muscle cannot be explained by a single mechanism that influences the output of the motor-unit population.
Authors:
Anna M Taylor; Evangelos A Christou; Roger M Enoka
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Publication Detail:
Type:  Journal Article; Research Support, U.S. Gov't, Non-P.H.S.; Research Support, U.S. Gov't, P.H.S.     Date:  2003-04-17
Journal Detail:
Title:  Journal of neurophysiology     Volume:  90     ISSN:  0022-3077     ISO Abbreviation:  J. Neurophysiol.     Publication Date:  2003 Aug 
Date Detail:
Created Date:  2003-08-07     Completed Date:  2003-10-01     Revised Date:  2007-11-14    
Medline Journal Info:
Nlm Unique ID:  0375404     Medline TA:  J Neurophysiol     Country:  United States    
Other Details:
Languages:  eng     Pagination:  1350-61     Citation Subset:  IM    
Affiliation:
Department of Integrative Physiology, University of Colorado, Boulder, Colorado 80309-0354, USA.
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MeSH Terms
Descriptor/Qualifier:
Adult
Computer Simulation
Electromyography
Female
Humans
Isometric Contraction / physiology*
Male
Muscle, Skeletal / physiology*
Reference Values
Grant Support
ID/Acronym/Agency:
AG-09000/AG/NIA NIH HHS

From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine


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