Document Detail

Reflex facilitation during the stretch-shortening cycle.
MedLine Citation:
PMID:  10818339     Owner:  NLM     Status:  MEDLINE    
Maximal torque during the concentric phase of a movement has been shown to be enhanced by prior eccentric muscle actions, a movement strategy referred to as the stretch-shortening cycle. Although the mechanical basis for this enhancement is well established, the neural component is not. We hypothesized that brief high-frequency bursts of spindle afferent discharge during the eccentric phase of the stretch-shortening cycle could be one mechanism for facilitating the volitional drive. To test this hypothesis, three sets of experiments were done. In the first (N=15), we demonstrated that both the peak and mean EMG of the soleus (S) and lateral gastrocnemius (LG) muscles were considerably greater during a reciprocal hopping (RHOP) task than for maximum isometric contractions (MIVCs). In the second experiment, we tested whether the dynamic nature of the RHOP or the eccentric phase of the RHOP contributed to the EMG potentiation. Peak and mean EMG produced with a concentric hop (CHOP), in which the lengthening phase of the hop was eliminated, were compared with that produced with the RHOP and MIVCs conditions (N=7). The RHOP produced greater peak EMG than either the CHOP or the MIVCs while the mean EMG for both hopping conditions was considerably more than the MIVCs. In the final experiment, we attempted to mimic the brief high-frequency burst of spindle afferent activity during the lengthening phase of the stretch-shortening cycle in the absence of muscle length changes. High-frequency (100 Hz) afferent stimulation (HFS) was delivered during MIVCs. At rest, the HFS produced negligible EMG activity but when superimposed over MIVCs produced a marked potentiation of the S EMG over values obtained during MIVCs alone. Evidence that HFS synchronizes the EMG associated with volitional activation is also provided. We conclude that a substantial but brief facilitation and possible synchronization of the neural drive is provided by the spindle afferents during the eccentric phase of the stretch-shortening cycle.
M H Trimble; C G Kukulka; R S Thomas
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Publication Detail:
Type:  Comparative Study; Journal Article    
Journal Detail:
Title:  Journal of electromyography and kinesiology : official journal of the International Society of Electrophysiological Kinesiology     Volume:  10     ISSN:  1050-6411     ISO Abbreviation:  J Electromyogr Kinesiol     Publication Date:  2000 Jun 
Date Detail:
Created Date:  2000-07-12     Completed Date:  2000-07-12     Revised Date:  2006-11-15    
Medline Journal Info:
Nlm Unique ID:  9109125     Medline TA:  J Electromyogr Kinesiol     Country:  ENGLAND    
Other Details:
Languages:  eng     Pagination:  179-87     Citation Subset:  IM    
Department of Physical Therapy, University of Florida, Gainesville,FL 32610-0154, USA.
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MeSH Terms
Electric Stimulation
Motor Neurons / physiology
Muscle Contraction / physiology*
Muscle Spindles / physiology*
Muscle, Skeletal / innervation,  physiology*
Reflex, Stretch / physiology*

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