Document Detail

Review of first trial responses in balance control: Influence of vestibular loss and Parkinson's disease.
MedLine Citation:
PMID:  21435732     Owner:  NLM     Status:  Publisher    
The reaction to an unexpected balance disturbance is unpractised, often startling and frequently associated with falls. This everyday situation can be reproduced in an experimental setting by exposing standing humans to sudden, unexpected and controlled movements of a support surface. In this review, we focus on the responses to the very first balance perturbation, the so-called first trial reactions (FTRs). Detailed analysis of FTRs may have important implications, both for clinical practice (providing new insights into the pathophysiological mechanisms underlying accidental falls in real life) and for understanding human physiology (what triggers and mediates these FTRs, and what is the relation to startle responses?). Several aspects of the FTRs have become clear. FTRs are characterized by an exaggerated postural reaction, with large EMG responses and co-contracting muscles in multiple body segments. This balance reaction is associated with marked postural instability (greater body sway to the perturbation). When the same perturbation is repeated, the size of the postural response habituates and the instability disappears. Other issues about FTRs remain largely unresolved, and these are addressed here. First, the functional role of FTRs is discussed. It appears that FTRs produce primarily increased trunk flexion during the multi-segmental response to postural perturbations, thus producing instability. Second, we consider which sensory signals trigger and modulate FTRs, placing specific emphasis on the role of vestibular signals. Surprisingly, vestibular signals appear to have no triggering role, but vestibular loss leads to excessive upper body FTRs due to loss of the normal modulatory influence. Third, we address the question whether startle-like responses are contributing to FTRs triggered by proprioceptive signals. We explain why this issue is still unresolved, mainly because of methodological difficulties involved in separating FTRs from 'pure' startle responses. Fourth, we review new work about the influence of perturbation direction on FTRs. Recent work from our group shows that the largest FTRs are obtained for toe-up support surface rotations which perturb the COM in the posterior direction. This direction corresponds to the directional preponderance for falls seen both in the balance laboratory and in daily life. Finally, we briefly touch upon clinical diagnostic issues, addressing whether FTRs (as opposed to habituated responses) could provide a more ecologically valid perspective of postural instability in patients compared to healthy subjects. We conclude that FTRs are an important source of information about human balance performance, both in health and disease. Future studies should no longer discard FTRs, but routinely include these in their analyses. Particular emphasis should be placed on the link between FTRs and everyday balance performance (including falls), and on the possible role played by startle reactions in triggering or modulating FTRs.
J H J Allum; K-S Tang; M G Carpenter; L B Oude Nijhuis; B R Bloem
Publication Detail:
Type:  JOURNAL ARTICLE     Date:  2011-3-22
Journal Detail:
Title:  Human movement science     Volume:  -     ISSN:  1872-7646     ISO Abbreviation:  -     Publication Date:  2011 Mar 
Date Detail:
Created Date:  2011-3-25     Completed Date:  -     Revised Date:  -    
Medline Journal Info:
Nlm Unique ID:  8300127     Medline TA:  Hum Mov Sci     Country:  -    
Other Details:
Languages:  ENG     Pagination:  -     Citation Subset:  -    
Copyright Information:
Copyright © 2011 Elsevier B.V. All rights reserved.
Dept. of ORL, University of Basel, Basel, Switzerland.
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