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This Article Full Text Full Text (PDF) All Versions of this Article: 101/6/2802    most recent 90945.2008v1 Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Google Scholar Articles by Oude Nijhuis, L. B. Articles by Bloem, B. R. PubMed PubMed Citation Articles by Oude Nijhuis, L. B. Articles by Bloem, B. R.

Directional Sensitivity of "First Trial" Reactions in Human Balance Control

¹Department of Otorhinolaryngology, University Hospital, Basel, Switzerland; and ²Department of Neurology, Donders Institute for Brain, Cognition and Behavior, Center for Neuroscience and ³Epidemiology and Biostatistics, Radboud University Nijmegen Medical Centre, Nijmegen, The Netherlands

Submitted 11 September 2008; accepted in final form 4 March 2009

Support-surface movements are commonly used to examine balance control. Subjects typically receive a series of identical or randomly interspersed multidirectional balance perturbations and the atypical "first trial reaction" (evoked by the first perturbation) is often excluded from further analysis. However, this procedure may obscure vital information about neurophysiological mechanisms associated with the first perturbation and, by analogy, fully unexpected falls. We studied first trial reactions, aiming to clarify their directional impact on postural control and to characterize the underlying neurophysiological substrate. We instructed 36 subjects to maintain balance following support-surface rotations in six different directions. Perturbations in each direction were delivered in blocks, consisting of 10 serial stimuli. Full body kinematics, surface reactive forces, and electromyographic (EMG) responses were recorded. Regardless of direction, for the very first rotation, displacement of the center of mass was 15% larger compared with the ensuing nine identical rotations (P < 0.0001). This first trial reaction immediately reemerged whenever a new perturbation direction was introduced. First trial reactions (and near-falls) were greatest for backward-directed rotations and smallest for laterally directed rotations. This directional dependence coincided with early changes in vertical head accelerations. First trial reactions in EMG responses involved larger amplitudes in general and earlier muscle response onsets in upper body muscles. These findings show that first trial reactions are associated with significantly increased postural instability, mainly due to increased response amplitudes. Although rapid habituation occurs following presentation of identical stimuli, subjects immediately become unstable again when the perturbation direction suddenly changes. Excessive responses due to a failure to combine proprioceptive and vestibular cues effectively may explain this instability seen with first trials, particularly when falling backward.