Identification of Human Postural Control Mechanisms using a Model-based Interpretation of Experimental Results

Robert Peterka
Department of Physiology & Pharmacology, Oregon Health and Science University

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     Last modified: April 28, 2007
     Presentation date: 08/10/2007 10:10 AM in MCC
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Abstract
This presentation will describe how we have characterized the dynamic behavior of the human postural control system, how relatively simple control-system models can be used to interpret experimental results and to gain insight into the mechanisms contributing to postural control, and how these models can be used to understand abnormal postural behavior. Our lab has extensive experience using wide-bandwidth, continuous pseudorandom rotational stimuli applied to the support surface upon which the test subject stands, or to the visual surround viewed by the subject. Using a frequency-domain, transfer-function analysis, we find that body sway evoked by any particular stimulus can be quite accurately represented by a linear control-system model. However, overall dynamic behavior is nonlinear in that relatively less body sway is evoked by larger amplitude stimuli. This decrease in response sensitivity with increasing stimulus amplitude can be explained by a sensory integration mechanism that increases the contribution of vestibular cues, and decreases the contribution of proprioceptive or visual cues with increasing stimulus amplitude. A negative-feedback model-based interpretation of this ?sensory re-weighting? accounts for postural behavior over a frequency range of about 0.1 to 1 Hz. To account for lower frequency postural behavior, we postulate the contribution of a positive feedback mechanism that senses the control effort and then generates a corrective action. This ?effort control? mechanism ultimately results in a reduction of the overall energy required for stance control. Finally, the contributions of stretch reflexes, muscle activation, and muscle mechanics are needed to account for higher frequency behavior. When all of these mechanisms are included in a postural control model, the model-predicts responses to transient perturbations (rapid surface translations or rotations) that closely match experimental responses reported in the traditional literature. This close match suggests that the mechanisms involved in the continuous regulation of posture are also responsible for the ?motor programs? generated in response to transient disturbances. Furthermore, insights into the causes of abnormal postural control can be gained by comparison of model predictions to experimental results. Examples will demonstrate how the model can be used to understand the limitations imposed by a bilateral loss of vestibular function and by failure of the sensory-reweighting mechanism.