MULTIVARIATE CHANGES IN COORDINATION OF POSTURAL CONTROL FOLLOWING SPACEFLIGHT

Postural and gait instabilities in astronauts returning from spaceflig ht are thought to result from in-flight adaptation of central nervous system processing of sensory inputs from the vestibular, proprioceptiv e, and visual systems. We hypothesized that reorganization of posture control relying on these multiple inputs would result in not only grea ter amounts of sway, but also changes in interjoint coordination. We t ested this hypothesis by examining the multivariate characteristics of postural sway and comparing the postural control gain used for mainte nance of upright stance during the altered sensory conditions of the S ensory Organization Test (EquiTest, Neurocom Intl.). We used the covar iance of hip and ankle kinematics as a measure of joint motion and int erjoint coordination, and then utilized discriminant analysis to furth er examine these characteristics in a group of 10 first-time astronaut s. In five of the six conditions, the most important difference was an increased relative utilization of the hip strategy, which would not b e evident using conventional balance measures such as peak or root-mea n-square sway. This finding was supported by indications of increased hip torque gains relative to lower extremity and neck motion in at lea st four conditions (p < 0.05). In contrast, ankle torque gains to thes e motions did not appear to change. These results suggest that after s paceflight, astronauts exhibit significant multivariate changes in mul tijoint coordination, of which increased sway is only one component. T hese changes are consistent with reweighting of vestibular inputs and changes in control strategy in a multivariable control system. (C) 199 8 Published by Elsevier Science Ltd. All rights reserved.