1997 ASB Predoctoral Award PaperMultivariate changes in coordination of postural control following spaceflight
Introduction
Conventional measures of postural sway, such as root-mean-square (rms) motion of the center of mass or center of pressure, are univariate in the sense that they provide single quantities summarizing overall motion of the body. The multijoint dynamics of the human body and the difficulty of simultaneously controlling these joints would suggest that in addition to stabilizing the center of mass, the central nervous system is confronted with the problem of coordinating the joints. We propose that sway is actually a multivariate response, involving multiple joints and multiple sensors (Kuo et al., 1998). A loss of one or more sensors then implies not only an increase in sway, but also an alteration in joint coordination. By examining the multivariate response, we can characterize postural sway more specifically and gain clues about the nature or cause of a balance problem such as postural instability following spaceflight.
Astronauts display a variety of postural difficulties upon returning to earth, including an inability to maintain a stable posture with eyes closed, using a wide stance to stand and walk, feeling sensations of lateral acceleration while walking, and an inability to detect small changes in head position (Kenyon and Young, 1986). One explanation of these observations is that gain on otolith signals may be reduced, consequently leading to a decrease in the ability to sense linear acceleration. An alternative hypothesis is that otolith signals are reinterpreted to represent only linear acceleration rather than pitch or roll of the head (Parker et al., 1985). Maintenance of this reinterpretation following spaceflight is maladaptive, resulting in postural instability with eyes closed and increased reliance on visual information for orientation (Young et al., 1984). Previous studies have assessed these changes in postural control following spaceflight with single summary measures (Homick and Reschke, 1977; Kenyon and Young, 1986; Paloski et al., 1992; Young et al., 1984).
There is evidence that hip responses to triggered perturbations are altered following spaceflight. Reschke et al. (1984)found evidence of an immediate strategy change in postflight postural responses, including more motion of the waist relative to the shoulders and deceased damping in the eyes closed condition. Anderson et al. (1986)found that in preflight testing, the astronauts moved shoulders and hips in phase with each other, while in postflight testing the subjects showed a more disjoint behavior, specifically with more hip movement. They suggested that a change in posture with more flexion at the hip may be the easiest way to move about in weightlessness. Alternatively, increased hip motions postflight may be due to the fact that motions about the waist would permit faster acceleration of the COM and thus faster correction of postural displacements (Kuo and Zajac, 1993). Kenyon and Young (1986)suggested that astronauts had more hip motion post-flight in an effort to minimize the motion of the head.
It is unclear whether these changes are limited to pre-programmed triggered responses, or if they also occur in the feedback loop active during quiet stance. If quiet stance is affected, it is reasonable to expect that changes could occur not only in amount of sway, but also interjoint coordination, because each sensor has a unique contribution to control of each joint (Kuo et al. 1998). Moreover, it is possible that coordinative changes could alternatively be attributed to adaptations which affect the detection of motion, or to changes in control gains.
We used multivariate analysis of postural sway to test for changes in posture control during quiet stance following spaceflight. Multivariate measures make it possible to differentiate between increases in sway and changes in interjoint coordination. Altered sensory conditions were employed to test whether these changes could be attributed to altered perception of self-motion, while indicators of control gains were used to test for changes in control separate from perception. We also compared the multivariate results with conventional univariate measures to test whether there were postural changes which could not be accounted for using only conventional measures.
Section snippets
Subjects and data collection
The Sensory Organization Test (EquiTest, Neurocom International) is regularly used to examine peak-to-peak sway in altered sensory environments. In this test, also known as dynamic posturography, subjects stand quietly on a computer-controlled movable platform under six sensory conditions which alter the available visual and proprioceptive information (Table 1). The support platform rotates about the axis of the ankles and is synchronized with sagittal plane sway of the subject. By maintaining
Increase in sway was statistically significant with a notable increase in hip strategy
Univariate (COP, COM) and multivariate results indicated significant differences between pre- and post-spaceflight assessments for all six conditions of the Sensory Organization Test (Fig. 2Fig. 3). Paired t-tests indicated significant (p<0.03) increases in sway between pre- and post-spaceflight tests across all six SOT conditions. Fig. 2aFig. 2b illustrates the mean rms of COP and COM sway across the six conditions, respectively. Similarly, paired t2-tests indicated significant
Discussion
Changes in postural control (Collins et al., 1995) following spaceflight are multivariate in nature, as shown by the covariance ellipses that have increases along different dimensions (Fig. 3). This indicates not only a change in the amount of sway, but a change in joint coordination as well. For all altered sensory conditions except SOT 5, the multivariate measures showed the most significant increase in hip strategy. For only one condition (5), was the increase in COM motion more significant
Acknowledgements
This work was supported in part by the National Science Foundation under grant IBN-951184, the National Institutes of Health under grant 1R29DC02312-01A1, the Whitaker Foundation, the NASA Extended Duration Orbiter Medical Project (EDOMP) DSO-605, and the NASA Graduate Student Researchers Program.
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