Abstract
Anxiogenic settings lead to reduced postural sway while standing, but anxiety-related balance may be influenced by the location of postural threat in the environment. We predicted that the direction of threat would elicit a parallel controlled manifold relative to the standing surface, and an orthogonal uncontrolled manifold during standing. Altogether, 14 healthy participants (8 women, mean age = 27.5 years, SD = 8.2) wore a virtual reality (VR) headset and stood on a matched real-world walkway (2 m × 40 cm × 2 cm) for 30 s at ground level and simulated heights (elevated 15 m) in two positions: (1) parallel to walkway, lateral threat; and (2) perpendicular to walkway, anteroposterior threat. Inertial sensors measured postural sway acceleration (e.g., 95% ellipse, root mean square (RMS) of acceleration), and a wrist-worn monitor measured heart rate coefficient of variation (HR CV). Fully factorial linear-mixed effect regressions (LMER) determined the effects of height and position. HR CV moderately increased from low to high height (p = 0.050, g = 0.397). The Height × Position interaction approached significance for sway area (95% ellipse; β = − 0.018, p = 0.062) and was significant for RMS (β = − 0.022, p = 0.007). Post-hoc analyses revealed that sagittal plane sway accelerations and RMS increased from low to high elevation in parallel standing, but were limited when facing the threat during perpendicular standing. Postural response to threat varies depending on the direction of threat, suggesting that the control strategies used during standing are sensitive to the direction of threat.
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Availability of data and material
All data are available here: https://github.com/keithlohse/Gait_VR/tree/master/standing_balance.
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All analysis code are available here: https://github.com/keithlohse/Gait_VR/tree/master/standing_balance.
Notes
HTC Vive versions were switched to version 2.0 after the first six participants to increase the number of lighthouses (i.e., infrared tracking stations) from two to four and improve tracking quality.
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Acknowledgements
We would like to acknowledge the hard work of the students that made this work possible including Mindie Clark, Nick Kreter, and Ashlee McBride.
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PCF was supported by the Eunice Kennedy Shiver National Institute of Child Health and Human Development of the National Institutes of Health under Award Number K12HD073945. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institute of Health.
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WRY, AMW, PCF, KRL and BF contributed to the study conception and design. Material preparation and data collection were performed by TER. Hypothesis development and data analysis were completed by TER and PCF. Statistical analysis was conducted by TER and KRL. The first draft of the manuscript was written by TER and all authors commented and contributed to all versions of the manuscript. Figures and manuscript revisions were managed by TER, BF, and PCF. All authors read and approved the final manuscript.
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The authors have no conflict of interest. Material has been reviewed by the Walter Reed Army Institute of Research. There is no objection to its presentation and/or publication. The opinions or assertions contained herein are the private views of the author, and are not to be construed as official, or as reflecting true views of the Department of the Army or the Department of Defense. The investigators have adhered to the policies for protection of human subjects as prescribed in AR 70–25.
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Communicated by Francesco Lacquaniti.
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Raffegeau, T.E., Fawver, B., Young, W.R. et al. The direction of postural threat alters balance control when standing at virtual elevation. Exp Brain Res 238, 2653–2663 (2020). https://doi.org/10.1007/s00221-020-05917-5
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DOI: https://doi.org/10.1007/s00221-020-05917-5