Postural Equilibrium Control is one of eleven International Bedrest Standard Measures conducted on healthy subjects at the :envihab facility in Cologne, Germany. The Postural Equilibrium Standard Measure will assess changes in motor coordination and sensory integration through Sensory Organization Tests (SOT) and Motor Control Tests (MCT). The SOT will objectively assess one’s ability to make effective use of (or suppress inappropriate) visual, vestibular, and proprioceptive information for balance control. The MCT will assess the crewmember's ability to quickly and automatically recover from unexpected support surface perturbations. The purpose of the standard measures project is to populate a repository with data that enables high-level monitoring of countermeasure effectiveness and meaningful interpretation of health and performance outcomes. These standard measures will support future, hypothesis-driven research that enables planetary missions by providing an optimized minimal set of measures captured from subjects. These measures will be available to other studies under data sharing agreements as well as undergo analysis to understand risk posture.
APPROACH:
Postural stability will be evaluated using a computerized dynamic posturography system. Motor coordination, sensory integration, and balance control performance will be measured before and after bed rest. Each subject will complete one pre-bed rest test session and one post-bed rest test.
During these sessions, subjects will stand on a movable, force-sensing, support surface and within the movable visual enclosure of the Equitest system. Movements of the support surface and/or visual enclosure, under precise computer control, will be used to modify the sensory conditions and/or to impose unexpected perturbations. Throughout each SOT and MCT trial, subjects will be instructed to maintain stable upright posture with arms folded across the chest. External auditory orientation cues are masked by white noise supplied through headphones. The SOTs assess the subject’s ability to make effective use of visual, vestibular, and somatosensory information for maintaining upright stance.
During some trials, the support surface and/or visual surround will be moved in relation to the subject's sway, referred to as sway-referencing. Postural sway will be measured during 20 second trials, including combinations of somatosensory conditions (fixed-support, sway-referenced support) and visual conditions (eyes open, eyes closed, sway-referenced vision). Center-of-mass sway angles are estimated from instantaneous anterior-posterior (AP) and medial-lateral (ML) center-of-force positions computed from force transducers mounted within the Equitest force plates. The primary outcome variable is the Equilibrium (EQ) score derived from a subject's peak-to-peak (p-p) AP sway during each trial relative to a theoretical stability limit of 12.5 degrees (maximum p-p sway). A subject swaying to the limits of stability will result in a very low score. The highest possible score is 100, which indicates no sway. A score of zero is assigned to all falls/stopped trials. Postural performance is repeated with head movements which are made in the pitch plane at plus or minus 20 degrees, paced by a sinusoidal auditory tone at 0.33Hz with eyes closed, and a fixed-support or sway-referenced support surface.
The subject will also be challenged by sudden base-of-support perturbations (forward and backward translations). Perturbations will be presented sequentially at randomly varied intervals. The primary outcome variable for perturbations was path length which is the distance subjects’ center of pressure traveled from the time of perturbation to the time of stability.
The test conditions are:
- Standard eyes open Romberg test-visual surround and support surface fixed
- Standard eyes closed Romberg test-support surface fixed
- Sway-referenced visual surround with fixed support surface
- Sway-referenced support surface with fixed visual surround
- Sway-reference support surface with eyes closed
- Dynamic head tilt with fixed support
- Dynamic head tilt with sway-referenced support
- Translation: base of support moves underneath, dependent on height of subject (forward and backward)
The scheduled days of collection are as follows:
- One day prior to mission
- The day mission ends
RESULTS:
The :envihab International Bedrest Standard Measures Test Battery is not a hypothesis-driven study, but rather a collection of data made available to future researchers and for trend analysis. As such, there are no specific hypotheses, outcome measures, or statistical analyses associated with the skeleton of the project. The questionnaire will capture a "snapshot" of optimized data that can be investigated by researchers in the future.
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