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Principal Investigator:
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Conrad Wall, III, Ph.D.
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Organization:
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Harvard -- Massachusetts Eye and Ear Institute
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Project Ended:
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2000
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Adapting to microgravity is not the only balance difficulty astronauts' face. Major postflight problems include difficulties with standing, walking, turning corners, climbing stairs and other activities that require stability of upright posture and gaze. These difficulties inhibit astronauts' ability to stand up, bail out, or escape from the vehicle during emergencies and to function effectively when leaving the space/shuttlecraft after flight. Thus it is important to understand the cause of these profound impairments of posture, gaze and locomotion stability in many returning astronauts (and in vestibular patients), and determine how they can be quantified.
Any developed countermeasure must be tested to determine its effect on gait stability, particularly under those conditions that are most troublesome following spaceflight. These countermeasures must be tested with valid and reliable tools.
This project's aims were to develop quantitative, parametric approaches for assessing gaze stability and spatial orientation during normal gait and when gait is perturbed. It has produced two new findings that are key to the understanding of human locomotion and a novel way of characterizing locomotor disturbances that are described below.
1. Understanding movements of the eyes, head, and body during locomotion. The ability to see objects clearly during locomotion is important for preventing collisions, falls, trips, and clumsy maneuvers. Clear vision requires that the image of the visual object of interest remains steady upon the retina. This ability to stabilize a retinal image is known to be compromised upon a change in G level after a prolonged exposure to a previous G level (for example, Earth return from a microgravity orbit). To understand just why there is blurred vision in returning astronauts, it is necessary to understand jointly during locomotion: the motion of the head in space, the motion of the eye in the head and the relationship of the visual target to the person. Two important results from our research involve the relationship of head motion to walking speed and the relationship of eye motion to visual target distance.
Trunk and head movements were characterized over a wide range of walking speeds to determine the relationship between stride length, stepping frequency, vertical head translation, pitch rotation of the head, and pitch trunk rotation as a function of gait velocity. The results suggest that two mechanisms are utilized to maintain a stable head fixation distance over the optimal range of walking velocities. The relative contribution of each mechanism to head movement depends on the frequency of head movement and consequently on walking velocity. From consideration of the frequency characteristics of the compensatory head pitch, we infer that compensatory head pitch movements may be produced predominantly by the angular vestibulo-collic reflex (aVCR) at low walking speeds and by the linear vestibulo-collic reflex (lVCR) at the higher optimal speeds of walking.
Eye and head movements were also characterized during locomotion while the subject viewed near and distant targets. For near targets eye velocity was essentially in phase with head pitch velocity. Eye velocity increasingly lagged head pitch as target distance increased, and was compensatory at 2.0 meters. For far targets, the gain and phase of eye re head pitch velocity indicated that the angular vestibulo-ocular reflex (aVOR) was generating the eye movement response. For near targets, decreasing target distance would augment the lVOR gain, and the eye velocity phase suggested that the linear vestibulo-ocular reflex (lVOR) was generating the vertical eye movement response.
The significance of the studies is that we have gained considerable insight into the compensatory and orienting functions of the vestibulo-collic and vestibulo-ocular reflexes during locomotion. Through this has come the development of countermeasure assessment criteria which can now be applied in studying behaviors that have proven to be difficult following exposure to microgravity.
2. Understanding the orientation mechanism during locomotion. One of our working hypothesis was that the above-mentioned "profound impairments of posture, gaze and locomotion stability" are caused by alterations in compensatory and orientation mechanisms that are generated in the central vestibular system from motion inputs. During exposure to altered gravity, the motion inputs from the otolith organs are "distorted" compared to the on-Earth conditions. These distortions, in turn, cause both inappropriate body head and eye movements and an altered sense of orientation, which degrades stability during locomotion.
We compared motions of the body during walking along a straight line with body motions while walking along a curved path. In the latter condition subjects accelerate in toward the direction of the curve which introduces an inertial component which may or may not effect measures of their body orientation in space. Our data show that compensatory eye, head and body movements stabilize gaze during straight walking, while orienting mechanisms direct the eyes, head and body to tilts of the resultant of gravitational and centripetal acceleration in space during turning. This finding in normal subjects can now be compared to subjects with known impairments in their balance system or to returning astronauts to determine whether or not such individuals can successfully align parts of their bodies in an appropriate way while turning.
3. Characterizing the recovery trajectory to disturbances of locomotion. Analysis of perturbed gait provides a means of evaluating the success of measures designed to counter loss of balance due to disease or exposure to microgravity. We measured several body segment variables (head, sternum, legs) and especially their trajectories in response to mechanical perturbations that were precisely delivered in time, magnitude, and direction to the foot.
In normal individuals, the recovery trajectory show a large initial displacement due to the disturbance and subsequently crosses the baseline at the second step to show a slight underdamped response at the third step with a return to, or near, the baseline by the fourth step. In contrast, the recovery trajectory for a pilot vestibulopathic patient shows a distinctive different pattern which takes several more paces to recover.
The development of an experimental paradigm that introduces a calibrated disturbance to the foot during the support phase of normal locomotion provides a means for the objective quantification of locomotor response dynamics that are known to be altered in astronauts upon return from exposure to microgravity but for which no current test exists. Returning astronauts whose orientation mechanism has been distorted and patients having vestibulopathies that may well affect their orientation mechanism are expected to have longer recovery trajectories than healthy normals.
Satisfaction of hypotheses, objectives, and specific aims of original proposal.
The first three of the original aims of this project and their hypotheses were successfully accomplished and produced the three key findings mentioned above. A fourth aim was truncated due to lack of a suitable patient population. A fifth aim was consolidated with an aim in another project. The final aim, which involved development of methodology, was integrated into the first three aims.
Implications for Critical Path Risk reduction and links to health research on Earth
The results from this project apply directly to the Critical Path Risk of impaired neuromuscular strength upon return to positive G leading to increased occurrence of falls and fractures during emergency egress and escape - a Type II risk. We have defined some situations that occur in every day locomotion in which astronauts returning from microgravity and in which patients having subtle vestibulopathies are apt to have trouble but which there are no objective measures currently available to quantify their performance. From these, we have increased our understanding of some of the fundamental processes that govern factors like gaze and head position while moving. There is now the potential for developing more meaningful and sensitive tests of balance function that can be applied to astronauts for countermeasure assessment, and to patients with balance anomalies.
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