Postural reflexes, balance control, and functional mobility with long-duration head-down bed rest.

INTRODUCTION: Spaceflight has functionally significant effects on sensorimotor behavior, but it is difficult to separate the effects of ascending somatosensory changes caused by postural muscle and plantar surface unloading from descending visual-vestibular neural changes. To differentiate somatosensory changes from graviceptor changes in post-spaceflight sensorimotor behavior, bed rest may serve as an exclusionary analog to spaceflight. METHODS: Four separate tests were used to measure changes in sensorimotor performance: 1) the monosynaptic stretch reflex (MSR); 2) the functional stretch reflex (FSR); 3) balance control parameters associated with computerized dynamic posturography (CDP); and 4) a functional mobility test (FMT). RESULTS: A mixed model regression analysis showed significant increases in median MSR start and peak latencies, while the median FSR latency showed no significant increase. Median MSR peak magnitude showed a significant increase during the middle bed rest period (19-60 d). There were no significant effects of bed rest on balance control, but some indication that dynamic head movements may affect posture after bed rest. Time to complete the course for the FMT increased significantly with bed rest. DISCUSSION: The four primary tests indicate that long-duration head-down bed rest, through unloading and modification of the body's support surface, serves as an exclusionary analog for sensorimotor responses to spaceflight. Furthermore, the data suggest that procedures designed to alleviate modifications to the sensory substrate serving the soles of the feet may provide a countermeasure to help maintain support afferentation of the postural muscles.

Preflight virtual reality training as a countermeasure for space motion sickness and disorientation.

INTRODUCTION: Research suggests that preflight training in virtual reality devices can simulate certain aspects of microgravity and may prove to be an effective countermeasure for space motion sickness (SMS) and spatial disorientation (SD). It is hypothesized that exposing subjects preflight to variable virtual orientations, similar to those encountered during spaceflight, will reduce the incidence and/or severity of SMS and SD. METHODS: Subjects were assigned to either a variable training (VT) or nonvariable training (NVT) condition to perform a simple navigation and switch activation task in a virtual space station. VT subjects performed the task starting in several different orientations, whereas NVT subjects always performed the task starting in the same orientation. On a separate day, all subjects then performed the same task in a transfer of training session starting from a novel orientation. RESULTS: When exposed to the novel test orientation, VT subjects performed the tasks more quickly (12%) and with fewer nausea symptoms (53%) than during the training session, compared with NVT subjects who performed more slowly (6%) and with more nausea symptoms (28%). Both VT and NVT conditions were effective in reducing the number of wall hits in the novel orientation (39% and 34%, respectively). DISCUSSION: These results demonstrate the effectiveness of using variable training in a virtual environment for reducing nausea and improving task performance in potentially disorienting surroundings, and suggest that such training may be developed into an effective countermeasure for SMS, SD, and associated performance decrements that occur in spaceflight.

Effect of 30-min +3 Gz centrifugation on vestibular and autonomic cardiovascular function.

INTRODUCTION: Repeated exposure to increased +Gz enhances human baroreflex responsiveness and improves tolerance to cardiovascular stress. However, it is not known whether such enhancements might also result from a single, more prolonged exposure to increased +Gz. Our study was designed to investigate whether baroreflex function and orthostatic tolerance are acutely improved by a single prolonged exposure to +3 Gz, and moreover, whether changes in autonomic cardiovascular function resulting from exposure to increased +Gz are correlated with changes in otolith function. METHODS: We exposed 15 healthy human subjects to +3 Gz centrifugation for up to 30 min or until symptoms of incipient G-induced loss of consciousness (G-LOC) ensued. Tests of autonomic cardiovascular function both before and after centrifugation included: 1) power spectral determinations of beat-to-beat R-R intervals and arterial pressures; 2) carotid-cardiac baroreflex tests; 3) Valsalva tests; and 4) 30-min head-up tilt tests. Otolith function was assessed during centrifugation by the linear vestibulo-ocular reflex and both before and after centrifugation by measurements of ocular counter-rolling and dynamic posturography. RESULTS: Of the 15 subjects who underwent prolonged +3 Gz, 4 were intolerant to 30 min of head-up tilt before centrifugation but became tolerant to such tilt after centrifugation. The Valsalva-related baroreflex as well as a measure of the carotid-cardiac baroreflex were also enhanced after centrifugation. No significant vestibular-autonomic relationships were detected beyond a vestibular-cerebrovascular interaction reported earlier in a subset of seven participants. CONCLUSIONS: A single prolonged exposure to +3 Gz centrifugation acutely improves baroreflex function and orthostatic tolerance.

Changes in gastric myoelectric activity during space flight.

The purpose of the present study was to examine postprandial myoelectric activity of the stomach and gastric activity associated with space motion sickness using electrogastrography. Three crewmembers participated in this investigation. Preflight, subjects exhibited normal postprandial responses to the ingestion of a meal. Inflight, crewmembers exhibited an abnormal decrease in the power of the normal gastric slow wave after eating on flight day 1, but had a normal postprandial response by flight day 3. Prior to and during episodes of nausea and vomiting, the electrical activity of the stomach became dysrhythmic with 60-80% of the spectral power in the bradygastric and tachygastric frequency ranges. These findings indicate that gastric motility may be decreased during the first few days of space flight. In addition, changes in the frequency of the gastric slow wave associated with space motion sickness symptoms are consistent with those reported for laboratory-induced motion sickness.

Predicting motion sickness during parabolic flight.

BACKGROUND: There are large individual differences in susceptibility to motion sickness. Attempts to predict who will become motion sick have had limited success. In the present study, we examined gender differences in resting levels of salivary amylase and total protein, cardiac interbeat intervals (R-R intervals), and a sympathovagal index and evaluated their potential to correctly classify individuals into two motion sickness severity groups. METHODS: Sixteen subjects (10 men and 6 women) flew four sets of 10 parabolas aboard NASA's KC-135 aircraft. Saliva samples for amylase and total protein were collected preflight on the day of the flight and motion sickness symptoms were recorded during each parabola. Cardiovascular parameters were collected in the supine position 1-5 days before the flight. RESULTS: There were no significant gender differences in sickness severity or any of the other variables mentioned above. Discriminant analysis using salivary amylase, R-R intervals and the sympathovagal index produced a significant Wilks' lambda coefficient of 0.36, p=0.006. The analysis correctly classified 87% of the subjects into the none-mild sickness or the moderate-severe sickness group. CONCLUSIONS: The linear combination of resting levels of salivary amylase, high-frequency R-R interval levels, and a sympathovagal index may be useful in predicting motion sickness severity.