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Weightlessness can affect the vestibular function, primarily the otolith function, both directly due to the loss of gravitational influence and by eliminating the support and minimizing proprioceptive afferentation, influencing indirectly through the central integrative multisensory structures of the central nervous system (CNS), where the convergence of afferent signals of various sensory modality (primarily visual, vestibular, support and motor) occurs. All this prompted the study of the influence of various afferentations on the otolith-ocular reflex under the conditions of real and simulated weightlessness. Using the video oculography method, 58 Russian cosmonauts, members of long-term missions to the ISS (125–219 days, ~175 days on average), were examined by a static torsional otolith-cervical-ocular reflex (OCOR) with a head tilt to the shoulder at an angle of 30 deg, the otolith-ocular reflex (OOR) when the body is tilted at an angle of 30 deg, and the otolith-ocular reflex when exposed to centrifugal acceleration using a centrifuge (OORCF). Nine out of 58 cosmonauts were examined in microgravity (175 days) as part of the Virtual space experiment (Stage 1). The study of simulation of the spaceflight (SF) factors involved 30 subjects during five- or seven-day horizontal “dry” immersion (DI) and 12 subjects in 60-day head-down tilt (–8 deg) bed rest (BR). The procedure and the corresponding hardware and software used both in studies with the participation of cosmonauts and in model experiments were completely identical. In weightlessness, throughout the whole SF, all the cosmonauts surveyed had a significant OCOR decrease with the presence (from 11 to 55%, depending on the flight day) of the atypical (inversion or absence) otolith-ocular reflex. After SF, on days 1–4 of examination, 80% of the cosmonauts exhibited a significant OCOR/OOR decrease; 34% of the cosmonauts had atypical (inversion or absence of the otolith reflex) vestibular reactions, while no differences were found between OCOR and OOR. Statistically significant differences were found between the values ​of the OCOR/OOR and OORCF reflexes before and on days 4–5, 9–12 after SF. On days 2–3 after SF, no difference between OORCF and OCOR/OOR was observed. The return of the otolith reflex to the baseline took place only on day 9 after SF. The OCOR/OOR study on days 1–3 after DI showed that the otolith-ocular reflex in 47% of those examined was significantly reduced; in 13% of the subjects atypical reactions (absence or inversion of the reflex) were recorded. On days 4–6 after DI, OCOR/OOR returned to the baseline in all subjects. Analysis of OCOR/OOR after the BR experiment showed that only 14% of the surveyed had a significant decrease in OCOR/OOR on day 1 after the experiment. The atypical form of OCOR/OOR (inversion or complete absence of the otolith reflex) after BR was not revealed in contrast to the testers after DI. The results obtained demonstrated the influence of nonvestibular afferentation on the intensity of the otolith-ocular reflex and its dependence on other sensory inputs.

According to the results of the experiment performed on board the International Space Station with participation of 15 Russian cosmonauts, comparative analysis of efficacy of two models of preventive measures used by the Russian members of long-term space missions is carried out: intense interval training in the aerobic–anaerobic power zone (recommended model) and continuous low-intensity exercises in the aerobic power zone of muscle activity energy supply. Interval training in space flight provided the maintenance of the level of physical performance close to the pre-flight level as indicated by the maximum running speed, physiological value of work, and the lactate level after performing the standard load. We describe putative mechanisms of counteraction to adaptive rearrangement of the propulsion system in zero gravity and expand understanding of the laws governing human body’s interaction with Earth’s gravitational field. The research results presented in this paper show the high preventive efficacy of interval training compared with regular aerobic training, which is very important now in the time of searching for means and methods of prevention of hypogravitational alterations during interplanetary missions.

The aim of the study was to examine effects of long-duration exposure to weightlessness on characteristics of the vertical gaze fixation reaction (GFR). The subjects were to perform the target acquisition task on visual stimuli that appeared at a distance of 16 deg. up- and down from the primary position in a random order. Experiments were performed before launch, during flight and after landing. Before flight time of gaze fixation reaction did not exceed 650 ms. During space flight (SF) it extended up to 900–1000 ms and more. The velocities of head movement in space decreased, but the velocities of eye counterrotation decreased to a lesser degree. This difference resulted in sharp increase of vertical vestibular ocular reflex (VOR) gain (up to 4.3 values in one of the cosmonauts) during the 1st month of flight; further it decreased reaching the values of 0.5–0.7 on the 5th month of SF. After landing vertical VOR gain increased greatly again. These results in the vertical axis are in agreement with the data of Kozlovskaya et al., which showed in experiments with monkeys that horizontal VOR gain increased together with redundant inadequate responses of vestibular nucleus on vestibular stimulation and that in the course of adaptation to these conditions central nervous system inhibited vestibular input from the motor control system.

Balance function is dramatically deteriorated after exposure to microgravity. The purpose of the present study was to investigate the role and the contribution of different gravity sensory systems to the development of balance impairment after long-term spaceflights. Postural perturbations (pushes to the chest) of the threshold, medium, and sub-maximal intensities were produced in eight cosmonauts before, and on the day 3, 7, and 11 following spaceflight. Postural corrective responses were analyzed by anterior-posterior body sway fluctuation and electromyographic activity of leg muscles. The characteristics of the postural corrective responses changed significantly on the day 3 following spaceflight: the amplitude of posterior sway caused by perturbation of threshold intensity was increased reaching 135% of preflight value; the corrective responses lasted more than 6 s in 50% of all trials, while it did not last more than 4 s in 96% before spaceflight. The EMG responses were characterized by increased contribution of medium- and long-latency reactions. On the day 11 following spaceflight, most of the characteristics of postural corrective responses were close to preflight values. We assumed that the balance alterations after spaceflight are caused by changes in weightlessness of functions of two main gravity sensory systems, namely, weight-bearing and vestibular one. The deficit of weight-bearing afferentation triggers a decline of the extensors’ muscle tone, while changes of vestibular function cause a decline of accuracy of postural corrections.

The results of studies on oculomotor reactions, both spontaneous and induced by visual and vestibular stimuli, that were performed during missions of Russian orbital complexes are described. It is demonstrated that abnormal sensorimotor reactions (spontaneous nystagmus; disturbance of the tracking function of the eyes; and decrease and increase in the tonic and dynamic vestibular excitability, respectively) are appropriate reactions of sensory systems during weightlessness. Abnormal sensorimotor reactions during spaceflights are individualized with respect to the strength, pattern of expression, time of development, and duration and dynamics of adaptation processes. Periods of adaptation (the initial, disintegration, and adaptation periods and alternation of compensation and decompensation periods) of sensorimotor functions to the conditions of weightlessness are determined. The possible mechanisms of neurosensory disturbances are described. These are otolith deafferentation, channel–otolith conflict, interlabyrinth asymmetry, intersensory mismatch, sensory deprivation, and suppression of vestibular afferent signals inadequate to new conditions.

International audience; Actual and simulated microgravity induces hypovolemia and cardiovascular deconditioning, associated with vascular dysfunction. We hypothesized that vasoconstriction of skin microcirculatory bed should be altered following 7 days of simulated microgravity in order to maintain cardiovascular homeostasis during active standing. Eight healthy men were studied before and after 7 days of simulated microgravity modeled by dry immersion (DI). Changes of plasma volume and orthostatic tolerance were evaluated. Calf skin blood flow (laser-Doppler flowmetry), ECG and blood pressure signal during a 10-min stand test were recorded, and skin vascular resistance, central hemodynamics, baroreflex sensitivity and heart rate variability were estimated. After DI we observed increased calf skin vascular resistance in the standing position (12.0 +/- 1.0 AU-after- vs. 6.8 +/- 1.4 AU-before), while supine it was unchanged. Cardiovascular deconditioning was confirmed by greater tachycardia on standing and by hypovolemia (-16 +/- 3% at day 7 of DI). Total peripheral resistance and indices of cardiovascular autonomic control were not modified. In conclusion, unchanged autonomic control and total peripheral resistance suggest that increased skin vasoconstriction to standing involves rather local mechanisms-as venoarteriolar reflex-and might compensate insufficient vasoconstriction of other vascular beds.

International audience; Dry immersion, which is a ground-based model of prolonged conditions of microgravity, is widely used in Russia but is less well known elsewhere. Dry immersion involves immersing the subject in thermoneutral water covered with an elastic waterproof fabric. As a result, the immersed subject, who is freely suspended in the water mass, remains dry. For a relatively short duration, the model can faithfully reproduce most physiological effects of actual microgravity, including centralization of body fluids, support unloading, and hypokinesia. Unlike bed rest, dry immersion provides a unique opportunity to study the physiological effects of the lack of a supporting structure for the body (a phenomenon we call 'supportlessness'). In this review, we attempt to provide a detailed description of dry immersion. The main sections of the paper discuss the changes induced by long-term dry immersion in the neuromuscular and sensorimotor systems, fluid-electrolyte regulation, the cardiovascular system, metabolism, blood and immunity, respiration, and thermoregulation. The long-term effects of dry immersion are compared with those of bed rest and actual space flight. The actual and potential uses of dry immersion are discussed in the context of fundamental studies and applications for medical support during space flight and terrestrial health care.

The human motor system responds to weightlessness by the slowing of movement. It has been suggested that deficits in visuo-motor co-ordination cause this effect. We studied the mechanisms of the slowing of movement in three long-term missions to the Russian space station Mir. In particular, the role of vision in the control of movement in microgravity has been studied in these experiments on seven cosmonauts, pre-, in-, and post-flight. The cosmonauts made arm movements to visual targets under the following conditions of visual control: no visual control, interrupted visual control, and undisturbed visual control. The results showed that the slowing of movement during weightlessness was manifested by decreases of peak velocity and peak acceleration, was not associated with a prolongation of the movement phase of deceleration, and was not affected by manipulation of the conditions of visual control. The slowing of movement tended to subside after the months of the flight and completely disappeared within days after the landing. Accuracy of the movements strictly depended on the constraints imposed on the vision and remained unaffected in-flight. The data presented demonstrate that the slowing of movement in microgravity is not directly related to deficits in sensori-motor co-ordination and is not associated with a reduction of the accuracy of movement. The strategy for motor control in microgravity seems to be directed towards the generation of smooth movements and the maintenance of their accuracy.

The system of countermeasure of microgravity effects has been developed in Russia that allowed to perform safely long-term space flights. This system that includes different means and methods such as special regimens of physical exercises, axial loading ("Pingiun") and antigravity suits, low body negative pressure device (LBNP, "Chibis") and "cuffs" and others has been used with certain variations at certain stages of flight in 27 successfully accomplished space flights that lasted from 60 to 439 days. The pre-, in- and postflight studies performed in 57 crew members of these flights have shown that the system of countermeasure is effective in preventing or diminishing to a great extent almost all the negative effects of weightlessness in flights of a year and more duration and that the intensity and duration of changes recorded in different body systems after flights do not correlate significantly to flight durations, correlating strongly to the volume and intensity of physical exercises used during flight and especially during concluding stage of it.

One of the most important fields of research being conducted in space medicine concerns the effect of weightlessness on the human locomotor system and its control systems in the central nervous system. Under terrestrial conditions, the earth’s gravitational field constantly influences all sensory motor functions. Only by adapting to gravity is it possible for man to assume an erect posture. Studies of human motor functions under microgravity offer new possibilities for analyzing the sensory motor systems and their influence on posture and motion.

The basic goal of medical investigations during and after the 366-day mission was to accumulate data about physiological responses to such a long exposure to microgravity. In flight, cardiovascular and other systems were examined in detail and the efficacy of countermeasures used was assessed. After flight, physiological systems were also followed very carefully. According to the preliminary data, the medical results obtained during and after flight give evidence that man can well adapt to a year-long space flight, maintaining good health and adequate work capacity. The readaptation process was very similar to that observed after shorter flights (6-II months). As compared to former flights, no new or qualitatively different changes in the vital systems of the body were seen. The observations indicate that the duration of manned space missions can be further increased.