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Abstract The aim of this study was to analyze the mechanisms of the development of hypogravitational hyperreflexia in the motoneuron pool of gravity-dependent muscles such as the gastrocnemius and soleus muscles of the leg under the conditions of five-day “dry” immersion in healthy volunteers using the method of transcranial and trans-spinal magnetic stimulation. The essence of the method lies in the stimulation of the areas of interest (motor areas of the cerebral cortex and lumbosacral thickening) with an electromagnetic stimulus. The study involved 10 subjects at the age of 29.9 ± 6.9 years, with no history of movement disorders or neurological diseases. The excitability of the motor neuron pool in both muscles was judged by the values of the thresholds and amplitudes of the motor response caused by transcranial and trans-spinal magnetic stimulations. A general pattern manifested in a significant decrease in thresholds and an increase in the amplitudes of motor responses caused by trans-spinal magnetic stimulation in both muscles gas been discovered. Specifically, the threshold of spinal evoked motor responses in both muscles decreased by 20%, and the amplitude increased by 150% after the end of immersion. The data obtained during the experiment confirm the spinal nature of the origin of hypogravitational hyperreflexia.

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.

The study of eye hydrodynamics following five-day “dry” immersion (DI) was aimed at evaluating the influence of compensatory body hypohydration on the intraocular hydrodynamic balance. For the first time the data of electronic eye tonography before and after modeled microgravity were compared. The study involved ten male volunteers 24 to 40 years of age without ophthalmologic pathologies. Electronic eye tonography results and hypohydration dynamics were assessed by fluid balance. Electronic tonography was performed before DI and on post-DI days 1 and 7 to measure the true intraocular pressure and to calculate the intraocular fluid production rate (F), coefficient of aqueous humor outflow ease, Bekker coefficient, and trophic coefficient (TC). On post-DI day 1, F reduction was observed in eight volunteers (60% of cases, 12 eyes). Level of the F reduction varied from 40% of the baseline value (5 eyes) to abnormally low 54% (7 eyes) underlying trophic changes. TC went down in 60% of cases (12 eyes); in 50% of cases (6 eyes) TC reduction was clinically significant. By the seventh day after DI, intraocular hydrodynamics was comparable with the baseline. It is hypothesized that the reflectory hypohydration during DI (the Gauer–Henry reflex) is responsible for changes in the intraocular hydrodynamics; the depth of these changes is determined by the degree of compensatory hypothalamic influence upon the antidiuretic hormone-angiotensin II system.

Electromyostimulation plays an important role along with other means of rehabilitation and training in sports, clinical and space medicine. Many laboratories in the world are involved in the development of new modes and hardware for electrical stimulation of skeletal muscles and in the study of the physiological effects of electromyostimulation. In the clinical setting, chronic low-frequency, low-intensity stimulation is usually applied with a view to preparing muscle tissue for cardiomyoplastic surgery, correcting the state of a paralyzed muscle, and to rehabilitating patients with the hypokinetic syndrome. This review is devoted to the study of the physiological basis (mechanisms of action) of chronic low-frequency stimulation. In the early 1970s, a model of chronic low-frequency electrical stimulation of the muscles of laboratory animals was developed and widely studied. As a result of numerous studies it was established that such stimulation led to an increase in muscle performance, fatigue resistance, the activities of oxidative enzymes and other structural and metabolic markers, as well as to a sustained fast-to-slow muscle fiber transformation. Similar data were obtained in the studies of healthy volunteers. Low-frequency stimulation had a beneficial effect on the atrophied disused muscle. The review examines the advantages and disadvantages of low-frequency electromyostimulation in relation to deconditioned muscle rehabilitation.

This review is dedicated to the memory of Inesa Kozlovskaya, whose contributions to gravitational physiology was and will continue to be decisive. Dr. Kozlovskaya has developed the concept of gravity-dependent motor control and substantiated the role of supporting afferentation in postural – tonic regulation. It was shown that the support afferentation plays the leading role in the control of the tonic muscle system and regulation of postural synergies. In this review the modern mechanisms of posture-locomotion integration as well as the mechanisms of sensory-motor regulation based on stimulation of foot and muscle receptors in combination with spinal cord stimulation will be considered. Based on the results presented in this review the concept of neurorehabilitation is proposed to be considered as the implementation of various neuromodulations aimed at regulating the functional state of the injured nervous system. The concept is based on the interaction of the processes of regulation of the functional state of the injured brain and sensory input during executing of motor tasks.

A review of the currently available ideas about the role of gravitational factor in the activity of the sensorimotor and cardiovascular systems, as well as new fundamental problems and questions for space medicine and physiology, is presented. The review presents data on the embryogenesis of animals under conditions of weightlessness, the evolution of the motor and cardiovascular systems and the peculiarities of their functioning under conditions of gravity, as well as in the change of gravitational load. Much attention is paid to the results of unique studies in modeling gravitational unloading on Earth: antiorthostatic hypokinesia, dry immersion and suspension, which made it possible to study the mechanisms of regulation of various body systems under conditions of altered gravity. Terrestrial organisms have learned to function in the gravitational field. Almost all systems of their body are gravitationally dependent. However, the extent and mechanisms of this dependence have long remained unclear. Space flights have opened up the possibility of studying the activity of living systems in the absence of gravity. Among the factors mediating the effect of weightlessness on the motor system, changes in the activity of sensory systems occupy an important place. Under the Earth conditions, the afferent support of motion control systems is polyreceptive: this is vision, and the vestibular apparatus, supporting and muscular afferentations. In zero gravity, the activity of some channels is completely eliminated (support afferentation), others are distorted (vestibular apparatus), and still others are weakened (proprioception). Similar processes occur in the cardiovascular system: with the loss of the pressure gradient caused by gravity, profound changes occur in the structure and functioning of the heart and vessels, both resistive and capacitive. The question of how much the various changes occurring in the cardiovascular system are associated with the disappearance of the gravitationally dependent pressure gradient is still open. It is not possible to solve all the problems of gravitational physiology In space flights. Therefore, various methods have been developed for simulating gravitational unloading on Earth. New data on the mechanisms of changes occurring in the sensorimotor system were obtained by comparing flight data and data obtained in model experiments. The fundamental problem for the gravitational physiology of cardiovascular system is the degree of correspondence of the changes observed in laboratory animals and under model conditions (antiorthostatic hypokinesia, immersion, suspension) with the changes that are recorded in real space flight in humans. This problem is specially discussed in the review. At the same time, in the light of the upcoming interplanetary expeditions, many questions remain unresolved, in particular, the problems of post-flight readaptation of the motor and cardiovascular systems to gravity conditions. This is a fight against loss of strength, endurance, orthostatic instability. The development and improvement of a system for preventing the negative effects of space flight factors is impossible without understanding the mechanisms of development of the observed changes.

The paper discusses the findings of studying neuroplastic transformations in the brain cortex owing to stroke patients therapy using soft multimodel exoskeleton complex (MEC) REGENT in comparison with activation of the cortex structures controlling locomotion in healthy people. The MEC course applied to hemiparetic patients increases walk speed; changes in the activity zones detected by functional magnetic resonance imaging (fMRI) attest to the positive trajectory of neuroplastic processes, i.e. activation in the precentral gyrus (primary motor cortex), secondary association cortex (inferior parietal lobule) on the damaged hemisphere, and right-side primary sensorimotor cortex. Analysis of the functional connectivity between the areas of interest before and after the MEC therapy elicited significant changes in the inter- and intra-hemispheric connections. This positive cortical reorganization has its origin in reduction of excitory interactions between the secondary associative areas (inferior parietal lobules in both hemispheres) and alleviation of the inhibitory interaction between the inferior parietal lobule and primary right-side sensorimotor cortex in the damaged hemisphere.

The review is an attempt to describe and give a meaning to the accumulated data about the mechanisms controlling the structure and functionality of the postural muscle the almost continuous work of which makes it possible for the humans and animals to exist actively on Earth's surface. A great bulk of these data was obtained, described and systematized by professor I.B. Kozlovskaya and her pupils. A body of the most interesting facts and regularities was documented in other laboratories and research centers, quite often under the influence of ideas suggested by I.B. Kozlovskaya. The concept of the tonic system, that is, an integral physiological apparatus comprising not only slow and fast muscular fibers and small controlling motoneurons but also a complex of the brain (up to and including the striatum and motor cortex) and sensory mechanisms, constitutes the most important parts of her theoretical legacy. The fundamental conclusion of this review is that the gravity-dependent tonic contracting activity of the postural muscle controlled by the nervous system and afferent mechanisms is key to maintaining its structure, signal pathways and mechanic properties crucial for its constant anti-gravity activity.

A joint NASA and Institute of Biomedical Problems (IBMP) study, designated as the Field Test, was designed to investigate how the immediate post-flight effects of long duration spaceflight impact performance on vestibular and sensorimotor-related functional tasks. One unique feature of this study was the use of functional tests typically associated with everyday living to track recovery. These tasks are also expected of crewmembers during recovery operations and include sit-to-stand and prone-to-stand transitions, translating objects, and jumping down from a platform. This paper provides an overview of the methodology used for the Field Test. As an example of the functional deficits observed, we present results for a seat egress and walking task that involved turning 180 degrees and stepping over obstacles. We observed significant increases in the time-to-complete this task on landing day. We recommend this task being incorporated into Standard Measures to evaluate the efficacy of countermeasures. The decrements observed at landing day have implications for the deconditioned crew during exploration missions where crewmembers will be unassisted after landing on a planetary surface.

It is believed that real movements and those performed mentally (imaginary movements) are functionally similar. This hypothesis is based on the use of the imaginary movement approach for the investigation of the neurophysiological mechanisms of real movements of a person, as well as for rehabilitation and optimization of motor functions. A large body of evidence in favor of this hypothesis has been obtained for upper limb movements, whereas data on lower limb movements are quite scarce. The aim of this review is to generalize the published data that demonstrate the similarity of the mechanisms and brain structures involved in the organization of imaginary and real locomotion.

Inesa Benediktovna Kozlovskaya is the head of a branch of the Institute of Biomedical Problems of the Russian Academy of Sciences, Doctor of Medical Sciences, Professor, Corresponding Member of the Russian Academy of Sciences, Honored Scientist and Engineer of the Russian Federation, a specialist in gravitational physiology, a participant in biomedical investigations during the spaceflights of the orbital stations Salyut-7, Mir, and ISS, as well as the flights of Bion biosatellites. She is one of the developers of the system of counter measures against unfavorable changes in the body in manned flights. She founded the school of the gravitational physiology of movements. Her scientific achievements are awarded with Russian and international prizes and awards. The editorial board of the journal sincerely congratulates Prof. Kozlovskaya on her ninetieth birthday and wishes her well-doing and many more years of active life.

This review addresses the use of robotic devices in the rehabilitation of poststroke and posttrauma patients as a rehabilitation technology which has developed rapidly over the last decade. The types of devices used are described – manipulators and exoskeletons – along with the clinical protocols for their use and the effectiveness of rehabilitation procedures. Particular attention is paid to the neurophysiological basis of the rehabilitation potential of this technology, including analysis of measures of plastic rearrangements of the brain. Results obtained from state-of-the-art rehabilitation technology using hand exoskeletons controlled by brain–computer interfaces based on kinesthetic motor imagery are considered.

Neuromuscular electrical stimulation (NMES) of large muscle groups in patients with stable chronic heart failure (CHF) increases physical work capacity and muscle strength. The safety and efficacy of short courses of NMES in patients with decompensated CHF has not been studied. The aim of the study was to compare the impact of leg NMES, interval bicycle training, and conventional treatment on the functional capacity and quality of life in patients hospitalized with decompensation of CHF. A total of 51 patients (84.3% of men; mean age, 62.5 ± 3.3 years) hospitalized with decompensated CHF were divided into three groups: optimal pharmacological therapy (OPT) + three weeks of NMES of thigh and shin muscles starting within the first three days after admission (n = 10), OPT + three weeks of bicycle exercise training (BET) starting on the fifth to seventh day (n = 20), and the control group of OPT only (n = 21). At baseline and after the three-week symptom-limited cardiopulmonary exercise test, 6-min walk test, Duke Activity Status Index (DASI) and quality of life assessment by the Minnesota Living with Heart Failure Questionnaire (MLHFQ) were performed. Patients in the NMES and BET groups presented with similar increases in the 6-min walk distance, plus 65.0 (50.0; 112.5) and 53.0 (51.0; 78; 0) m; DASI, plus 8.6 (5.5; 11.8) and 8.0 (4.5; 9.0) points; and VO2peak, plus 1.9 (0.3; 3.2) and 2.2 (0.7; 3.2) mL/(min kg), respectively. In the control group, only the 6-min walk distance increased significantly (+21 m). The improvement in the MLHFQ score was observed in all the three groups: maximal in the BET group and minimal in the NMES group. Three weeks of leg muscle electrical stimulation and bicycle training provide similar improvement in physical capacity, daily activity, and the quality of life in patients hospitalized for severe CHF. The efficacy of NMES is comparable with low to moderate intensity interval bicycle training and well tolerated by patients.

We analyzed the signaling processes initiating proteolytic events in the human soleus muscle during short-term exposure under the non-weight-bearing conditions. Dry immersion (DI) was used to induce weight deprivation in the m. soleus for 3 days. Western blotting was used to determine the level of insulin receptor substrate 1 (IRS-1), total and phosphorylated neuronal NO synthase (nNOS), and adenosine monophosphate-activated protein kinase (AMPK), which control the anabolic and catabolic signaling pathways, and the level of cytoskeletal protein desmin and Са2+-activated protease calpain. By day 3 of DI, calpain- dependent proteolysis manifests itself by reductions in both the total content and level of nNOS phosphorilation. The rate of AMPK phosphorylation was significantly decreased.

This review discusses the functioning of the motor system under normal and reduced gravity. Analysis of the experimental data led to the conclusion that all changes in the functioning of tonic muscular system are related to each other. When transiting to the state of microgravity, changes are caused by one common factor, namely a sharp decrease in the activity of support afferent input, specifically oriented to the perception and analysis of gravitational loads and firmly embedded in the mechanisms of postural synergism organization. We analyzed data obtained in studies on the activation of cortical areas of the brain during the stimulation of support afferents in order to test the hypothesis that such stimulation in both healthy subjects and patients with neurologic deficiency leads to activation of both the sensory and motor cortex involved in supraspinal control of the movement of the lower limbs, in particular when walking.

The present experiments were designed to gain additionally insight into how the spinal networks process direct spinal stimulation and peripheral sensory inputs to control posture and locomotor movements. We have developed a plantar pressure stimulation system that can deliver naturalistic postural and gait-related patterns of pressure to the soles of the feet to simulate standing and walking, thereby activating and/or modulating the automated spinal circuitry responsible for standing and locomotion. In the present study we compare the patterns of activation among selected motor pools and the kinematic consequences of these activation patterns in response to patterned heel-to-toe mechanical stimulation of the soles of the feet, and/or transcutaneous electrical spinal stimulation, for postural and locomotion regulation. The studies were performed in healthy individuals (n = 12) as well as in subjects (n = 2) with motor complete spinal cord injury. We found that plantar pressure stimulation and/or spinal stimulation can effectively facilitate locomotor output in the subjects placed with their legs in gravity neutral position. We have shown synergistic effects of combining sensory and spinal cord stimulation, suggesting that the two networks are different, but complementary. Also we provide evidence that plantar stimulation could serve as a novel neuro-rehabilitation tool alone or as part of a multi-modal approach to restoring motor function after complete paralysis due to SCI.

Nowadays it is widely believed that the animal motor system historically evolved under the powerful pressure of gravity forces. By the late 1970s, many manifestations of the microgravity effects on the motor system had already been known. At the same time, the basic sensorimotor relationships during exposure to zero-gravity remained unexplored. The article considers the main results of the studies of the scientific school of I.B. Kozlovskaya regarding the roles of the gravitational forces in the functioning of the tonic motor system in humans and other mammals. In these studies, it was demonstrated that the muscle tonic system is relatively independent and possesses its own structures and mechanisms at every level—from receptors to effectors. The support afferent input plays the main role in the regulation of the postural tonic system. The withdrawal of the support afferentation leads to the decline of the tonic motor units activity in extensor muscles and the alteration of the motor units recruitment patterns in the spinal cord. The decline of the tonic activity of the extensor motoneurons triggers on the development of the sensorimotor effects of microgravity including atony and muscle atrophy.

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 effects of additional dynamic visual stimuli (retinal optokinetic stimulation (ROKS)) on the visual–manual tracking (VMT) indicators in the absence of support afferentation and with a reduced level of proprioceptive afferentation were determined using a model of horizontal “dry” immersion. The accuracy of the VMT of jerky and smooth (linear, pendular, and circular) movements represented by visual dot stimuli was evaluated in all 18 participants aged 19–31 before, during and after their exposure to a five- to seven-day immersion bath. The eye movements were recorded by electrooculography, while the hand movements were recorded by a joystick with a biological visual feedback (the current angle of the joystick handle was imaged on the screen). Computerized visual stimulation tests were presented, through virtual reality glasses, to subjects in the absence and against the background ROKS. We analyzed the temporal and the amplitude- and velocity-related visual and manual tracking (VT and MT) characteristics, including the efficiency (e) and gain (g) coefficients as the ratios between the amplitudes and velocities of eye/hand movements and the amplitude of stimulus movements. The efficiency and gain coefficients of both VT and MT without ROKS were significantly decreased against the baseline during the entire period including three days of immersion and 3 post-immersion days. The most pronounced worsening was observed in the VT parameters. Whereas the VT and MT parameters remained unchanged against the threshold ROKS before the immersion, they were improved during and after the immersion (the improvement was significant on the fifth to seventh day of immersion and on the thirdthird post-immersion day, compared to the test indicators on the clean screen). The most pronounced impact of ROKS was observed in the VT parameters. The vestibular function (VF) was evaluated by videooculography before and after immersion. We analyzed the static torsional otolith-cervicalocular reflex (OCOR), dynamic vestibular-cervical-ocular reactions (VCOR), vestibular reactivity (VR), and spontaneous eye movements (SpEM). A significant decrease in OCOR (gOCOR was 0.1, compared to the background gOCOR value of 0.25) was detected alongside a simultaneous significant increase in the VCOR/VR parameters in 28% of subjects on day R + 1 after immersion. Correlational has been found between the parameters of VT and MT, as well as between those of VF and VT, but no correlation has been found between the VF and MT characteristics. The results have shown that the removal of support afferentation and the minimization of proprioceptive afferentation more affected the accuracy of VT rather than that of MT. The correlational links between the studied parameters against the background of ROKS were not only preserved, but also intensified. The obtained results confirm the development of sensory deprivation (and afferent deficit) under the exposure to an immersion bath and indicate the approach to correcting the sensory deprivation through additional ROKS.

The mechanisms underlying the locomotion recovery in poststroke patients remain unknown. Navigated transcranial magnetic stimulation (nTMS) is a new method to evaluate the functional state of the motor system. Using of the “Regent” soft exoskeleton complex (SEC) allow to correct walking pattern significantly. The aim of this study was to evaluate the capability of nTMS to assess changes in gait cortical control using SEC in poststroke patients. 14 patients of the subcortical stroke (the mean age was 53.0 years, the mean time from stroke onset was 14.2 months) received 10 training sessions with SEC. The patients received nTMS before and after the sessions, as well as they were clinically evaluated by the Fugl–Meyer scale section for lower extremity and a 10-m walk test. Whereas a reliable reduction in the time of walking for 10 m was recorded after the sessions with the application of SEC, the Fugl-Meyer scale assessment remained unchanged. During nTMS, a reduction was recorded in the average latency of evoked motor response from the affected hemisphere, as well as various patterns of changes in the size and localization of cortical representations of the leg muscles. We have concluded that the nTMS method allowed us to identify the individual patterns of changes in the cortical representations of leg muscles as a result of the use of SEC in post-stroke patients with injuries in a group of locomotor system elements, thus identifying not only the fact of the undergoing neuroplastic processes, but also their direction.

The effect of a treatment course with a Regent multimodal complex exoskeleton (MCE) on the reorganization of cortical locomotor zones was studied in 14 patients with post-stroke hemiparesis, mainly in the chronic stage of the disease. Specific activation zones were identified prior to treatment in the primary sensorimotor and supplementary motor areas and the inferior parietal lobules of both affected and healthy hemispheres by functional magnetic resonance imaging (fMRI) used in a special passive sensorimotor paradigm. After a treatment course with the MCE, temporal characteristics of walking were found to improve, which was accompanied by a decrease in the activation zones of the inferior parietal lobules, especially in the healthy hemisphere, and a significant increase in the activation zones of the primary sensorimotor and supplementary motor areas. Significant changes in intrahemispheric and interhemispheric interactions were revealed by analyzing the functional connectivity of the zones under study before and after a course of treatment with the MCE.

The study discusses Russia’s system of preventive countermeasures and its general principles based on a successful experience accumulated by many crews of long-duration missions on Russian space stations. The preventive system tested during the exploitation of the International Space Station (ISS) was described in detail. For example, differences in training workouts performed by cosmonauts allowed specialists to distinguish two exploitation phases on the ISS. The recorded locomotion workouts performed by cosmonauts displayed departures from the onboard documentation. The concluding section of the paper discusses prospects for developing a system of preventive countermeasures for interplanetary missions.

BACKGROUND The purpose of this work was to investigate the brain's rhythmic activity during a simulated microgravity condition (namely dry immersion). METHODS During dry immersion, which lasted for 5 d, nine subjects (healthy men, 20 to 29 yr of age) were individually placed in a tub (2.2 × 1.1 × 0.85 m) filled with water (temperature was kept constant at about 33°C). Subject floated in the tub without bodily support in the supine horizontal position, but isolated from the water by waterproof material. Resting state EEGs were registered at the fourth or fifth day of dry immersion. Under the control conditions, resting state EEGs were registered while subjects laid in a supine position on a couch. RESULTS Compared to the control condition, EEG power in the alpha range (8-13 Hz) was greater in dry immersion; this effect was distributed across the whole scalp. No effects of dry immersion were found for the beta, delta, or theta frequency bands. CONCLUSION The results of the study, similar to those obtained in a real spaceflight, indicate that support withdrawal is an important contributor to brain activity alterations in weightlessness.Lazarev IE, Tomilovskaya ES, Kozlovskaya IB. Resting state brain activity during long-term dry immersion. Aerosp Med Hum Perform. 2018; 89(7):642-647.

The effects of mechanical stimulation of the sole support zones on the state of the m. soleus moto-neuron pool in humans have been studied under 7-day support unloading conditions, which were simulated using a dry immersion (DI) model. The excitability of the m. soleus motoneuron pool was estimated by the H-reflex amplitude normalized by the maximal M-wave amplitude before, during, and after immersion exposure. The results were compared for two groups of volunteers, i.e. a control group, which was subjected only to immersion exposure, and a test group, in which mechanical stimulation in locomotion regimens was applied daily during DI. The relative H-reflex amplitude increased during immersion in the control group, whereas such alterations were not detected in the test group.

The experiments reported here showed that decerebrate cats can actively maintain posture during standing and walking. On standing, postural corrections consisted of redistribution of extensor muscle activity in response to perturbations. Correcting reactions during walking included changes in the durations of the swing and transfer phases of the locomotor cycle, modulation of supporting force reactions, and modification of flexor and extensor function. Detailed analysis of correlations between muscle activity, supporting force reactions, and the kinematics of truncal and hindlimb movements showed that the motor system of the decerebrate animal can use a combination of feedback and feedforward to regulate dynamic balance during locomotion. Furthermore, balance was rapidly restored after impairment due to stumbling or perturbing influences. The intraspinal neural networks and somatosensory afferent input from the limbs can effectively regulate balance during walking and standing, without involvement of the vestibular and visual systems. After interruption of the connection with the motor centers of the stem and forebrain in decerebrate and spinal animals, these networks were in a suppressed state but could be activated by epidural and sensory stimulation substituting for tonic supraspinal drive.