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The results of extended comparative investigations into the hypogravity effects on the human motor system in space flight and ground-based experiments evidenced that dry immersions (DI) is the most adequate model of microgravity as time of development, degree and depth of structural and functional motor disorders in DI are very close to what is observed in real microgravity. High intensity and speed of the hypogravity effects during immersion as compared with bed rest hypokinesia the only difference from which is the level of support removal provided insight into the leading (trigger) role of support removal in genesis of the microgravity-induced syndromes of muscular deconditioning and hypogravitational ataxia. Involvement and paths of support afferentation within the muscular system were experimentally studied and verified. Much less understood are processes changing the system mechanisms. These issues as well as some nontrivial approaches to elimination of the negative effects of hypogravity were the subject of the multisided immersion experiment the results of which are presented in this book of the journal.

In two series of experiments aimed at studying the possibility to alleviate the hypokinetic syndrome in primates with centrifuge-produced artificial gravity (AG) one group of animals was immobilized in the head-down position (HDI, -5 degrees) for 28 d and the other was immobilized and also subjected to regular centrifugation (HDI+AG). In the first series, the animals were rotated at 1.2-1.6 g (+Gz) for 30-40 min 5 times/wk and in the second series, at 1.2 g (+Gz) for 30 min 2 or 3 times/wk. AG abolished the HDI-induced locomotion and coordination disturbances within a short timeframe. Besides, it increased the lymphoid activity and erythropoiesis, and limited growth of haematocrit parameters and shifts in protein metabolism during rehabilitation. Centrifugation reduced amounts of extra-cellular fluids and circulating blood in leg muscles, atrophy of both types of muscle fibers, loss of fiber capillaries, and saved aerobic production of muscles. These effects were less deep in the first series. By the criterion of AG tolerance, centrifugation regime in the second series turned to be more favorable. According to these results, AG (+Gz) from 1.2 to 1.6 g can be used as a preventive to the hypokinetic syndrome in macaques (Macaca mulatta) with due precautions against negative cumulative effects of repeated rotation.

Antiorthostaic hypokinesia (AOHK) was found to entail a considerable diminishing of the control system's precision abilities. In the first 14-30 days of the AOHK (the 1st stage), variability of interimpike intervals (ISI) was sharply increased as well as the degree of synchronisation of the motor units' (MU) activity; starting from the 30th day (the 2nd stage), a regular diminishing of the ISI occurred and the MU synchronisation disappeared. The data obtained suggest the different nature of the precision disorders during the 1st and the 2nd stages of the AOHK: a reflex responses to support unloading and the atrophic processes in the muscles, respectively.

The system of posture regulation was evaluated during 120-d head-down bedrest (-6 degrees). Functioning of the system was evaluated by the characteristics of corrective posture responses to pushes in the chest breaking the body equilibrium. Analyzed were parameters of amplitude and time of corrective responses, and strength and range of stimulus intensity. Results of the investigation showed that HD bedrest materially disturbs vertical stability and impairs effectiveness of the posture corrective mechanisms. Based on the data of this investigation and findings of previous efforts we can confidently distinguish two phases in development of disturbances in vertical stability during long-term HD bedrest associated with different leading mechanisms. On the first phase (15-30-d into the experiment), the principle posture problems are reduced tone of the anti-gravity musculature. On the second phase, particularly beginning on day 60, muscular atrophy processes become leading factors of postural disturbances. Apparently, these mechanisms, along with changed vestibular input, considerably impact vertical stability, especially after extended space mission.

The dynamics of atrophy in the muscles-extensors of the leg and thigh during long-term hypokinesia was studied with the model of head-down tilt (HDT -6 degrees) in a series of two experiments (120 and 60 days in duration) with participation of men and women. Dynamics of the cross-sectional area of human muscular fibers was characterized by similar atrophy of the fast and slow fibers on the first stage of HDT. Later on, the size of fast fibers in m. soleus and m. gastrocnemius lateralis reached plateau whereas the size of slow fibers continued to diminish. These were only fast fibers that underwent further atrophy in locomotor m. vastus lateralis. A supposition can be made that the controversy between literature data on the character of atrophy of muscular fibers in experiments with human subjects and rodents is primarily linked with different rates of atrophy developed by muscles in various species; besides, investigators describe different stages of one process. Analysis of the benefit from preventive exercises for the muscular atrophy control during HDT indicated that locomotor (with loading restraint belts) and resistive exercises can preclude slow fibers atrophy under the conditions of hypokinesia.

Time dynamics of variations in the spinal reflex mechanisms was evaluated during 120-d HDT (-6 degrees). Already within the first days in HDT the knee and Achilles reflexes were noted to sharply become easy to fulfil, the conclusion made from a significant drop of thresholds, an appreciable rise of amplitudes of reflex responses and an expansion of the range of stimulations. However, dynamics of these variations was diverse. The abrupt drop of reflex thresholds determined already on HDT day 2 was persistent throughout the experiment, whereas maximal rise of the tendon reflex amplitude was seen on the initial phase of HDT (days 15-30). Later on, a clear-cut downward trend of the electromyographic amplitude was observed. The experimental data suggest that hyperreflexia during HDT is one of the manifestations of partial deafferentation of the motoneuron pool due to diminution of the proprioceptive afflux. At the same time, hyperactivity of the reflex mechanisms is masked by the peripheral muscular effects of the same factor, i.e. HDT.