Impairments in balance control are inevitable following exposure to microgravity. However, the role of particular sensory system in postural disorders at different stages of the exposure to microgravity still remains unknown. We used a method called Dry Immersion (DI), as a ground-based model of microgravity, to elucidate the effects of 6-h of load-related afferent inputs on kinematic characteristics of postural corrective responses evoked by pushes to the chest of different intensities during upright standing. The structure of postural corrective responses was altered following exposure to DI, which was manifested by: (1) an increase of the ankle and knee flexion during perturbations of medium intensity, (2) the lack of the compensatory hip extension, as well as diminished knee and ankle flexion with a further increase of the perturbation intensity to submaximal level. We suggest that the lack of weight-bearing increases the reactivity of the balance control system, whereas the ability to scale the responses proportionally to the perturbation intensity decreases. Disrupted neuromuscular coordination of postural corrective responses following DI can be attributed to adaptive neural modifications on the spinal and cortical levels. The present study provides evidence that even a short-term lack of load-related afferent inputs alters kinematic patterns of postural corrective responses, and can result in decreased balance control. Because vestibular input is not primarily affected during the DI exposure, our results indicate that activity and the state of the load-related afferents play critical roles in balance control following real or simulated microgravity.

The study focused on the effects of duration of a single aerobic exercise session of moderate intensity (60% of {ie296-1}) on the activation of PGC-1α gene expression-regulating signaling kinases and the expression of the genes that regulate mitochondrial biogenesis and play a role in regulating catabolism. Nine athletes ({ie296-2} 59 mL/min/kg) cycled on a ergometer for 30, 60, and 90 min. A exercise-induced increase in PGC-1α gene expression was found to occur without activation of the kinases AMPK, p38 MAPK, and CaMKII. A comparable increase in PGC-1α gene expression was observed after 60- and 90-min exercises, while VEGFA gene expression increased only after 90-min exercise. Even 90-min exercise of the given intensity did not activate the FOXO1-E3 ubiquitin ligase pathway, nor did it increase the expression of catabolism-regulating genes.

Гипертрофический эффект силовых физических упражнений связывают с влиянием как механических, так и метаболических стимулов. При использовании упражнений в условиях ограниченного кровоснабжения работающих мышц, то есть в условиях, когда метаболические сдвиги усиливаются, тренировочный эффект может достигаться при использовании гораздо меньших внешних отягощений (20% максимальной произвольной силы (МПС)). Были сопоставлены эффекты 8-недельной обычной высокоинтенсивной (80–85% МПС) силовой тренировки и тренировки с низкой интенсивностью (50% МПС) без расслабления работающих мышц. Оказалось, что обычная тренировка приводит к несколько большему приросту силовых возможностей и размеров тренируемых мышц, чем тренировка без расслабления. При обычной тренировке происходит преимущественное увеличение площади, занимаемой на срезе мышечными волокнами II типа, а при тренировке без расслабления – волокнами I типа. Упражнения без расслабления вызывают более выраженное увеличение секреции в кровь гормона роста, инсулино-подобного фактора роста-1 и кортизола. После обычных силовых упражнений и упражнения без расслабления происходит различное изменение экспрессии гена-регулятора миогенеза миостатина. Силовая тренировка (50% МПС) без расслабления – эффективное средство для роста силы и гипертрофии мышц, которое может найти применение в восстановительной медицине, спорте и физической культуре, как тренировочный режим с относительно низкими нагрузками.

Hypertrophic effect of strength training is known to originate from mechanical and metabolic stimuli. During exercise with restricted blood supply ofworking muscles, that is under conditions of intensified metabolic shifts, training effect may be achieved with much lower external loads (20% of one repetition maximum (1 RM)). The aim of the study was to compare the effects of 8 wks high-intensity (80-85% MVC) strength training and low-intensity (50% 1 RM) training without relaxation. The high-intensity strength training leads to somewhat higher increments in strength and size of trained muscles than training without relaxation. During high-intensity training an increase of area occupied by type II fibers at muscle cross section prevails while during training without relaxation - an increase of area occupied by type I fibers takes place. An exercise session without relaxation leads to a more pronounced increase in secretion of growth hormone, IGF-1 and cortisol. Expression of gene regulating myogenesis (Myostatin) is changed in different ways after high-intensity strength exercise session and after exercise session without relaxation. Low-intensity strength training (50% 1 RM) without relaxation is an effective way for inducing increases of strength and size of trained muscles. This low intensive type of training may be used in restorative medicine, sports and physical culture.

The lateral stiffness and electromyogram characteristics of the resting calf flexor and extensor muscles have been studied in 18 subjects during a seven-day immersion with and without mechanical stimulation of the foot support zones. It has been shown that as early as day 1 of support deprivation, the lateral stiffness steadily decreases in the m. soleus and, in contrast, drastically increases in the m. tibialis anterior. A mechanical stimulation of the foot support zones decreases the rate and degree of the changes observed in both muscles. The tight correlation of the changes in the lateral stiffness with the muscle activity suggests a significant dependence of these changes on the electromyogram characteristics at rest.

The paper summarizes the results of experimental studies advocating for the leading role of support afferentation in control of the functional organization of the tonic muscle system. It is shown that transition to supportless conditions is followed by a significant decline of transverse stiffness and maximal voluntary force of postural (extensor) muscles limiting their participation in locomotion and increasing involvement of phasic muscles. Mechanical stimulation of the support zones of the soles under the supportless conditions eliminates all the above-mentioned effects, including changes in transverse stiffness and maximal voluntary forces of postural muscles, and consequent loss of influence of postural muscles in the locomotor activity. It is suggested that support afferentation, facilitating (support is present) or suppressing (support is absent) the tonic motor units (MUs) activities, defines the coordination patterns of postural synergies, and ensures the optimal strategy of corrective postural responses.

The results of studies of the effects of mechanostimulation of the soles’ support zones on the effects of microgravity in the motor system are presented. It was shown that mechanostimulation of the soles support zones in regimen of slow and fast walking, being used daily during 7 days dry immersion, eliminates fully or suppress considerably all the microgravity effects. In subjects in which stimulation was applied six times a day by 20min every hour the decrease of force–velocities properties and atrophic changes in the leg extensors after the exposure to microgravity were not revealed. Their transverse stiffness was only slightly lowered and the amplitude of electromyographic activity at rest stayed unchanged. The level of orthostatic deficiency in this group was also lower than in the group without stimulation. Thus presented experimental results in full agreement with previous studies allow to conclude that support afferentation plays the leading role in gravitational deprivation of the activity of tonic muscle system and that adequate mechanostimulation of the soles support zones can be used as a countermeasures mean in weightlessness.

Stiffness of m. soleus (Sol.) and m. tibialis anterior (TA) was evaluated in 16 volunteers during exposure to 7-days dry immersion alone and to the combination of immersion and mechanic stimulation of foot support zones. It was shown that Sol. stiffness decreased progressively starting from day-1 of immersion, whereas TA stiffness, on the contrary, made a sharp rise. Mechanic stimulation of foot support zones slowed down the rate and extent of changes in both muscles.

Posture disorders are an inevitable consequence of exposure to microgravity . However, the role of different sensorimotor and sensory factors on postural function at different stages of the exposure to microgravity still remains unknown. The results obtained in a 6 hr dry immersion (DI) study where chest pushes served as a pre- and post-immersion perturbation, and DI was used as an analog of microgravity suggest that in addition to vestibular contributions, postural control may be related to a reduction of support loading and consequent decline of the tone of anti-gravitational muscles. Analysis of postural video data in response to chest pushes obtained before and after DI indicate that the structure of corrective responses was modified so that postural perturbations from threshold to moderate pushes showed a significant rise in the amplitude of ankle and knee angular displacement. With push intensity near the submaximal level, equilibrium was maintained by the elimination of excessive degrees of freedom; as manifested by the restriction of the hip joints mobility when coupled with a reduction of the knee and ankle displacement. These results suggest that DI increases the sensitivity of the posture control system by making posture control more rigid reflecting a change of the weight bearing receptors.