[Characteristics of postural corrective responses before and after long-term spaceflight].

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% ofpreflight 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.

[Effect of 120 days head-down tilt (HDT) on characteristics of tendinous reflexes]. / Vliianie 120-sutochnoi antiortostaticheskoi gipokinezii na kharakteristiki sukhozhil'nykh refleksov.

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.

[The effect of 120 day anti-orthostatic hypokinesia on the status of the posture regulation systems]. / Vliianie 120-sutochnoi antiortostaticheskoi gipokinezii na sostoianie sistem poznogo regulirovaniia cheloveka.

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.