[The influence of the leg load and the support mobility under leg on the anticipatory postural adjustment].

Anticipatory postural adjustment is an essential part of equilibrium maintainance during standing in human. So changes in stance condition could affect both control of equilibrium and anticipatory adjustment. Anticipatory changes in the stabilogram of each leg were studied in standing subject during the early stage of quick right arm lifting while legs were on two separated supports. The center of pressure (CP) movement was analyzed in three variants of experiment: both legs on immovable support, with only right leg on the movable support and with only left leg on the moveable support. In each standing condition subject stood with symmetrical load on two legs or with the load voluntary transferred to one leg. The anticipatory CP shift depended on the mobility of the support under the leg and on loading of the leg. While standing on unmovable supports with symmetrical load on the legs before lifting of the right arm CP of right leg shifted backward and CP of left leg--forward. While standing with one leg on movable support the anticipatory CP shift of this leg was small and did not depend on the load on the leg. However the shift of CP of the leg that was placed on the unmovable support depended on the load in the same way as in the case when both legs were on unmovable supports. Results suggested that since on movable support the support and proprioceptive afferent flow from distal part of the leg that was did not supply unambiguous information about body position, the role of distal joints in posture control is reduced.

[Influence of Achilles tendon vibration on the human vertical posture during standing with asymmetrical leg loading].

The shift of center of pressure (CP) of body and CP of each leg was studied during Achilles tendon vibration of one or both legs while subject was standing with symmetrical load on the legs or with the load transferred on one leg. The CP shift of standing subject during unilateral Achilles tendon vibration depended both on the side of the tendon vibration and on the leg load. When standing with a load transferred on one leg the shift of common CP was larger than when the vibration was applied to the loaded leg. The CP shift of one leg was greater if the vibration, and the load was applied to it. Vibration of unloaded leg caused a CP shift in the contralateral loaded leg. In this case, the vibration of left unloaded leg caused no noticeable CP shift of left leg, while the vibration of the unloaded right leg caused CP shift of right foot. In the same conditions of load and vibration the CP displacement of right leg was larger than the CP shift of left foot. It can be assumed that the change in the load on the leg and unilateral vibration of leg muscles change of the internal representation of the vertical body axis, which affects the CP position of one leg during the muscles vibration.

[Influence of the movable support under one leg on human vertical posture during standing with asymmetric load on legs].

The posture in standing subjects was studied when the legs were placed on supports of different degrees of mobility, as well as, when a part of body weight was voluntary transferred to one leg. The aim of these experiments was to explore how the mobility of support under the feet affects the balance and how this influence could be changed by the load distribution between the legs during standing. When both legs were on rigid immovable supports, the posture maintaining was accomplished by control of center of pressure (CP) of both legs. When the subject transferred the weight on one foot the posture was maintained mainly due to the control of CP of loaded leg. When the legs were on the supports of different mobility, the balance was maintained by the control of CP of the leg on the immovable support. This result was observed both when the subject stood with symmetric load on the legs and when the load was transferred to one leg. Even when the leg was unloaded but was placed on immovable support its CP moved more in comparison with the CP of loaded leg on movable support. The results show that the support mobility under the legs is a factor that determines the mechanisms of the posture maintenance, and this factor is more significant than the load distribution between the legs. So, we can conclude that the upright posture is maintained accounting the physical properties of the supports under the feet.

[Characteristics of vertical posture maintenance during standing with asymmetrical legs loading].

We studied the motion of the common center of pressure (CP) and of right and left foot CP, while maintaining a vertical posture when standing with symmetrical legs loading and with the transfer a part of weight to the right and left leg. It was shown that when standing with symmetrical legs loading the single leg CP motion occurred along a straight line with small side deviations in contrast to common CP motion, which was along an irregular curve, which often changed the direction and covered some area. When weight was transferred on one leg the motion of CP of loaded leg was similar to the motion of CP of that leg during standing with symmetrical load. The motion of CP of unloaded leg was along an irregular curve. During standing with weight transferred on one leg, the correlation between the CP movements of the left and right foot was reduced compared to standing with symmetrical load on thelegs. The velocity of loaded leg CP increased in the sagittal direction, but did not change inthe frontal direction. The velocity of unloaded leg CP motion did not change in the sagittal direction, and increased in the frontal. It was suggested that during standing with asymmetrical load on legs the involvement of the leg in maintaining the vertical posture depends on the load supported by this leg.

[Visual illusions during the electrical stimulation of the labyrinth]. / Zritel'nye illiuzii pri élektricheskoi stimuliatsii labirinta.

Transcutaneous electrical stimulation of the labyrinth induced apparent motion of a stationary light source in darkness in normal subjects. This effect is similar to the oculogyric illusion induced by rotatory vestibular stimulation. Monaural anodal stimulation of the right labyrinth evoked apparent movement of the target to the left, whereas cathodal stimulation induced opposite illusion. The threshold current was 0.35-0.6 mA. Binaural bipolar stimuli induced illusory target motion directed to the side of the cathode, the threshold decreased 1.5-2.5 times. Binaural monopolar stimulation induced vertical apparent displacement of a target, the threshold being 1.4-3.0 mA. The amplitude and velocity of illusory target motion increased with current. The subject eyes began to move with much higher currents than those necessary for illusory sensation. It is therefore suggested that the visual illusion is related not to vestibulo-visual interaction but to vestibular effects on the spatial perception system.

[The dependence of the changes in vestibular postural reactions on the information content of visual feedback]. / Zavisimost' izmenenii vestibuliarnykh poznykh reaktsii ot informatsionnogo soderzhaniia zritel'noi obratnoi sviazi.

Electrical stimulation of the labyrinth in standing subjects induced the body sway predominantly in the frontal plane. Stabilographic response included both early (latency 120-200 ms) and late (200-500 ms) components. Their magnitudes depended on the visual control condition. Maximal responses were recorded in the eyes-closed condition. The response decreased when subjects maintained erect posture with their eyes closed, when fixing a stable visual target, and when tracking the frontal stabilogram displayed on an oscilloscope screen (visual feedback). In all the conditions the early component decreased by 10-20 percent whereas the late one decreased by 50-70 percent. Visual fixation of a small light stationary relative to the head did not influence the response. On the other hand, information on the direction of the expected body sway given in the visual fixation condition resulted in a considerable and approximately equal decrease of the two components (by 70-80 percent). It is concluded that the early and late components of the vestibulo-motor response are mediated via different mechanisms with specific temporal and functional characteristics.

[Changes in the direction of vestibulomotor responses in the process of adaptation to prolonged static head turning in man]. / Izmeneniia napravleniia vestibulomotornykh reaktsii v protsesse privykaniia k dlitel'nomu staticheskomu povorotu golovy u cheloveka.

Vestibulomotor responses were studied in normal blindfold subjects in the upright posture during adaptation to maintaining the head turned to the left up to the extreme position for 10 min. The head was maintained in that position either passively or actively. In 5 out of 12 subjects tested, adaptation to the unusual head position resulted in a gradual decrease of the appreciated angle of the head position. The error in appreciating the head position reached as much as 70-80 degrees. The direction of the vestibulomotor response was found to change in parallel with the appreciated head orientation. Thus, under mismatch between the perceived and actual head positions the direction of the vestibulomotor response corresponded to the spatial perception rather than to the actual head orientation.

[Effect of real and illusory movements on the human vestibulomotor reaction]. / Vliianie real'nykh i illiuzornykh dvizhenii na vestibulomotornuiu reaktsiiu u cheloveka.

Lateral stabilographic responses to galvanic labyrinth stimulation were studied in healthy subjects in the standing posture. The responses increased during the body tilt forward performed either voluntarily or involuntarily as a result of vibration of the tibialis anterior muscles. The illusory body tilt forward induced by the vibration of the triceps surae muscles under mechanical fixation of the trunk resulted in similar increase of the vestibulomotor response. It is concluded that the enhancement of the vestibulomotor responses during voluntary movements is mediated by the spatial perception system.

[Spatial perception and vestibulomotor reactions in man]. / Prostranstvennoe vospriiatie i vestibulomotornye reaktsii u cheloveka.

Postural responses to transcutaneous galvanic stimulation of the right labyrinth were recorded by means of a stabilograph in normal human subjects in various head positions as well as under the illusion of head and trunk rotation induced by vibration of the gluteus maximum muscle. The direction of the vestibulo-motor response was determined by the position of the head: in a normal head position the body swayed in the frontal plane, whereas with the head turned 90 degrees it moved in the sagittal plane. During the illusory head and trunk 90 degrees rotation the responses were sagittally directed like those recorded in the real head-turned-sideways position. When the vibration did not induce the illusion of the head rotation with respect to the feet, the direction of the postural response was determined by the real head orientation. It is concluded that the spatial perception system is involved in the control of spatially oriented vestibulo-motor responses.