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Levik YuS

Publications and source records attributed to Levik YuS.

10 recordsLinked to original sources
PubMed1995

Kinesthetic reference for human orthograde posture.

Humans with occluded vision were subjected to superslow tilts of the supporting platform, producing the inclination of the subject's body in the sagittal plane, but subthreshold for the most vestibular and proprioceptive phasic reactions. Two types of perturbation were used: sinusoidal tilts (frequency 0.007 Hz, amplitude 1.5 degrees) and ramps (amplitude 1.0 and 0.25 degrees, angular velocity 0.04 degrees/s). During slow sinusoidal tilts of the platform, the ankle angle and body position undergo periodical changes, but these changes have significant phase lead relative to the platform movement: 119 +/- 26 for ankle angle and 55 +/- 19 degrees for body sway. Gains were about 0.9 for both parameters. Large phase shift (tens of seconds) indicated a long delay in compensation of body inclination by ankle joint. The ramp tilt produced an initial body deviation followed by a slow (seconds or tens of seconds) approach of body position to a new steady level after the termination of ramp. Large slow body movements were superimposed with small irregular oscillations (about 10% of the amplitude of large displacements) of higher frequency. These oscillations resembled normal stabilograms on a stationary support. Thus, the usual process of stabilization of body gravity center was continued, though not around a fixed set-point but relative to a slowly changing position. Data obtained support the hypothesis that, besides operative control assigned to compensate deviations from a reference position, the system of postural control includes at least one additional level, which elaborates this reference using information about mutual position of body links, muscular torques and interaction with the support on the basis of criteria taking into account the energy cost of standing and demands for stability and security.

Adult↗
PubMed1994

The contribution of foot deformation to the changes of muscular length and angle in the ankle joint during standing in man.

This study was designed to evaluate the extent of foot deformation in healthy subjects during standing on an immobile support and during slow tilts of the support platform by 1 deg. The angle in ankle joint was evaluated by two methods: as an angle of skin inclination relative to the platform and as an angle, calculated on the basis of recording of the projective length of the soleus muscle. It was shown that the real changes of the angle in ankle joint during standing on an immobile platform were up to 2 times smaller than the changes of angular position of the shin relative to vertical axis. However, considerable intersubject variability was observed in this respect. During slow tilts of the support platform a marked divergence was observed in the shape of recordings of two "ankle angles" in subjects with high foot compliance. The vertical displacements of the calcaneus recorded by means of a clamp rigidly fixed at the heel were 0.5 +/- 0.3 mm (the range 0.1-1 mm) for each degree of body deviation in the forward or backward direction. In 12 subjects, the average foot compliance was 0.04 +/- 0.03 deg/Nm (maximal value 0.1 deg/Nm). It can be assumed that the mechanical properties of the foot can appreciably influence the afferent outflow during maintenance of orthograde posture in man.

Ankle Joint↗
PubMed1993

Egocentric references and human spatial orientation in microgravity. I. Perception of complex tactile stimuli.

This paper is devoted to the results of the "tactile matrix" experiment performed during the second French-Soviet spaceflight (project Aragatz). The perception of the orientation of complex tactile stimuli (letters and digits) applied to different skin areas under varied conditions was studied. The task of interpretation of complex tactile stimuli was not affected by the absence of the gravitational vertical, although this task is closely associated with mechanisms for the perception of body configuration, as well as the spatial orientation of different body parts. The number of errors made under conditions of weightlessness was often even less than on Earth. The results confirm the high stability of the egocentric reference system and provide evidence that this system is based on a body scheme which cannot be easily modified by changing external conditions.

Humans↗
PubMed1993

Egocentric references and human spatial orientation in microgravity. II. Body-centred coordinates in the task of drawing ellipses with prescribed orientation.

This article describes the results of the "ellipses" experiment conducted during the second French-Soviet spaceflight (project Aragatz). The realization of oriented motor tasks, on the basis of internal body representation and without visual feedback, was chosen as a paradigm for studying the determinants of spatial orientation under weightlessness. The process of drawing ellipses in the air, using arm movements with axes parallel or perpendicular to the longitudinal body axis, was studied under normal gravity and in weightlessness, and recorded using a video computer motion-analyzing system (Kinesigraph). On Earth, the experiments were performed in standing and lying positions, and in flight, in the erect position with the feet fixed to the floor. In general, performance of the task in microgravity was not disturbed. Under conditions of spaceflight, the longitudinal ellipse was inclined forward in accordance with the inclination of the whole body relative to the fixed feet. On Earth, the angle between the long axes of longitudinal and transverse ellipses deviated from 90 degrees by 20-30 degrees. The same deviation persisted under microgravity conditions. The distinctive features of ellipses traced by individual subjects were also preserved. It is concluded that an egocentric reference system ensures normal performance of sensorimotor tasks in the absence of a gravitational reference.

Adaptation, Psychological↗
PubMed1993

The suppression of cervico-ocular response by the haptokinetic information about the contact with a rigid, immobile object.

Horizontal eye movements were recorded in eight healthy subjects during super-slow trunk rotation with respect to the space-stationary head. In some trials, subjects simultaneously indicated their perception of self-motion by means of a joystick. Over the frequency range employed (0.007-0.05 cycles per second, +/- 20 degrees), all subjects perceived the relative motion of head and trunk as a head rotation with respect to the stationary trunk. Eye movements were observed which were in phase with imaginary head rotation; their amplitude exceeded the amplitude of actual body rotation. The grasping of a rigid ground-based handle (1) produced a sensation of trunk rotation in space, (2) suppressed the sensation of imaginary head rotation in space and (3) gave rise to a significant decrease in amplitude of eye movements. The grasping of a stiff rod with non-zero compliance did not produce these effects. It is concluded that eye movements in response to body rotation with respect to the fixed head are not purely reflex reactions, but are influenced by the internal representation of body motion.

Eye Movements↗
PubMed1992

Some properties of linear relaxation in unfused tetanus of human muscle.

The linear relaxation (LR) was studied in isometric unfused tetanus (UT) of the human flexor digitorum sublimis muscle. With a decrease of the force level, the shoulder on the relaxation curve, as measured from the last stimulus, shifted to the right. The length of the linear portion itself weakly depended on activation level. When steady force changed from 100 to 40-50% of the maximum, the slope of LR decreased only by 15 +/- 4%. At smaller force levels the slope began to increase. LR can probably also be hidden in the twitch. With increased tetanus duration, LR becomes longer and slower at all force levels. LR was markedly diminished in contraction on the steep part of the exponential relaxation after a smooth tetanus. Its full recovery needed a train of 4-5 pulses (near 1 s) at the new stationary level. The form of the response to the additional pulse given during relaxation remained approximately constant during the most of LR portion and differed markedly before and after it. LR did not have direct relation to fatigue: in the first UT LR was always slower and longer than in subsequent ones.

Adult↗