Characteristics of the maintenance of the upright posture in subjects touching an external object while standing on a moving or immobile platform

Postural sway was compared for humans touching an external object while standing on an immobile or slowly moving posturographic platform. When the platform moves, the central nervous system may interpret the movement of the point of the contact with the external object as the movement of the body relative to the support or as the movement of the support itself. Thus, the information concerning the body position that is provided by the touch becomes ambiguous. It was demonstrated that contact with an external object during standing on an unstable support leads to a decrease in support sway. When a subject stands on a moving platform, this decrease is smaller than in the case of an immobile platform. Contact with an external object causes a decrease in postural responses to shank muscle vibrations on an immobile platform. On a moving platform, this decrease is nonsignificant. The change in postural sway depending on the unambiguity of afferent information is discussed in terms of the interaction between afferent signals of different modalities on the basis of the body scheme in subjects maintaining balance.Translated from Fiziologiya Cheloveka, Vol. 31, No. 1, 2005, pp. 59–65.Original Russian Text Copyright © 2005 by Kazennikov, Shlykov, Levik.     

Comparative analysis of the dynamics of human postural control during fixation and pursuit of a visual target

The properties of the system maintaining the upright posture were compared in different states of the oculomotor system: during target fixation and horizontal fast and slow pursuit (0.1 and 0.01 Hz), recording the trajectories of the center of pressure in the frontal and the sagittal planes. Methods of nonlinear analysis were applied to assess the similarity in pairwise comparisons. The overall similarity of the frontal plane dynamics proved to be higher than that of the sagittal plane dynamics. However, differences were revealed in fast pursuit versus slow pursuit or fixation in the frontal but not in the sagittal plane. Such differences may reflect the different inertia of the oculomotor and the balance control systems. In general, the results are consistent with the current notions on the two orthogonal subsystems of postural control.     

Vibration-Induced Frontal Postural Reactions in Humans Standing on Unstable Supports of Various Types

The authors studied the influence of the direction of support stability the vibration-induced frontal postural reactions of healthy humans during unilateral vibration of three lateral muscles, namely, the long peroneal muscle, tensor muscle of fascia lata, and abdominal external oblique muscle. The subjects stood on a movable blotter- shaped support. Its base was cylindrical or spheric; its height was 24 cm, and its base radius was 40 cm. The platform turn angle and sagittal and frontal horizontal shift of the upper part of the subjects' bodies were recorded. Reacting to vibrations, the subjects tilted their bodies contralaterally to the vibrated muscle, irrespective of the support type. These reactions were strongest when the subjects stood on a stable support and were weakest when the support was unstable. The reaction values were also influenced by the distance between the vibrated muscle and general center of gravity of the body. As a rule, all other things being equal, vibration-induced reaction strengthened when the stimulated muscle was closer to the center of gravity. The authors concluded that vibration-induced frontal reactions of a standing human depend on the support properties and reflect the features of the participation of the lateral muscles in the standing posture regulation. The information received from the lateral muscular receptors by the control system may be used in different manners, depending on the internal body–support interaction model formed by the CNS.     

Relationship between postural oscillations on an unstable support and variations of the force of grip on a hand-held object

The relationship between postural oscillations and variations of the force of grip on a finger-held object while keeping balance on an unstable support was studied. The distribution median of the intervals between peaks in the sagittal stabilogram and the nearest peaks in the grip force recording from one and the same test proved smaller than that obtained from different tests. The proportion of intervals shorter than 175 ms was greater in real records than in “fictitious” ones. Thus, there is a connection between changes in the grip force and postural oscillations, and the balance control system can use signals from finger skin receptors to improve the stability of an upright posture.     

Effects of reversal in the human equilibrium regulation system

The postural reactions induced in man by bilateral vibration of the tibial muscles under various conditions were investigated: sitting with feet not in contact with support; sitting in an unstable position on a narrow seat; sitting with feet in contact with different types of support (suspended platform, turntable, etc.); standing with an asymmetrical load applied to the body. It was demonstrated that local vibration of the tibial muscles can induce not only activation of the vibrated muscle or of its antagonist (local effects), but excitation of the extensors and flexors of the knee joint (nonlocal effects) or remote muscle groups participating in the realization of various postural synergies. The specific activation of the muscles is determined by a number of factors, among which the most important are the reciprocal position of the members of the body and the interaction of the feet with the support. The reflection of the configuration of the body in the system of internal representation also plays an important role.Institute for Problems of Information Transmission, Russian Academy of Sciences, Moscow. Translated from Neirofiziologiya, Vol. 24, No. 4, pp. 462–470, July–August, 1992.     

Effects of Modulations of the Visual Conditions on Subjects’ Perception of Their Own and Another Person’s Hand

According to current concepts, the execution of expedient actions well-coordinated in space becomes possible owing to the creation of a system for internal representation, which includes a body coordinate system, in the central nervous system. The goal of this study was to assess the effects induced by the exclusion of vision and a left-right inversion in visual space on the accuracy in the internal representation of hands and on aimed arm movements. The study cohort included 16 participants aged from 18 to 25 years. The experiment consisted of two test series. In the first series, a subject placed his/her left hand under a transparent plexiglass screen. Upon the experimenter’s command, the subject had to indicate the position of his/her left wrist and the terminal phalanges of the thumb, middle and little fingers with his/her right index finger on the plexiglass, which was accompanied by the corresponding marks displayed on the screen. The positional accuracy in the subject’s perception of his/her own hand position was recorded in the conditions of a leftright inversion of visual space, which were created by wearing prismatic spectacles and the exclusion of visual control. In the second case, the subject’s left hand was replaced on the table under the transparent screen by a similarly shaped left hand belonging to another person. It has been shown that the positions of the middle fingertip and the wrist were sufficiently precisely perceived by the subject through prismatic spectacles. However, the position of the tips of the thumb and little finger relative to the axis connecting the wrist and the terminal phalanx of the middle finger (the hand axis) was perceptually inverted. The accuracy of the indication was reduced for all fingers when the eyes were closed. In testing another person’s hand, a left–right inversion in the visual space created an illusory 90° turn of the hand’s axis and an illusory bias relative to the wrist towards elongation in the marker points corresponding to another person’s fingers. We can suggest that when the alien hand replaced the subject’s own hand, in accordance with the modulations in the motor task conditions, the egocentric system of coordinates was replaced by the allocentric system. The role of vision in the execution of spatially oriented and accurate hand movements increased in this case.