Yu. S. Levik

Kinesthetic reference for human orthograde posture

Humans with occluded vision were subjected to superslow tilts of the supporting platform, producing the inclination of the subjects 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.

Locomotor‐like movements evoked by leg muscle vibration in humans

We attempted to elicit automatic stepping in healthy humans using appropriate afferent stimulation. It was found that continuous leg muscle vibration produced rhythmic locomotor‐like stepping movements of the suspended leg, persisting up to the end of stimulation and sometimes outlasting it by a few cycles. Air‐stepping elicited by vibration did not differ from the intentional stepping under the same conditions, and involved movements in hip and knee joints with reciprocal electromyogram (EMG) bursts in corresponding flexor and extensor muscles. The phase shift between evoked hip and knee movements could be positive or negative, corresponding to ‘backward’ or ‘forward’ locomotion. Such an essential feature of natural human locomotion as alternating movements of two legs, was also present in vibratory‐evoked leg movements under appropriate conditions. It is suggested that vibration evokes locomotor‐like movements because vibratory‐induced afferent input sets into active state the central structures responsible for stepping generation.

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 selfmotion by means of a joystick. Over the frequency range employed (0.007–0.05 cycles per second, ±20°), 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.

The effects of moving sound images on postural responses and the head rotation illusion in humans.

The results of pilot studies on the effects of sound images moving in the horizontal plane on poststimulus responses and the head rotation illusion are presented. These phenomena are demonstrated to occur.

Relative Roles of the Ankle and Hip Muscles in Human Postural Control in the Frontal Plane during Standing

The main goal was to evaluate the relative roles of the ankle and hip muscles in human postural control in the frontal plane during normal upright standing. Experiments were designed to compare upright standing with and without the involvement of the ankle joint. The results demonstrated that standing balance in the frontal plane depended largely on the hip muscles and just slightly on the ankle muscles, which performed only small adjusting movements in the frontal plane. During quiet standing, the human body swayed in the frontal plane as a two-component inverted pendulum or, when no ankle joint torque was permitted, as an inverted pendulum consisting of only one component.

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.

Additional afferent signals in the human upright posture control system

Equilibrium maintenance was estimated in a subject standing with the eyes closed while holding a small weight. The experiments were performed on a movable platform in the form of a seesaw. Loads of 200, 500, and 1000 g were held by the subjects between the forefinger and the thumb, with the arm bent at the elbow. The rate of change in the length of the sagittal stabilogram and the root mean square deviation of the center of pressure from the equilibrium position were less when a subject was holding a load while standing on a movable support. The rate of change in the stabilogram length was the lowest (44.5 ± 6.8 mm/s) if the subject stood holding a 1000-g load and was 52.6 ± 9.2 mm/s without a load (p < 0.05, paired T-test). At the same time, when these loads were fixed on a mechanical holder attached to the trunk and simulating an arm bent at the elbow, there was no significant change in stabilogram parameters. Apparently, postural sway reduction is associated with the fact that the system of equilibrium maintenance can control the upright posture using an uncommon afferent input, namely, modulation of afferent signals induced by inertial interaction of an object and the fingers.

Eye Movements Induced by Changes in the Internal Representation of Body Posture

Oculomotor responses to body rotation were investigated in subjects standing with the eyes closed. A rotatable platform was used to provide body rotation relative to the space-stationary head or upper part of the body (fixation of the head; the head and the shoulders; and the head, the shoulders, and the pelvis). A slow rotation of the body about the longitudinal axis by ±6.5° within 10–150 s evoked an illusion of the upper part of the body turning in space, while the moving footplate was perceived as stationary in space. This illusion was accompanied by marked eye movements in the direction of the illusory rotation. In subjects grasping a rigid ground-based handle, the perception of body movements corresponded to the actual rotation of body parts. In this case, the amplitude of eye movements was substantially lower. It was concluded that the eye movement pattern depends not only on the actual relative movement of the body segments but also on the perception of this movement relative to the extrapersonal space.

Spectral analysis of the human body sway during standing on firm and compliant surfaces under different visual conditions

Effects of different visual conditions on the vertical posture maintenance were compared in subjects standing on a firm or compliant surface. These visual conditions included a motionless visual environment (MVE), eyes-closed condition (EC), and a virtual visual environment (VVE). The VVE consisted of two planes: the foreground and background. The foreground displayed a room window with adjacent walls, and the background was represented by an aqueduct with the adjacent landscape. The VVE was destabilized by inducing either the cophased or the antiphased relation between the foreground of the visual scene and the body sway. We evaluated changes in the amplitude spectra of two elementary variables calculated from the trajectories of the plantar center of pressure (CoP) displacements in the anteroposterior and lateral directions, namely, the trajectories for the center of gravity projections on the support (the CG variable) and the differences between the CoP and CG trajectories (the CoP–CG variable).The CG trajectory was considered as a controlled variable, and the difference between the CoP and CG trajectories were considered as a variable related to the body acceleration and reflecting changes in the resultant stiffness in ankle joints. The rootmean-square (RMS) values for the spectra of both variables calculated from the body sway in the anteroposterior direction in standing on a firm support decreased proportionately with antiphased relation between the foreground and the body sway and increased with the cophased relation, compared with the RMS calculated for the MVE conditions. RMS for the spectra of the CG variable in the cophased relation were nearly the same, as in standing with eyes closed (EC), while the RMS for the spectra of the CoP–CG variable were significantly less than with EC. The body sway during standing on a compliant support significantly increased in both the anteroposterior and the lateral directions under all visual conditions. RMS for the spectra of both variables with EC increased considerably higher than in the cophased relation. Furthermore, the RMS for the spectra of the CG variable calculated from the body sway in the lateral direction on a compliant support was substantially higher in the antiphased relation than in the cophased relation, whereas the RMS for the spectra of the CoP–CG variable under both conditions had similar values. The analysis of body sway and the results under some visual conditions have shown that the amplitude characteristics of the CG and CoP–CG variables changed not always proportionately with the passage from standing on a firm support to a compliant support. It is suggested that the found disproportion of changes in these two variables is probably associated with the contribution of another additional factor to the process of postural control, the passive elastic component of musculo-articular stiffness generated by fascial-tendon tissues.

Influence of a light tactile contact on vertical posture maintenance under the conditions of destabilization of visual environment

The influence of a light contact between index finger and a stationary external surface on the maintenance of upright posture in healthy subjects “immersed” in unstable virtual visual environment has been studied. Under these conditions, the subjects saw a screen with a visual scene consisting of a foreground and a background. In the foreground, there was a window of a room with the adjacent walls; in the background, there was an aqueduct with the adjacent terrain. The virtual visual environment was destabilized by setting inphase or antiphase couplings between the foreground and body oscillations. The analysis of upright posture maintenance was focused on the assessment of amplitude–frequency characteristics of two elementary variables calculated from the trajectories of the center of pressure of feet (CoP) in mediolateral and anteroposterior directions: the trajectory of vertical projection of the center of gravity (the CG variable) and the differences between the CoP and CG trajectories (the CoP-CG variable). Both in case of normal posture and the posture with a fingertip contact, the root mean square (RMS) values of the spectra of both variables were the lowest in motionless visual environment with antiphase coupling between the foreground and the body oscillations and the highest with inphase coupling and with eyes closed. In the cases with fingertip contact, the intensity of body oscillations in both directions was considerably lower; the influence of different visual conditions on RMS values of the spectra of both variables decreased. This effect was more significant for the CG variable. The frequency of body oscillations decreased as well. We observed the effect of tactile contact on the frequency of the spectra of both variables. The median frequencies of the spectra of the CoP-CG variable calculated from body oscillations in the anteroposterior and mediolateral directions increased under the conditions of tactile contact. On the contrary, the median frequencies of the spectra of the CG variable increased only for body oscillations in the mediolateral direction. Our results show that a light tactile contact (providing no mechanical support) significantly improves vertical posture maintenance, inter alia, under the conditions of destabilization of virtual visual environment. This improvement is provided by multidirectional and independent effects on the amplitude–frequency characteristics of elementary variables (CG and CoP-CG).