[Spectral Analysis of Sway of Human Body When Standing on Firm and Compliant Surfaces under Different Visual Conditions.]

The influence of different visual conditions on maintenance of vertical posture when standing on a firm and a compliant surface were compared. Motionless visual environment (MVE), closed eyes (CE) and virtual visual environment (VVE) were used as visual conditions. Displayed VVE included foreground and background. The foreground represented a room window with the walls adjoining to it, and the background - an aqueduct with the adjacent terrain. Destabilization of the VVE was produced by assignmentinphase or anti-phase relationship between the position of the foreground of visual scene and the body sway. We estimated changes in the amplitude spectra of two elementary variables calculated from trajectories of displacement of the center of pressure (CoP) in the anteroposterior and lateral directions, namely, trajectories of the center of gravity (CG variable) and differences between trajectories of CoP and CG (CoP-CG variable). The trajectory of CG was considered as a controlled variable, and a difference between CoP and CG trajectories - as the variable connected with acceleration of body and related to changes of resultant stiffness in ankle joints. In the condition of inphaserelation RMS of spectra of the CG variable were about the same, as when standing with CE, and RMS of spectra of the CoP-CG variable were significantly less, than at CE. When standing on acompliantsurface, body sway was increased significantly both in anteroposterior and lateral directions under all visual conditions. Moreover, during standing with CE RMS ofspectra of both variables were increased significantly more, than during standing with in phaserelation between the foreground of visual scene and the body sway. Furthermore, when standing on a compliant support RMS of spectra of CG variable, calculated from the body sway in the lateral direction, under the condition of antiphase relation were significantly greater than under the condition of inphaserelation, while RMS of spectra of CoP-CG variable had similar values underboth conditions. The results of the analysis of body sway showed that under somevisual conditions the transition from standing on a firm support to the standing on acompliant support have not causeda propor- tional change of amplitude characteristics of CG and CoP-CG variables. It is supposed that the revealed disproportion of changes of these two variables is probably connected with a contribution to posturalcontrol of one additional factor: the passive elastic component of muscular-articular stiffness generated by fascial-tendon tissue.

Postural instability enhances motor responses to transcranial magnetic stimulation in humans.

Does the state of postural instability require a high hierarchical level of posture control? Electromyographic (EMG) activity of leg muscles was recorded during transcranial magnetic stimulation (TMS) of the motor cortex and electrical stimulation of the tibial nerve (H-reflex) in healthy subjects standing on a rigid floor and on a rocking platform. In the soleus muscle, TMS-evoked EMG responses increased considerably (2.2+/-1.1 times) when balancing on the rocking platform, whilst the H-reflex tended to decrease. The effect of support instability was specific to the muscles participating in the posture control. The results suggest that postural instability might change the state and the role of the motor cortex in equilibrium maintenance.

Relative contribution of ankle and hip muscles in regulation of the human orthograde posture in a frontal plane.

It was investigated, whether the postural regulation in the frontal plane takes place mainly at the hip or at the ankle level. The elimination of ankle torque was achieved by providing a point support in the frontal plane. (Two boards were attached to subject feet, below each board a metal pipe 2 cm diameter was fixed, so, the subject stood on contrivances resembling 'skates'). The lateral displacements of breast and hip, the angle of 'skates' tilt and the characteristics of frontal stabilogram and electromyogram of two ankle muscles (m. peroneus and m. soleus) were compared in two situations: (1) during normal standing; (2) under the conditions of the exclusion of ankle torque from postural control. During normal standing the body behaved as two-link inverted pendulum. Transition from normal standing to standing on free 'skates' produced changes in the kinematics of body movement. Under the conditions of ankle torque exclusion (free 'skates') breast and hip of a subject moved in a frontal plane as a single unit (one-link inverted pendulum). During standing on free 'skates' the electromyographic activity of m. peroneus and m. soleus was the same as during normal standing (approximately 70-100 microV).

The direction of postural instability affects postural reactions to ankle muscle vibration in humans.

Postural instability changes the weighting of different types of sensory information and the state of the equilibrium maintenance system. We have recently found that the effect of Achilles tendon vibration (so-called 'vibration-induced falling') strongly diminishes during unstable posture. However, it is unclear whether the state of postural instability has directionally specific or non-specific effects on the attenuation of proprioceptive influences from ankle muscles. To check this hypothesis, we varied the direction of support mobility and applied shank muscle vibration to subjects standing on a rocking platform movable in the sagittal, frontal or both directions. Postural reactions were present only on those supports that were stable in the sagittal direction. We suggest that the direction of postural instability affects information processing from ankle muscle receptors.

Position sense during imperceptibly slow movements.

Proprioceptive input provides the nervous system with information related to body position and movement. This study investigated how normal humans perceive information related to the position of a limb when it is either at rest or moving very slowly, below the threshold for movement perception. Each subject's left hand, hidden from view, was horizontally translated in the frontal plane such that joint rotation was largely isolated to the shoulder. The translation speed was too slow for the subjects to detect movement. Subjects indicated their perception of hand position at 1- or 2-min intervals by pointing with the right index finger to the perceived location of the tip of the left middle finger. The constant error (i.e., bias) and variable error (i.e., SD of mean constant error) of each pointing movement was quantified. Partway through the trial, the direction of hand movement was reversed. In two additional experimental conditions, the subjects pointed at 1- or 2-min intervals at the stationary left hand, either when they assumed the hand would be moved or when they knew it would not. During slow movement, errors in the frontal and sagittal planes were found to be independent, and therefore, data analysis focused on the frontal plane, as this was the plane relevant to the subjects' task. All subjects performed the task sufficiently well to demonstrate a clear perception of hand location during the slow movement. The accuracy of perception was better when the left (target) hand was in its ipsilateral hemifield and, correspondingly, when the right hand pointed to its contralateral hemifield. There was no significant difference in constant error when the hand moved slowly, although there was a slightly higher variable error during slow movement than when the hand was stationary. Based on the similarity of results in trials with very slow and no hand movement, it was concluded that position-sense percepts are more accurately distinguished by the speed of movement rather than whether movement is occurring or not.

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.

Human equilibrium on unstable support: the importance of feet-support interaction.

Healthy humans maintained equilibrium on rocking supports (seesaw) of different curvatures and heights. We recorded platform tilt, horizontal displacements of the upper body, ankle joint angle and activity of ankle joint muscles. Subjects maintained balance by making seesaw rotations placing the support under the body's centre-of-gravity. Forward displacement was balanced by compensatory plantariflexion: thus the relation between muscle activity and ankle joint angle differed from that on a rigid floor. Mechanical analysis of stability showed that standing on low seesaws requires ankle torque increase during forward body shift (as on a rigid floor) and torque decrease on high seesaws (when the seesaw height exceeded its radius). In the latter case, balancing was impossible with eyes closed. The results suggest that directionally specific torque changes in response to centre-of-gravity shifts provide important information for maintenance of orthograde posture.

Muscle resistance to slow ramp weakly depends on activation level.

The mechanical response of human m. flexor pollicis longus to slow (3.2 degrees/s) linear stretch by 5.5 degrees was measured during sustained (45-60 s, 9-13.5 p.p.s.) unfused tetanus evoked by electrical stimulation. The stiffness increased during unfused tetanus. At the late phase of unfused tetanus it was 1.8 +/- 0.2 (mean +/- S.D.) times greater than at the early phase. The sensitivity of the isometric tension level to a short change in a stimulation frequency also increased. At the late phase of unfused tetanus force oscillations increased 1.2 +/- 0.2-fold during slow stretch or shortening and immediately reached a smaller amplitude after the cessation of length change. This was probably related to the friction and thixotropy in muscles. Muscle resistance to slow ramp depended only weakly on activation level. In the late phase of unfused tetanus the stiffness per unit force was 1.5 +/- 0.4 times greater at 9-13.5 p.p.s. than at 20-25 p.p.s. Thus, the relative value of muscle stiffness was greater for smaller activation levels typical for maintenance of posture. The enhancement of muscle stiffness during sustained unfused tetanus and a weak stiffness dependence on the activation level indicated a non-additivity of processes occurring in active muscle.

The influence of head rotation on human upright posture during balanced bilateral vibration.

During a balanced bilateral vibration of tensor fasciae latae muscles evoking no evident postural changes in a standing human the slow head turns resulted in a pronounced lateral common gravity centre displacements to the 'occipital' side. Neck influences were most prominent at intermediate tonic background and were lacking both during very weak and strong vibratory stimulation in contrast to usual tonic vibratory reflex which becomes stronger with the increase of frequency. Body sway was induced not only by actual head turns but also by illusory ones evoked by neck muscle vibration. The neck influences on standing posture are therefore present in healthy adults, but they require a definite level of tonic readiness for their manifestation and are incorporated in the whole system of body scheme mechanisms.

Head and trunk movements in the frontal plane during complex dynamic equilibrium tasks in humans.

Eight normal human subjects were asked to maintain monopodal equilibrium on a narrow beam (task 1) or bipodal equilibrium on an unstable rocking platform (task 2) for 5 s. Each task was performed under four experimental conditions: (1) in light, (2) in darkness, (3) in light while subject had to hold a full cup of water, and (4) as in 3, but with additional instructions to fix the gaze on the cup. The movements of the trunk and head in the frontal plane were recorded by means of a 50-Hz TV image analyzer that computed the coordinates of small reflective markers glued on the skin of the subjects. On the beam the trunk was inclined on the side of the supporting foot (13 +/- 9 degrees), on the rocking platform the mean trunk orientation during the tests was nearly vertical (2 +/- 7 degrees). Nevertheless, in both tasks the mean head position was the same and close to vertical: 1.5 +/- 4 degrees on the rocking platform and 1.5 +/- 5 degrees on the beam. For both tasks and all experimental conditions the head remained stabilized relative to vertical, despite large translations in the frontal plane. Standard deviations of head orientation from its mean value were 2.8 +/- 2 degrees for task 1 and 2 +/- 1.5 degrees for task 2. The changes of trunk orientation were significantly higher: 6.2 +/- 4.8 degrees and 4.5 +/- 4 degrees, respectively. The differences in angular stability of head and trunk, measured through the standard deviations of angular displacements, were especially pronounced in trials with large trunk movements. It was concluded that head angular stabilization, providing the central nervous system with necessary visual and vestibular references, is essential for effective dynamic postural control in the frontal plane during complex equilibrium tasks.