Maintenance Of Upright Stance Research Articles

Vertical ground reaction force oscillation during standing on hard and compliant surfaces: The "postural rhythm".

When a person stands upright quietly, the position of the Centre of Mass (CoM), the vertical force acting on the ground and the geometrical configuration of body segments is accurately controlled around to the direction of gravity by multiple feedback mechanisms and by integrative brain centres that coordinate multi-joint movements. This is not always easy and the postural muscles continuously produce appropriate torques, recorded as ground reaction force by a force platform. We studied 23 young adults during a 90 s period, standing at ease on a hard (Solid) and on a compliant support (Foam) with eyes open (EO) and with eyes closed (EC), focusing on the vertical component of the ground reaction force (VGRF). Analysis of VGRF time series gave the amplitude of their rhythmic oscillations (the root mean square, RMS) and of their frequency spectrum. Sway Area and Path Length of the Centre of Pressure (CoP) were also calculated. VGRF RMS (as well as CoP sway measures) increased in the order EO Solid ≈ EC Solid < EO Foam < EC Foam. The VGRF frequency spectra featured prevailing frequencies around 4–5 Hz under all tested conditions, slightly higher on Solid than Foam support. Around that value, the VGRF frequencies varied in a larger range on hard than on compliant support. Sway Area and Path Length were inversely related to the prevailing VGRF frequency. Vision compared to no-vision decreased Sway Area and Path Length and VGRF RMS on Foam support. However, no significant effect of vision was found on VGRF mean frequency for either base of support condition. A description of the VGRF, at the interface between balance control mechanisms and sway of the CoP, can contribute information on how upright balance is maintained. Analysis of the frequency pattern of VGRF oscillations and its role in the maintenance of upright stance should complement the traditional measures of CoP excursions in the horizontal plane.

Modeling Perturbed Posture Through An Adaptive Sliding Mode Approach.

Upright stance maintenance under perturbed condition is a complex phenomenon in which the Central Nervous System(CNS) is engaged to regulate the balance preventing subject to fall. Many models of unperturbed stance are present in literature. However, the necessity to model balance maintenance in presence of external disturbance is still an open problem. In this paper, a new model representing the human balance maintenance under perturbed condition is presented. An adaptive sliding mode approach is used to model the action played by the CNS, the control parameters are then optimized in order to match real and simulated data. The trend of optimized parameters seems to reveal the development of different postural strategies throughout the experimental trials.

Attentional cost in additional visual feedback protocols in healthy young subjects.

Additional visual feedback (VFB) is a technique allowing improved postural stability in young healthy individuals despite an increased muscular activity, the two trends being assessed through center-of-gravity (CGv) and differences between CGv and center-of-pressure (CP) movements (CP-CGv), respectively. These two opposing effects are likely explained by the respective contribution of automatic and voluntary controls and in turn the neural circuits involved. To specify these specific contributions, a dual-task protocol was set up, consisting in adding to VFB a navigation task performed at the maximum cognitive capacities of the subjects who were evaluated beforehand. Overall, the protocol comprises six conditions: three visual tasks (eyes open without VFB, VFBBW based on body-weight distribution, VFBCP based on CP displacements) associated with or without a cognitive task. Variances of CP-CGv and CGv movements, along the mediolateral (ML) and anteroposterior (AP) axes, and parameters from fractional Brownian motion modeling (transition point coordinates and scaling regimes to assess the level of deterministic or stochastic activity) were used to assess the postural behaviors. The results show that during VFBCP, the dual tasks protocol infers a decreased contribution of deterministic activity in CP-CGv movements, inducing decreased variances, and alters the correction of the CGv over the longest Δt but nonetheless without changing CGv variances. Disturbing the subject's attention during the VFBBW condition induces decreased CP-CGv and CGv movements along the ML and AP axes, respectively. These data demonstrate the high level of attention induced by VFB protocols. If the tonic postural activity, expressed through CP-CGv movements, decreases whatever VFB condition along both the ML and AP axes, the effects on CGv movement appear to be mostly related to the additional information (BW or CP) provided. Overall, if too much voluntary control in upright stance maintenance is detrimental for the magnitudes of the CP-CGv movements, it appears beneficial for those of the CGv movements. By emphasizing the role of automatic and voluntary controls in VFB protocols, these insights document the neural circuits involved in such protocols and specify their conditions of use.

Advantages and disadvantages of stiffness instructions when studying postural control

To understand the maintenance of upright stance, researchers try to discover the fundamental mechanisms and attentional resources devoted to postural control and eventually to the performance of other tasks (e.g., counting in the head). During their studies, some researchers require participants to stand as steady as possible and other simply ask participants to stand naturally. Surprisingly, a clear and direct explanation of the usefulness of the steadiness requirement seems to be lacking, both in experimental and methodological discussions. Hence, the objective of the present note was to provide advantages and disadvantages of this steadiness requirement in studies of postural control. The advantages may be to study fundamental postural control, to eliminate useless postural variability, to control spurious body motions and to control the participants’ thoughts. As disadvantages, this steadiness requirement only leads to study postural control in unnatural upright stance, it changes the focus of attention (internal vs. external) and the nature of postural control (unconscious vs. conscious), it increases the difficulty of a supposedly easy control task and it eliminates or reduces the opportunity to record exploratory behaviors. When looking carefully at the four advantages of the steadiness requirement, one can believe that they are, in fact, more disadvantageous than advantageous. Overall therefore, this requirement seems illegitimate and it is proposed that researchers should not use it in the study of postural control. They may use this requirement only if they search to know the limit until which participants can consciously reduce their postural sway.

The role of central vision in posture: Postural sway adaptations in Stargardt patients

The role of central and peripheral vision in the maintenance of upright stance is debated in literature. Stargardt disease causes visual deficits affecting the central field, but leaving unaltered a patient's peripheral vision. Hence, the study of this rare pathology gives the opportunity to selectively investigate the role of central vision in posture. Postural sway in quiet stance was analyzed in 10 Stargardt patients and 10 control subjects, in three different conditions: (1) eyes closed, (2) eyes open, gazing at a fixed target, and (3) eyes open, tracking a moving target. Stargardt patients outperformed controls in the condition with eyes closed, showing a reduced root mean square (RMS) of the medio-lateral COP displacement, while their performance was not significantly different from controls in the antero-posterior direction. There were no significant differences between patients and controls in open eyes conditions. These results suggest that Stargardt patients adapted to a different visual-somatosensory integration, relying less on vision, especially in the medio-lateral direction. Hence, the central vision seems to affect mostly the medio-lateral direction of postural sway. This finding supports the plausibility of the “functional sensitivity hypothesis”, that assigns complementary roles to central and peripheral vision in the control of posture.

Effects of Destabilization of Visual Environment Perception on the Maintenance of Upright Stance by Humans on Different Support Surfaces

We compared characteristics of the maintenance of human upright stance under conditions of a real visual environment (VE) and “immersion” into a virtual visual environment (VVE). The foreground of the latter corresponded to the window in the room, while the background was a view of the aqueduct with the adjacent terrain. Destabilization of the VVE was created by “coupling” of the foreground position with oscillations of the subject’s body within the sagittal plane. We measured elementary variables calculated according to the trajectory of the center of feet pressure (CFP); these variables were: (i) displacement of the vertical projection of the center of gravity (CGv) and (ii) difference between the positions of the CFP and CGv (variables CGv and CFP – CGv). When standing on a rigid support surface, the root mean square (RMS) of the spectra of oscillations of both variables decreased in the case of an antiphase relation between displacements of the VE foreground with oscillations of the body and increased in the case of an inphase relation between these variables, as compared with the RMS in the maintenance of upright stance under conditions of an immobile VE (ImVE). Under conditions of the inphase relation, however, there were no dramatic disorders in the vertical stance; maximum oscillations of the body in this case did not exceed values typical of the upright stance with the eyes closed (EC). When the upright stance was maintained on a squeezable support, body oscillations increased significantly under all visual conditions, and the difference between the RMS of the CGv spectra obtained for the conditions of the inphase relation and EC became statistically significant. In the case of standing on a squeezable support, RMSs of the CFP – CGv variable at the antiphase relation of the VVE foreground were greater than those at the inphase relation. At the same time, the RMS of the CGv spectra were, vice versa, greater at the inphase relation. Thus, upon variation of the conditions for the vertical stance maintenance, the amplitude characteristics of elementary variables (CGv and CFP – CGv) determining the CFP on a support can vary in both a parallel and an independent manner. These variables can be controlled not only by coupled but also by independent (uncoupled) mechanisms controlling their amplitude/frequency parameters.

A new measure of the CoP trajectory in postural sway: Dynamics of heading change

The maintenance of upright stance requires the simultaneous control of posture in both the anterior–posterior (AP) and medial–lateral (ML) dimensions. Postural sway is typically measured by quantifying the movement of the center of pressure (CoP) in the AP and ML dimensions independently. Metrics such as path length and 95% ellipse area have been developed to take into account movement in both the AP and ML directions, but these metrics only quantify the magnitude of the CoP movement. The movement of the CoP is technically a vector quantity with both magnitude and direction characteristics. The direction of displacement, or heading, of the CoP may provide further insight into the control of posture. Accordingly, we present a novel variable that describes the rate of change in direction of CoP displacement in two dimensions, the heading change (Δϕ), which is derived from the CoP heading (ϕ). We then compared the standard deviation (SD) and the dynamic structure characterized by sample entropy (SampEn) of the heading change time series to previously examined metrics presented in the literature (SD and SampEn of the AP and ML time series, path length, SD and SampEn of the CoP resultant magnitude time series) during a 60s single-leg stance performed by healthy participants and patients with a ruptured anterior cruciate ligament (ACL) prior to surgical intervention. Patients with an ACL rupture exhibited a different dynamic structure in Δϕ compared to healthy controls, t(14)=2.44, p=0.029, whereas none of the other metrics differed between groups (all p>0.05). The novelty and utility of Δϕ is that it characterizes directional changes of the CoP, whereas previously documented postural control analyses describe only changes in magnitude.

Utilização da informação visual no controle postural de crianças com paralisia cerebral

INTRODUÇÃO: As limitações impostas pela paralisia cerebral (PC) levam à persistência de distúrbios dos movimentos e da postura. Sendo assim, o controle postural tem um papel importante, pois a capacidade de manter a orientação postural é fundamental para realização das atividades de vida diária. OBJETIVOS: Investigar a utilização da informação visual no controle postural de crianças com PC e de crianças neurologicamente normais (NN). MATERIAIS E MÉTODOS: Foram avaliadas nove crianças com PC diplégica e nove crianças NN de ambos os sexos, entre 6 e 12 anos de idade. Todas as crianças permaneceram em pé e paradas dentro de uma sala móvel nas seguintes condições experimentais: sem movimento da sala com olhos abertos e com olhos fechados e com movimento da sala nas frequências de 0,2 e 0,5 Hz. O registro dos movimentos da sala e das crianças foi realizado por marcadores ativos afixados entre as escápulas das crianças e na parede da sala, respectivamente, e do sistema OPTOTRAK. RESULTADOS: As crianças com PC oscilaram mais em relação às crianças NN, e o movimento da sala influenciou a oscilação corporal de todas as crianças de maneira similar, sendo que todas elas foram influenciadas pela frequência em que a sala foi movimentada. CONCLUSÃO: Crianças com PC e NN são influenciadas pela manipulação da informação visual durante a manutenção da postura ereta e quieta e pela frequência em que o estímulo visual é apresentado.

Maintenance of the Upright Posture in Humans upon Manipulating the Direction and Delay of Visual Feedback

We examined the maintenance of the upright posture in subjects immersed in a 3D virtual visual environment, VVE. The latter consisted of two plans; the VVE foreground represented the image of a room window, while the background looked like an outside urban landscape. The position of the foreground (window) was linked, using the respective software, with body sways in the sagittal plane and shifted in accordance with body movements, while the background always remained immobile. By manipulating the direction and time delay of the relationship between the body sways and shifts of the VVE foreground, we tried to estimate the contribution of visual signals to postural control. For this purpose, we used the technique of frequency filtration of displacements of the center of feet pressure (CFP) and, in such a way, detected signals proportional to movements of the general center of gravity (CG) of the body (CG variable). After this, we calculated the difference between the CFP and CG shifts (CFP-CG-variable). The latter variable is known to be proportional to the horizontal acceleration; this is why it was used for estimation of the muscular torques correcting CG shifts. Analysis of changes in the above variables (CG and CFP-CG) demonstrated a clear dependence between the root mean square (RMS) of their amplitude spectra on the direction of feedback between body sways and shifts of the VVE foreground. With a synphase (SPh) pattern of the link between shifts of the body and those of the VVE foreground, the RMS values were closer to the range typical of standing with the eyes closed, while with an antiphase (APh) link, they were closer to the zone of values observed under conditions of normal vision. Introduction of time delays in feedback between sagittal body sways and shifts of the VVE foreground exerted nearly similar effects on the CG and CFP-CG variables. In both SPh and APh relations, 0.2- and 0.5-sec-long time delays resulted in increases in the RMS of the sway spectrum, while 0.8- and 1.0-sec-long delays led to RMS decreases. Thus, our findings show that the maintenance of upright stance under conditions of manipulations of the direction and time delay of visual feedback is performed, at least partly, with the use of an unstable VVE foreground as the reference. This can be considered proof of a special (frequently, dominating) role of the visual signals in postural control.

Experimental System for Investigation of Visual Sensory Input in Postural Feedback Control

The human postural control system represents a biological feedback system responsible for maintenance of upright stance. Vestibular, proprioceptive and visual sensory inputs provide the most important information into the control system, which controls body centre of mass (COM) in order to stabilize the human body resembling an inverted pendulum. The COM can be measured indirectly by means of a force plate as the centre of pressure (COP). Clinically used measurement method is referred to as posturography. In this paper, the conventional static posturography is extended by visual stimulation, which provides insight into a role of visual information in balance control. Visual stimuli have been designed to induce body sway in four specific directions – forward, backward, left and right. Stabilograms were measured using proposed single-PC based system and processed to calculate velocity waveforms and posturographic parameters. The parameters extracted from pre-stimulus and on-stimulus periods exhibit statistically significant differences.

Maintenance of upright stance in humans in normal and virtual visual environments

We recorded in 16 healthy subjects the sagittal and frontal components of the stabilogram when standing on a rigid motionless or movable (oscillating) platform under four conditions of visual control: (i) open eyes, OE; (ii) closed eyes, CE; (iii) central vision, CV, and (iv) virtual visual environment, VVE. Under the latter condition, subjects observed the 3D image of a room, which was generated by the computer; the image was adhered to head movements in such a manner that a peculiar connection for normal visual conditions between movements of the head and displacements of the visible visual environment was reproduced. Through a low-pass filtration of a trajectory of the center of pressure of feet (CPF), two elementary variables were received, horizontal motions of the center of gravity (CG) and the difference between the CPF and the CG (CPF-CG). Changes in these variables (CG and CPF-CG) were estimated using spectral analysis and subsequent calculation of the median frequency (MF) and root mean square value (RMS) of the spectra. The MFs of the spectra of the investigated variables were approximately identical under conditions of standing on oscillating and motionless supports and showed no clear dependence on various visual conditions. Unlike MFs, the RMSs of the spectra of body sways appeared more dependent on changes of conditions of standing and the mode of visual control (differing from each other in the higher sensitivity to modifications of conditions of standing and visual control). With standing on the motionless support, the RMSs of the spectra of both variables were the greatest under VVE and CE conditions and the smallest under OE condition. The body oscillations were considerably amplified under conditions of standing on a movable support, and a different pattern of visual influences on the RMS of both investigated variables was revealed. The RMSs had the greatest value under CE condition and were much smaller (50–40%) under other visual conditions, including the VVE condition. Therefore, our findings show that, under VVE conditions, visual sensory afferentation is ignored by cerebral structures controlling postural adjustments if standing occurs on a motionless support but is effectively used at the maintenance of upright stance on an oscillating support.

Quelles stratégies motrices de compensation les patients souffrant d’une sclérose en plaques sont-ils capables de développer pour contrôler leur équilibre ?

One of the main features of multiple sclerosis (MS) is the deterioration of motor pathway axons, and in some cases, sensory system axons. Consequently, experimental sensori-motor testing with the undisturbed upright stance paradigm might be useful. It can be hypothesized that the postural strategies could be differently affected depending on the degree of dysfunction of both sensory and motor tracts.Twelve and fifteen patients, classified from sensory clinical tests as ataxo-spastic (SEP-AS) or only spastic (SEP-S), respectively were included in this study and compared to 12 healthy adults matched for age. The postural strategies were assessed from the centre of pressure trajectories (CP), measured from the force platform on which the subjects were instructed to stand upright eyes open for a trial lasting 51.2 s. biomechanical modelling was applied to these trajectories to compute the movements of the centre of gravity (CG) and consequently, the vertical difference between the CP and then the CP-CG, two elementary movements known to characterize postural performance movements for CGv and horizontal acceleration communicated to the CG for the CP-CG movements, and consequently overall neuro-muscular activity. To estimate the relative contribution of each of these elementary movements, an analysis based on frequency parameters (RMS and MF) was conducted.Both SEP-AS and SEP-S groups demonstrate larger CG and CP-CG movements than the age paired individuals. However, some statistically significant differences has to be emphasised between the two MS subgroups but only for the CP-CG component: the RMS of these movements are largely increased for the SEP-AS group, as compared to the SEP-S one. Biomechanically, this feature expresses the necessity for these very patients to produce exaggerated horizontal forces, and thus an increased energy expenditure, to handle the CG movements. The lack of effect observed for the CG movements underlines the capacity for the SEP-AS group to set appropriate control mechanisms for counteracting these less favourable initial conditions.By demonstrating specific trends in the postural organisation aimed at controlling undisturbed upright stance maintenance, this study can be of interest for the practitioner by legitimating this experimental paradigm as a simple and non invasive way to diagnose appropriately the sensori-motor deficiency.

Performing Saccadic Eye Movements or Blinking Improves Postural Control

To determine the relationship between eye movement and postural control on an undisturbed upright stance maintenance protocol, 15 young, healthy individuals were tested in various conditions. These conditions included imposed blinking patterns and horizontal and vertical saccadic eye movements. The directions taken by the center of pressure (CP) were recorded via a force platform on which the participants remained in an upright position. The CP trajectories were used to estimate, via a low-pass filter, the vertically projected movements of the center of gravity (CGv) and consequently the difference CP-CGv. An analysis of the frequency shows that regular bilateral blinking does not produce a significant change in postural control. In contrast, performing saccadic eye movements induces some reduced amplitude for both basic CGv and CP-CGv movements principally along the antero-posterior axis. The present result supports the theory that some ocular movements may modify postural control in the maintenance of the upright standing position in human participants.

Effects of a Prior Stretching of the Plantarflexor Muscles on the Capacity to Control Upright Stance Maintenance in Healthy Adults

To assess whether prior stretching of a muscle can induce improved postural control, 15 healthy adults stood still upright with their eyes closed before and after a series of bilateral stretches of the triceps surae muscles. The analysis focused on the center of pressure (CP) and the vertical projection of the center of gravity (CGv) trajectories and their difference (CP - CGv). The prolonged stretching induced a forward shift of the mean position of the CGv. The frequency analysis showed a constancy of the amplitudes of both basic movements whereas an increased mean power frequency was seen for the CP - CGv movements. A fractional Brownian motion modeling of the trajectories indicates shortest time intervals and lower covered distances by the CGv before a change in its control occurs along the antero-posterior axis. This reorganization is thought to be a result of improved body movement detection, which allows postural control over the longest time intervals to be triggered more rapidly.

Influence of Stance and Support Surface Variations on Soleus Presynaptic Inhibition

Presynaptic inhibition (PI) is instrumental in enabling the nervous system to respond to environmental changes. Subjects who demonstrate little change in PI of the soleus muscle have been shown to have a decreased ability to adapt to environmental changes (i.e. uneven surface conditions). Little research has been done to examine changes in PI during various surface and stance conditions. PURPOSE: this study utilized H-reflex measures to examine extrinsic PI in the soleus muscle during single- and double-legged stance, under stable and unstable surface conditions. Design & settings: A 2 × 2 fully repeated measures design was employed with leg (single vs. double) and support surface (foam, no foam) serving as the independent variables. subjects: Ten healthy subjects (age = 23.±1.7 yr, mass = 75.2±6.4 kg, ht = 172.1±8.5 cm) with no known acute or chronic neurological or lower extremity musculoskeletal injury 6 months prior to the study served as volunteers. Measurements: The tibial nerve was stimulated using an extrinsic PI conditioning protocol. The ratio of the unconditioned H-reflex amplitude to the conditioned H-reflex amplitude represented extrinsic PI. All subjects completed a total of 7 trials for the conditioned and unconditioned soleus H-reflex response for each treatment. RESULTS: No interaction was found between leg and surface condition on soleus PI (p > 0.05). Main effects testing revealed that a significant increase in soleus PI for the single-legged stance compared the double-legged stance (P=0.001). Surface variation did not affect soleus PI (p > 0.05). CONCLUSIONS: The increase in soleus PI during a single-legged stance suggests inhibition of the motoneuron pool may aid in making postural corrections, while maintaining upright stance during less stable positions. Moreover, maintenance of upright stance on an unstable foam surface does not appear to influence extrinsic PI modulation of the soleus muscle.

Evidence for reflex and perceptual vestibular contributions to postural control.

Vestibular signals are known to have an important role in stance under specific conditions. Potentially these effects could be modulated by vestibular reflexes or by voluntary responses to perceived vestibular signals. Our preliminary aim was to confirm that vestibulospinal reflexes change in parallel with sway under different postural conditions, and then to determine whether any relationship was present between these reflexes and body sway within fixed postural conditions. Sixteen subjects (eight male, eight female) were tested in conditions assessing the effects of vision (eyes open or closed), support surface (firm or compliant), external support (with or without) and stance width (feet apart or together). Sway (centre of pressure) in the anteroposterior (AP) and mediolateral planes was measured using a force platform. A subgroup of 11 subjects (five male, six female) underwent testing to measure short (SL) and medium latency (ML) reflexes from soleus. Bipolar, transmastoid galvanic stimulation (1 mA, 200 ms) was administered while subjects stood in the most unstable of our conditions (eyes closed, compliant surface and feet together). In the final part, to assess possible perceptual contributions to body sway, short duration AP sway levels were measured and expressed in angular terms (sway in mrad, velocity in mrad s(-1)) in the 11 subjects for both our baseline (eyes open, firm surface and feet apart) and most unstable conditions. Average sway levels increased more than seven-fold between conditions and had significant, positive correlations with reported changes in mean vestibulospinal reflexes under similar conditions (overall r = 0.75, P < 0.001). However, the SL reflex for the subgroup of 11 subjects had a significant negative correlation (r = -0.71; P = 0.014) with the degree of AP sway in the condition with maximum reliance on vestibular inputs (eyes closed, compliant surface, and feet together). Under baseline conditions, 5/125 (4%) of the short-term AP sway displacements were above the threshold previously reported for the detection of imposed sway. In the unstable condition, when sway was increased, 43/138 (31%) of the short-term AP sway movements were above the threshold for perception of imposed body sway based on vestibular signals. Our results confirm that vestibulospinal reflexes appear to be acutely facilitated as body sway increases. For the most unstable condition, when non-vestibular information was absent or attenuated, subjects with larger SL reflexes had less AP sway, suggesting that the SL reflex acted to attenuate sway. Under the same condition, short duration sway levels increased such that 31% were above the previously published threshold for detection using vestibular afferents. We conclude that both vestibular reflexes and perceptual signals appear to have a specific role in the maintenance of upright stance, under conditions in which other sources of postural information are attenuated or absent.

Analyse biomécanique de la contribution relative de chacun des appuis dans le contrôle de la station debout non-perturbée

Upright undisturbed stance constitutes a fundamental attitude involving very cunning sensori-motor strategies. Until now, few analyses on postural control have been undertaken to explain the influence of both supports in this particular task. With the aim of analysing asymmetrical postures resulting from neurological (hemiplegia) or traumatic (amputation) origins, a new analysis and a reference framework are proposed, describing the influence of each support in upright stance maintenance. Ten healthy adults (eight men and two women ranging in age from 20 to 42 years) volunteered for this study. The subjects were required to remain as still as possible in upright stance. The control mechanisms for equilibrium maintenance were estimated by separate measures of the elementary and resultant centres of pressure through two force platforms. The analysis of the relationship between the elementary and resultant centres of pressure on the one hand and the centre of gravity on the other hand emphasises that both took place similarly and at the same time in the control of upright undisturbed stance. This control is done through two strategies that come into play at the level of the ankle and/or hip in the medio-lateral and antero-posterior directions, respectively. Furthermore, the various trajectories (elementary and resultant centres of pressure, centre of gravity) and analytical methods (frequential, correlations and cross-correlations) are compared. This methodology should be very interesting in the analysis of pathologies inferring postural asymmetry in weight distribution, since one could suppose that asymetric patients could be characterised by specific control of both supports.

Optimising the visual feedback technique for improving upright stance maintenance by delaying its display: behavioural effects on healthy adults.

The visual feedback technique (VFB) is recognized by several studies as a valuable tool for re-establishing the balance functions. However, one former study has highlighted the fact that the increased control induced by this technique infer both favourable (the amplitudes of the horizontal motions of the centre of gravity (CoG(h)) are diminished) and unfavourable features (the vertical difference between the CoG(h) motions and centre of pressure (CoP) trajectories are enhanced). One means to decrease these CoP-CoG(v) motions is to delay their display on the screen of the monitor. To assess these behavioural effects, 16 healthy adults were evaluated with various delays from 0 to 1200 ms. CoP displacements, measured through a force platform, were decomposed into two elementary motions: CoG(h) and the difference CoP-CoG(v). A fractional Brownian motion modelling of these motions allowed to determine from which distance and for how long the corrective process takes over and to what extent the motion is controlled. Compared to the VFB real time condition, increasing the delay induces some effects mainly on the CoP-CoG(v) motions which are largely diminished, the most striking effect appearing for delays exceeding 600 ms. Despite the lower forces these reduced amplitudes infer to control body sways, the amplitudes of the CoG(h) motions tend to increase slightly. Considered as a whole, whilst retaining the beneficial aspects of VFB without delay and significantly suppressing the unfavourable features, the data suggests that the method of delaying the screen display optimises the VFB technique.

Multivariate changes in coordination of postural control following spaceflight

Postural and gait instabilities in astronauts returning from spaceflight are thought to result from in-flight adaptation of central nervous system processing of sensory inputs from the vestibular, proprioceptive, and visual systems. We hypothesized that reorganization of posture control relying on these multiple inputs would result in not only greater amounts of sway, but also changes in interjoint coordination. We tested this hypothesis by examining the multivariate characteristics of postural sway and comparing the postural control gain used for maintenance of upright stance during the altered sensory conditions of the Sensory Organization Test (EquiTest, Neurocom Intl.). We used the covariance of hip and ankle kinematics as a measure of joint motion and interjoint coordination, and then utilized discriminant analysis to further examine these characteristics in a group of 10 first-time astronauts. In five of the six conditions, the most important difference was an increased relative utilization of the hip strategy, which would not be evident using conventional balance measures such as peak or root-mean-square sway. This finding was supported by indications of increased hip torque gains relative to lower extremity and neck motion in at least four conditions ( p<0.05). In contrast, ankle torque gains to these motions did not appear to change. These results suggest that after spaceflight, astronauts exhibit significant multivariate changes in multijoint coordination, of which increased sway is only one component. These changes are consistent with reweighting of vestibular inputs and changes in control strategy in a multivariable control system.

The contribution of vestibulo-spinal mechanisms to the maintenance of human upright posture.

The maintenance of upright stance in humans requires the integrative evaluation of vestibular, visual, and proprioceptive information. Recent experiments prove the existence of fast vestibulo-spinal reactions in humans. Their physiological significance for the compensation of sudden external displacements of the body is minor. For low frequency stabilization of the body, however, vestibulo-spinal afferents as well as vision and proprioception are important. Unilateral loss of vestibular function initially causes a tendency to fall to the ipsilateral side as well as nystagmus and vertigo. After the first days of compensation, the remaining functional disturbance can only be detected by exposing the patient to conflicting sensory reafferents or by reducing proprioceptive and visual inputs simultaneously. The loss of vestibular function not only alters low frequency stabilization and to some extent also reflex-like fast motor patterns, but also interferes with the selection of preprogrammed motor synergies.