Sensory supplementation system based on electrotactile tongue biofeedback of head position for balance control

The present study aimed at investigating the effects of an artificial head position-based tongue-placed electrotactile biofeedback on postural control during quiet standing under different somatosensory conditions from the support surface. Eight young healthy adults were asked to stand as immobile as possible with their eyes closed on two Firm and Foam support surface conditions executed in two conditions of No-biofeedback and Biofeedback. In the Foam condition, a 6-cm thick foam support surface was placed under the subjects’ feet to alter the quality and/or quantity of somatosensory information at the plantar sole and the ankle. The underlying principle of the biofeedback consisted of providing supplementary information about the head orientation with respect to gravitational vertical through electrical stimulation of the tongue. Centre of foot pressure (CoP) displacements were recorded using a force platform. Larger CoP displacements were observed in the Foam than Firm conditions in the two conditions of No-biofeedback and Biofeedback. Interestingly, this destabilizing effect was less accentuated in the Biofeedback than No-biofeedback condition. In accordance with the sensory re-weighting hypothesis for balance control, the present findings evidence that the availability of the central nervous system to integrate an artificial head orientation information delivered through electrical stimulation of the tongue to limit the postural perturbation induced by alteration of somatosensory input from the support surface.     

Controlling posture using a plantar pressure-based,tongue-placed tactile biofeedback system

The present paper introduces an original biofeedback system for improving human balance control, whose underlying principle consists in providing additional sensory information related to foot sole pressure distribution to the user through a tongue-placed tactile output device. To assess the effect of this biofeedback system on postural control during quiet standing, ten young healthy adults were asked to stand as immobile as possible with their eyes closed in two conditions of No-biofeedback and Biofeedback. Centre of foot pressure (CoP) displacements were recorded using a force platform. Results showed reduced CoP displacements in the Biofeedback relative to the No-biofeedback condition. The present findings evidenced the ability of the central nervous system to efficiently integrate an artificial plantar-based, tongue-placed tactile biofeedback for controlling control posture during quiet standing.     

Inter-individual variability in sensory weighting of a plantar pressure-based, tongue-placed tactile biofeedback for controlling posture

The purpose of the present experiment was to investigate whether the sensory weighting of a plantar pressure-based, tongue-placed tactile biofeedback for controlling posture could be subject to inter-individual variability. To achieve this goal, 60 young healthy adults were asked to stand as immobile as possible with their eyes closed in two conditions of No-biofeedback and Biofeedback. Centre of foot pressure (CoP) displacements were recorded using a force platform. Overall, results showed reduced CoP displacements in the Biofeedback relative to the No-biofeedback condition, evidencing the ability of the central nervous system to efficiently integrate an artificial plantar-based, tongue-placed tactile biofeedback for controlling posture during quiet standing. Results further showed a significant positive correlation between the CoP displacements measured in the No-biofeedback condition and the decrease in the CoP displacements induced by the use of the biofeedback. In other words, the degree of postural stabilization appeared to depend on each subject's balance control capabilities, the biofeedback yielding a greater stabilizing effect in subjects exhibiting the largest CoP displacements when standing in the No-biofeedback condition. On the whole, by evidencing a significant inter-individual variability in sensory weighting of an additional tactile information related to foot sole pressure distribution for controlling posture, the present findings underscore the need and the necessity to address the issue of inter-individual variability in the field of neuroscience.     

Can a plantar pressure–based tongue-placed electrotactile biofeedback improve postural control under altered vestibular and neck proprioceptive conditions?

We investigated the effects of a plantar pressure-based tongue-placed electrotactile biofeedback on postural control during quiet standing under normal and altered vestibular and neck proprioceptive conditions. To achieve this goal, 14 young healthy adults were asked to stand upright as immobile as possible with their eyes closed in two Neutral and Extended head postures and two conditions of No-biofeedback and Biofeedback. The underlying principle of the biofeedback consisted of providing supplementary information related to foot sole pressure distribution through a wireless embedded tongue-placed tactile output device. Center of foot pressure (CoP) displacements were recorded using a plantar pressure data acquisition system. Results showed that (1) the Extended head posture yielded increased CoP displacements relative to the Neutral head posture in the No-biofeedback condition, with a greater effect along the anteroposterior than mediolateral axis, whereas (2) no significant difference between the two Neutral and Extended head postures was observed in the Biofeedback condition. The present findings suggested that the availability of the plantar pressure-based tongue-placed electrotactile biofeedback allowed the subjects to suppress the destabilizing effect induced by the disruption of vestibular and neck proprioceptive inputs associated with the head extended posture. These results are discussed according to the sensory re-weighting hypothesis, whereby the CNS would dynamically and selectively adjust the relative contributions of sensory inputs (i.e. the sensory weights) to maintain upright stance depending on the sensory contexts and the neuromuscular constraints acting on the subject.     

Head position-based electrotactile tongue biofeedback affects postural responses to Achilles tendon vibration in humans

The purpose of the present experiment was to investigate whether postural responses to ankle proprioceptive perturbation Achilles tendon vibration were affected by the availability of augmented sensory information about head orientation/motion with respect to gravitational vertical, i.e., normally provided by the vestibular system. To achieve this goal, ten standing subjects were exposed to Achilles tendon vibration in two No Biofeedback and Biofeedback conditions. The No Biofeedback condition served as a control condition. In the Biofeedback condition, subjects performed the postural task using a head position-based electrotactile tongue-placed biofeedback system. Center of foot pressure (CoP) displacements were recorded using a force platform. Results showed that (1) Achilles tendon vibration increased CoP displacements in the No Biofeedback condition and (2) this destabilizing effect was less accentuated in the Biofeedback condition. These results are consistent with and discussed in terms of sensory re-weighting mechanisms involved in postural control. In the condition of Achilles tendon vibration, which renders ankle proprioceptive information less reliable for controlling posture, the central nervous system was able to integrate alternatively available augmented sensory information suitable and usable in upright postural control to reduce the destabilizing effect of the ankle proprioceptive perturbation.     

How a plantar pressure-based,tongue-placed tactile biofeedback modifies postural control mechanisms during quiet standing

The purpose of the present study was to determine the effects of a plantar pressure-based, tongue-placed tactile biofeedback on postural control mechanisms during quiet standing. To this aim, 16 young healthy adults were asked to stand as immobile as possible with their eyes closed in two conditions of No-biofeedback and Biofeedback. Centre of foot pressure (CoP) displacements, recorded using a force platform, were used to compute the horizontal displacements of the vertical projection of the centre of gravity (CoG _v ) and those of the difference between the CoP and the vertical projection of the CoG (CoP-CoG _v ). Analysis of the CoP-CoG _v displacements showed larger root mean square (RMS) and mean power frequencies (MPF) in the Biofeedback than in the No-biofeedback condition. Stabilogram-diffusion analysis further showed a concomitant increased spatial and reduced temporal transition point co-ordinates at which the corrective processes were initiated and an increased persistent behaviour of the CoP-CoG _v displacements over the short-term region. Analysis of the CoG _v displacements showed decreased RMS and increased MPF in the Biofeedback relative to the No-biofeedback condition. Stabilogram-diffusion analysis further indicated that these effects mainly stem from reduced spatio-temporal transition point co-ordinates at which the corrective process involving CoG _v displacements is initiated and an increased anti-persistent behaviour of the CoG _v displacements over the long-term region. Altogether, the present findings suggest that the main way the plantar pressure-based, tongue-placed tactile biofeedback improves postural control during quiet standing is via both a reduction of the correction thresholds and an increased efficiency of the corrective mechanism involving the CoG _v displacements.     

Postural destabilization induced by trunk extensor muscles fatigue is suppressed by use of a plantar pressure-based electro-tactile biofeedback

Separate studies have reported that postural control during quiet standing could be (1) impaired with muscle fatigue localized at the lower back, and (2) improved through the use of plantar pressure-based electro-tactile biofeedback, under normal neuromuscular state. The aim of this experiment was to investigate whether this biofeedback could reduce postural destabilization induced by trunk extensor muscles. Ten healthy adults were asked to stand as immobile as possible in four experimental conditions: (1) no fatigue/no biofeedback, (2) no fatigue/biofeedback, (3) fatigue/no biofeedback and (4) fatigue/biofeedback. Muscular fatigue was achieved by performing trunk repetitive extensions until maximal exhaustion. The underlying principle of the biofeedback consisted of providing supplementary information related to foot sole pressure distribution through electro-tactile stimulation of the tongue. Centre of foot pressure (CoP) displacements were recorded using a force platform. Results showed (1) increased CoP displacements along the antero-posterior axis in the fatigue than no fatigue condition in the absence of biofeedback and (2) no significant difference between the no fatigue and fatigue conditions in the presence of biofeedback. This suggests that subjects were able to efficiently integrate an artificial plantar pressure information delivered through electro-tactile stimulation of the tongue that allowed them to suppress the destabilizing effect induced by trunk extensor muscles fatigue.     

Vestibular and neck somatosensory weighting changes with trunk extensor muscle fatigue during quiet standing

The purpose of this study was to investigate whether vestibular and neck somatosensory weighting could change in conditions of trunk extensor muscle fatigue during quiet standing. To achieve this goal, 20 young healthy adults were asked to stand as still as possible in two conditions of No fatigue and Fatigue of trunk extensor muscles. In Experiment 1 (n = 10), the postural task was executed in two head conditions: Neutral and Head tilted backwards, recognised to degrade vestibular and neck somatosensory information. In Experiment 2 (n = 10), the postural task was executed in two stimulation conditions: No tactile stimulation and Tactile stimulation of the neck provided by the application of strips of adhesive bandage to the skin over and around the neck. The centre of foot pressure displacements (CoP) were recorded using a force platform. Results showed that (1) trunk extensor muscles fatigue increased CoP displacements under normal vestibular and neck somatosensatory conditions (Experiments 1 and 2), (2) this destabilizing effect of fatigue was exacerbated when vestibular and neck somatosensory information was altered (Experiment 1) and (3) this destabilizing effect of fatigue was suppressed when neck somatosensory information was neck was facilitated (Experiment 2). Taken together, results of Experiments 1 and 2 could be interpreted as an up-weighting of vestibular and neck somatosensory information for controlling posture during quiet standing following trunk extensor muscles fatigue.     

Tongue-placed tactile biofeedback suppresses the deleterious effects of muscle fatigue on joint position sense at the ankle

Whereas the acuity of the position sense at the ankle can be disturbed by muscle fatigue, it recently also has been shown to be improved, under normal ankle neuromuscular state, through the use of an artificial tongue-placed tactile biofeedback. The underlying principle of this biofeedback consisted of supplying individuals with supplementary information about the position of their matching ankle position relative to their reference ankle position through electrotactile stimulation of the tongue. Within this context, the purpose of the present experiment was to investigate whether this biofeedback could mitigate the deleterious effect of muscle fatigue on joint position sense at the ankle. To address this objective, sixteen young healthy university students were asked to perform an active ankle-matching task in two conditions of No-fatigue and Fatigue of the ankle muscles and two conditions of No-biofeedback and Biofeedback. Measures of the overall accuracy and the variability of the positioning were determined using the absolute error and the variable error, respectively. Results showed that the availability of the biofeedback allowed the subjects to suppress the deleterious effects of muscle fatigue on joint position sense at the ankle. In the context of sensory re-weighting process, these findings suggested that the central nervous system was able to integrate and increase the relative contribution of the artificial tongue-placed tactile biofeedback to compensate for a proprioceptive degradation at the ankle.     

Vestibular and somatosensory contributions to responses to head and body displacements in stance

The relative contribution of vestibular and somatosensory information to triggering postural responses to external body displacements may depend on the task and on the availability of sensory information in each system. To separate the contribution of vestibular and neck mechanisms to the stabilization of upright stance from that of lower body somatosensory mechanisms, responses to displacements of the head alone were compared with responses to displacements of the head and body, in both healthy subjects and in patients with profound bilateral vestibular loss. Head displacements were induced by translating two 1-kg weights suspended on either side of the head at the level of the mastoid bone, and body displacements were induced translating the support surface. Head displacements resulted in maximum forward and backward head accelerations similar to those resulting from body displacements, but were not accompanied by significant center of body mass, ankle, knee, or hip motions. We tested the effect of disrupting somatosensory information from the legs on postural responses to head or body displacements by sway-referencing the support surface. The subjects' eyes were closed during all testing to eliminate the effects of vision. Results showed that head displacements alone can trigger medium latency (48–84 ms) responses in the same leg and trunk muscles as body displacements. Nevertheless, it is unlikely that vestibular signals alone normally trigger directionally specific postural responses to support surface translations in standing humans because: (1) initial head accelerations resulting from body and head displacements were in opposite directions, but were associated with activation of the same leg and trunk postural muscles; (2) muscle responses to displacements of the head alone were only one third of the amplitude of responses to body displacements with equivalent maximum head accelerations; and (3) patients with profound bilateral vestibular loss showed patterns and latencies of leg and trunk muscle responses to body displacements similar to those of healthy subjects. Altering somatosensory information, by sway-referencing the support surface, increased the amplitude of ankle muscle activation to head displacements and reduced the amplitude of ankle muscle activation to body displacements, suggesting context-specific reweighting of vestibular and somatosensory inputs for posture. In contrast to responses to body displacements, responses to direct head displacements appear to depend upon a vestibulospinal trigger, since trunk and leg muscle responses to head displacements were absent in patients who had lost vestibular function as adults. Patients who lost vestibular function as infants, however, had near normal trunk and leg response to head displacements, suggesting a substitution of upper trunk and neck somatosensory inputs for missing vestibular inputs during development.     

Postural control during quiet standing following cervical muscular fatigue: effects of changes in sensory inputs

The purpose of the present experiment was to investigate the effects of cervical muscular fatigue on postural control during quiet standing under different conditions of reliability and/or availability of somatosensory inputs from the plantar soles and the ankles and visual information. To this aim, 14 young healthy adults were asked to sway as little as possible in three sensory conditions (No vision, No vision-Foam support and Vision) executed in two conditions of No fatigue and Fatigue of the scapula elevator muscles. Centre of foot pressure (CoP) displacements were recorded using a force platform. Results showed that (1) the cervical muscular fatigue yielded increased CoP displacements in the absence of vision, (2) this effect was more accentuated when somatosensation was degraded by standing on a foam surface and (3) the availability of vision allowed the individuals to suppress this destabilising effect. On the whole, these findings not only stress the importance of intact cervical neuromuscular function on postural control during quiet standing, but also suggest a reweigthing of sensory cues in balance control following cervical muscular fatigue by increasing the reliance on the somatosensory inputs from the plantar soles and the ankles and visual information.     

Effects of electrotactile vestibular substitution on rehabilitation of patients with bilateral vestibular loss

The present study evaluated the effectiveness of electrotactile tongue biofeedback (BrainPort^®) as a sensory substitute for the vestibular apparatus in patients with bilateral vestibular loss (BVL) who did not have a good response to conventional vestibular rehabilitation (VR). Seven patients with BVL were trained to use the device. Stimulation on the surface of the tongue was created by a dynamic pattern of electrical pulses and the patient was able to adjust the intensity of stimulation and spatially centralize the stimulus on the electrode array. Patients were directed to continuously adjust head orientation and to maintain the stimulus pattern at the center of the array. Postural tasks that present progressive difficulties were given during the use of the device. Pre- and post-treatment distribution of the sensory organization test (SOT) composite score showed an average value of 38.3 ± 8.7 and 59.9 ± 11.3, respectively, indicating a statistically significant improvement (p = 0.01). Electrotactile tongue biofeedback significantly improved the postural control of the study group, even if they had not improved with conventional VR. The electrotactile tongue biofeedback system was able to supply additional information about head position with respect to gravitational vertical orientation in the absence of vestibular input, improving postural control. Patients with BVL can integrate electrotactile information in their postural control in order to improve stability after conventional VR. These results were obtained and verified not only by the subjective questionnaire but also by the SOT composite score. The limitations of the study are the small sample size and short duration of the follow-up. The current findings show that the sensory substitution mediated by electrotactile tongue biofeedback may contribute to the improved balance experienced by these patients compared to VR.     

Differential integration of kinaesthetic signals to postural control

The purpose of the present experiment was to identify whether non-visual sensory cues involved in the maintenance of balance control could be weighted differently from one subject to another in condition during which kinaesthetic signals, stemming from the ankle proprioceptors and plantar pressure somatosensory sensors, were altered. A large population of blindfolded healthy young university students (n = 140) were asked to sway as little as possible on: (1) a firm support (Firm condition) and (2) an unstable support used to impair the exploitation of the kinematic ankle proprioceptive and plantar pressure somatosensation (Foam condition). Centre of foot pressure (CoP) displacements were recorded using a force platform. Analyses of the surface area, range, and mean velocity of the CoP displacements showed significant negative correlations between the postural sway observed in the Firm condition and the increase in postural sway observed in the Foam condition. In other words, the alteration of ankle proprioception had a greater destabilising effect in subjects exhibiting the smallest CoP displacements when standing in a normal proprioception condition. The present findings suggest that the exploitation of the kinaesthetic relationships to postural control varied from one subject to another, hence evidencing the need to introduce differential approach to assess the general impact of preferential modes of spatial referencing in postural control.     

Somatosensory loss increases vestibulospinal sensitivity

To determine whether subjects with somatosensory loss show a compensatory increase in sensitivity to vestibular stimulation, we compared the amplitude of postural lean in response to four different intensities of bipolar galvanic stimulation in subjects with diabetic peripheral neuropathy (PNP) and age-matched control subjects. To determine whether healthy and neuropathic subjects show similar increases in sensitivity to galvanic vestibular stimulation when standing on unstable surfaces, both groups were exposed to galvanic stimulation while standing on a compliant foam surface. In these experiments, a 3-s pulse of galvanic current was administered to subjects standing with eyes closed and their heads turned toward one shoulder (anodal current on the forward mastoid). Anterior body tilt, as measured by center of foot pressure (CoP), increased proportionately with increasing galvanic vestibular stimulation intensity for all subjects. Subjects with peripheral neuropathy showed larger forward CoP displacement in response to galvanic stimulation than control subjects. The largest differences between neuropathy and control subjects were at the highest galvanic intensities, indicating an increased sensitivity to vestibular stimulation. Neuropathy subjects showed a larger increase in sensitivity to vestibular stimulation when standing on compliant foam than control subjects. The effect of galvanic stimulation was larger on the movement of the trunk segment in space than on the body's center of mass (CoM) angle, suggesting that the vestibular system acts to control trunk orientation rather than to control whole body posture. This study provides evidence for an increase in the sensitivity of the postural control system to vestibular stimulation when somatosensory information from the surface is disrupted either by peripheral neuropathy or by standing on an unstable surface. Simulations from a simple model of postural orientation incorporating feedback from the vestibular and somatosensory systems suggest that the increase in body lean in response to galvanic current in subjects with neuropathy could be reproduced only if central vestibular gain was increased when peripheral somatosensory gain was decreased. The larger effects of galvanic vestibular stimulation on the trunk than on the body's CoM suggest that the vestibular system may act to control postural orientation via control of the trunk in space.     

Experimental neck muscle pain impairs standing balance in humans

Impaired postural control has been reported in patients with chronic neck pain of both traumatic and non-traumatic etiologies, but whether painful stimulation of neck muscle per se can affect balance control during quiet standing in humans remains unclear. The purpose of the present experiment was thus to investigate the effect of experimental neck muscle pain on standing balance in young healthy adults. To achieve this goal, 16 male university students were asked to stand upright as still as possible on a force platform with their eyes closed in two conditions of No pain and Pain of the neck muscles elicited by experimental painful electrical stimulation. Postural control and postural performance were assessed by the displacements of the center of foot pressure (CoP) and of the center of mass (CoM), respectively. The results showed increased CoP and CoM displacements variance, range, mean velocity, and mean and median frequencies in the Pain relative to the No pain condition. The present findings emphasize the destabilizing effect of experimental neck muscle pain per se, and more largely stress the importance of intact neck neuromuscular function on standing balance.     

Attentional demands associated with the use of a light fingertip touch for postural control during quiet standing

The purpose of the present experiment was to investigate whether and how using a light fingertip touch for postural control during quiet standing requires additional attentional demands. Nine young healthy university students were asked to respond as rapidly as possible to an unpredictable auditory stimulus while maintaining stable seated and upright postures in three sensory conditions: vision, no-vision and no-vision/touch. Touch condition involved a gentle light touch with the right index finger on a nearby surface at waist height. Center of foot pressure (CoP) displacements were recorded using a force platform. Reaction times (RTs) values were used as an index of the attentional demand necessary for calibrating the postural system. Results showed decreased CoP displacements in both the vision and no-vision/touch conditions relative to the no-vision condition. More interestingly, a longer RT in the no-vision/touch than in the vision and no-vision conditions was observed. The present findings suggest that the ability to use a light fingertip touch as a source of sensory information to improve postural control during quiet standing is attention demanding.     

Postural adjustments in sitting humans following external perturbations: muscle activity and kinematics

There are several controversies concerning the organization and induction of postural adjustments in standing humans. Some investigators suggest the responses are triggered by somatosensory inputs (especially from the ankle in standing subjects), while others emphasize the vestibular input induced by head acceleration. We examined postural responses in sitting subjects in order to describe the muscle activation pattern during various perturbations and to test whether somatosensory or vestibular stimulation elicited the responses. The kinematics and EMG patterns in respons to perturbations caused by movements of the support surface were studied in adults. The postural muscle activation following a backward sway was mainly the same, whether it was elicited by a forward translation or a legs-up rotation. This is remarkable, since, except for pelvis rotation, the movements of all body segments including the head differed in the two conditions. Furthermore, a second experiment showed that the direction of the initial head movement could be reversed with retainment of the same postural muscle activation pattern. The results suggest that somatosensory signals derived from the backward rotation of the pelvis, and not vestibular information from the head, trigger postural responses during sitting. There was a slight but consistent difference in the muscle activation pattern, whether the backward sway was elicited by a forward translation or legs-up rotation. The difference seemed to reflect the sensory information from head and other body parts (except the pelvis). This finding allowed us to speculate in a central pattern generator for postural adjustments containing two levels. At the first level, a simple format of the muscle activation would be generated; at the second level, the centrally generated pattern could be shaped and timed by interaction from the entire somatosensory, vestibular, and visual input.     

Reduced postural sway during quiet standing by light touch is due to finger tactile feedback but not mechanical support

It is well known that a light and voluntary touch with a fingertip on a fixed surface improves postural stability during quiet standing. To determine whether the effect of the light touch is due to the tactile sensory input, as opposed to mechanical support, we investigated the light touch effect on postural stability during quiet standing with and without somatosensory input from the fingertip. Seven young subjects maintained quiet standing on a force platform with (LT) and without (NT) lightly touching a fixed surface, and with (TIS) and without (CON) the application of tourniquet ischemia, which removed the tactile sensation from the fingertip. The mean velocity of centre of pressure (CoP) was calculated to assess the postural sway in each condition. The mean velocity of CoP was significantly smaller in the LT condition compared to the NT condition only under the CON condition, whereas the light touch effect was not significant under the TIS condition. We found that the reduction of the horizontal ground reaction force due to the light touch was about 20%, which was approximately equivalent to the reduction of mean velocity of CoP in the LT condition compared to the NT condition. Since the fingertip contact force was relatively large compared to the horizontal ground reaction force, one could say that the light touch effect might be due to the mechanical support provided by the contact itself. However, we demonstrated experimentally that light touch effects were diminished due to loss of finger tactile feedback induced by the tourniquet ischemia, but not due to the mechanical support provided by the light touch. One possible reason is the lack of feedback information in controlling posture, and the other is the altered control of the arm induced by the loss of tactile feedback.     

Postural strategies associated with somatosensory and vestibular loss

Summary This study examines the roles of somatosensory and vestibular information in the coordination of postural responses. The role of somatosensory information was examined by comparing postural responses of healthy control subjects prior to and following somatosensory loss due to hypoxic anesthesia of the feet and ankles. The role of vestibular information was evaluated by comparing the postural responses of control subjects and patients with bilateral vestibular loss. Postural responses were quantified by measuring 1) spatial and temporal characteristics of leg and trunk EMG activation; 2) ankle, knee, and hip joint kinematics, and 3) surface forces in response to anterior and posterior surface translations under different visual and surface conditions. Results showed that neither vestibular nor somatosensory loss resulted in delayed or disorganized postural responses. However, both types of sensory deficits altered the type of postural response selected under a given set of conditions. Somatosensory loss resulted in an increased hip strategy for postural correction, similar to the movement strategy used by control subjects while standing across a shortened surface. Vestibular loss resulted in a normal ankle strategy but lack of a hip strategy, even when required for the task of maintaining equilibrium on a shortened surface. Neither somatosensory nor vestibular loss resulted in difficulty in utilizing remaining sensory information for orientation during quiet stance. These results support the hypothesis that cutaneous and joint somatosensory information from the feet and ankles may play an important role in assuring that the form of postural movements are appropriate for the current biomechanical constraints of the surface and/or foot. The results also suggest that vestibular information is necessary in controlling equilibrium in a task requiring use of the hip strategy. Thus, both somatosensory and vestibular sensory information play important roles in the selection of postural movement strategies appropriate for their environmental contexts.     

Postural coordination modes and transition: dynamical explanations

While research to date has been successful in quantifying postural behaviour, this paper examines the causes of transition between postural coordination mode using dynamical variables and, by inference, efficient control strategies underlying postural behaviour. To this end, six subjects in bipedal stance were instructed to maintain a constant distance between their head and a visual target that oscillated along the line of sight. Within sessions, participants were exposed to gradual changes in increasing target motion frequency. Kinematic results showed a sudden transition between in-phase and anti-phase postural coordination modes in visual target tracking. The dynamical analysis pointed out that (1) the center of pressure (CoP) position parameter is a crucial parameter in the determination of the adopted coordination mode, (2) the change occurred in response to limits bordered by the system: the interaction between equilibrium constraints (A/P displacements of CoP), physiological limits (net joint moments) support the emergence of different postural behaviours and, (3) finally, the anti-phase mode presents a better distribution of muscular moment between hip and ankle joints and is more effective to achieve high frequency oscillations with limited CoP displacements.