Upright stance on a single vs double seesaw: are automatic and voluntary components similarly involved in balance control?

The relevance of seesaw devices in postural evaluation and training is gathering growing evidence due to its sensory-motor specificity. Nonetheless, the physiological specificities resulting from the dissociation or not of the seesaws (single vs double) still need to be investigated, in particular by assessing the respective contribution of automatic and voluntary components in the postural control. A protocol based on attention disturbance through a dual-task paradigm was set to establish this contribution. The general assumption was that the larger the dual-task effects, the larger the voluntary component contribution. Based on the larger postural sway induced by the dissociated seesaws, it is expected that the larger dual-task effect occurs in that latter case. The subjects were required to stand with eyes closed on solid ground (SG), a single (SS) and a double (DS) seesaw device while mentally solving or not a navigation task. The movements of the seesaw, placed on a double force platform, were assessed through a frequency analysis of the resultant center-of-pressure displacements along both mediolateral and anteroposterior axes. A larger contribution (p < 0.05) of the voluntary component was observed when subjects were standing on the DS seesaw, especially along the mediolateral axis. This trend is thought to prepare for a step initiation and would be related to the threat encountered by the subjects. In contrast, performing a dual-task protocol does not affect the antero-posterior sway whatever the support conditions. These data give specific pointers for the relative cognitive demand in the postural strategies induced by the double seesaw device.

Standing on a Double-Seesaw Device is an Easy Way to Modify the Coordination Between the Two Feet for Controlling Upright Stance: Assessment Through Weight-Bearing Asymmetry.

Healthy young subjects were instructed to modify their weight-bearing asymmetry when standing on a double-seesaw device. The results indicated decreased and unchanged amplitudes in the center-of-pressure movements under the unloaded and loaded legs, respectively. In addition, a concomitant increased contribution of the more loaded leg and a decreased contribution of the pressure distribution mechanism along the mediolateral axis were observed in the production of the resultant center of pressure, its amplitude remaining constant. Thus, contrary to what was previously reported for stance control on solid ground, one of the main characteristics of a double-seesaw device, by preventing increased amplitudes on the loaded side during weight-bearing asymmetry, would be to facilitate a greater independency of the feet in the stance control process.

Upright standing after stroke: How loading-unloading mechanism participates to the postural stabilization.

Postural strategies employed by hemiparetic stroke patients need to be better understood to guide rehabilitation. Of the two complementary mechanisms used to stabilize the standing posture, loading-unloading (LU) and pressure distribution (PD), it is hypothesized that the former would be predominantly used. To this aim, posturographic assessments, through a dual force-platform, were performed in 30 Hemiparetics tested 3 months after a unilateral stroke, and 30 matched healthy Controls. Original indices (from 0 to 1) were calculated to assess LU and PD contributions. The results show that along the mediolateral axis, the LU contribution was very high and similar in Hemiparetics and in Controls (0.80 ±â€¯0.07 vs 0.76 ±â€¯0.09 a.u; p > 0.05), indicating a predominant hip involvement. Along the anteroposterior axis, the PD contribution was very close to 1 in controls (0.96 ±â€¯0.03 a.u.) indicating an exclusive ankle involvement. Despite a lower contribution in Hemiparetics (0.88 ±â€¯0.11 a.u.; p < 0.01), the indices were surprisingly always above 0.5, meaning that ankle movements remain predominant for controlling postural sways along the anteroposterior axis in all patients even those with severe clinical deficits. However the PD contribution appeared larger in patients with light or moderate deficits of the sensitivity (r = -0.532; p < 0.01) or the motor command (r = -0.513; p < 0.01). These results indicate that postural stabilization of hemiparetic persons remains controlled by a PD mechanism along the anteroposterior axis, even in those combining poor distal motor command and deep sensory loss. This ankle control, piloted by the more-loaded non-paretic limb, would therefore be preferred to a hip control through lateral trunk motion. This should be considered when defining the objectives of the postural rehabilitation after stroke.

Biomechanical Study of Tandem Stance in Healthy Young Adults: Effects of Weight-Bearing and Limb Dominance.

Postural strategies of healthy young adults to control tandem stance were investigated through the calculation of the relative contribution of the two loading-unloading (LU) and pressure-distribution (PD) mechanisms and the two legs for controlling the resultant center-of-pressure (CPRes) displacements. Weight-bearing asymmetry and limb dominance were also studied. For antero-posterior and medio-lateral CPRes displacements, LU and PD mechanisms mainly contribute, respectively. A significant LU contribution is nonetheless observed for medio-lateral control, due to a lateral gap between the CP positions under each foot despite a strict sagittal alignment for the two feet. Moreover, for medio-lateral control, the respective involvement of the two legs is related to the level of weight-bearing asymmetry, whereas the front leg mainly controls the antero-posterior sway. By specifying potential effects of body-weight asymmetry and limb dominance, to a better testing of patients with deficiencies in lateral sway control.

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.

Postural control in healthy young adults using a double seesaw device.

Postural control on single and double seesaws was investigated in young healthy adults required to stand as still as possible on two side-by-side seesaws favoring pitch motion and lying on two separate force platforms. The device offers the possibility to get associated or dissociated seesaws and, if dissociated, to induce asymmetric patterns for the centers-of-pressure (CP) under both left and right feet by using different radii for the two seesaws. Substituting a parallelepiped volume to one seesaw offering a firm contact to one foot is also possible. The results indicated that dissociating the two seesaws led to increased resultant CP (CPRes) and vertically projected center-of-gravity movements (CGv) only along the mediolateral axis, whereas a slight decreasing tendency characterized these movements along the antero-posterior axis. When standing on two independent seesaws with different radii, significantly larger CP displacements were seen along the antero-posterior axis under the foot lying on the more stable support, i.e., the seesaw with the longer radius or the parallelepiped volume. In these two asymmetrical conditions, the CPRes output results from a compensatory mechanism, i.e. larger movements under one foot to compensate for the decreased movements occurring under the opposite foot. This postural control strategy is aimed at allowing sufficient CPRes displacements in order to appropriately secure balance. Because of the complex sensorimotor coordination induced, involving differentially in certain cases both legs, the double seesaw device can be viewed as a possible tool for challenging postural control by inducing asymmetrical patterns between left and right feet CP movements.

Interaction between postural asymmetry and visual feedback effects in undisturbed upright stance control in healthy adults.

OBJECTIVES: The technique of additional visual feedback has been shown to significantly decrease the center of pressure (CP) displacements of a standing subject. Body-weight asymmetry is known to increase postural instability due to difficulties in coordinating the reaction forces exerted under each foot and is often a cardinal feature of various neurological and traumatic diseases. To examine the possible interactions between additional visual feedback and body-weight asymmetry effects, healthy adults were recruited in a protocol with and without additional visual feedback, with different levels of body-weight asymmetry. METHODS: CP displacements under each foot were recorded and used to compute the resultant CP displacements (CPRes) and to estimate vertically projected center of gravity (CGv) and CPRes-CGv displacements. Overall, six conditions were randomly proposed combining two factors: asymmetry with three BW percentage distributions (50/50, 35/65 and 20/80; left/right leg) and feedback (with or without additional VFB). RESULTS: The additional visual feedback technique principally reduces CGv displacements, whereas asymmetry increases CPRes-CGv displacements along the mediolateral axis. Some effects on plantar CP displacements were also observed, but only under the unloaded foot. Interestingly, no interaction between additional visual feedback and body-weight asymmetry was reported. DISCUSSION: These results suggest that the various postural effects that ensue from manipulating additional visual feedback parameters, shown previously in healthy subjects in various studies, could also apply independently of the level of asymmetry. CONCLUSION: Visual feedback effects could be observed in patients presenting weight-bearing asymmetries.

How does wearing a lumbar orthosis interfere with gait initiation?

The interaction between medical devices and the human body must be evaluated in standardised laboratory tests. Since wearing a lumbar orthosis is assumed to reduce lower back mobility and reinforce trunk movement control through imposed lordosis, this device is expected to affect gait initiation which requires trunk and pelvic rotations. Thirteen healthy subjects were asked to initiate gait without orthosis (control) and orthosis with or without lordosis constraints. The biomechanical parameters usually reported for gait initiation were studied and no statistically significant effects were found. Indeed, the duration of the anticipation, and execution phases and maximal instantaneous velocity of centre of gravity at the end of the first step were not modified by the experimental conditions. The lack of interference underlines the robustness of the gait initiation parameters, which therefore may lead subjects to adopt adaptive strategies to retain this invariance. Future experiments should be conducted to highlight these strategies. Practitioner Summary: The aim of this study was to investigate the effect of various lumbar orthosis characteristics on gait initiation organisation. The results, based on a dynamic analysis of balance strategies, showed that the medical device had no repercussions on movement control. Several explanations are proposed, which should be validated by future studies.

How providing more or less time to solve a cognitive task interferes with upright stance control; a posturographic analysis on healthy young adults.

Contrasted postural effects have been reported in dual-task protocols associating balance control and cognitive task that could be explained by the nature and the relative difficulty of the cognitive task and the biomechanical significance of the force platform data. To better assess their respective role, eleven healthy young adults were required to stand upright quietly on a force platform while concomitantly solving mental-calculation or mental-navigation cognitive tasks. Various levels of difficulty were applied by adjusting the velocity rate at which the instructions were provided to the subject according to his/her maximal capacities measured beforehand. A condition without any concomitant cognitive task was added to constitute a baseline behavior. Two basic components, the horizontal center-of-gravity movements and the horizontal difference between center-of-gravity and center-of-pressures were computed from the complex center-of-pressure recorded movements. It was hypothesized that increasing the delay should infer less interaction between postural control and task solution. The results indicate that both mental-calculation and mental-navigation tasks induce reduced amplitudes for the center-of-pressure minus center-of-gravity movements, only along the mediolateral axis, whereas center-of-gravity movements were not affected, suggesting that different circuits are involved in the central nervous system to control these two movements. Moreover, increasing the delays task does not infer any effect for both movements. Since center-of-pressure minus center-of-gravity expresses the horizontal acceleration communicated to the center-of-gravity, one may assume that the control of the latter should be facilitated in dual-tasks conditions, inferring reduced center-of-gravity movements, which is not seen in our results. This lack of effect should be thus interpreted as a modification in the control of these center-of-gravity movements. Taken together, these results emphasized how undisturbed upright stance control can be impacted by mental tasks requiring attention, whatever their nature (calculation or navigation) and their relative difficulty. Depending on the provided instructions, i.e. focusing our attention on body movements or on the opposite diverting this attention toward other objectives, the evaluation of upright stance control capacities might be drastically altered.

Postural mechanisms to control body displacements in the performance of lateral gaze shifts.

Medialateral postural control mechanisms (bodyweight distribution and center of pressure location) have been studied in static conditions. Our objective was to determine how these mechanisms are adjusted to perform voluntary movements, in our case 80° lateral gaze shifts at 0.125 Hz and 0.25 Hz. In healthy, young adults, we expected body marker (neck, lower back) and center of pressure displacements to be significantly greater in gaze shift conditions than in the stationary gaze condition. To explain these changes in center of pressure displacement, the amplitude contribution of both mechanisms was expected to increase significantly. All these results were found accordingly. Unexpectedly, the active contribution of the bodyweight distribution mechanism was negatively related to body marker displacements in the gaze shift conditions (ns in stationary condition). Moreover, changes in the contribution of the mechanisms were statistically weaker in effect size than changes in body displacement. However, the participants were not unstable because they performed the visual tasks as requested. We propose that the strength of medialateral postural control mechanisms may not only be strengthened to control challenging ML stance conditions but also slightly weakened to allow the performance of adequate body motions in ongoing tasks.

The contribution of body weight distribution and center of pressure location in the control of mediolateral stance.

The study investigated the mediolateral control of upright stance in 16 healthy, young adults. The model analyzed the body weight distribution and center of pressure location mechanisms under three stance width conditions (feet close, under standard condition, and apart). Our first objective was to discuss some methodological requirements to investigate the contribution of both mechanisms by means of two platforms. It is proposed that both the amplitude contribution (in variability analyses) and active contribution (in cross-correlation analyses) need to be studied distinctively. These analyses may be concerned with the strength and the degree of active contributions, respectively. Based on this theoretical proposition, we expected and found that the amplitude contribution of both mechanisms was higher and lower in wide and narrow stances compared with that in the standard stance, respectively. Indeed, the closer the two reaction forces, the lower their mechanical contribution. As expected, the active contribution of both mechanisms was significantly lower and higher in wide and narrow stances, respectively. Indeed, the further the feet apart, the less active both mechanisms needed to be to control mediolateral stance. Overall, only the center of pressure location mechanism really changed its significant contribution to control mediolateral stance under the three conditions. The result is important because this mechanism is known to be secondary, weaker than the body weight distribution mechanism to control mediolateral stance. In practical terms, these findings may explain why the mediolateral variability of center of pressure displacement was significantly higher in narrow stance but not lower in wide stance.

How an acute mastering of balance on a seesaw can improve the relationship between "static" and "dynamic" upright postural control.

Postural control ability has been widely evaluated using undisturbed upright stance protocols. However, standing on a seesaw may offer additional insights due to changes in the available sensory information and the amplification of the motor command resulting from the translational and rotational movement of the device. These two tasks share close biomechanical and neurophysiological principles. To highlight their possible linkage, 32 young healthy adults participated in this study, which consisted of testing postural performance while standing on a firm surface or on a seesaw producing rolling or pitching movements. The results showed increased CP displacements along the seesaw's pitching or rolling axis and also along the perpendicular axis. However, comparing the two tasks can be difficult because of discrepancies in the ability to rapidly master the new constraints brought about by the seesaw. To highlight the role played by adaptation, 15 subjects of the whole sample participated in a complementary protocol consisting of a 20-min training session aimed at improving the mastery of the seesaw producing pitch motions. The relationship between the amplitudes of the CP displacements between "static" and "dynamic" tasks was investigated. Interestingly, whereas no statistically significant linear correlation was found before training, several significant correlations were found after pitch training for AP displacements. By emphasizing the key role played by short-term adaptation in standing performance, these results are likely to have potential implications regarding the conception of standardized tests aimed at evaluating postural ability in healthy or disabled subjects.

How additional visual feedback of the movements between center of pressure and vertically projected center of gravity can be used by hemiparetic patients.

PURPOSE: In healthy adults, the beneficial effect of providing additional visual feedback (VFB) with a delay of either the center of pressure (CP) or the difference between CP movements and center of gravity (CP-CGv) movements was previously demonstrated and explained by their capacity to decrease their CP-CGv movements. The present study seeks to demonstrate whether similar trends could be observed in hemiparetic patients. METHODS: Posturographic measurements were recorded in 56 hemiparetic patients in 3 VFB conditions: (1) CP in real time (VFBCP0), (2) CP with a 600-ms delay (VFBCP600), and (3) CP-CGv with a 600-ms delay (VFBCP-CG600). A condition without VFB (eyes open [EO]) was also included. The individual capacity to lower CP-CGv movements as compared to the reference VFBCP0 condition was assessed using dependency coefficients (ie, average surface covered by the CP-CGv displacements in an experimental condition relative to the VFBCP0 condition) in the VFBCP600, VFBCP-CG600, and EO conditions. RESULTS: Reduced CP-CGv movements were only obtained for half of the patients for the conditions with delay or without VFB. Significant linear correlations revealed that the most successful patients are characterized by reduced CP-CGv movements in these conditions and in the EO condition. In contrast, a poor correlation was found with the patients' clinical scores. CONCLUSION: Based on the behavioral profile of a patient able to use these VFB techniques effectively, this study emphasizes the complementary nature of the various conditions proposed by the VFB technique and provides insights for establishing rehabilitation protocols based on this VFB technique.

When might a cane be necessary for walking following a stroke?

BACKGROUND: For individuals with lateral postural imbalance after stroke, the decision to adopt a cane for walking often is not based on objective findings. OBJECTIVE: The authors investigated the explanatory value of 2 posturographic criteria for lateral postural imbalance on the walking abilities of poststroke subjects. METHODS: Indices of postural asymmetry (percentage of body weight on the less loaded lower limb) and instability (mediolateral variance of center-of-pressure displacements) were measured in 40 healthy individuals and 52 patients (mean 94.2 days after first hemispheric stroke), who stood still on a double force platform. Cut-off values (mean ± 2 standard deviations) were calculated and compared. The predictive value of both postural indices on walking abilities with or without a cane was analyzed. RESULTS: Of the patients, 34.6% were unstable along the mediolateral axis (variance >7 mm(2)), and 44.2% were asymmetrical (body weight <40%); 30% needed a technical aid and 35% walked without a cane. The probability of being able to walk without a cane was less than 5% if the paretic lower limb was not loaded more than 40%. The postural instability index was less informative. CONCLUSIONS: This study suggests that patients who do not load more than 40% of their body weight on their paretic lower limb may benefit from the prescription of a cane.

How performing a repetitive one-legged stance modifies two-legged postural control.

The proprioceptive cues in the control of movement is recognized as playing a major role in postural control. However, little is known about its possible increased contribution to postural control consecutive to repetitive muscular activations. To test this, the short-term effects induced by a 1-legged exercise on 2-legged postural control with the eyes closed were assessed in healthy subjects. The center-of-pressure (CP) displacements obtained using a force platform were split into 2 elementary movements: center-of-gravity vertical projection (CGv) and the difference (CP - CGv). These movements assessed the net postural performance and the level of neuromuscular activity, respectively, and were processed afterward (a) through variances, mean velocity, and the average surface covered by the trajectories and (b) a fractional Brownian motion (fBm) modeling. The latter provides further information about how much the subject controls the movements and the spatiotemporal relation between the successive control mechanisms. No difference was found using the classical parameters. In contrast, fBm parameters showed statistically significant changes in postural control after 1-legged exercises: The spatial and temporal coordinates of the transition points for the CG movements along the anteroposterior axis are decreased. Because the body movement control does not rely on visual or vestibular cues, this ability to trigger the corrective process of the CG movements more quickly in the postexercise condition and once a more reduced distance has been covered emphasizes how prior muscular activation improves body movement detection. As a general rule, these data show that the motor systems control body motions better after repetitive stimulation of the sensory cues. These insights should be of interest in physical activities based on a precise muscular length control.

Assessment of appropriate ankle-foot orthoses models for patients with Charcot-Marie-Tooth disease.

OBJECTIVE: : The purpose of this study was to demonstrate to what extent ankle-foot orthoses improve posture and gait control in patients with Charcot-Marie-Tooth disease and to identify the most appropriate characteristics of ankle-foot orthoses for patients regarding their clinical characteristics. DESIGN: : Twenty-six Charcot-Marie-Tooth patients were recruited. Clinical data (such as levels of sensory and muscular deficits) and posture and gait capacities were collected in three randomized experimental conditions (wearing ordinary shoes or with plastic and elastic orthoses). Several subgroups of patients, constituted using predictive value analysis, were associated using the probabilities of enhancing posture and gait control while wearing the various models of orthoses. RESULTS: : Compared with ordinary shoes alone, adding plastic ankle-foot orthoses partially improved both gait and posture control, whereas wearing elastic orthoses only partially affected the more dynamic gait control. Furthermore, the choice between the two models can be clarified by taking into account distal lower limb muscle capacity. CONCLUSIONS: : Ankle-foot orthosis prescription appears relevant for improving balance and gait performance in Charcot-Marie-Tooth patients, particularly when the model adequately compensates for specific muscle deficits. This study also provides objective arguments for making adequate bracing.

Visual feedback of force platform displacements for balance control training: what postural ability do healthy subjects have to develop to decrease the difference between center of pressure and center of gravity movements?

Past studies have emphasized the beneficial effect of additional visual feedback (VFB) on the capacity of healthy adults to decrease the amplitudes of the center-of-pressure minus center-of-gravity (CP-CG(v)) movements. To better assess these capacities, 56 subjects were asked to stand still on a force platform and to use the visual information provided. Dependency coefficients, based on their capacity to lower their CP-CG(v) movements and therefore relax their lower limb muscles, as well as parameters aimed at characterizing their postural strategies were measured across VFB conditions including (1) CP displacements in real time (VFB(CP0)), (2) CP displacements with a 600-ms delay (VFB(CP600)), and (3) CP-CG(v) displacements with a 600-ms delay (VFB(CP-CG600)). A non-VFB condition (eyes open) was also included. Several linear correlations were used to specify the relation between subjects' capacity to relax, compared with the VFB(CP0) condition, across the three remaining conditions. The data highlight the complementary nature of the VFB conditions and establish the postural control behaviors necessary to use these VFB protocols efficiently.

Undisturbed stance control in healthy adults is achieved differently along anteroposterior and mediolateral axes: evidence from visual feedback of various signals from center of pressure trajectories.

Provided through the screen of a monitor, the participant's resultant center of pressure (CPRes) movements from a force platform device, modified the postural performance of a healthy individual. However, these effects could largely vary with the axis that researchers consider (mediolateral [ML] or anteroposterior [AP]), because they know these controls are involved in 2 distinct ankle and hip mechanisms. To demonstrate this organization, the author tested a group of healthy adults in several conditions that gave the whole or some part of the information in the CPRes displacements. Compared with the CPRes feedback, left and right plantar CP or body weight distribution feedback deteriorated the control of the vertically projected center of gravity (CGv) along the ML and AP axes, whose amplitudes increased, respectively. These data highlight the primary role of loading or unloading and pressure variations in the achievement of postural control along each ML or AP axis, respectively. It is interesting that merging these 2 pieces of information (CPRes displacements) helped participants optimize their postural performance.

Relation between postural control assessment with eyes open and centre of pressure visual feedback effects in healthy individuals.

Visual feedback (VFB) of the resultant centre of pressure (CP(Res)) is a potentially interesting technique for rehabilitation purposes. However, all past studies have not been unanimous in supporting its utility in a physical therapy program. The present study was therefore undertaken with the main aim of assessing whether insights from postural control without additional feedback could be gained to explain the possible VFB effects. The CP(Res) displacements of 65 healthy adults were analysed in two conditions (eyes-open and VFB) through various classical parameters (including surface and variances along each medio-lateral and antero-posterior axes) and fractional Brownian motion (fBm) modelling. The results indicate that 69% of the sample were dependent on VFB, i.e. that the surface covered by the CP(Res) displacements was smaller during VFB than in the eyes-open condition. The comparison of the two subgroups (i.e. those who were dependant on VFB and those who were not) brought out significant differences in many classical and fBm parameters. In addition, the correlation between the degree of VFB dependency and the values measured during the eyes-open condition indicates that the more the subjects are VFB-dependent, the less they lean forward, the smaller the variance and the better the CP(Res) displacements are controlled over the longest time intervals. By specifying the links between the postural performance in an eyes-open standard condition and the degree of VFB dependency, a number of clues help identify the reasons for which some subjects do not succeed in using this technique, hence explaining the controversy about its use as a rehabilitation tool.

How spreading the forefeet apart influences upright standing control.

Resultant center-of-pressure (CP) displacements result along mediolateral (ML) and anteroposterior (AP) axes from strategies mobilizing hips and ankles, respectively, and thus, should be largely influenced by the angles between the feet. To assess this relation and the effects of foot position on postural performance, 9 healthy young adults were tested. The main results, as the forefeet are spread farther apart (from 30 degrees in endorotation to 120 degrees in exorotation), indicate (1) a larger contribution of the estimated ankle mechanisms in the generation of the CP trajectories along the ML axis, (2) increased variances along the longitudinal axis of the feet, (3) a constant longitudinal pattern of the CP trajectories under each foot whose main axis displays a progressively increased angle with the inner borders of the feet, and (4) increased variances for CP displacements along both ML and AP axes. These data emphasize the importance of foot positioning in stance control, especially along the ML axis where spreading the forefeet apart progressively increases the contribution of the mechanisms mobilizing the ankles.