Moving beyond quiet stance: Applicability of the inverted pendulum model to stooping and crouching postures

Background

Currently, it is unknown whether the inverted pendulum model is applicable to stooping or crouching postures. Therefore, the aim of this study was to determine the degree of applicability of the inverted pendulum model to these postures, via examination of the relationship between the centre of mass (COM) acceleration and centre of pressure (COP)–COM difference.

Methods

Ten young adults held static standing, stooping and crouching postures, each for 20 s. For both the anterior–posterior (AP) and medio-lateral (ML) directions, the time-varying COM acceleration and the COP–COM were computed, and the relationship between these two variables was determined using Pearson?s correlation coefficients. Additionally, in both directions, the average absolute COM acceleration, average absolute COP–COM signal, and the inertial component (i.e., −I/Wh) were compared across postures.

Results

Pearson correlation coefficients revealed a significant negative relationship between the COM acceleration and COP–COM signal for all comparisons, regardless of the direction (p<0.001). While no effect of posture was observed in the AP direction (p=0.463), in the ML direction, the correlation coefficients for stooping were different (i.e., stronger) than standing (p=0.008). Regardless of direction, the average absolute COM acceleration for both the stooping and crouching postures was greater than standing (p<0.002).

Conclusion

The high correlations indicate that the inverted pendulum model is applicable to stooping and crouching postures. Due to their importance in completing activities of daily living, there is merit in determining what type of motor strategies are used to control such postures and whether these strategies change with age.     

Visual and somatosensory contributions to infant sitting postural control

Abstract

There are a limited number of studies that have investigated sitting posture during infancy and the contribution of the sensory systems. The goal of this study was to examine the effects of altered visual and somatosensory signals on infant sitting postural control. Thirteen infants (mean age?±?SD, 259.69?±?16.88?days) participated in the study. Initially, a single physical therapist performed the Peabody Developmental Motor Scale to determine typical motor development. Then the child was placed onto a force platform under four randomized conditions: (a) Control (C) – sat independently on the force plate, (b) Somatosensory (SS) – Sat independently on a foam pad (low density), (c) Visual (VS) – sat independently on the force plate while the lights were turned off creating dim lighting, and (d) Combination of b and c (NVSS). Center of pressure (COP) data from both the anterior-posterior (AP) and the medial-lateral (ML) directions were acquired through the Vicon software at 240?Hz. The lights off conditions, both VS and NVSS, lead to increased Root Mean Square (RMS) and Range values in the AP direction, as well as increased Lyapunov Exponent (LyE) values in the ML direction. Altered visual information lead to greater disturbances of sitting postural control in typically developing infants than altered somatosensory information. The lights off conditions (VS and NVSS), unveiled different control mechanisms for AP and ML direction during sitting. Thus, the present findings confirm the dominance of vision during the early acquisition of a new postural accomplishment.     

Time-to-boundary measures of postural control during single leg quiet standing

A novel approach to quantifying postural stability in single leg stance is assessment of time-to-boundary (TTB) of center of pressure (COP) excursions. TTB measures estimate the time required for the COP to reach the boundary of the base of support if it were to continue on its instantaneous trajectory and velocity, thus quantifying the spatiotemporal characteristics of postural control. Our purposes were to examine: (a) the intrasession reliability of TTB and traditional COP-based measures of postural control, and (b) the correlations between these measures. Twenty-four young women completed three 10-second trials of single-limb quiet standing on each limb. Traditional measures included mean velocity, standard deviation, and range of mediolateral (ML) and anterior-posterior (AP) COP excursions. TTB variables were the absolute minimum, mean of minimum samples, and standard deviation of minimum samples in the ML and AP directions. The intrasession reliability of TTB measures was comparable to traditional COP based measures. Correlations between TTB and traditional COP based measures were weaker than those within each category of measures, indicating that TTB measures capture different aspects of postural control than traditional measures. TTB measures provide a unique method of assessing spatiotemporal characteristics of postural control during single limb stance.     

Synergistic co-activation in multi-directional postural control in humans

To examine the muscle synergies of multi-directional postural control, we calculated the target-directed variance fraction (η) and net action direction of each muscle using the electromyogram-weighted averaging (EWA) method. Subjects stood barefoot on a force platform and maintained their posture by producing a center of pressure (COP) in twelve target directions. Surface electromyograms were recorded from 6 right-sided muscles: tibialis anterior (TA), soleus (SOL), lateral gastrocnemius (LG), medial gastrocnemius (MG), fibularis longus (FL), and gluteus medius (GM). η was calculated from COP with duration of 20-s, during which the COP was relatively constant. The EWA method was applied to the EMG and the two COP components to estimate the net action direction of each muscle. The results showed that η values in all directions did not cross the 0.8 threshold. This suggests that human postural control is achieved by synergistic co-activation. The EWA revealed that the net action directions of TA, SOL, LG, MG, and GM were 277.6°, 71.1°, 87.7°, 94.0°, and 2.2°, respectively. This suggests that postural maintenance by muscle synergy can be attributed to the relevant muscles having various action directions. These results demonstrate that muscle synergies can be investigated using COP fluctuations.     

A point of application study to determine the accuracy,precision and reliability of a low-cost balance plate for center of pressure measurement

Changes in postural sway measured via force plate center of pressure have been associated with many aspects of human motor ability. A previous study validated the accuracy and precision of a relatively new, low-cost and portable force plate called the Balance Tracking System (BTrackS). This work compared a laboratory-grade force plate versus BTrackS during human-like dynamic sway conditions generated by an inverted pendulum device. The present study sought to extend previous validation attempts for BTrackS using a more traditional point of application (POA) approach. Computer numerical control (CNC) guided application of ∼155 N of force was applied five times to each of 21 points on five different BTrackS Balance Plate (BBP) devices with a hex-nose plunger. Results showed excellent agreement (ICC > 0.999) between the POAs and measured COP by the BBP devices, as well as high accuracy (<1% average percent error) and precision (<0.1 cm average standard deviation of residuals). The ICC between BBP devices was exceptionally high (ICC > 0.999) providing evidence of almost perfect inter-device reliability. Taken together, these results provide an important, static corollary to the previously obtained dynamic COP results from inverted pendulum testing of the BBP.     

Time course and variability of tendinous vibration-induced postural reactions in forward and backward directions

Mechanical vibration of tendons induces large postural reactions (PR-VIB) but little is known about how these reactions vary within and between subjects. We investigated the intra- and inter-individual variability of PR-VIB and determined the reliability of center of pressure (COP) measures. Bipodal postural control (eyes closed) of 30 healthy adults were evaluated using a force platform under 02 conditions: bilateral VIB of the tibialis anterior (TA) and Achilles tendons (ACH-T) at 80 Hz. Each condition consisted of 03 trials of 30 s duration (Baseline: 10 s; VIB: 10 s; POST-VIB: 10 s). The Amplitude and Velocity of the COP in the antero-posterior/medio-lateral (AP/ML) directions were recorded and analyzed according to 5 time-windows incremented every 2 s of vibration (i.e. the first 2 s; 4 s; 6 s; 8 s & 10 s), whereas the COP position/AP was monitored every 0.5 s. All postural parameters increased significantly during TA and ACH-T vibration compared to the Baseline. The reliability of the COP measures showed good ICC scores (0.40-0.84) and measurement errors that varied depending on the duration of VIB time-windows. The COP position/AP reveals a lower intra- and inter-subject variability of PR-VIB in the first 2 s of VIB. The metrological characteristics of PR-VIB should be investigated further to guide their future use by clinicians and researchers.     

The influence of foot position on standing balance

To test the hypothesis that variations in foot position would significantly affect standing balance, we studied ten normal subjects on a Kistler force platform which measured the travel and center of pressure displacement. With the feet together there was substantially more mediolateral (ML) travel than with the axes of the feet 15, 30 or 45 cm apart and the mean ML position of the center of pressure was displaced toward the right; there was no consistent effect on anteroposterior (AP) travel or position. As the right foot was placed 10 and 30 cm forward or back, the least amount of ML and AP travel occurred with the feet even or at 10 cm either direction; the mean AP and ML position moved toward the foot which was placed more posteriorly. Of the five foot angles ranging from toes-out 45 degrees to toes-in 45 degrees, the extent of ML and AP travel was lowest in the toes-out 25 degrees position and greatest in the toes-in 45 degrees position; the mean AP and ML position was farthest forward and to the right with toes-in 45 degrees. These findings have implications for the prosthetic replacement of the lower limbs, sports, ergonomics and postural sway studies.     

Footwear and Foam Surface Alter Gait Initiation of Typical Subjects

Gait initiation is the task commonly used to investigate the anticipatory postural adjustments necessary to begin a new gait cycle from the standing position. In this study, we analyzed whether and how foot-floor interface characteristics influence the gait initiation process. For this purpose, 25 undergraduate students were evaluated while performing a gait initiation task in three experimental conditions: barefoot on a hard surface (barefoot condition), barefoot on a soft surface (foam condition), and shod on a hard surface (shod condition). Two force plates were used to acquire ground reaction forces and moments for each foot separately. A statistical parametric mapping (SPM) analysis was performed in COP time series. We compared the anterior-posterior (AP) and medial-lateral (ML) resultant center of pressure (COP) paths and average velocities, the force peaks under the right and left foot, and the COP integral x force impulse for three different phases: the anticipatory postural adjustment (APA) phase (Phase 1), the swing-foot unloading phase (Phase 2), and the support-foot unloading phase (Phase 3). In Phase 1, significantly smaller ML COP paths and velocities were found for the shod condition compared to the barefoot and foam conditions. Significantly smaller ML COP paths were also found in Phase 2 for the shod condition compared to the barefoot and foam conditions. In Phase 3, increased AP COP velocities were found for the shod condition compared to the barefoot and foam conditions. SPM analysis revealed significant differences for vector COP time series in the shod condition compared to the barefoot and foam conditions. The foam condition limited the impulse-generating capacity of COP shift and produced smaller ML force peaks, resulting in limitations to body-weight transfer from the swing to the support foot. The results suggest that footwear and a soft surface affect COP and impose certain features of gait initiation, especially in the ML direction of Phase 1.     

Center of pressure and center of mass behavior during gait initiation on inclined surfaces: A statistical parametric mapping analysis

This study analyzed gait initiation (GI) on inclined surfaces with 68 young adult subjects of both sexes. Ground reaction forces and moments were collected using two AMTI force platforms, of which one was in a horizontal position and the other was inclined by 8% in relation to the horizontal plane. Departing from a standing position, each participant executed three trials in the following conditions: horizontal position (HOR), inclined position at ankle dorsi-flexion (UP), and inclined position at ankle plantar-flexion (DOWN). Statistical parametric mapping analysis was performed over the entire center of pressure (COP) and center of mass (COM) time series. COP excursion did not show significant differences in the medial-lateral (ML) direction in both inclined conditions, but it was greater in the anterior-posterior (AP) direction for both inclined conditions. COP velocities are smaller in discrete portions of GI for the UP and DOWN conditions. COM displacement was greater in the ML direction during anticipatory postural adjustments (APA) in the UP condition, and COM moves faster in the ML direction during APA in the UP condition but slower at the end of GI for both the UP and the DOWN conditions. The COP-COM vector showed a greater angle in the DOWN condition. We observed changes for COP and COM in GI in both the UP and the DOWN conditions, with the latter showing changes for a great extent of the task. Both the UP and the DOWN conditions showed increased COM displacement and velocity. The predominant characteristic during GI on inclined surfaces, including APA, appears to be the displacement of the COM.     

Functional postural responses after perturbations in multiple directions in a standing man: a principle of decoupled control

The objective of this study was to assess functional postural responses by analyzing the net joint torques (NJT) in the ankles and the hips resulting from perturbations delivered in multiple directions to subjects standing quietly. A total of eight subjects were standing on two force platforms while an apparatus randomly delivered controlled perturbations at the level of the pelvis in eight directions: anterio-posterior (AP), medio-lateral (ML), and four combinations of these principal directions. Perturbations were repeated five times in each direction for six conditions (i.e., three different perturbation strengths and three different feet orientations). The comparison of the averaged ankle sum NJT (AP) responses showed that the time courses of the responses elicited by a perturbation acting only in the AP direction were identical to those elicited by a combination of two corresponding AP and ML perturbations. In contrast the observed averaged ankle NJT (ML) responses did not follow the same similarity. The comparison of the averaged ankle and hip sum NJT (ML) responses revealed that the time courses of the responses elicited by a perturbation acting only in the ML direction were identical to those elicited by a combination of two corresponding AP and ML perturbations. These findings were invariable of the experimental conditions and were consistent among all the eight subjects. Thereby, we conclude that the ankle sum NJT (AP) and the ankle and hip sum NJT (ML) are the global variables being controlled. This shows that CNS controls the recovery from the multiple direction perturbations of moderate strength by decoupling the AP-ML postural space into two orthogonal directions (AP and ML).     

Effects of window size on ankle joint stiffness calculation during quiet standing: How the rule changes the result

Measuring ankle joint stiffness (AJS) during quiet standing QS using an inverted pendulum model typically involves a single calculation covering the entire period of QS. This study compared AJS using the same 20.0s set of QS postural sway data but employing seven different calculation windows (0.25s, 0.5s, 1.0s, 2.0s, 5.0s, 10.0s and 20.0s). AJS was calculated for both anterio-posterior AP and medio-lateral ML directions of sway. Postural sway data from 19 subjects were used to calculate mean±SD and time-normalized AJS over the same 20s period of QS. Statistical power of this study was 0.99. The AJS had ICCs ranging from 0.47 to 0.85 with coefficient of variations ranging from 11.1% to 31.8%. There were significant differences in AJS between window sizes (P<0.0001) for both directions of sway. Specifically, AJS calculated by 1.0s windows was significantly larger (P<0.01) than others, except 0.5s, while the AJS of the largest two windows 10.0s and 20.0s were significantly smaller (P<0.01) than all others in both directions of sway. In conclusion, it is recommended that 1.0s windows be used to calculate AJS and that stiffness analyzed as a continuous signal offers a more complete picture of how AJS behaves during QS.     

Changes in postural sway and its fractions in conditions of postural instability

We investigated changes in postural sway and its fractions associated with manipulations of the dimensions of the support area. Nine healthy adults stood as quietly as possible, with their eyes open, on a force plate as well as on 5 boards with reduced support area. The center of pressure (COP) trajectory was computed and decomposed into rambling (Rm) and trembling (Tr) trajectories. Sway components were quantified using RMS (root mean square) value, average velocity, and sway area. During standing on the force plate, the RMS was larger for the anterior-posterior (AP) sway components than for the mediolateral (ML) components. During standing on boards with reduced support area, sway increased in both directions. The increase was more pronounced when standing on boards with a smaller support area. Changes in the larger dimension of the support area also affected sway, but not as much as changes in the smaller dimension. ML instability had larger effects on indices of sway compared to AP instability. The average velocity of Rm was larger while the average velocity of Tr was smaller in the AP direction vs. the ML direction. The findings can be interpreted within the hypothesis of an active search function of postural sway. During standing on boards with reduced support area, increased sway may by itself lead to loss of balance. The findings also corroborate the hypothesis of Duarte and Zatsiorsky that Rm and Tr reveal different postural control mechanisms.     

Development and validation of a subject-specific moving-axis tibiofemoral joint model using MRI and EOS imaging during a quasi-static lunge

The aims of this study were to introduce and validate a novel computationally-efficient subject-specific tibiofemoral joint model. Subjects performed a quasi-static lunge while micro-dose radiation bi-planar X-rays (EOS Imaging, Paris, France) were captured at roughly 0°, 20°, 45°, 60°, and 90° of tibiofemoral flexion. Joint translations and rotations were extracted from this experimental data through 2D-to-3D bone reconstructions, using an iterative closest point optimization technique, and employed during model calibration and validation. Subject-specific moving-axis and hinge models for comparisons were constructed in the AnyBody Modeling System (AMS) from Magnetic Resonance Imaging (MRI)-extracted anatomical surfaces and compared against the experimental data. The tibiofemoral axis of the hinge model was defined between the epicondyles while the moving-axis model was defined based on two tibiofemoral flexion angles (0° and 90°) and the articulation modeled such that the tibiofemoral joint axis moved linearly between these two positions as a function of the tibiofemoral flexion. Outside this range, the joint axis was assumed to remain stationary. Overall, the secondary joint kinematics (ML: medial–lateral, AP: anterior-posterior, SI: superior-inferior, IE: internal-external, AA: adduction-abduction) were better approximated by the moving-axis model with mean differences and standard errors of (ML: −1.98 ± 0.37 mm, AP: 6.50 ± 0.82 mm, SI: 0.05 ± 0.20 mm, IE: 0.59 ± 0.36°, AA: 1.90 ± 0.79°) and higher coefficients of determination (R²) for each clinical measure. While the hinge model achieved mean differences and standard errors of (ML: −0.84 ± 0.45 mm, AP: 10.11 ± 0.88 mm, SI: 0.66 ± 0.62 mm, IE: −3.17 ± 0.86°, AA: 11.60 ± 1.51°).     

Physical characterization of the therapeutic device Posturomed as a measuring device--presentation of a procedure to characterize balancing ability]

Training measures to improve neuromuscular coordination are becoming ever more popular for both prevention and rehabilitation, not only in athletes but also patients receiving joint replacements. Numerous proprioceptive training measures and devices are used to train the sense of balance. Parameters suitable for quantifying the results of therapy are largely lacking. Herein, a simple method for quantifying the balancing on one leg using the therapeutic device (Posturomed) commonly employed to train balance in the upright stance. The horizontal movements of the oscillating suspended platform were recorded in two orthogonal directions using a non-contact measurement system. To simulate disturbance of the upright stance, a mechanical deflection device was applied to the platform. The physical characterization of the measuring system was done using rigid masses. 13 volunteers adopting a one-legged stance were investigated. The measured displacement in the medio-lateral (ML) and AP directions were used to establish a balance index. Examination of the oscillatory behaviour of the platform revealed the path signal to be a suitable parameter for analyzing the platform movements. Differences in balance characteristics between AP and ML movements could be quantified. Frontal disturbance of the upright stance is equilibrated more effectively than lateral disturbances. Combined with a non-contact path measuring system the therapeutic device is suitable for characterizing balancing ability in an upright one-legged stance. To obtain more detailed information on the neuromuscular mechanisms involved, further studies are needed.     

Stooping,crouching, and standing – Characterizing balance control strategies across postures

BackgroundWhile stooping and crouching postures are critical for many activities of daily living, little is known about the balance control mechanisms employed during these postures. Accordingly, the purpose of this study was to characterize the mechanisms driving net center of pressure (COP_Net) movement across three postures (standing, stooping, and crouching) and to investigate if control in each posture was influenced by time.MethodsTen young adults performed the three postures for 60 s each. Kinetic signals were collected via a force platform under each foot. To quantify mechanisms of control, correlations (Correl_LR) were calculated between the left and right COP trajectories in the anterior-posterior (AP) and medio-lateral (ML) directions. To examine the potential effects of time on balance control strategies, outcomes during the first 30 s were compared to the last 30 s.ResultsCorrel_LR values did not differ across postures (AP: p = 0.395; ML: p = 0.647). Further, there were no main effects of time on Correl_LR (AP: p = 0.976; ML: p = 0.105). A significant posture-time interaction was observed in the ML direction (p = 0.045) characterized by 35% decreases in Correl_LR over time for stooping (p = 0.022).ConclusionThe dominant controllers of sway (i.e., AP: ankle plantar/dorsi flexors; ML: hip load/unload mechanism) are similar across quiet stance stooping, and crouching. Changes in ML control strategies over time suggests that fatigue could affect prolonged stooping more so than crouching or standing.     

Gender‐specific Effect of Obesity on Balance

Obesity modifies the body geometry by adding mass to different regions and it influences the biomechanics of activities of daily living. Weight influences postural stability, but there is no consensus as to whether the different fat distribution in males and females produces gender‐related effects on balance. The aim of this study was to investigate the effect of body weight increases on postural performance in males and females. A total of 22 obese females (BMI: 41.1 ± 4.1 kg/m²) and 22 obese males (BMI: 40.2 ± 5 kg/m²) were analyzed during a static posture trial on a force platform in standardized conditions. Twenty healthy subjects (10 females, 10 males) constituted the control group. We computed the following parameters related to the center of pressure (CoP): velocity and displacements along the antero‐posterior (AP) and medio‐lateral axis (ML). We found several statistically significant differences between healthy and obese men, in particular regarding the AP and ML CoP parameters, which were correlated to body weight (r = 0.36–0.58). The comparison between healthy and obese females pointed out statistically significant differences in AP parameters and no significant differences in ML displacements. Body weight was found to correlate with AP parameters (r = 0.36–0.74), but not with ML displacements. The increased body mass seems to produce AP instability in both genders and ML destabilization only in males. Rehabilitation programs should take these findings into account by adopting specific interventions to improve ML control in obese males, and through weight loss and strengthening of ankle flexors/extensors in both genders.     

Investigation of balance strategy over gait cycle based on margin of stability

This study was conducted to investigate the balance strategy of healthy young adults through a gait cycle using the margin of stability (MoS). Thirty healthy young adults participated in this study. Each performed walking five times at a preferred speed and at a fast speed. The MoS was calculated over a gait cycle by defining the base of support (BoS) changes during a gait cycle. The MoS was divided into medial/lateral and anterior/posterior components (ML MoS and AP MoS). The central values and the values at 12 gait events of the MoS were compared. Positive/negative integration of ML MoS (ML MoS_POS and ML MoS_NEG, respectively) and the average ML/AP MoS over a cycle (ML/AP MoS_mean) were significantly lower at a fast gait than at a preferred gait. ML/AP MoS were lower at a fast speed than at the preferred speed, except for the ML MoS immediately before left heel strike (pre left HS) and right and left heel strike (HS). ML/AP MoS were significantly lower immediately before heel strike (pre-HS) than in other gait events, regardless of walking speed. It was suggested that pre-HS is the most unstable moment in both ML/AP directions and a crucial moment in control of gait stability. The results presented above might be applicable as basic data regarding dynamic stability of healthy young adults through a gait cycle for comparisons with elderly people and patients with orthopedic disorders or neurological disorders.     

Using Low Levels of Stochastic Vestibular Stimulation to Improve Balance Function

Low-level stochastic vestibular stimulation (SVS) has been associated with improved postural responses in the medio-lateral (ML) direction, but its effect in improving balance function in both the ML and anterior-posterior (AP) directions has not been studied. In this series of studies, the efficacy of applying low amplitude SVS in 0–30 Hz range between the mastoids in the ML direction on improving cross-planar balance function was investigated. Forty-five (45) subjects stood on a compliant surface with their eyes closed and were instructed to maintain a stable upright stance. Measures of stability of the head, trunk, and whole body were quantified in ML, AP and combined APML directions. Results show that binaural bipolar SVS given in the ML direction significantly improved balance performance with the peak of optimal stimulus amplitude predominantly in the range of 100–500 μA for all the three directions, exhibiting stochastic resonance (SR) phenomenon. Objective perceptual and body motion thresholds as estimates of internal noise while subjects sat on a chair with their eyes closed and were given 1 Hz bipolar binaural sinusoidal electrical stimuli were also measured. In general, there was no significant difference between estimates of perceptual and body motion thresholds. The average optimal SVS amplitude that improved balance performance (peak SVS amplitude normalized to perceptual threshold) was estimated to be 46% in ML, 53% in AP, and 50% in APML directions. A miniature patch-type SVS device may be useful to improve balance function in people with disabilities due to aging, Parkinson’s disease or in astronauts returning from long-duration space flight.     

Modelling 3D control of upright stance using an optimal control strategy

A 3D balance control model of quiet upright stance is presented, based on an optimal control strategy, and evaluated in terms of its ability to simulate postural sway in both the anterior–posterior and medial–lateral directions. The human body was represented as a two-segment inverted pendulum. Several assumptions were made to linearise body dynamics, for example, that there was no transverse rotation during upright stance. The neural controller was presumed to be an optimal controller that generates ankle control torque and hip control torque according to certain performance criteria. An optimisation procedure was used to determine the values of unspecified model parameters including random disturbance gains and sensory delay times. This model was used to simulate postural sway behaviours characterised by centre-of-pressure (COP)-based measures. Confidence intervals for all normalised COP-based measures contained unity, indicating no significant differences between any of the simulated COP-based measures and corresponding experimental references. In addition, mean normalised errors for the traditional measures were < 8%, and those for most statistical mechanics measures were ～3–66%. On the basis these results, the proposed 3D balance control model appears to have the ability to accurately simulate 3D postural sway behaviours.     

How experienced alpine-skiers cope with restrictions of ankle degrees-of-freedom when wearing ski-boots in postural exercises

The present study investigates the mechanisms underlying changes in postural strategy that occur to compensate for mechanical ankle joint restrictions induced by wearing ski-boots during postural exercises. Fourteen experienced skiers were asked to stand as still as possible in a stable (STA) posture and in 2 postures with instability in the medio/lateral and antero/posterior (ML and AP postures) direction. Postural tasks were performed with eyes open or closed and while wearing or not wearing ski-boots. The electromyographic (EMG) activity of representative lower limb muscles and positions of centre-of-foot pressure (COP) were recorded and analyzed. Our results illustrated enhanced postural performances with ski-boots in the STA posture, whereas postural performances remained unchanged when wearing ski-boots in the ML and AP postures. Analysis of COP sways in the frequency domain did not illustrate any modification in the contribution of different neuronal loops when the study subjects wore ski-boots. EMG showed that the mechanical effects of wearing ski-boots were compensated by changes in postural strategy through the reorganization of muscle coordination, made possible by inherent redundancies in the human body. The preservation of postural performances, despite restrictions of ankle degrees-of-freedom induced by ski-boots, emphasizes the subjects’ capacity to exploit the additional support provided by ski-boots by adequately adjusting muscle coordination to control posture in different balance conditions.