Effect of ageing and vision on limb load asymmetry during quiet stance

Although the identification and characterization of limb load asymmetries during quiet standing has not received much research attention, they may greatly extend our understanding of the upright stance stability control. It seems that the limb load asymmetry factor may serve as a veridical measure of postural stability and thus it can be used for early diagnostic of the age-related decline in balance control. The effects of ageing and of vision on limb load asymmetry (LLA) during quiet stance were studied in 43 healthy subjects (22 elderly, mean age 72.3+/-4.0 yr, and 21 young, mean age 23.9+/-4.8 yr). Postural sway and body weight distribution were recorded while the subject was standing on two adjacent force platforms during two 120 s trials: one trial was performed with the eyes open (EO), while the other trial was with the eyes closed (EC). The results indicate that LLA was greater in the old adults when compared with the young control subjects. The LLA values were correlated with the postural sway magnitudes especially in the anteroposterior direction. Eyes closure which destabilized posture resulted in a significant increase of body weight distribution asymmetry in the elderly but not in the young persons. The limb load difference between EO and EC conditions showed a significantly greater effect of vision on LLA in the elderly compared to the young subjects. The observed differences in the LLA may be attributed to the decline of postural stability control in the elderly. Ageing results in the progressive decline of postural control and usually the nervous system requires more time to complete a balance recovery action. To compensate for such a deficiency, different compensatory strategies are developed. One of them, as evidenced in our study, is preparatory limb unload strategy (a stance asymmetry strategy) which could significantly shorten reaction time in balance recovery.     

Postural changes after sustained neck muscle contraction in persons with a lower leg amputation

Lower leg amputation generally induces asymmetrical weight-bearing, even after rehabilitation treatment is completed. This is detrimental to the amputees’ long term quality of life. In particular, increasing strains on joint surfaces that receive additional weight load causes back and leg pain, premature wear and tear and arthritis. This pilot study was designed to determine whether subjects with lower leg amputation experience postural post-effects after muscle contraction, a phenomenon already observed in healthy subjects, and whether this could improve the weight-bearing on their prosthesis.Fifteen subjects with a unilateral lower leg amputation and 17 control subjects volunteered to participate in this study. Centre of pressure (CP) position was recorded during standing posture, under eyes closed and open conditions. Recordings were carried out before the subjects performed a 30-s voluntary isometric lateral neck muscle contraction, and again 1 and 4 min after the contraction.Postural post-effects characterized by CP shift, occurred in the medio-lateral plane in the majority of the amputated (7/15 eyes closed, 9/15 eyes open) and control (9/17 eyes closed, 11/17 eyes open) subjects after the contraction. Half of these subjects had a CP shift towards the side of the contraction and the other half towards the opposite side. In four amputated subjects tested 3 months apart, shift direction remained constant. These postural changes occurred without increase in CP velocity.Thus, a 30-s voluntary isometric contraction can change the standing posture of persons with lower leg amputation. The post-effects might result from the adaptation of the postural frame of reference to the proprioceptive messages associated with the isometric contraction.     

Age-related changes of human balance during quiet stance

Certain aspects of balance control change with age, resulting in a slight postural instability. We examined healthy subjects between 20-82 years of age during the quiet stance under static conditions: at stance on a firm surface and/or on a compliant surface with eyes either open or closed. Body sway was evaluated from centre of foot pressure (CoP) positions during a 50 sec interval. The seven CoP parameters were evaluated to assess quiet stance and were analyzed in three age groups: juniors, middle-aged and seniors. The regression analysis showed evident increase of body sway over 60 years of age. We found that CoP parameters were significantly different when comparing juniors and seniors in all static conditions. The most sensitive view on postural steadiness during quiet stance was provided by CoP amplitude and velocity in AP direction and root mean square (RMS) of statokinesigram. New physiological ranges of RMS parameter in each condition for each age group of healthy subjects were determined. Our results showed that CoP data from force platform in quiet stance may indicate small balance impairment due to age. The determined physiological ranges of RMS will be useful for better distinguishing between small postural instability due to aging in contrast to pathological processes in the human postural control.     

Assessment of influence of breath holding and hyperventilation on human postural stability with spectral analysis of stabilographic signal

The influence of breath holding and voluntary hyperventilation on the traditional stabilometric parameters and the frequency characteristics of stabilographic signal was studied. We measured the stabilometric parameters on a force platform (“Ritm”, Russia) in the 107 healthy volunteers during quiet breath, voluntary hyperventilation (20 seconds) and maximal inspiratory breath holding (20 seconds). Respiratory frequency, respiratory amplitude and ventilation were estimated with the strain gauge. We found that antero-posterior and medio-lateral sway amplitude and velocity as well as sway surface during breath holding and during quiet breathing were the same, so breath holding didn’t influence the postural stability. However, the spectral parameters in the antero-posterior direction shifted to the high frequency range due to an alteration of the respiratory muscles’ contractions during breath holding versus quiet breath. Voluntary hyperventilation caused a significant increase of all stabilographic indices that implied an impairment of the postural stability. We also found that the spectral indices shifted toward the high-frequency range, and this shift was much greater compared to that during breath holding. Besides, amplitudes of the spectral peaks also increased. Perhaps, such change of the spectral indices was due to distortion of the proprioceptive information because of increased excitability of the nerve fibers during hyperventilation. Maximal inspiratory breath holding caused an activation of the postural control mechanisms. It was manifested as an elevation of the sway oscillations’ frequency with no postural stability changes. Hyperventilation led to the greatest strain of the postural control and to a decrease of the postural stability, which was manifested as an increase of center of pressure oscillations’ amplitude and frequency.     

Small,movement dependent perturbations substantially alter postural control strategy in healthy young adults

Postural control is commonly investigated by observing responses to perturbations. We developed a perturbation paradigm mimicking self-generated errors in weight shifting, which are a common cause of falling among older adults. Our aim was to determine the effects of this small, but complex, perturbation on postural sway of healthy young adults and evaluate the role of vision and cognition during movement dependent perturbations. Fifteen participants stood hip-width apart with their eyes open, closed and while performing two different cognitive tasks. Participants were continuously perturbed by medial-lateral (ML) support surface translations corresponding to, and hence doubling, their own center of mass sway. We analyzed the standard deviation (SD), root mean square (RMS), range, and mean power frequency (MPF) of center of pressure displacements. ML postural sway increased due to the perturbation (SD p ≤ .001, range p < .001, RMS p ≤ .001, MPF p < .001). Cognitive load increased the ML sway range (p = .048). Lack of vision increased ML MPF (p = .001) and anterior-posterior (AP) range (p < .001), SD (p < .001), and RMS (p = .001). Significant interaction of vision with the perturbation was found for the ML range (p = .045) and AP SD (p = .018). The perturbation specifically affected ML postural sway. Increased MPF is indicative of a postural control strategy change, which was insufficient for fully controlling the increased sway. Despite being small, this type of perturbation appears to be challenging for young adults.     

Low-frequency force steadiness practice in plantar flexor muscle reduces postural sway during quiet standing

The purpose of this study was to assess the effect of low-frequency force steadiness practice in the plantar flexor muscles on postural sway during quiet standing. Healthy young 21 men (21±1 yrs) were randomly assigned to a practice group (n=14) and a nonexercising control group (n=7). Practice groups were divided by frequency of practice: 7 participants practiced once a week, and the other 7 twice a week, for 4 weeks. Steadiness practice required practice group to 5 sets of 60-s contraction at levels corresponding to 10% and 20% maximal voluntary contraction (MVC) in the plantar flexor muscles. The 4-week-long practice period reduced the force fluctuations (assessed as the standard deviation (SD) of the outputted force during steady isometric plantar flexion) and postural sway (assessed as SD of the center of mass velocity during quiet standing). However, these practice effects were not significantly affected by the practice frequencies (1 vs. 2 sessions per week) examined in this study. Further, a linear regression analysis revealed the association between prepractice postural sway and the relative change in postural sway by the practice (r=-0.904) in the practice group. These results suggest that the steadiness practice in plantar flexor muscles improves postural stability during quiet standing, even though the practice is low-frequency (once a week) and low-intensity (within 20% MVC). These practice effects are dependent on prepractice postural stability. Further, the present results have provided the functional significance of force fluctuation in lower limb muscles.     

Characteristic Features of Disorders of the Upright Posture in Patients with Poststroke Hemipareses

Characteristic features of upright posture maintenance and mechanisms of postural disorders in poststroke hemiparetic patients were studied using a bilateral force platform. The following features of postural disorders were revealed in the patients tested: an increase in the velocity and amplitude of the center-of-pressure (CP) sway as compared to in healthy subjects, an absolute decrease in the half-cycles of the CP sway, asymmetry of weight bearing by both feet, and a shift of the center of pressure of an affected foot towards the toe. The disturbance of stability of the vertical posture in such patients is to a greater extent associated with weight-bearing asymmetry. It was shown that the character of the CP sway is mainly determined by a disorder of the sensory motor control, whereas damage to the efferent pathways is responsible for the postural asymmetry. Increase in the muscle tone restricts the sway amplitude. Thus, several forms of postural instability are characteristic of hemiparetic patients. Predominantly sensory, motor, or tonic disorders are responsible for these disturbances of stability.     

Chaos in Balance: Non-Linear Measures of Postural Control Predict Individual Variations in Visual Illusions of Motion

Visually-induced illusions of self-motion (vection) can be compelling for some people, but they are subject to large individual variations in strength. Do these variations depend, at least in part, on the extent to which people rely on vision to maintain their postural stability? We investigated by comparing physical posture measures to subjective vection ratings. Using a Bertec balance plate in a brightly-lit room, we measured 13 participants'' excursions of the centre of foot pressure (CoP) over a 60-second period with eyes open and with eyes closed during quiet stance. Subsequently, we collected vection strength ratings for large optic flow displays while seated, using both verbal ratings and online throttle measures. We also collected measures of postural sway (changes in anterior-posterior CoP) in response to the same visual motion stimuli while standing on the plate. The magnitude of standing sway in response to expanding optic flow (in comparison to blank fixation periods) was predictive of both verbal and throttle measures for seated vection. In addition, the ratio between eyes-open and eyes-closed CoP excursions during quiet stance (using the area of postural sway) significantly predicted seated vection for both measures. Interestingly, these relationships were weaker for contracting optic flow displays, though these produced both stronger vection and more sway. Next we used a non-linear analysis (recurrence quantification analysis, RQA) of the fluctuations in anterior-posterior position during quiet stance (both with eyes closed and eyes open); this was a much stronger predictor of seated vection for both expanding and contracting stimuli. Given the complex multisensory integration involved in postural control, our study adds to the growing evidence that non-linear measures drawn from complexity theory may provide a more informative measure of postural sway than the conventional linear measures.     

Biomechanical evaluation of the relationship between postural control and body mass index

Postural stability is crucial in maintaining body balance during quiet standing, locomotion, and any activities that require a high degree of balance performance, such as participating in sports and dancing. Research has shown that there is a relationship between stability and body mass. The aims of this study were to examine the impact that two variables had on static postural control: body mass index (BMI) and gender. Eighty healthy young adults (age=21.7±1.8 yr; height=1.65±0.09 m; mass=67.5±19.0 kg) participated in the study and the static postural control was assessed using the Biodex Balance System, with a 20 Hz sampling rate in the bipedic stance (BLS) and unipedic stance (ULS) for 30s. Five test evaluations were performed for each balance test. Postural control was found to be negatively correlated with increased adiposity, as the obese BMI group performed significantly poorer than the underweight, normal weight and overweight groups during BLS and ULS tests. The underweight, normal weight and overweight groups exhibited greater anterior-posterior stability in postural control during quiet stance. In addition, female displayed a trend of having a greater postural sway than male young adults, although it was evidenced in only some BMI groups. This study revealed that BMI do have an impact on postural control during both BLS and ULS. As such, BMI and gender-specific effects should be taken into consideration when selecting individuals for different types of sporting activities, especially those that require quiet standing.     

Postural Stability in Young Adults with Down Syndrome in Challenging Conditions

To evaluate postural control and performance in subjects with Down syndrome (SwDS), we measured postural sway (COP) in quiet stance in four 20-second tests: with eyes open or closed and on hard or foam surface. Ten SwDS and eleven healthy subjects participated, aged 29.8 (4.8) and 28.4 (3.9), respectively. The time-series recorded with the sampling rate of 100 Hz were used to evaluate postural performance (COP amplitude and mean velocity) and strategies (COP frequency, fractal dimension and entropy). There were no intergroup differences in the amplitude except the stance on foam pad with eyes open when SwDS had larger sway. The COP velocity and frequency were larger in SwDS than controls in all trials on foam pad. During stances on the foam pad SwDS increased fractal dimension showing higher complexity of their equilibrium system, while controls decreased sample entropy exhibiting more conscious control of posture in comparison to the stances on hard support surface. This indicated that each group used entirely different adjustments of postural strategies to the somatosensory challenge. It is proposed that the inferior postural control of SwDS results mainly from insufficient experience in dealing with unpredictable postural stimuli and deficit in motor learning.     

The influence of adipose tissue location on postural control

A growing body of evidence suggests, that excessive body weight is inseparably connected with postural instability. In none of previous studies, body weight distribution has been considered as a factor, which may affect results of a static posturography. The purpose of the present study is to quantify some center of foot pressure (COP) characteristics in 40 obese women with android type of obesity (waist-to-hip ratio - WHR ≥ 0.85, BMI: 37.5 ± 5.4) and 40 obese women with gynoid type of obesity (WHR < 0.85, BMI: 36.9 ± 5.1). Variables of postural sway were acquired while subjects were standing quietly on a force plate with eyes open and closed. Both in the sagittal and frontal plane sway range, average velocity, and maximal velocity of COP were calculated. Moreover, the total average velocity and total maximal velocity of the COP displacement were computed.Women with abdominal obesity showed a larger sway range in the anterior-posterior plane with eyes open (p < 0.0282) and eyes closed conditions (p < 0.0115) and a greater maximal COP velocity to compare with subjects with gynoidal obese type (p < 0.0112) with eyes closed condition.The postural stability in obese women from the biomechanical point of view is strongly dependent on body distribution. Women with the abdominal obesity type may be exposed to a greater risk of postural instability as compare to women with gynoid fat distribution.     

Effects of two passive back-support exoskeletons on postural balance during quiet stance and functional limits of stability

While occupational back-support exoskeletons (BSEs) are considered as potential workplace interventions, BSE use may compromise postural control. Thus, we investigated the effects of passive BSEs on postural balance during quiet upright stance and functional limits of stability. Twenty healthy adults completed trials of quiet upright stance with differing levels of difficulty (bipedal and unipedal stance; each with eyes open and closed), and executed maximal voluntary leans. Trials were done while wearing two different BSEs (SuitX™, Laevo™) and in a control (no-BSE) condition. BSE use significantly increased center-of-pressure (COP) median frequency and mean velocity during bipedal stance. In unipedal stance, using the Laevo™ was associated with a significant improvement in postural balance, especially among males, as indicated by smaller COP displacement and sway area, and a longer time to contact the stability boundary. BSE use may affect postural balance, through translation of the human + BSE center-of-mass, restricted motion, and added supportive torques. Furthermore, larger effects of BSEs on postural balance were evident among males. Future work should further investigate the gender-specificity of BSE effects on postural balance and consider the effects of BSEs on dynamic stability.     

Physiological complexity and system adaptability: evidence from postural control dynamics of older adults

The degree of multiscale complexity in human behavioral regulation, such as that required for postural control, appears to decrease with advanced aging or disease. To help delineate causes and functional consequences of complexity loss, we examined the effects of visual and somatosensory impairment on the complexity of postural sway during quiet standing and its relationship to postural adaptation to cognitive dual tasking. Participants of the MOBILIZE Boston Study were classified into mutually exclusive groups: controls [intact vision and foot somatosensation, n = 299, 76 ± 5 (SD) yr old], visual impairment only (<20/40 vision, n = 81, 77 ± 4 yr old), somatosensory impairment only (inability to perceive 5.07 monofilament on plantar halluxes, n = 48, 80 ± 5 yr old), and combined impairments (n = 25, 80 ± 4 yr old). Postural sway (i.e., center-of-pressure) dynamics were assessed during quiet standing and cognitive dual tasking, and a complexity index was quantified using multiscale entropy analysis. Postural sway speed and area, which did not correlate with complexity, were also computed. During quiet standing, the complexity index (mean ± SD) was highest in controls (9.5 ± 1.2) and successively lower in the visual (9.1 ± 1.1), somatosensory (8.6 ± 1.6), and combined (7.8 ± 1.3) impairment groups (P = 0.001). Dual tasking resulted in increased sway speed and area but reduced complexity (P < 0.01). Lower complexity during quiet standing correlated with greater absolute (R = -0.34, P = 0.002) and percent (R = -0.45, P < 0.001) increases in postural sway speed from quiet standing to dual-tasking conditions. Sensory impairments contributed to decreased postural sway complexity, which reflected reduced adaptive capacity of the postural control system. Relatively low baseline complexity may, therefore, indicate control systems that are more vulnerable to cognitive and other stressors.     

Vertical heterophoria and postural control in nonspecific chronic low back pain

The purpose of this study was to test postural control during quiet standing in nonspecific chronic low back pain (LBP) subjects with vertical heterophoria (VH) before and after cancellation of VH; also to compare with healthy subjects with, and without VH. Fourteen subjects with LBP took part in this study. The postural performance was measured through the center of pressure displacements with a force platform while the subjects fixated on a target placed at either 40 or 200 cm, before and after VH cancellation with an appropriate prism. Their postural performance was compared to that of 14 healthy subjects with VH and 12 without VH (i.e. vertical orthophoria) studied previously in similar conditions. For LBP subjects, cancellation of VH with a prism improved postural performance. With respect to control subjects (with or without VH), the variance of speed of the center of pressure was higher, suggesting more energy was needed to stabilize their posture in quiet upright stance. Similarly to controls, LBP subjects showed higher postural sway when they were looking at a target at a far distance than at a close distance. The most important finding is that LBP subjects with VH can improve their performance after prism-cancellation of their VH. We suggest that VH reflects mild conflict between sensory and motor inputs involved in postural control i.e. a non optimal integration of the various signals. This could affect the performance of postural control and perhaps lead to pain. Nonspecific chronic back pain may results from such prolonged conflict.     

Wearing shoes increasing dorsiflexion improves short-term but not long-term balance control in young healthy adults

Wearing sport shoes inducing ankle dorsiflexion has been shown to alter the biomechanical specificities of the stretched muscles. The possible effect over the short and long term upon the sensorial capacities induced by such stretching has not been addressed yet. Fourteen healthy individuals were involved to assess the proprioceptive repercussion and their effects upon postural control strategies. Postural control and proprioceptive assessment were measured twice: when receiving sport shoes inducing ankle dorsiflexion and 18 days later. Proprioceptive effects were assessed using an ad-hoc device through which the seated and blindfolded subjects were required to reposition their feet in a starting position after the ankles were passively displaced to dorsiflexed and plantarflexed positions. Center-of-gravity horizontal displacements (CG_v), estimated from center-of-pressure (CP) displacements, and CP–CG_v displacements were measured through a force platform during upright quiet stance maintenance. The initial session was recorded with the subjects barefoot and wearing the shoes with a set of chocks with 0° (horizontal) and −5° (dorsiflexion) tilting angles. The second session included only barefoot performance in horizontal and dorsiflexion conditions. Dorsiflexion had no immediate effect on the postural control strategies along the anteroposterior axis. In contrast, barefoot or wearing shoes, stability was increased along the mediolateral axis during the dorsiflexion conditions. No ankle proprioceptive or postural change was observed after wearing the shoes for 18 days. Wearing dorsiflexion sport shoes induces short-term effects probably by inducing a backward tilt of the pelvis. A muscular adaptation likely prevents this effect from being prolonged.     

Undisturbed stance on a double seesaw: Interaction between asymmetries of the center-of-pressure patterns under each foot and weight-bearing

Both center-of-pressure (CP) displacements under each foot and relative body-weight distribution intervene in the production of resultant CP movements. To better understand their respective involvement, a protocol was set up for young healthy individuals consisting in standing on a double seesaw, favoring pitch motions and laying on a dual-force platform. The postural control effects induced by two types of asymmetry, weight-bearing and the CP movement patterns, were investigated. These asymmetries were achieved by associating two seesaws with two different lengths for the radii of the ridges and by requiring specific body-weight distributions. The results indicate that the postural strategies, aimed at controlling anteroposterior sway, are related to the subjects’ capacity to minimize the CP displacements under the less stable support, whatever load is applied. In contrast, the degree of involvement of the more stable support must be viewed as a complement used to secure the appropriate motor output, i.e., the resultant CP movements. Within this objective, both the applied load and the CP amplitudes under the more stable support are taken into account. These data provide additional insights into the compensatory mechanisms between the interactions between the two feet, which are used to produce the adequate resultant CP movements and therefore upright stance control. The specificity of the double seesaw that can induce asymmetric CP patterns and/or asymmetric body-weight distribution makes it a legitimate contender to be used as a rehabilitation device for patients with neurological and/or traumatic diseases.     

Human postural response to lower leg muscle vibration of different duration

Body lean response to bilateral vibrations of soleus muscles were investigated in order to understand the influence of proprioceptive input from lower leg in human stance control. Proprioceptive stimulation was applied to 17 healthy subjects by two vibrators placed on the soleus muscles. Frequency and amplitude of vibration were 60 Hz and 1 mm, respectively. Vibration was applied after a 30 s of baseline. The vibration duration of 10, 20, 30 s respectively was used with following 30 s rest. Subjects stood on the force platform with eyes closed. Postural responses were characterized by center of pressure (CoP) displacements in the anterior-posterior (AP) direction. The CoP-AP shifts as well as their amplitudes and velocities were analyzed before, during and after vibration. Vibration of soleus muscles gradually increased backward body tilts. There was a clear dependence of the magnitude of final CoP shift on the duration of vibration. The amplitude and velocity of body sway increased during vibration and amplitude was significantly modulated by duration of vibration as well. Comparison of amplitude and velocity of body sway before and after vibration showed significant post-effects. Presented findings showed that somatosensory stimulation has a long-term, direction-specific influence on the control of postural orientation during stance. Further, the proprioceptive input altered by soleus muscles vibration showed significant changes in postural equilibrium during period of vibration with interesting post-effects also.     

Stance Postural Strategies in Patients with Chronic Inflammatory Demyelinating Polyradiculoneuropathy

Introduction

Polyneuropathy leads to postural instability and an increased risk of falling. We investigated how impaired motor impairment and proprioceptive input due to neuropathy influences postural strategies.

Methods

Platformless bisegmental posturography data were recorded in healthy subjects and patients with chronic inflammatory demyelinating polyradiculoneuropathy (CIDP). Each subject stood on the floor, wore a head and a hip electromagnetic tracker. Sway amplitude and velocity were recorded and the mean direction difference (MDD) in the velocity vector between trackers was calculated as a flexibility index.

Results

Head and hip postural sway increased more in patients with CIDP than in healthy controls. MDD values reflecting hip strategies also increased more in patients than in controls. In the eyes closed condition MDD values in healthy subjects decreased but in patients remained unchanged.

Discussion

Sensori-motor impairment changes the balance between postural strategies that patients adopt to maintain upright quiet stance. Motor impairment leads to hip postural strategy overweight (eyes open), and prevents strategy re-balancing when the sensory context predominantly relies on proprioceptive input (eyes closed).     

Sway-dependent modulation of the triceps surae H-reflex during standing.

Previous research has shown that changes in spinal excitability occur during the postural sway of quiet standing. In the present study, it was of interest to examine the independent effects of sway position and sway direction on the efficacy of the triceps surae Ia pathway, as reflected by the Hoffman (H)-reflex amplitude, during standing. Eighteen participants, tested under two different experimental protocols, stood quietly on a force platform. Percutaneous electrical stimulation was applied to the posterior tibial nerve when the position and direction of anteroposterior (A-P) center of pressure (COP) signal satisfied the criteria for the various experimental conditions. It was found that, regardless of sway position, a larger amplitude of the triceps surae H-reflex (difference of 9-14%; P = 0.005) occurred when subjects were swaying in the forward compared with the backward direction. The effects of sway position, independent of the sway direction, on spinal excitability exhibited a trend (P = 0.075), with an 8.9 +/- 3.7% increase in the H-reflex amplitude occurring when subjects were in a more forward position. The observed changes to the efficacy of the Ia pathway cannot be attributed to changes in stimulus intensity, as indicated by a constant M-wave amplitude, or to the small changes in the level of background electromyographic activity. One explanation for the changes in reflex excitability with respect to the postural sway of standing is that the neural modulation may be related to the small lengthening and shortening contractions occurring in the muscles of the triceps surae.