Posturographic Standards for Optimal Control of Human Standing Posture.

Static posturography is a simple non-invasive technique commonly used in contemporary labs and clinics to quantify the central nervous system adaptive mechanisms involved in the control of posture and balance. Its diagnostic value, however, is quite limited due to the lack of posturographic standards for the stable posture. To solve this problem, in this research, we aimed to establish reference values for the stable human posture using our novel parameters of static posturography including the sway anteroposterior directional index (DIAP), the mediolateral directional index (DIML), the stability vector amplitude (SVamp), and the stability vector azimuth (SVaz). Towards this end, in a population of young (mean age 22 yrs), healthy able-bodied volunteers (50 males and 50 females), trajectories of postural sway, based upon the center-of-pressure (COP), were assessed. The experiment consisted of ten 60 s trials that were carried out 5 times while subjects were standing quietly on the force plate with eyes open (EO test) and 5 times with eyes closed (EC test). Results showed that in young healthy subjects, regardless of gender, the basic variables of COP remained at the following levels: SVamp = 9.2 ± 1.6 mm/s, SVaz = 0.9 ± 0.1 rad, and directional indices DIAP = 0.7 ± 0.05, DIML = 0.56 ± 0.06. Some of the measures were sensitive to visual input (EC trials), and showed a weak to moderate correlation with anthropometric features. These measures can be recommended as reference values that characterize the most stable erect posture.

New methods of posturographic data analysis may improve the diagnostic value of static posturography in multiple sclerosis.

BACKGROUND: Early and accurate diagnosis of multiple sclerosis (MS) is crucial for its effective treatment. In MS diagnostic, neuronal networks that control posture and movement are of particular importance, which performance can be assessed using static posturography. Unfortunately, most of the commercially available posturographic platforms are not equipped with the appropriate procedures. METHODS: To solve this problem, the postural sway trajectories have been recorded in 55 MS patients while standing quiet with eyes open (EO), and then with eyes closed (EC). The trajectories were analyzed using our novel methods of postural sway parametrization, including sway stability vector (SV), anteroposterior and mediolateral sway indices (DIAP and DIML). RESULTS: The results exhibited unique postural sway patterns that may be attributed to MS. Our novel parametrization methods of postural sway showed pathology specific increase of the postural sway velocity in EC tests. Additionally, we documented the abnormal alterations of the anteroposterior (AP) and the mediolateral (ML) sway indices that were also uniquely dependent on visual input. In EC tests, patients exhibited a characteristic pattern of sway increase in both AP and ML directions that correlated with the advance of the disease as measured by the EDSS Kurtzke scale and Functional System Scores. CONCLUSIONS: The applied in the present study our novel posturographic metrics give the assessment a diagnostic value. It allows us to recommend the static posturography test as a simple and safe supplementary clinical tool in the diagnosis of MS. In the assessment of MS pathology or the effects of its treatment, the impact of vision on the sway stability vector seems the most important factor.

Maturation of the postural control in adolescent girls: A 3-year follow-up study.

BACKGROUND: Stable posture is a manifestation of the appropriate functioning of the neuromuscular system that is essential for proper motor development and control. Balance and stability of the erect posture are shaped during the entire childhood to culminate in its full efficiency in adolescent subjects. METHODS: In this 3-year follow-up study, the process of the postural control maturation has been assessed in a group of 18 girls at the transition period between childhood to adolescence. Their balance and postural stability control were assessed using standard static posturography supplemented by two postural stability tests: the rising-on-toes (ROT), and the maximum forward lean (MFL), all performed with (EO) and without vision (EC). Balance control was analyzed with the sway vector (SV) and sway directional indices, whereas the anteroposterior trajectories of the center-of- pressure (COP) during forward-leaning and the raise-on-toes tests were used to determine changes in postural stability control. RESULTS: The study documented that stability control in girls aged 11-13 is shaped according to their own pace of development. Their postural sway was characterized by the lower COP velocity but very sensitive to visual input. The directional sway measures remained at the same level for the entire period of observation. MFL and ROT tests provided similar information on postural stability and its dependence on visual input. These tests allow for more thorough assessment of postural stability to compare with quiet stance testing. SIGNIFICANCE: Subtle changes in postural control in adolescents could be assessed based on the results of combined static and dynamic tests. In particular, the ROT test can be recommended for the assessment of postural stability.

Step Response of Human Motor System as a Measure of Postural Stability in Children.

Postural sway is a product of the neuromuscular system that is commonly used in contemporary labs and clinics for the assessment of postural stability. In this study, we analyzed the transient responses of the neuromuscular system during the rise-on-toes (ROT) movement in eighteen 11 yrs old girls. Their center of pressure (COP) trajectories were recorded with standard force-platform during the transition from quiet stance to standing on toes. To assess the robustness of children's postural stability, we compared the ROT trajectories while the movement was performed with and without vision. Our results confirmed that the dynamic characteristics of the COP step response were significantly modified by visual feedback. In particular, the ROT test performed with eyes closed (EC) was characterized by a four-fold increase of COP chaotic oscillations at the target (tiptoe) position. This resulted in a substantial increase in the movement's index of difficulty (ID) thus to achieve adequate accuracy of the target-oriented movement the COP velocity was decreased accordingly. This inherent strategy of the brain controller allowed for precise positioning of the COP within the reduced size of the target. In conclusion, the dynamics of the ROT movement is always precisely adjusted to the stability of the upright posture, and thus, the dynamic characteristics of the COP step response are also sensitive measures of postural stability and the ROT can be recommended as a useful test for this assessment in the general population.

Trunk forward flexion mobility in reference to postural sway in women after delivery: A prospective longitudinal comparison between early pregnancy and 2- and 6-month postpartum follow-ups.

BACKGROUND: It has been documented that pregnancy-related increased connective tissue laxity may persist postpartum; however, it is still unclear for how long. This longitudinal study aimed to compare total trunk forward flexion mobility in women between their first trimester of pregnancy and at 2- and 6-month postpartum follow-ups. We also searched for a correlation between women's trunk flexibility and their postural stability in the sagittal plane. METHODS: Seventeen healthy women participated in the study. Data were collected at their 7-12 weeks gestation appointments and at 6-10 and 25-28 weeks postpartum. At each session, the women performed a finger floor distance test, and data were collected on their waist circumference and BMI. The women's center of foot pressure mean velocity in the anterior-posterior direction was computed from 30-s long quiet-standing trials on a stationary force plate. FINDINGS: Total trunk forward flexion mobility was significantly higher at 2 and 6 months postpartum compared to that in early pregnancy (P < 0.05). At 6 months postpartum, a moderate negative correlation between finger floor distance test values and their anterior-posterior center of foot pressure mean velocity was observed (r = -0.6, P < 0.05). INTERPRETATION: Increased total trunk flexibility may be present in women 6 months postpartum. During that period, women with higher trunk flexibility may be more likely to present higher anterior-posterior postural sway velocity in quiet standing.

Nigrostriatal interaction in the aging brain: new therapeutic target for Parkinson's disease.

Parkinson's disease (PD) is a progressive neurodegenerative disorder of unclear etiology and pathogenesis. Research results gathered to date support the hypothesis that the motor symptoms of the disease result from the gradual loss of midbrain dopamine neurons residing in the substantia nigra pars compacta (SNpc). Recent discoveries, however, significantly expand this knowledge indicating that the primary source of the PD pathogenesis may be located both in the SNpc as well as in the GABAergic striatum. Newly discovered striatal neurogenesis - normally a lifelong process - determines the efficiency of nigrostriatal interaction. Deficient neurogenesis within the striatum followed by a decline in the GABAergic/dopaminergic interaction results in progressive disconnection of the dopaminergic input, which initiates a 'vicious circle' cascade of neuronal damage. Effects of both deficient striatal neurogenesis and age-related neurodegeneration within the striatum accumulate, resulting in a progressive decline in the control functions of the basal ganglia, loss of dopaminergic neurons, and occurrence of PD clinical symptoms. Functional and pharmacological control of these dynamic relationships may result in treatments that are more effective with fewer side-effects.

Parkinson's Disease and Neurodegeneration: GABA-Collapse Hypothesis.

Neurodegenerative diseases constitute a heterogeneous group of age-related disorders that are characterized by a slow but irreversible deterioration of brain functions. Evidence accumulated over more than two decades has implicated calcium-related homeostatic mechanisms, giving rise to the Ca(2+) hypothesis of brain aging and, ultimately, cell death. Gamma-aminobutyric acid (GABA) is the main inhibitory neurotransmitter within the central (CNS), peripheral and enteric nervous systems. It appears to be involved in a wide variety of physiological functions within and outside the nervous system, that are maintained through a complex interaction between GABA and calcium-dependent neurotransmission and cellular metabolic functions. Within CNS the Ca(2+)/GABA mechanism stabilizes neuronal activity both at cellular and systemic levels. Decline in the Ca(2+)/GABA control initiates several cascading processes leading to both weakened protective barriers (in particular the blood-brain barrier) and accumulations of intracellular deposits of calcium and Lewy bodies. Linking such a vital mechanism of synaptic transmission with metabolism (both at cellular and tissue level) by means of a common reciprocal Ca(2+)/GABA inhibition results in a fragile balance, which is prone to destabilization and auto-destruction. The GABA decline etiology proposed here appears to apply to all human neurodegenerative processes initiated by abnormal intracellular calcium levels. Therefore, the original description of Parkinson's disease (PD) as due to the selective damage of dopaminergic neurons in the mesencephalon should be updated into the concept of a severe multisystemic neurodegenerative disorder of the nervous system, whose clinical symptoms reflect the localization and progression of the most advanced GABA pathology. A future and more complete therapeutic approach to PD should be aimed first at slowing (or stopping) the progression of Ca(2+)/GABA functional decline.

The use of force-plate posturography in the assessment of postural instability.

Force-plate posturography is a simple method that is commonly used in the contemporary laboratory and clinic to assess postural control. Despite the obvious advantages and popularity of the method, universal standards for posturographic tests have not been developed thus far: most postural assessments are based on the standard spatiotemporal metrics of the center-of-foot pressure (COP) recorded during quiet stance. Unfortunately, the standard COP characteristics are strongly dependent on individual experimental design and are susceptible to distortions such as the noise of signal digitalization, which often makes the results from different laboratories incomparable and unreliable. The COP trajectories were recorded in subjects standing still, with eyes open (EO) and then, with eyes closed (EC). The 168 subjects were divided into 3 experimental groups: young adults, older adults, and patients with Parkinson's disease. Three novel output measures: the sway directional index (DI), the sway ratio (SR), and the sway vector (SV) were applied to assess the postural stability in the experimental groups. The controlled variables: age, pathology, and visual conditions, uniquely affected the output measures. The basic attributes of the SV: its reference position, magnitude, and azimuth, provided a unique set of descriptors for postural control that allowed me unambiguously to differentiate the decline in postural stability caused by natural ageing and Parkinson's disease. As shown in previous investigations, the SV attributes, when optimally filtered with a low-pass filter, were highly independent of the trial length and the sampling frequency, and were unaffected by the sampling noise. In conclusion, the SV may be recommended as the useful standard in static posturography.

Directional measures of postural sway as predictors of balance instability and accidental falls.

Despite the obvious advantages and popularity of static posturography, universal standards for posturographic tests have not been developed thus far. Most of the center-of-foot pressure (COP) indices are strongly dependent on an individual experimental design, and are susceptible to distortions, which makes results of their analysis incomparable. In this research, we present a novel approach to the analysis of the COP trajectory based on the directional features of postural sway. Our novel output measures: the sway directional indices (DI) and sway vector (SV) were applied to assess the postural stability in the group of young able-bodied subjects. Towards this aim, the COP trajectories were recorded in 100 students standing still for 60 s, with eyes open (EO) and then, with eyes closed (EC). Each record was subdivided then into 20, 30 and 60 s samples. Interclass correlation coefficients were calculated from the samples. The controlled variables (visual conditions) uniquely affected the output measures, but only in case of proper signal pretreatment (low-pass filtering). In filtering below 6 Hz, the DI and SV provided a unique set of descriptors for postural control. Both sway measures were highly independent of the trial length and the sampling frequency, and were unaffected by the sampling noise. Directional indices of COP filtered at 6 Hz showed high to very high reliability, with ICC range of 0.7-0.9. Results of a single 60 s trial are sufficient to reach acceptable reliability for both DI and SV. In conclusion, the directional sway measures may be recommended as the primary standard in static posturography.

Adaptive changes in spatiotemporal gait characteristics in women during pregnancy.

Spatiotemporal gait cycle characteristics were assessed at early (P1), and late (P2) pregnancy, as well as at 2 months (PP1) and 6 months (PP2) postpartum. A substantial decrease in walking speed was observed throughout the pregnancy, with the slowest speed (1±0.2m/s) being during the third trimester. Walking at slower velocity resulted in complex adaptive adjustments to their spatiotemporal gait pattern, including a shorter step length and an increased duration of both their stance and double-support phases. Duration of the swing phase remained the least susceptible to changes. Habitual walking velocity (1.13±0.2m/s) and the optimal gait pattern were fully recovered 6 months after childbirth. Documented here adaptive changes in the preferred gait pattern seem to result mainly from the altered body anthropometry leading to temporary balance impairments. All the observed changes within stride cycle aimed to improve gait safety by focusing on its dynamic stability. The pregnant women preferred to walk at a slower velocity which allowed them to spend more time in double-support compared with their habitual pattern. Such changes provided pregnant women with a safer and more tentative ambulation that reduced the single-support period and, hence, the possibility of instability. As pregnancy progressed a significant increase in stance width and a decrease in step length was observed. Both factors allow also for gait stability improvement.

Static Postural Stability in Women during and after Pregnancy: A Prospective Longitudinal Study.

This longitudinal study aimed to compare static postural stability in women between early pregnancy, advanced pregnancy, and at 2 and 6 months postpartum. Forty-five pregnant women were enrolled and 31 completed the protocol. Data were collected at 7-16 and 34-39 weeks gestation, and at 6-10 and 26-30 weeks postpartum. For each subject, the center of foot pressure path length and mean velocity (with directional subcomponents) were computed from 30-s long quiet-standing trials on a stationary force plate with eyes open or closed. The body mass, stance width, and sleep duration within 24 h before testing were also recorded. Static postural stability was not different between pregnancy and postpartum, except for the anterior posterior sway tested in the eyes-closed condition, which was significantly increased in late pregnancy compared to that at 2 and 6 months postpartum. Pregnant/postpartum women's body mass weakly positively correlated with anterior-posterior sway in the eyes-closed condition and their stance width weakly positively correlated with the anterior-posterior sway in the eyes-open condition. No effect of sleep duration on postural sway was found. Our findings indicate that under visual deprivation conditions women in advanced pregnancy may have decreased static stability compared to their non-pregnant state.

Comparison of static postural stability in exercising and non-exercising women during the perinatal period.

BACKGROUND: The purpose of the study was to determine whether women who exercised during and after pregnancy had better static postural stability compared to those who did not exercise. MATERIAL AND METHODS: Posturographic tests were performed in 31 women at 34-39 weeks gestation, and again at 6-10 weeks postpartum. The center of pressure mean velocity (with directional subcomponents) and sway area were computed from 30-s quiet standing trials on a stationary force plate with eyes open or closed. The women were surveyed about their lifestyle and physical activity in the perinatal period. Based on the survey, 12 of the women were assigned as regular exercisers and 19 as non-exercisers. A Mann-Whitney U test was used to compare data of the exercisers and the non-exercisers in their advanced pregnancy and again at 2 months postpartum. RESULTS: Postural sway measures were not significantly different between the exercisers and the non-exercisers in advanced pregnancy and at 2 months postpartum (p>0.05). CONCLUSIONS: Individually performed physical activity during the perinatal period did not affect pregnant/postpartum women's postural stability characteristics of quiet standing.

Assessment of postural stability in young healthy subjects based on directional features of posturographic data: vision and gender effects.

Directional characteristics of postural sway and their impact on postural stability control (PSC) still need to be clarified. To assess the directional characteristics of PSC, center-of-pressure (COP) while standing quiet was recorded and analyzed in 46 young healthy subjects. The participants completed five 60-s trials with 'eyes open' and five with 'eyes closed'. The control asymmetry was characterized using three standardized COP measures: mean sway velocity (V), sway ratio (SR), and sway directional index (DI). The COP data were low-pass filtered at 10 Hz. In such conditions the V, SR and DI revealed consistent and unique differences in both the anteroposterior (AP) and the mediolateral (ML) controls. Each of the COP measures showed the involvement of different neuromuscular and biomechanical mechanisms in the stabilization of standing posture. The DI documented a very stable proportion between frontal and saggital controls, with roughly 60% of the swaying movements being performed in the AP direction. The AP DI was significantly higher in comparison with the ML value (0.71±0.04 vs. 0.55±0.06). This proportion remained at the same level in both experimental groups and it was only slightly affected by the visual conditions. Analysis of the SR confirmed that to maintain stable stance the neuromuscular system must allocate 50% more effort to control AP stability. The results also documented gender-related differences in postural stability. Generally the female subjects appeared to have lower postural stability as evidenced by higher COM and COP velocities. Consequently the higher SR values observed in the female group indicate a need for higher neuromuscular efforts to maintain stable posture. The introduced here set of COP measures proved to be a valuable standard for static posturography, creating the opportunity for improved and reliable assessment of postural stability.

Assessment of postural control in patients with Parkinson's disease: sway ratio analysis.

Analysis of the postural stability impairments in neurodegenerative diseases is a very demanding task. Age-related declines in posturographic indices are usually superimposed on effects associated with the pathology and its treatment. We present the results of a novel postural sway ratio (SR) analysis in patients with Parkinson's disease (PD) and age-matched healthy subjects. The sway ratios have been assessed based upon center of foot-pressure (CP) signals recorded in 55 parkinsonians (Hoehn and Yahr: 1-3) and 55 age-matched healthy volunteers while standing quiet with eyes open (EO) and then with eyes closed (EC). Complementing classical sway measure abnormalities, the SR exhibited a high discriminative power for all controlled factors: pathology, vision, and direction of sway. Both the anteroposterior (AP) and mediolateral (ML) sway ratios were significantly increased in PD patients when compared to the control group. An additional SR increase was observed in the response to eyes closure. The sway ratio changes documented here can be attributed to a progressive decline of a postural stability control due to pathology. In fact, a significant correlation between the mediolateral SR under EO conditions and Motor Exam (section III) score of the UPDRS was found. The mediolateral sway ratios computed for EO and EC conditions significantly correlated with the CP path length (r = .87) and the mean anteroposterior CP position within the base of support (r = .38). Both indices reflect postural stability decline and fall tendency # in parkinsonians. The tremor-type PD patients (N=34) showed more pronounced relationships between the mediolateral SR and selected items from the UPDRS scale, including: falls (Kendall Tau=.47, p < .05), rigidity (.45, p < .05), postural stability (retropulsion) (.52), and the Motor Exam score (.73). The anteroposterior SR correlated only with tremor (Kendal Tau = .77, p < .05). It seems that in force plate posturography the SR can be recommended as a single reliable measure that allows for a better quantitative assessment of postural stability impairments.

Impact of excess body weight on walking at the preferred speed.

The implications of a long-lasting mechanical load on the locomotor activity are poorly understood. The objective of the present studies was to determine an impact of excess body weight on basic spatiotemporal gait measures and to test the hypothesis that leg swing phase may account for a load-related adaptation of the stride characteristics. To this end the basic spatial and temporal stride measures were assessed in 100 obese and 36 lean women (age range between 18 and 67 years) walking with their self-selected pace on a 10-meter long and 1 meter wide instrumented pathway. Among the subjects there were: 44 with class I obesity, 27 with class II obesity, and 29 with class III. Subjects' stance and swing times as well as the stride lengths were recorded by means of contact copper-film electrodes attached to a sole of subject' footwear. The acquired gait measures were used then to compute: a mean velocity of walking, double support times and a mean velocity of a foot during swing phase. Data analysis showed that subjects from every experimental groups walked with a very similar speed (1.08 +/- 0.2 m/s) and cadence (106 +/- 10 steps/min). Their stance time was not affected by body weight and it remained at the mean level of 746 +/- 90 ms for all groups. The temporal stride characteristics and the stance-to-swing ratio were, however, substantially modified in obese individuals due to attenuation of the swing time. As a consequence, the remaining normalized (i.e., expressed as percentage of gait cycle time) phases of stride: the stance and the double support were relatively longer. While the swing time negatively correlated with the body mass index (BMI), the normalized stance and the double support exhibited strong positive correlation (r=0.46) with the BMI. The increase of leg swing velocity seems the main and unique adaptation mechanism that is utilized in the preferred walking gait in obese women.

Acoustic startle and disruption of prepulse inhibition by dizocilpine in selectively bred mice.

In this study we examined the relationship between genetically produced differences in the magnitude of prepulse inhibition (PPI) of the acoustic startle response (ASR) and stress induced swim analgesia in genetically different strains of mice. Prepulse inhibition of the ASR and its changes due to dizocilpine (MK-801) injection were studied in 180 mice. The animals used in this study were obtained from our colony of 54 generation, Swiss-Webster mice selectively bred for high and low magnitude of analgesia. Three month old male mice of the high analgesia (HA) and the low analgesia (LA) lines, in addition to randomly bred controls (C) were used in the experiment. Thirty minutes before the ASR session the mice were injected intraperitoneally with saline or with 0.15, 0.25, 0.5 mg÷kg of dizocilpine maleate. Prepulses suppressed the acoustic startle response in all lines in a prepulse intensity-dependent manner, but only the differences between the weakest and the strongest prepulses appeared significant. Two-way ANCOVA performed separately for each line revealed a significant effect of dizocilpine and prepulse intensity. Only in the HA line, however, the disruption of PPI from the injection of dizocilpine was evidenced by significant treatment by prepulse interaction. That means the prepulses decreased ASR significantly less in dizocilpine- treated animals than in saline-treated animals. The results confirmed that the mouse lines manifesting differential ASR magnitudes along with different degrees of PPI sensitivity to dizocilpine, might be suitable for pharmacogenetic studies on the glutaminergic mechanism of the startle response.

Differential startle magnitude in mice selected for high and low swim analgesia is not related to difference in nociception.

The acoustic startle response (ASR) elicited by 110 dB 10-ms pulses was studied in relation to pain sensitivity in mouse lines selectively bred for high (HA) and for low (LA) swim analgesia. The magnitudes of ASR, similarly as hot-plate latencies, differed between the lines in the rank order HA is greater than unselected controls (C) greater than LA. The animals' nociception did not change after the ASR session consisting of a sequence of 20 acoustic stimuli. Morphine hydrochloride (5 and 10 mg per kg i.p.) increased hot-plate latencies in the order of HA greater than C greater than LA, and was not effective on ASR magnitude in HA as well as in C mice. In the LA line, 10 mg per kg of morphine slightly attenuated ASR, but caused only a little analgesia. We conclude that (1) the difference in ASR between the selected lines is inversely correlated with the difference in pain sensitivity; (2) the magnitude of ASR is not altered by morphine analgesia; (3) the procedure of ASR using brief acoustic pulses is not stressful enough to elicit a form of stress analgesia. The lack of a direct relationship between the readiness to startle and pain sensation may be beneficial for an animal's survival in dangerous situations. It is beneficial when the startle to a warning signal precedes defensive behaviors and it often must be effectuated in a state of decreased nociception.

Effects of excessive body weight on postural control.

Research that evaluated both static and dynamic stability was performed, to clarify the impact of excessive body weight on postural control. The spontaneous center of foot pressure (CP) motion during quiet stance and a range of forward voluntary CP displacements were studied in 100 obese, and 33 lean women. Characteristics of postural sway were acquired while the subjects were standing quiet on a force plate with eyes open (EO) and with eyes closed (EC). Their anterior range of CP voluntary displacements was assessed upon a range of maximal whole body leanings which were directed forward. A substantial reduction of postural sway was observed in all patients which had increased body weight. Main postural sway parameters i.e., the total path length as well as its directional components were negatively correlated with the body mass and body mass index (BMI). The range of a whole body voluntary forward leaning, did not exhibit any significant change in patients with an obesity grade of I and II. Such a deficit was, however, found in subjects with a body mass index above 40. In conclusion, the increased body weight imposed new biomechanical constraints, that resulted in functional adaptation of the control of the erect posture. This functional adaptation was characterized by a reduced postural sway associated with a substantial reduction of the dynamic stability range in subjects with BMI>40.

Sway ratio - a new measure for quantifying postural stability.

In the search of a reliable postural stability index, two sway time series: the center-of-mass (COM) and the center-of-foot pressure (COP) were recorded simultaneously in elderly subjects standing quiet with eyes open and with eyes closed. From a battery of commonly use sway measures, only the anteroposterior COM and the COP path lengths proved their high sensitivity and discriminative power to the imposed vision conditions. Based upon these indices, a new measure - sway ratio (SR) - was computed, as the COP-to-COM path length ratio. The measure can easily distinguish vision vs. no vision in the elderly. The SR can be successfully accessed base upon the COP signal only. In contrast to traditional sway indices, the SR as a relative measure is insensitive to the length of sampled record and to the signal sampling frequency. Its magnitude can be interpreted as an average amount of balance controlling motor activity that coincides with a unit COM displacement. The SR is recommended as a reliable measure that allows for assessment of postural stability.