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

REPEATABILITY OF SWAY MEASURES IN UPPER EXTREMITY WEIGHT-BEARING

Background

Analysis of upper extremity weight bearing ability is important for athletes as some function largely in a closed chain capacity (e.g., wrestling, football, gymnastics); also, all require closed chain upper extremity function during strength and conditioning. Additionally, in a rehabilitation setting, closed chain upper extremity functional testing is often used as a return to play criterion. Lower extremity sway measures (biomechanical and clinical) have been published widely and have established reliability and validity; however, the reliability of upper extremity sway biomechanical measures has not been investigated to date.

Hypothesis/Purpose

The purpose of this study was to determine the repeatability of a variety of force plate measurements during an upper extremity task in an athletic population. It was hypothesized that variables measuring upper extremity sway in a closed kinetic chain position would have excellent reliability.

Study Design

Cross-sectional.

Methods

All data were collected using a force plate system with commercially available software. Four hundred and ninety healthy Division I athletes were tested for both their dominant and non-dominant upper extremity at one of two testing sessions. Subjects were instructed to stay as still as possible while maintaining a full plank position with one upper extremity on the force plate and the contralateral upper extremity behind their back. Two, 20-second trials were performed for each extremity. Variables measured included average sway velocity (ASV), sway velocity in medial-lateral (SVML) and anterior-posterior (SVAP) directions, sway velocity at 1^st and 2^nd time intervals for AP (VAP1 and 2) and ML (VML1 and 2) directions, and sway frequency in the AP direction for 1^st and 2^nd time intervals (FreAP1 and 2). Intraclass correlation coefficients (ICC_(2,1)) and their 95% confident intervals were calculated for all force plate variables for 980 limbs.

Results

No difference was seen between left and right extremities for any measure (p > 0.05). ICC's ranged from 0.61-0.90 for all variables, indicating moderate to excellent reliability for all variables.

Conclusion

Upper extremity sway biomechanical variables using a force plate system have moderate to excellent reliability. These results are important prior to validation and clinical utilization of these measures including baseline testing, return to play guidelines, and injury prevention parameters.

Level of Evidence

3.

Postural symmetry evaluation using phase approximations of the follow-up CoP trajectories

This paper presents a method of postural symmetry evaluation implementing the so-called follow-up posturography. The method boils down to the assessment of similarity of the phase approximation of the counter-clockwise follow-up CoP (Center of Pressure) trajectory and the mirror image against the y-axis of the phase approximation corresponding to the clockwise follow-up CoP trajectory. The usability of the presented approach was tested on the data collected in the group of 30 patients rehabilitated after total hip arthroplasty. The observed difference between the values of the proposed postural symmetry coefficient obtained at the end and at the beginning of the rehabilitation program was statistically significant (p < 0.001). These values, however, were not significantly correlated with the values of postural symmetry coefficients computed in static posturography. Lack of significant correlations between the coefficients supports the reasoning that the new postural symmetry evaluation method quantifies symmetry of posture in terms of dynamic mechanisms, which are not manifested in the case of static posturography. As a major advantage of the herein discussed approach one can distinguish its potential to evaluate postural symmetry in dynamic conditions using relatively inexpensive single-plate posturographic platform.

Center of pressure displacement characteristics differentiate fall risk in older people: A systematic review with meta-analysis

Falling is the second most prevalent cause of accidental death in the world. Currently available clinical tests to assess balance in older people are insufficiently sensitive to screen for fall risk in this population. Laboratory tests that record the center of pressure (COP) trajectory could overcome this problem but despite their widespread use, the choice of COP trajectory features for use as a biomarker of fall risk lacks consensus. This systematic review and meta-analysis aimed at identifying the best COP characteristics to predict risk of falling in older adults. More than 4000 articles were screened; 44 (7176 older adults) were included in this study. Several COP parameters emerged as good indices to discriminate fallers from non-fallers. From sensitivity analysis, Sway area per unit time, anteroposterior mean velocity, and radial mean velocity were the best traditional features. In this study, identification of older people with a high fall risk was demonstrated using quiet-standing recordings. Such screening would also be useful for routine follow-up of balance changes in older fallers in clinical practice.

Posturography Comparison and Discriminant Analysis Between Individuals With and Without Chronic Low Back Pain

OBJECTIVE

The aims of this study were to evaluate the center of pressure (CoP) in individuals with chronic low back pain (LBP) compared with matched controls and perform discriminant analysis to detect which CoP variables differentiate the groups.

METHODS

Thirty-two participants with LBP and 33 matched controls were evaluated on a force plate in a bipedal static position for 30 seconds in 2 conditions: eyes open (EO) and eyes closed (EC). Two discriminant analyzes were performed to detect which CoP variables could discriminate between groups.

RESULTS

Those with LBP had higher values (ie, poorer balance) for most variables compared with the control group. With EO, total displacement of sway (TDS) was as follows: LBP group (median [25%-75%]) 31.77 (26.39-41.79) cm, control group 27.21 (22.29-31.78) cm, P = .008 and area: LBP group 3.31 (2.33-4.68) cm², control group 1.77 (1.3-2.71) cm². With EC, TDS was as follows: LBP group 49.6 (39.65-68.15) cm, control group 38.77 (30.36; 45.65) cm, P = .003 and area: LBP group 4.68 (2.6-7.28) cm², control group 2.4 (2.1-3.34) cm². The discriminating variables in the EO condition were the TDS for the LBP group and the anteroposterior mean velocity for the control group, while in the EC condition they were mediolateral dispersion and area for the LBP group.

CONCLUSION

Individuals with chronic LBP had worse postural control performance than matched controls, and it is possible to characterize those with and without LBP with CoP variables.

Agreement of Three Posturographic Force Plates in the Assessment of Postural Stability

This study was designed to assess how the results obtained for three different posturographic platforms agreed with each other in an assessment of static postural stability. The study included 111 young healthy participants. A measurement of postural stability was made for each participant, with their eyes open and then closed, on each platform in a random order. The Romberg ratio, path length, and center of pressure (COP) area were analyzed. For all measures, significant differences (p < 0.05) were observed among the three force plates. The highest Spearman's rank correlation was observed between Alfa vs. CQStab2P (0.20 to 0.38), and the lowest between Alfa vs. AccuGait (-0.19 to 0.09). Similar results were obtained for the concordance correlation coefficient (0.10 to 0.22 for Alfa vs. CQStab2P and -0.6 to 0.02 for Alfa vs. AccuGait). Bland-Altman analysis for values standardized (z-scores) against AccuGait indicated a low level of agreement between compared platforms, with the largest error between AccuGait vs. Alfa, and a slightly lower error between AccuGait vs. CQStab2P or Alfa vs. CQStab2P. The 95% limits of agreement ranged from 2.38 to 7.16 (Alfa vs. AccuGait), 2.09 to 5.69 (CQStab2P vs. AccuGait), and 1.39 to 7.44 (AccuGait vs. Alfa) in COP length with eyes open and COP length Romberg ratio, respectively. Special care is recommended when comparing values relating to COPs from different devices that are analyzed by different software. Moreover, unperturbed stance tests among young healthy adults can be questioned as a valid postural control parameter.

Demonstration of the Effect of Centre of Mass Height on Postural Sway Using Accelerometry for Balance Analysis

The effect of center of mass (COM) height on stand-still postural sway analysis was studied. For this purpose, a measurement apparatus was set up that included an accelerometry device attached to a rod: three plumb lines, positioned at 50, 75 and 100 cm to an end of the rod, each supported a plumb bob. Using a vice mechanism, the rod was inclined from vertical (0 degree inclination) in steps of 5 degrees to 90 degrees. For each inclination, the corresponding inclination angle was manually measured by a protractor and the positions of the three plumb bobs on the ground surface were also manually measured using a tape measure. Algebraic operations were used to calculate the inclination angle and the associated displacements of the plumb bobs on the ground surface from the accelerometry data. For each inclination angle, the manual and accelerometry calculated ground displacement were close. The height of COM, where the measurement was taken, affected the projected displacement on the ground surface. The COM height had a nonlinear double-effect relationship with sway as it can affect both the angle and projected sway. Normalization of the COM height was used to reduce this effect for comparison purposes.

Reliability and robustness of optimization properties for stabilization of the upright stance as determined using posturography

Diagnostic value of static posturography depends on its methodological features, measurement properties, and on computational methods that extract meaningful information from the postural sway i.e. the center-of-pressure (CoP) displacements. In this study, we assessed the reliability and robustness of the postural system based on the optimization properties of the CoP signal: descending, local and global stability, and convergence. For the analysis, we used CoP data from 146 participants (104 [71%] female, age 46 ± 23 years, body mass index 23.6 ± 3.4 kg/m²) recorded while standing quietly on a foam surface without visual input. Reliability was estimated using the intraclass correlation coefficient from a single (ICC_2,1) and averaged (ICC_2,3) measurements. Robustness was assessed through main and interaction effects for the signal duration (60, 30 s), sampling frequency (100, 50 Hz), and lowpass filtering cutoff frequency (10, 5 Hz). The observed reliability depended on the use of average or single measurements as it was excellent for the stability property (ICC_2,k ≥ 0.772); excellent-to-acceptable (ICC_2,3 ≥ 0.540) or excellent-to-unacceptable (ICC_2,1 ≥ 0.281) for the descending property; and excellent-to-unacceptable (ICC_2,3 > 0.295; ICC_2,1 > 0.122) for the convergence property. Robustness analysis showed large main effects of signal duration (ω² ≤ 0.834, p < 0.001), sampling frequency (ω² ≤ 0.526, p < 0.001), and the lowpass filter cutoff frequency (ω² ≤ 0.523, p < 0.001) on the optimization properties; but all two-way and three-way effects varied from medium to trivial. Reliability is thus excellent to acceptable for deriving the descending, stability, and convergence properties from the average of three measurements. Those optimization properties are robust to the interaction but not the main effects of methodological sources of variation of posturography.

Balance Control in Obese Subjects during Quiet Stance: A State-of-the Art

Obese individuals are characterized by a reduced balance which has a significant effect on a variety of daily and occupational tasks. The presence of excessive adipose tissue and weight gain could increase the risk of falls; for this reason, obese individuals are at greater risk of falls than normal weight subjects in the presence of postural stress and disturbances. The quality of balance control could be measured with different methods and generally in clinics its integrity is generally assessed using platform stabilometry. The aim of this narrative review is to present an overview on the state of art on balance control in obese individuals during quiet stance. A summary of knowledge about static postural control in obese individuals and its limitations is important clinically, as it could give indications and suggestions to improve and personalize the development of specific clinical programs.

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.

Asymmetric dynamic center-of-pressure in Parkinson's disease

BACKGROUND

Gait disturbance gradually worsens as Parkinson's disease (PD) progresses, which significantly affects the quality of life of PD patients. Treadmill-based gait analysis systems can measure gait parameters including the dynamic center-of-pressure (COP) trajectory during ambulation. In this study, we hypothesized that altered dynamic COP changes are new gait characteristics for PD patients.

METHODS

Dynamic COP parameters and classic spatiotemporal parameters were obtained for each patient using a treadmill-based system at the maximal comfortable treadmill speed (MCTS). We compared dynamic COP parameters between 44 PD patients and 31 controls, correlated these parameters with clinical and spatiotemporal data, and adjusted for age and MCTS to determine whether the parameters were independent from the treadmill speed. We also evaluated characteristics of COP parameters in relation to the more and less affected sides in PD patients.

RESULTS

During treadmill walking the length of the COP trajectory in the stance phase was decreased, an effect that was more prominent on the more affected side in PD patients. COP parameters related to this change were significantly altered in patients when compared to controls. Asymmetry of the COP trajectories compared between both feet was identified as a significant gait characteristic after adjusting for age and MCTS. The overlaid graphical display of dynamic COP trajectory in PD patients showed "distorted butterfly with asymmetric wing" feature.

CONCLUSION

Dynamic COP asymmetry provides a new and intuitive way to analyze gait abnormalities of PD patients. Further studies with prospective designs will substantiate the clinical usefulness of this feature of gait.

Frequency analyses of posturography using logarithmic translation

Background: Power-spectral analysis of the centre of pressure (CoP) frequencies of posturography provides exponentially approximated distributions, whereas logarithmic translation enables linear approximation.Objectives: Frequency analyses were adopted for posturography of healthy subjects and patients with spinocerebellar degeneration (SCD) using logarithmic translation to determine its clinical usefulness for managing the elderly and patients with disequilibrium.Material and methods: We included 172 healthy subjects and 47 SCD patients. Posturography was performed with the eyes fixated and closed, with and without foam rubber. The power-spectral data of the CoP were obtained with the maximum entropy method. Power-spectral data were logarithmically translated for quantitative evaluation.Results: For teenagers, high-frequency fluctuations were dominant and attributable to proprioceptive compensation due to immature postural control. In elderly populations, the increased frequency in the lateral direction was characterised by three peaks indicating postural disturbances attributed to three sensory inputs. The disappearance of one peak in the anteroposterior fluctuation indicates a decrease in vestibular contribution. The foam rubber and the closed-eye condition enhanced fluctuations in two peaks. There were differences in power-spectral distributions of two peaks between the healthy subjects and SCD patients.Conclusions: Logarithmic power-spectral data distribution could provide an age- and disease-specific novel and visually-comprehensible parameter.

Posturographic measures did not improve the predictive power to identify recurrent falls in community-dwelling elderly fallers

OBJECTIVE

This study aimed to evaluate if posturography can be considered a recurrent fall predictor in elderly individuals.

METHODS

This was a cross-sectional study. A total of 124 subjects aged 60 to 88 years were evaluated and divided into two groups-the recurrent fallers (89) and single fallers (35) groups. Patients' sociodemographic characteristics were assessed, and clinical testing was performed. The functional test assessment instruments used were timed up and go test (TUGT), Berg Balance Scale (BBS), five times sit-to-stand test, and Falls Efficacy Scale (to measure fear of falling). Static posturography was performed in a force platform in the following three different situations-eyes open (EO), eyes closed (EC), and EO dual task.

RESULTS

There were significant differences between the single and recurrent fallers groups regarding the fear of falling, the Geriatric Depression Scale score, the mean speed calculated from the total displacement of the center point of pressure (COP) in all directions with EO, and the root mean square of the displacement from the COP in the mediolateral axis with EC. Based on the hierarchical logistic regression model, none of the studied posturographic variables was capable of significantly increasing the power of differentiation between the recurrent and single fallers groups. Only TUGT with a cognitive distractor (p<0.05) and the BBS (p<0.01) presented with significant independent predictive power.

CONCLUSION

TUGT with a cognitive distractor and the BBS were considered recurrent fall predictors in elderly fallers.

Postural stability in Parkinson's disease patients' wives and in elderly women leading different lifestyles

The study aimed to determine postural stability of Parkinson's disease (PD) patients' wives in comparison with women differing in their lifestyle. (PD) patients' wives (n = 44), homemakers (n = 41), and female students of the University of the Third Age (n = 43) performed balance tests on a stabilometric platform. The PD patients' wives were characterized by significantly (p < 0.001) higher values of mean velocity sway than the homemakers and students (approximately 3.5 and 5 mm/s, respectively) and performed worst in displacement velocity and sway range in both sagittal and frontal plane. The results indicate that the wives of PD patients need support in the area of health training targeted at improving their standing stability.

Center of pressure characteristics from quiet standing measures to predict the risk of falling in older adults: a protocol for a systematic review and meta-analysis

BACKGROUND

Falling is the most common accident of daily living and the second most prevalent cause of accidental death in the world. The complex nature of risk factors associated with falling makes those at risk amongst the elderly population difficult to identify. Commonly used clinical tests have limitations when it comes to reliably detecting the risk of falling, but existing laboratory tests, such as force platform measurements, represent one method of overcoming this lack of a test. Despite their widespread use, however, Center of Pressure (COP) signal analysis techniques vary and there is currently no consensus on which features should be used diagnostically. Our objective is to identify, through a systematic review and meta-analysis, the COP characteristics of older adults (≥ 60 years old) during quiet bipedal stance which will allow fallers to be distinguished from non-fallers.

METHODS

The systematic review will include both prospective and retrospective articles. Five databases will be searched: PubMed, Cochrane CENTRAL, EMBASE, and ScienceDirect. In addition, a search of gray literature will be performed using Google Scholar and ClinicalTrials.gov. Searches will be circumscribed to include only older adults (aged over 60 years) who underwent a bipedal quiet standing measure of their balance and for whom the number of falls was reported. Two authors will independently assess the risk of bias for each included article using a 26-item checklist. Funnel plots will be drawn to attest of possible publication biases for each COP parameters. The results will be synthesized descriptively and a meta-analysis will be undertaken. When trial methodological heterogeneity is too great for pooling of the data into a meta-analysis, evidence strength will be evaluated using best evidence analysis.

DISCUSSION

Despite the numerous advantages of posturography, the diversity of studies exploring balance in older fallers has led to uncertainty regarding the method's ability to reliably identify fall-prone older adults. It is expected that the findings from this systematic review will help clinicians use bipedal quiet standing measures as a diagnostic test and allow researchers to explore COP characteristics to create better models for fall prevention care.

SYSTEMATIC REVIEW REGISTRATION

PROSPERO CRD42018098671.

Posturographic characteristics of the standing posture and the effects of the treatment of obesity on obese young women

To determine the impact of body weight on quiet standing postural sway characteristics in young women, this research compared spontaneous oscillations of the center of foot pressure (COP) between 32 obese (BMI: 36.4 ± 5.2 kg/m²), and 26 normal-weight (BMI: 21.4 ± 1.5 kg/m²) women and assessed the influence of obesity treatment and body weight reduction on postural sway. Trajectories of the COP were assessed while the subjects were standing quietly with eyes open (EO) and closed (EC). Both in the sagittal (AP) and frontal (ML) planes the sway range, average velocity, and maximal velocity of COP were calculated. Moreover, the total average and maximal velocities were computed. In the obese group, the tests were performed twice–before and after the obesity treatment. A greater (18% in EC) AP sway range and a substantial reduction of ML sway (25% in EO, 22% in EC) were observed in the obese women. The total COP velocities (average and maximal) were decreased in obese women (20% and 20% in EO) as well as the velocities in the frontal plane (EO: 33%, 41%; EC: 34%, 40%). Body weight reduction resulted in significant changes in postural sway. The following parameters increased: ML sway range (28% in EO), average (20% in EO, 16% in EC) and maximal ML (20% in EO) velocities. The results indicate that young obese women in the habitual standing position are characterized by the destabilizing influence of mass in the sagittal plane only in the absence of a visual control. This effect is dominated by the stabilizing mass effect in the frontal plane, which affects overall postural stability when standing. The reduction of body mass enables a decrease in ML static stability, likely due to natural changes in the base of support while standing.

Is the dopaminergic loss associated with gait and postural impairments in subjects with Parkinson's disease at different motor stages?

Gait and postural control deficiencies in Parkinson's disease (PD) involve several specific motor aspects. The aim of this study was to identify and compare the main changes in gait kinematics and postural control with dopaminergic loss in the striatum region. This is a cross-sectional study that included 42 individuals with PD at different motor stages, according to the Hoehn & Yahr scale (H&Y). Motor subsection of the Movement Disorder Society-Unified Parkinson Disease Rating Scale-part III (MDS-UPDRS III) was used to evaluate general motor aspects. Gait kinematics was assessed using a three-dimensional motion capture system. Postural control was assessed by stabilometry using force platforms. Dopamine depletion was verified through 99mTc-TRODAT-1 (SPECT-CT) examination. We included 12, 15 and 15 individuals classified as H&Y I, II and III, respectively. We identified worse values of dopamine transporter uptake, MDS-UPDRS III, gait parameters (velocity, step length and stride length) and center of pressure displacement as the disease progressed. Our results indicate that higher dopaminergic loss and gait and postural control deficits occur between the H&Y levels II and III.

Postural balance and rifle stability in a standing shooting position after specific physical effort in biathletes

The aim of the study was to evaluate the effect of maximal specific physical effort on postural balance and rifle stability in biathletes in a standing shooting position. The study included 10 junior elite biathletes. The measurements were taken with the Vicon system and AMTI force platform. Postural balance and rifle stability characteristics were determined at rest as well as 1 and 5 min post maximal specific physical effort which was performed on a ski ergometer and continued until exhaustion. Maximal physical effort exerted a significant effect on all examined postural sway and rifle sway characteristics. The duration of the post-exercise changes was longer than 5 minutes. Higher post-effort rifle sway was observed in the vertical direction than in the across the shooting line direction. Post-effort postural balance impairment in the shooting line was much greater than in the across the shooting line direction. Moreover, a strong correlation was found between postural balance and rifle stability. Maximal physical effort influenced postural balance and rifle stability during aiming. Rifle sway during aiming in a standing shooting position seems to be coordinated with the postural sway of the biathlete's body. Thus, an increase in postural sway contributes to greater sway and lesser stability of the rifle.

Comparing Balance Performance on Force Platform Measures in Individuals with Parkinson's Disease and Healthy Adults

INTRODUCTION

Postural instability is a known contributing factor to balance dysfunction and increased fall risk in those with Parkinson's disease (PD). Computerized posturography employing a force platform system provides objective, quantitative assessments of postural control impairments. This study examines balance performance as measured by force platform (FP) tests in persons with PD compared to age-matched healthy adults. Secondarily, we examine if these FP measures provide diagnostic and clinically meaningful information about the underlying balance impairments in the PD population.

METHODS

Participants-42 individuals with PD (Hoehn and Yahr stage = 2.33 ± 0.77) and 55 age-matched healthy adults-were assessed on three standardized balance measures on a computerized force platform system. Between groups, comparisons of FP performance were analyzed using independent t-test. Within the group, comparisons for the PD cohort were analyzed using ANOVA for comparing disease stage and Mann-Whitney U test for PD subtypes.

RESULTS

The PD cohort demonstrated significantly greater postural instability on the sensory organization test (SOT) measures (P=0.013, CI-95% = 1.286 to 10.37) and slower movement velocity on the limits of stability (LOS) test (P=0.001, CI-95% = 0.597 to 1.595) than the healthy cohort, suggesting that these tests were sensitive to detect sensory integration and voluntary postural control deficits in the PD cohort. Within the PD group, the SOT differentiated between H&Y stages 1-3. The motor control test (MCT) detected changes in reactive postural control mainly in later disease stages. All three FP tests distinguished between PD subtypes, with the Posture Gait Instability subtype demonstrating poorer balance performance than Tremor Dominant subtype.

CONCLUSION

These findings suggest FP measures provide clinically meaningful, diagnostic information in the examination of balance impairments in individuals with PD. FP measures may inform clinicians regarding intrinsic balance deficits and guide them in designing targeted balance interventions to reduce fall risk in persons with PD.

Nonlinear dynamics analysis of the human balance control subjected to physical and sensory perturbations

Postural control after applying perturbation involves neural and muscular efforts to limit the center of mass (CoM) motion. Linear dynamical approaches may not unveil all complexities of body efforts. This study was aimed at determining two nonlinear dynamics parameters (fractal dimension (FD) and largest Lyapunov exponent (LLE)) in addition to the linear standing metrics of balance in perturbed stance. Sixteen healthy young males were subjected to sudden rotations of the standing platform. The vision and cognition during the standing were also interfered. Motion capturing was used to measure the lower limb joints and the CoM displacements. The CoM path length as a linear parameter was increased by elimination of vision (p<0.01) and adding a cognitive load (p<0.01). The CoM nonlinear metric FD was decreased due to the cognitive loads (p<0.001). The visual interference increased the FD of all joints when the task included the cognitive loads (p<0.01). The slightly positive LLE values showed weakly-chaotic behavior of the whole body. The local joint rotations indicated higher LLEs. Results indicated weakly chaotic response of the whole body. Increase in the task difficulty by adding sensory interference had difference effects on parameters. Linear and nonlinear metrics of the perturbed stance showed that a combination of them may properly represent the body behavior.

Postural stability and risk of falls per decade of adult life – a pilot study

A gradual loss of function in the balance system may begin in the fourth decade of life. The effects of this process become visible in old age, when problems with postural stability contribute to falls, making it an important social problem. Early detection of this dysfunction is essential for minimizing the risk of age-related falls, one of the main causes of hospitalization or even death in older adults. The aim of this study was to evaluate somatic factors that may result in the deterioration in postural stability and determine the age range in which the first changes in stability occur. The study included healthy non-sporting adults aged from 20 to 70 years. Four tests based on the Biodex Balance System were used to determine static postural stability, dynamic postural stability, risk of falling and stability limits. The obtained results showed that dysfunctions of dynamic balance appeared significantly earlier than static balance dysfunctions, i.e. as early as at 50 years of age, and then gradually increased. Higher BMI and the percentage and absolute fat content significantly increased the risk of falls and also adversely affected the results of dynamic stability tests.  

Muscle coactivation: definitions, mechanisms, and functions

The phenomenon of agonist-antagonist muscle coactivation is discussed with respect to its consequences for movement mechanics (such as increasing joint apparent stiffness, facilitating faster movements, and effects on action stability), implication for movement optimization, and involvement of different neurophysiological structures. Effects of coactivation on movement stability are ambiguous and depend on the effector representing a kinematic chain with a fixed origin or free origin. Furthermore, coactivation is discussed within the framework of the equilibrium-point hypothesis and the idea of hierarchical control with spatial referent coordinates. Relations of muscle coactivation to changes in one of the basic commands, the c-command, are discussed and illustrated. A hypothesis is suggested that agonist-antagonist coactivation reflects a deliberate neural control strategy to preserve effector-level control and avoid making it degenerate and facing the necessity to control at the level of signals to individual muscles. This strategy, in particular, allows stabilizing motor actions by covaried adjustments in spaces of control variables. This hypothesis is able to account for higher levels of coactivation in young healthy persons performing challenging tasks and across various populations with movement impairments.

Posturography and dynamic pedobarography in lame dogs with elbow dysplasia and cranial cruciate ligament rupture

BACKGROUND

The usefulness of studying posture and its modifications due to locomotor deficiencies of multiple origins has been widely proven in humans. To assess its suitability in the canine species, static posturography and dynamic pedobarography were performed on lame dogs affected with unilateral elbow dysplasia and cranial cruciate ligament rupture by using a pressure platform. With this objective, statokinesiograms and stabilograms, the percentage of pressure distribution between limbs, paw area, mean pressure, and peak pressure, were obtained from lame and sound dogs. These data were compared with Peak Vertical Force values originated from a force platform in the same recording sessions.

RESULTS

Significant differences were found in the parameters mentioned above between sound and lame dogs and limbs.

CONCLUSIONS

Posturography and pedobarography are useful and reliable for the monitoring of fore and hindlimb lameness in dogs, providing a new set of parameters for lameness detection.

Estimation of Posturographic Trajectory Using k-Nearest Neighbors Classifier in Patients with Rheumatoid Arthritis and Osteoarthritis

Rheumatoid arthritis (RA) and osteoarthritis (OA) are common rheumatic diseases and account for a significant percentage of disability. Posturography is a method that assesses postural stability and quantitatively evaluates postural sways. The objective of this study was to estimate posturographic trajectories applying pattern recognition algorithms. To this end, k-nearest neighbors (k-NN) classifier was used to differentiate between healthy subjects and patients with OA and RA. The following parameters of trajectories were computed: radius of sways, developed area, total length, and two directional components of sways: length of left-right and forward-backward motions. Posturographic tests were applied with eyes open and closed, and with biofeedback control. We found that in RA, the radius of sways, the trajectory area, and the biofeedback coordination were related to the patients' condition. The trajectory dynamics in OA patients were smaller compared to those in RA patients. The smallest misclassification errors were observed after feature selection in the biofeedback test compared with the eyes open and closed tests. We conclude that the estimation of posturographic trajectory with k-NN classifier could be helpful in monitoring the condition of RA patients.

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.

Unintentional drifts during quiet stance and voluntary body sway

We explored unintentional drifts in voluntary whole-body sway tasks following the removal of visual feedback. The main hypothesis was that the unintentional drifts were produced by drifts of referent coordinates for salient performance variables. Young healthy subjects stood quietly on a force platform and also performed voluntary body sway at 0.5 Hz both in the anterio-posterior and medio-lateral directions. Visual feedback on the center of pressure (COP) coordinate was provided and then turned off. During quiet stance trials, the subjects matched the initial COP coordinate with a target shifted by 3 cm anterior, posterior, left, or right from the coordinate during natural standing and activated the right tibialis anterior to 30% of its maximal voluntary contraction. During cyclical voluntary sway task, the nominal sway amplitude was always 4 cm while the midpoint was at one of the four mentioned locations. Removing visual feedback caused COP drifts during quiet stance trials that were consistent across trials performed by a subject but could be in opposite directions across subjects; there was a consistent drop in the activation level of tibialis anterior. During voluntary body sway, removing visual feedback caused a consistent increase in the voluntary sway amplitude and a drift of the midpoint that was consistent within but not across subjects. Motor equivalent and non-motor equivalent inter-cycle motion components were quantified within the space of muscle groups (muscle modes) under visual feedback and at the end of the period without visual feedback. Throughout the trial, there were large motor equivalent motion components, and they increased over the period without visual feedback. The results corroborate the idea that referent coordinate drifts at different levels of the control hierarchy can lead to unintentional drifts in performance. It suggests that directions of COP drifts are defined by two main factors, drift of the body referent coordinate toward the actual coordinate (that can lead to fall) and an opposite drift to ensure body motion to a safer location. Analysis of motor equivalence suggests that postural stability is not compromised during unintentional drifts in performance in contrast to earlier studies of multi-finger tasks. This may be due to the vital importance of postural stability for everyday actions.

Static Posturography: A New Perspective in the Assessment of Lameness in a Canine Model

The aim of this study was to assess the static posturography in dogs as a useful tool for diagnosis of lameness by means of the use of a pressure platform. For this purpose, a series of different parameters (pressure distribution, area of support, mean pressure, maximum pressure and statokinesiograms) were obtained from five lame dogs with unilateral elbow osteoarthritis treated with plasma rich in growth factors. Data were obtained before and 3 months after treatment, and results were compared with a control group of sound dogs of similar conformation. Significant differences were found in the above mentioned parameters between sound and lame limbs. Improvement after 3 months of treatment was also detected, demonstrating that this multi-parametric technique is an effective and reliable method for the assessment of lameness in dogs.

Use of Mobile Device Accelerometry to Enhance Evaluation of Postural Instability in Parkinson Disease

OBJECTIVE

To determine the accuracy of inertial measurement unit data from a mobile device using the mobile device relative to posturography to quantify postural stability in individuals with Parkinson disease (PD).

DESIGN

Criterion standard.

SETTING

Motor control laboratory at a clinic.

PARTICIPANTS

A sample (N=28) of individuals with mild to moderate PD (n=14) and age-matched community-dwelling individuals without PD (n=14) completed the study.

INTERVENTIONS

Not applicable.

MAIN OUTCOME MEASURES

Center of mass (COM) acceleration measures were compared between the mobile device and the NeuroCom force platform to determine the accuracy of mobile device measurements during performance of the Sensory Organization Test (SOT). Analyses examined test-retest reliability of both systems and sensitivity of (1) the equilibrium score from the SOT and (2) COM acceleration measures from the force platform and mobile device to quantify postural stability across populations.

RESULTS

Metrics of COM acceleration from inertial measurement unit data and the NeuroCom force platform were significantly correlated across balance conditions and groups (Pearson r range, .35 to .97). The SOT equilibrium scores failed to discriminate individuals with and without PD. However, the multiplanar measures of COM acceleration from the mobile device exhibited good to excellent reliability across SOT conditions and were able to discriminate individuals with and without PD in conditions with the greatest balance demands.

CONCLUSIONS

Metrics employing medial-lateral movement produce a more sensitive outcome than the equilibrium score in identifying postural instability associated with PD. Overall, the output from the mobile device provides an accurate and reliable method of rapidly quantifying balance in individuals with PD. The portable and affordable nature of a mobile device with the application makes it ideally suited to use biomechanical data to aid in clinical decision making.

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.