Stability boundaries and lateral postural control in parkinson's disease

A static posturography guide to implementing time-to-boundary

The emergence of time-to-boundary provides an advanced representation of the spatiotemporal characteristics of postural control through the estimation of the time required for the center of pressure to reach the boundary of the base of support. Time-to-boundary has demonstrated its utility in several healthy and clinical adoptions; however, unknown inconsistencies among studies exist. Text and graphical representations understandably highlight idealistic standards, but new investigators to this measure are forced to wade through the same potential pitfalls that others have addressed, but the field has neglected to concatenate. The purpose of this communication is to share recent methodological advancements made to enhance time-to-boundary and describe the components of the time-to-boundary code that is being made publicly accessible for the first time. We anticipate future researchers who wish to apply this methodology to their data processing toolbox could utilize our script in full, with any deviations in potential future developments noted in clear fashion. Historically, researchers (including ourselves) have had to interpret text-based descriptions of the existing literature into quantitative steps in a computational mathematics script. In contrast to fixed process measures that do not require investigator input (e.g., path length), time-to-boundary poses two distinct but connected challenges to investigators. The coding process itself can be a hurdle for novices or practitioners. Second, transferring logical considerations such as robust, objective event detection routines must be defended in the review process. This comprehensive guide to time-to-boundary, as used in our applications, should enhance adoption and advance the comprehension of postural control.

Older adults and individuals with Parkinson's disease control posture along suborthogonal directions that deviate from the traditional anteroposterior and mediolateral directions

A rich and complex temporal structure of variability in postural sway characterizes healthy and adaptable postural control. However, neurodegenerative disorders such as Parkinson's disease, which often manifest as tremors, rigidity, and bradykinesia, disrupt this healthy variability. This study examined postural sway in young and older adults, including individuals with Parkinson's disease, under different upright standing conditions to investigate the potential connection between the temporal structure of variability in postural sway and Parkinsonism. A novel and innovative method called oriented fractal scaling component analysis was employed. This method involves decomposing the two-dimensional center of pressure (CoP) planar trajectories to pinpoint the directions associated with minimal and maximal temporal correlations in postural sway. As a result, it facilitates a comprehensive assessment of the directional characteristics within the temporal structure of sway variability. The results demonstrated that healthy young adults control posture along two orthogonal directions closely aligned with the traditional anatomical anteroposterior (AP) and mediolateral (ML) axes. In contrast, older adults and individuals with Parkinson's disease controlled posture along suborthogonal directions that significantly deviate from the AP and ML axes. These findings suggest that the altered temporal structure of sway variability is evident in individuals with Parkinson's disease and underlies postural deficits, surpassing what can be explained solely by the natural aging process.

Good times, bad times? An evaluation of event detection strategies in time to boundary postural assessments

Time-to-boundary (TtB) is a popular balance metric that identifies minimum reaction times available to correct balance challenges during quiet standing. Minimum event criteria is a critical methodological consideration to determine physiologically relevant TtB outcomes yet selection methodology appears inconsistent and/or vaguely defined across studies. This study aimed to identify a robust, objective methodology to select meaningful TtB outcomes. Ninety-seven healthy adults stood quietly on a force platform with eyes open and feet together. Anterior-posterior (AP) and medial-lateral (ML) center-of-pressure data from 150 s were utilized to compute a TtB series. The MATLAB findpeaks function identified minima with and without a time delay following selected events and/or a vertical axis threshold. An individualized time delay excluded excessively large values that hold no clinically relevant information, and this effect was enhanced by a vertical threshold at 22 s. The absolute minimum TtB was unaffected by any findpeaks criteria. The recommendations implicated by these results will help improve clarity and consistency among TtB studies, thereby enhancing the applicability of clinical findings.

Directional virtual time-to-contact: A new measure for investigating temporal, spatial, and control aspects of postural balance control

Virtual time-to-contact (VTC) is a promising approach for investigating postural balance control. However, current VTC calculation approaches are limited as they (1) cannot be used to evaluate directional components of balance, and (2) only assess a single, temporal aspect of balance control. This study introduces a new approach for VTC calculation, namely directional VTC, expanding VTC to assess temporal, spatial, and control aspects of balance. Three case studies were conducted across varying populations and conditions as a proof-of-concept of the presented method. The first study examined quiet stance on a firm surface in people with Parkinson's disease (PD; n = 10) in comparison to their healthy peers (n = 10). The second and third studies assessed balance control of healthy individuals under challenging environments. Ten healthy individuals participated in standing tasks on compliant ground surfaces, while another ten on oscillatory ground surfaces, all simulated by a dual-axis robotic platform. Preliminary results not only provided a closer look at balance control with multiple aspects, including temporal, spatial, and control aspects, but also showed how different aspects of balance changed due to neurological diseases (Case Study I) or challenging standing grounds (Case Studies II and III). This study advances our understanding of posture biomechanics and its clinical applications.

Characterization of trial duration in traditional and emerging postural control measures

Researchers may select from varied technological and practical options when evaluating balance. Methodological choices inform the quantitative outcomes observed and allow practitioners to diagnose balance abnormalities. Past investigations have differed widely on sampling duration, and these discrepancies hinder comparisons among studies and confidence in outcomes where trials were excessively short. This study aimed to identify necessary trial lengths for common and emerging center of pressure-based measures. We hypothesized that dependent variables would fluctuate over time but eventually reach a stable magnitude. Ninety-seven apparently healthy adults performed quiet standing for 180-seconds (s) with eyes (A) open and (B) closed on a force platform. Anterior-posterior and medial-lateral elements of the center of pressure were used to calculate velocity, time-to-boundary, and Hurst exponents using 15, 30, 90, 120, 150, and 180 s of data. Two-way repeated measures ANOVAs were used to differentiate postural measures over time and between vision conditions. Outcomes were considered stable when significant changes in the measure were no longer observed in the time factor. Dependent measures stabilized for velocity between 60 and 120 s, time-to-boundary between 120 and 150 s, and the Hurst exponent between 30 and 120 s. Velocity measures stabilized quicker with eyes open, whereas vision had no effect or the eyes closed condition was faster to stabilize in time-to-boundary and detrended fluctuation analysis measures. We conclude that 150 s of standing data is sufficient to capture a broad range of postural stability outcomes regardless of vision condition.

The Feasibility of Using the Virtual Time-to-Contact Measure of Postural Stability to Examine Postural Recovery in People With Diabetes Mellitus

This study aimed to examine the feasibility of using time-to-contact measures during the perturbation protocol in people with diabetes mellitus. Three-dimension motion capture and force data were collected during 0.5-s perturbations in four directions (forward, backward, right, and left) and at two accelerations (20 and 40 cm/s2) to compute the time-to-contact. Time-to-contact analysis was divided into three phases: perturbation, initial recovery, and final recovery. The statistical analysis showed the main effects of Direction and Phase (p < .01) as well as a Direction by Phase interaction (p < .01). Backward perturbation with lower acceleration and backward/forward perturbation with higher acceleration had deleterious effects on postural stability in people with diabetes mellitus.

Effects of dual-tasking on time-to-boundary during stance in people with PD: A preliminary study

BACKGROUND

Quiet stance is impacted by Parkinson's disease and dual-tasking. Recently developed outcomes such as the time-to-boundary provide unique insight into balance by integrating center of pressure position with base of support. However, little is known about the effects of Parkinson's disease on time-to-boundary. In particular, the effects of distracting cognitive tasks, and how people with Parkinson's disease prioritize balance and cognitive tasks are poorly understood.

METHODS

14 people with Parkinson's disease and 13 controls completed quiet standing and cognitive Stroop tasks separately (single-task) and together (dual-task). 2-dimentional, medio-lateral, and anterior-posterior time-to-boundary were calculated via force-plate data. Traditional sway outcomes, including sway area and path length, were also calculated. Cognitive performance was measured as the verbal reaction time after auditory stimulus delivery. Prioritization was assessed by taking the difference between cognitive and postural dual-task interference.

FINDINGS

Time-to-boundary was worse in Parkinson's disease compared to controls (2-dimentional: p = .019; anterior-posterior: p = .062; medio-lateral: p = .012). Medio-lateral time-to-boundary, but not anterior-posterior, was significantly worse during dual-tasking than single-tasking (p = .024). Neurotypical adults tended to prioritize cognition over medio-lateral postural outcomes.

INTERPRETATION

People with Parkinson's disease exhibit worse time-to-boundary than their neurotypical peers, and medio-lateral outcomes were sensitive to single to dual-task performance changes. Further, participants generally showed cognitive prioritization, such that cognitive performance was less impacted than medio-lateral postural outcomes by dual-tasking.

The assessment of center of mass and center of pressure during quiet stance: Current applications and future directions

This perspective article provides a brief review of our understanding of how center of pressure (CoP) and center of mass (CoM) are traditionally utilized to measure quiet standing and how technological advancements are allowing for measurements to be derived outside the confines of a laboratory setting. Furthermore, this viewpoint provides descriptions of what CoP and CoM outcomes may reflect, a discussion of recent developments in selected balance outcomes, the importance of measuring instantaneous balance outcomes, and directions for future questions/research. Considering the enormous number and cost of falls annually, conclusions drawn from this perspective underscore the need for more cohesive efforts to advance our understanding of balance performance. As we refine the technology and algorithms used to portably assess postural stability, the question of which measurement (i.e. CoP or CoM) to utilize seems to be highly dependent on the question being asked. Further, the complexity of the question appears to span multiple disciplines and cultivate exploration of the intrinsic mechanisms of stability. Recently developed multi-dimensional methods for assessing balance performance may provide additional insight into balance, improving our ability to predict balance impairments and falls outside the laboratory and in the clinic. However, additional work will be necessary to understand the clinical significance and predictive capacity of these outcomes in various fall-prone populations.

Gait alterations on irregular surface in people with Parkinson's disease

BACKGROUND

Persons with Parkinson's disease are at high risk for fall-related injuries with a large proportion of falls occurring while walking, especially when the walking environments are complex. The aim of this study was to characterize gait parameters on irregular surface for persons with Parkinson's disease.

METHODS

Three-dimensional gait analysis was conducted for nine persons with Parkinson's disease and nine healthy age-matched adults on both regular and irregular surfaces. Repeated ANOVA and paired t-test were performed to determine the effect of surface and group for spatiotemporal, kinematic and stability variables.

FINDINGS

Individuals with Parkinson's disease showed a larger ratio of reduction for speed, cadence and step length than controls when the surface changed from regular to irregular. The ankle transverse range of motion and root mean square of trunk acceleration increased on irregular surface for both groups. Additionally, individuals with Parkinson's disease demonstrated a decreased knee sagittal range of motion and trunk frontal and transverse range of motion compared with controls, especially on the irregular surface.

INTERPRETATION

The irregular surface posed a greater challenge to maintain balance and stability for individuals with Parkinson's disease. A relatively small knee range of motion in the sagittal plane and large root mean square of trunk acceleration increased the potential fall risk for individuals with Parkinson's disease. This information improves the understanding of parkinsonian gait adaptations on irregular surfaces and may guide gait training and rehabilitation interventions for this high fall-risk population.

Motor Subtypes of Parkinson's Disease Can Be Identified by Frequency Component of Postural Stability

Parkinson’s disease (PD) can be divided into two subtypes based on clinical features—namely tremor dominant (TD) and postural instability and gait difficulty (PIGD). This categorization is important at the early stage of PD, since identifying the subtypes can help to predict the clinical progression of the disease. Accordingly, correctly diagnosing subtypes is critical in initiating appropriate early interventions and tracking the progression of the disease. However, as the disease progresses, it becomes increasingly difficult to further distinguish those attributes that are relevant to the subtypes. In this study, we investigated whether a method using the standing center of pressure (COP) time series data can separate two subtypes of PD by looking at the frequency component of COP (i.e., COP position and speed). Thirty-six participants diagnosed with PD were evaluated, with their bare feet on the force platform, and were instructed to stand upright with their arms by their sides for 20 s (with their eyes open and closed), which is consistent with the traditional COP measures. Fast Fourier transform (FFT) and wavelet transform (WT) were performed to distinguish between the motor subtypes using the COP measures. The TD group exhibited larger amplitudes at the frequency range of 3–7 Hz when compared to the PIGD group. Both the FFT and WT methods were able to differentiate the subtypes. COP time series information can be used to differentiate between the two motor subtypes of PD, using the frequency component of postural stability.

Sensor assisted self-management in Parkinson's disease: A feasibility study of ambulatory posture detection and feedback to treat stooped posture

INTRODUCTION

A stooped posture is one of the characteristic motor symptoms of patients with Parkinson's disease, and has been linked to impairments in daily activities and quality of life. We aimed to test the efficacy, safety, practical utility and user-friendliness of a posture correction and vibrotactile trunk angle feedback device (the UpRight) in the home setting of patients with Parkinson's disease with a stooped posture. It was hypothesized that ambulatory use of the UpRight would be safe, feasible and result in a less stooped posture, i.e. a lower trunk angle during daily activities.

METHODS

15 patients wore the UpRight during a baseline period of 1 week (no feedback), followed by an intervention period of 1 week (feedback).

RESULTS

We found a significant decrease (average -5,4°) in trunk angle from baseline period to intervention period without the occurrence of adverse events. In addition, patients found the device usable and beneficial to posture.

CONCLUSION

Use of the feedback and correction device has a positive effect on ambulatory trunk angles. The device appears to be both safe and useful for self-management of stooped posture in patients with Parkinson's Disease.

Time-to-Boundary Function to Study the Development of Upright Stance Control in Children

Background:

The development of postural control across the primary school time horizon is a complex process, which entails biomechanics modifications, the maturation of cognitive ability and sensorimotor organization, and the emergence of anticipatory behaviour. Postural stability in upright stance has been thus object of a multiplicity of studies to better characterize postural control in this age span, with a variety of methodological approaches. The analysis of the Time-to-Boundary function (TtB), which specifies the spatiotemporal proximity of the Centre of Pressure (CoP) to the stability boundaries in the regulation of posture in upright stance, is among the techniques used to better characterize postural stability in adults, but, as of now, it has not yet been introduced in developmental studies. The aim of this study was thus to apply this technique to evaluate the development of postural control in a sample population of primary school children.

Methods:

In this cross-sectional study, upright stance trials under eyes open and eyes closed were administered to 107 healthy children, divided into three age groups (41 for Seven Years' Group, Y7; 38 for Nine Years' Group, Y9; 28 for Eleven Years' Group, Y11). CoP data were recorded to calculate the Time-to-Boundary function (TtB), from which four spatio-temporal parameters were extracted: the mean value and the standard deviation of TtB minima (M_min, Std_min), and the mean value and the standard deviation of the temporal distance between two successive minima (M_dist, Std_dist).

Results:

With eyes closed, M_min and Std_min significantly decreased and M_dist and Std_dist increased for the Y7 group, at Y9 M_min significantly decreased and Std_dist increased, while no effect of vision resulted for Y11. Regarding age groups, M_min was significantly higher for Y9 than Y7, and Std_min for Y9 was higher than both Y7 and Y11; M_dist and Std_dist resulted higher for Y11 than for Y9.

Conclusion:

From the combined results from the spatio-temporal TtB parameters, it is suggested that, at 9 years, children look more efficient in terms of exploring their limits of stability than at 7, and at 11 the observed TtB behaviour hints at the possibility that, at that age, they have almost completed the maturation of postural control in upright stance, also in terms of integration of the spatio-temporal information.

Postural Control in Young People with Visual Impairments and Various Risks of Falls

Introduction

Early diagnosis of postural control deficiencies facilitates implementation of an individual rehabilitation plan to prevent falls. The aim of the study was to assess the risk of falling in individuals with visual impairments, and to compare performance-based and theoretical limits of stability in subjects with various risks of falling.

Methods

The study was comprised of 23 participants with severe visual impairments. The risk of fall was assessed with the Step Test. Performance-based limits of stability were measured with the Advanced Mechanical Technology, Inc. (AMTI) platform, and theoretical limits of stability were calculated based on the height of the center of mass (COM) and maximum body sway leaning angles.

Results

COM displacement values, corresponding to performance-based limits of stability, in individuals with visual impairments whose risk of falling was classified as high were significantly lower than the theoretical values (p ≤ .05). Similar differences were not observed in participants with visual impairments whose risk of falling was assessed as low. Individuals from the low-risk group showed significantly higher values of performance-based limits of stability than the participants from the high-risk group (p ≤ .01 for medio-lateral direction and p ≤ .05 for forward-backward direction).

Discussion

Nearly half of young people with visual impairments are at increased risk of falling. Such persons showed lower performance-based limits of stability than participants with visual impairments with a low risk of falling. Performance-based limits of stability in individuals with visual impairments with a high risk of falling are lower than their theoretical limits of stability.

Implications for practitioners

The fact that nearly half of young people with visual impairments are at an increased risk of falling necessitates implementation of preventive measures in this group, as well as among individuals who are blind.

Effect of Biomechanical Constraints on Neural Control of Head Stability in Children With Moderate to Severe Cerebral Palsy

BACKGROUND

External support has been viewed as an important biomechanical constraint for children with deficits in postural control. Nonlinear analysis of head stability may be helpful to confirm benefits of interaction between external trunk support and level of trunk control.

OBJECTIVE

The purpose of this study was to compare the effect of biomechanical constraints (trunk support) on neural control of head stability during development of trunk control.

DESIGN

This was a quasi-experimental repeated-measures study.

METHODS

Data from 15 children (4-16 years of age) with moderate (Gross Motor Function Classification System [GMFCS] IV; n=8 [4 boys, 4 girls]) or severe (GMFCS V; n=7 [4 boys, 3 girls]) cerebral palsy (CP) were compared with previous longitudinal data from infants with typical development (TD) (3-9 months of age). Kinematic data were used to document head sway with external support at 4 levels (axillae, midrib, waist, and hip). Complexity, predictability, and active degrees of freedom for both anterior-posterior and medial-lateral directions were assessed.

RESULTS

Irrespective of level of support, CP groups had lower complexity, increased predictability, and greater degrees of freedom. The effect of support differed based on the child's segmental level of control. The GMFCS V and youngest TD groups demonstrated better head control, with increased complexity and decreased predictability, with higher levels of support. The GMFCS IV group had the opposite effect, showing decreased predictability and increased complexity and degrees of freedom with lower levels of support.

LIMITATIONS

Infants with typical development and children with CP were compared based on similar segmental levels of trunk control; however, it is acknowledged that the groups differed for age, cognitive level, and motor experience.

CONCLUSIONS

The effect of external support varied depending on the child's level of control and diagnostic status. Children with GMFCS V and young infants with TD had better outcomes with external support, but external support was not enough to completely correct for the influence of CP. Children with GMFCS IV performed worse, with increased predictability and decreased complexity, when support was at the axillae or midribs, suggesting that too much support can interfere with postural sway quality.

Transitions of postural coordination as a function of frequency of the moving support platform

This study was set-up to investigate the multi-segmental organization of human postural control in a dynamic balance task. The focus was on the coupling between the center of mass (CoM) and center of pressure (CoP) as a candidate collective variable that supports maintaining balance on a sinusoidal oscillating platform in the medial-lateral (ML) plane and was continuously scaled up and then down across a frequency range from 0.2Hz to 1.2Hz. The CoM-CoP coordination changed from in-phase to anti-phase and anti-phase to in-phase at a critical frequency (∼0.4Hz to 0.6Hz, respectively) in the scaling up or down of the support surface frequency, showed hysteresis as a function of the direction of frequency change and critical fluctuations at the transition region. There was evidence of head motion independent of CoM motion at the higher platform frequencies and a learning effect on several of the dynamic indices over 2days of practice. The findings are consistent with the hypothesis of CoM-CoP acting as an emergent collective variable that is supported by the faster time scale motions of the joints and their synergies in postural control.

Postural Stability Margins as a Function of Support Surface Slopes

This investigation examined the effects of slope of the surface of support (35°, 30°, 20°, 10° Facing(Toe) Down, 0° Flat and 10°, 20°, 25° Facing (Toe) Up) and postural orientation on the margins of postural stability in quiet standing of young adults. The findings showed that the center of pressure—CoP (displacement, area and length) had least motion at the baseline (0° Flat) platform condition that progressively increased as a function of platform angle in both facing up and down directions. The virtual time to collision (VTC) dynamics revealed that the spatio-temporal margins to the functional stability boundary were progressively smaller and the VTC time series also more regular (SampEn–Sample Entropy) as slope angle increased. Surface slope induces a restricted stability region with lower dimension VTC dynamics that is more constrained when postural orientation is facing down the slope. These findings provide further evidence that VTC acts as a control variable in standing posture that is influenced by the emergent dynamics of the individual-environment-task interaction.

What Could Posturography Tell Us About Balance Problems in Parkinson's Disease?

OBJECTIVE

Impaired balance in patients with Parkinson's disease (PD) leads to loss of balance and frequent falls. Computerized dynamic posturography allows the assessment of stance tasks whereas mobile posturography analyzes the balance in free-field conditions, where falls among PD patients commonly occur (e.g. sitting down or standing up). The aim of the present study is to assess postural stability in PD patients with both techniques.

STUDY DESIGN

Prospective study.

SETTING

University Hospitals, ambulatory care (outpatient clinic).

PATIENTS

Thirty-three patients diagnosed with idiopathic PD.

INTERVENTION

Balance assessment.

MAIN OUTCOME MEASURES

Dizziness handicap inventory (DHI), activities-specific balance confidence scale (ABC), composite score of sensory organization test (SOT), results of free-field body sway analysis (standard balance deficit test (SBDT)), or geriatric SBDT.

RESULTS

PD patients showed a significantly higher sway in the roll direction in almost all of the SBDT conditions. Also, pathological sway compared with normative values was more prominent in complex tasks. There is a significant correlation between the different objective variables of the postural study (SOT and SBDT) and the ABC, but not with the DHI. Finally, the percentage of PD patients with a pathological score in SOT-composite score was 54.5% whereas in SBDT-composite score it was significantly higher (93.9%).

CONCLUSION

Mobile posturography is more accurate in depicting the reality of balance impairment in PD patients than platform posturography. Also, ABC relates better than DHI to the significant psychological consequences of balance impairments. An increased lateral trunk sway seems to be a key factor of postural instability in PD patients.

The effects of visual feedback during a rhythmic weight-shifting task in patients with Parkinson's disease

Augmented visual feedback (VF) may offer benefits similar to those of rhythmic external cues in alleviating some mobility-related difficulties in individuals with Parkinson's disease (PD). However, due to an impaired ability to reweigh sensory information under changing circumstances, subjects with PD may be rather vulnerable to incongruity of visual information. In the present study, we investigated whether VF is indeed effective in improving motor functioning in a weight-shifting task during upright stance, and whether subjects with PD are affected more by incongruent VF than healthy controls. Participants performed sideways swaying motions based on tracking of real-time and delayed VF - the first providing congruent, and hence more accurate, visual information than the latter. We analyzed center-of-pressure signals patterns for 28 individuals with PD and 16 healthy, age- and gender-matched controls by estimating task accuracy, movement pattern variability, and normalized movement amplitude. For conditions without feedback and with real-time feedback, subjects with PD performed lateral swaying motions with greater error (F(1, 42)=12.065, p=.001) and with more variable movement patterns than healthy controls (F(1, 24)=113.086, p<.001). Error change scores revealed that patients with PD were nevertheless still able to use VF to improve tracking performance (t(24)=-2.366, p=.026). However, whereas controls were able to adapt to a certain amount of visual incongruity, patients with PD were not. Instead, movement amplitude was significantly reduced in this group (F(1.448, 60.820)=17.639, p<.001). By reducing movement amplitude, subjects with PD appear to resort to a 'conservative' strategy to minimize performance breakdown.

Comparison of the Fullerton Advanced Balance Scale, Mini-BESTest, and Berg Balance Scale to Predict Falls in Parkinson Disease

BACKGROUND

The correct identification of patients with Parkinson disease (PD) at risk for falling is important to initiate appropriate treatment early.

OBJECTIVE

This study compared the Fullerton Advanced Balance (FAB) scale with the Mini-Balance Evaluation Systems Test (Mini-BESTest) and Berg Balance Scale (BBS) to identify individuals with PD at risk for falls and to analyze which of the items of the scales best predict future falls.

DESIGN

This was a prospective study to assess predictive criterion-related validity.

SETTING

The study was conducted at a university hospital in an urban community.

PATIENTS

Eighty-five patients with idiopathic PD (Hoehn and Yahr stages: 1-4) participated in the study.

MEASUREMENTS

Measures were number of falls (assessed prospectively over 6 months), FAB scale, Mini-BESTest, BBS, and Unified Parkinson's Disease Rating Scale.

RESULTS

The FAB scale, Mini-BESTest, and BBS showed similar accuracy to predict future falls, with values for area under the curve (AUC) of the receiver operating characteristic (ROC) curve of 0.68, 0.65, and 0.69, respectively. A model combining the items "tandem stance," "rise to toes," "one-leg stance," "compensatory stepping backward," "turning," and "placing alternate foot on stool" had an AUC of 0.84 of the ROC curve.

LIMITATIONS

There was a dropout rate of 19/85 participants.

CONCLUSIONS

The FAB scale, Mini-BESTest, and BBS provide moderate capacity to predict "fallers" (people with one or more falls) from "nonfallers." Only some items of the 3 scales contribute to the detection of future falls. Clinicians should particularly focus on the item "tandem stance" along with the items "one-leg stance," "rise to toes," "compensatory stepping backward," "turning 360°," and "placing foot on stool" when analyzing postural control deficits related to fall risk. Future research should analyze whether balance training including the aforementioned items is effective in reducing fall risk.

Rethinking energy in parkinsonian motor symptoms: a potential role for neural metabolic deficits

Parkinson’s disease (PD) is characterized as a chronic and progressive neurodegenerative disorder that results in a variety of debilitating symptoms, including bradykinesia, resting tremor, rigidity, and postural instability. Research spanning several decades has emphasized basal ganglia dysfunction, predominantly resulting from dopaminergic (DA) cell loss, as the primarily cause of the aforementioned parkinsonian features. But, why those particular features manifest themselves remains an enigma. The goal of this paper is to develop a theoretical framework that parkinsonian motor features are behavioral consequence of a long-term adaptation to their inability (inflexibility or lack of capacity) to meet energetic demands, due to neural metabolic deficits arising from mitochondrial dysfunction associated with PD. Here, we discuss neurophysiological changes that are generally associated with PD, such as selective degeneration of DA neurons in the substantia nigra pars compacta (SNc), in conjunction with metabolic and mitochondrial dysfunction. We then characterize the cardinal motor symptoms of PD, bradykinesia, resting tremor, rigidity and gait disturbance, reviewing literature to demonstrate how these motor patterns are actually energy efficient from a metabolic perspective. We will also develop three testable hypotheses: (1) neural metabolic deficits precede the increased rate of neurodegeneration and onset of behavioral symptoms in PD; (2) motor behavior of persons with PD are more sensitive to changes in metabolic/bioenergetic state; and (3) improvement of metabolic function could lead to better motor performance in persons with PD. These hypotheses are designed to introduce a novel viewpoint that can elucidate the connections between metabolic, neural and motor function in PD.

Instrumented functional reach test differentiates individuals at high risk for Parkinson's disease from controls

The functional reach (FR) test as a complex measure of balance including limits of stability has been proven to differentiate between patients with Parkinson’s disease (PD) and controls (CO). Recently, it has been shown that the instrumentation of the FR (iFR) with a wearable sensor may increase this diagnostic accuracy. This cross-sectional study aimed at investigating whether the iFR has the potential to differentiate individuals with high risk for PD (HRPD) from CO, as the delineation of such individuals would allow for, e.g., early neuromodulation. Thirteen PD patients, 13 CO, and 31 HRPD were investigated. HRPD was defined by presence of an enlarged area of hyperechogenicity in the mesencephalon on transcranial sonography and either one motor sign or two risk and prodromal markers of PD. All participants were asked to reach with their right arm forward as far as possible and hold this position for 10 s. During this period, sway parameters were assessed with an accelerometer (Dynaport, McRoberts) worn at the lower back. Extracted parameters that differed significantly between PD patients and CO in our cohort [FR distance (shorter in PD), anterior–posterior and mediolateral acceleration (both lower in PD)] as well as JERK, which has been shown to differentiate HRPD from CO and PD in a previous study, were included in a model, which was then used to differentiate HRPD from CO. The model yielded an area under the curve of 0.77, with a specificity of 85%, and a sensitivity of 74%. These results suggest that the iFR can contribute to an assessment panel focusing on the definition of HRPD individuals.

Comparing the Fullerton Advanced Balance Scale with the Mini-BESTest and Berg Balance Scale to assess postural control in patients with Parkinson disease

OBJECTIVES

To validate the Fullerton Advanced Balance (FAB) Scale for patients with idiopathic Parkinson disease (PD); and to compare the FAB Scale with the Mini-Balance Evaluation Systems Test (Mini-BESTest) and Berg Balance Scale (BBS).

DESIGN

Observational study to assess concurrent validity, test-retest, and interrater reliability of the FAB Scale in patients with PD and to compare the distribution of the scale with the Mini-BESTest and BBS.

SETTING

University hospital in an urban community.

PARTICIPANTS

Patients with idiopathic PD (N=85; Hoehn and Yahr stages 1-4).

INTERVENTIONS

Not applicable.

MAIN OUTCOME MEASURES

FAB Scale, Mini-BESTest, BBS, timed Up and Go test, Unified Parkinson's Disease Rating Scale, and visual analog scale.

RESULTS

Interrater (3 raters) and test-retest (3±1 d) reliability were high for all scales (ICCs≥.95). The FAB Scale was highly correlated with the Mini-BESTest (Spearman ρ=.87) and timed Up and Go test item of the Mini-BESTest (Spearman ρ=.83). In contrast with the BBS, the FAB Scale and Mini-BESTest have only minimal ceiling effects. The FAB Scale demonstrated the most symmetric distribution when compared with the Mini-BESTest and BBS (skewness: FAB scale: -.54; Mini-BESTest: -1.07; BBS: -2.14).

CONCLUSIONS

The FAB Scale is a valid and reliable tool to assess postural control in patients with PD. No ceiling effect was noted for the FAB Scale. Although the items of the FAB Scale are more detailed when compared with the Mini-BESTest, interrater and test-retest reliability were excellent. The scale is a promising tool to detect small changes of the postural control system in individuals with PD.

Evaluating runners with and without anterior knee pain using the time to contact the ankle joint complexes' range of motion boundary

BACKGROUND

Little biomechanical evidence exists to support the association between excessive foot pronation and anterior knee pain (AKP). One issue could be the way excessive pronation has been defined. Recent evidence has suggested that evaluating pronation in the context of the joint's available range of motion (ROM, anatomical threshold) provides greater insight on when pronation contributes to injury. Theoretically, quantifying the amount of time the joint has to respond before reaching end range (neuromuscular threshold) could provide additional insight. Therefore the purpose of this study was to use a neuromuscular threshold, the time to contact (TtC) the ankle joint complex's ROM boundary, to evaluate runners with and without AKP.

METHODS

Nineteen healthy and seventeen runners with AKP had their ROM and running biomechanics evaluated. The TtC was calculated using each individual's angular distance from end range (eversion buffer) and eversion velocity. Data were recorded over ten stance phases and evaluated using a one way analysis of variance and 95% confidence intervals.

RESULTS

Runners with AKP had significantly shorter TtC the joint's ROM boundary when compared to healthy runners (64.0 ms vs. 35.6 ms, p=0.01). While not statistically significant, this shorter TtC was in large part due to having a smaller eversion buffer, however velocity was found to have a substantial influence on the TtC of select individuals. These results provide evidence that a link between pronation and AKP exists when using anatomical and neuromuscular based thresholds.

Effect of elastic taping on postural control deficits in subjects with healthy ankles, copers, and individuals with functional ankle instability

BACKGROUND

Ankle sprains are the most common injury among physically active people, with common sequelae including repeated episodes of giving way, termed functional ankle instability. Copers are a cohort in ankle research comprised of those who have sprained their ankle but have not suffered any further dysfunction. The use of an elastic tape, Kinesio Tape, in sports medicine practice has recently gained popularity and may help improve postural control deficits related to functional ankle instability. The purpose of this study was to examine the immediate and prolonged effects of Kinesio Taping on postural control in healthy, coper, and unstable ankles as measured through single-limb balance on a force plate.

METHODS

Sixty physically active, college-aged participants (72.5 ± 9.7 cm, 74.2 ± 16.2 kg, 21.5 ± 2.6 years) were stratified into healthy, coper, or unstable groups using the Cumberland Ankle Instability Tool (CAIT) combined with their history of ankle injury. Dependent variables included time-to-boundary (TTB) measures and traditional center of pressure (COP) measures in both the mediolateral (frontal) and anteroposterior (sagittal) planes. Testing was performed prior to tape application, immediately after application of the tape, 24 hours following tape application, and immediately after tape removal.

RESULTS

Significant differences between groups were observed for COP standard deviation and range in the sagittal plane. Significant differences between tape conditions for TTB absolute minima and standard deviation were also noted. Post hoc testing revealed large to medium effect sizes for the group differences and very small effect sizes for the differences between conditions.

CONCLUSIONS

Our study did not reveal decisively relevant changes following application of Kinesio Tape to the ankle. However, we did observe sagittal plane postural control deficits in subjects with ankle instability measured through summary COP variables over 20-second trials.

CLINICAL RELEVANCE

Ankle instability is a concern for many clinicians. Kinesio Taping, although a popular form of clinical intervention, remains understudied. Evidence from this study does not support the use of Kinesio Taping for improving postural control deficits in those with ankle instability.

A Systems Perspective on Postural and Gait Stability: Implications for Physical Activity in Aging and Disease

Postural instability, falls, and fear of falling that accompany frailty with aging and disease form major impediments to physical activity. In this article we present a theoretical framework that may help researchers and practitioners in the development and delivery of intervention programs aimed at reducing falls and improving postural stability and locomotion in older individuals and in those with disability due to disease. Based on a review of the dynamical and complex systems perspectives of movement coordination and control, we show that 1) central to developing a movement-based intervention program aimed at fall reduction and prevention is the notion that variability can play a functional role and facilitate movement adaptability, 2) intervention programs aimed at fall reduction should focus more on coordination and stability boundary measures instead of traditional gait and posture outcome variables, and 3) noise-based intervention techniques using stochastic resonance may offer external aids to improve dynamic balance control.

A cane improves postural recovery from an unpracticed slip during walking in people with Parkinson disease

BACKGROUND

Little is known about the effects of use of a cane on balance during perturbed gait or whether people with Parkinson disease (PD) benefit from using a cane.

OBJECTIVES

The purpose of this study was to evaluate the effects of cane use on postural recovery from a slip due to repeated surface perturbations in individuals with PD compared with age- and sex-matched individuals who were healthy.

DESIGN

This was a prospective study with 2 groups of participants.

METHODS

Fourteen individuals with PD (PD group) and 11 individuals without PD (control group) walked across a platform that translated 15 cm rightward at 30 cm/s during the single-limb support phase of the right foot. Data from 15 trials in 2 conditions (ie, with and without an instrumented cane in the right hand) were collected in random order. Outcome measures included lateral displacement of body center of mass (COM) due to the slip and compensatory step width and length after the perturbation.

RESULTS

Cane use improved postural recovery from the first untrained slip, characterized by smaller lateral COM displacement, in the PD group but not in the control group. The beneficial effect of cane use, however, occurred only during the first perturbation, and those individuals in the PD group who demonstrated the largest COM displacement without a cane benefited the most from use of a cane. Both PD and control groups gradually decreased lateral COM displacement across slip exposures, but a slower learning rate was evident in the PD group participants, who required 6, rather than 3, trials for adapting balance recovery.

LIMITATIONS

Future studies are needed to examine the long-term effects of repeated slip training in people with PD.

CONCLUSIONS

Use of a cane improved postural recovery from an unpracticed slip in individuals with PD. Balance in people with PD can be improved by training with repeated exposures to perturbations.

Redução do limite de estabilidade direção-específica em indivíduos leve a moderadamente afetados pela doença de Parkinson

A instabilidade postural na doença de Parkinson (DP) tem sido associada a uma diminuição do limite de estabilidade (LE) na direção ântero-posterior (AP). Entretanto, ainda que possíveis alterações do LE na direção látero-lateral (LL) tenham sido sugeridas, tal direção não tem sido avaliada nos estudos com DP, principalmente quando o teste de limite de estabilidade (TLE) envolve movimentos intencionais que deslocam o centro de massa corporal (CMC). O objetivo do presente estudo foi investigar o LE na postura de pé durante movimentos voluntários que promovem deslocamento do CMC nas direções AP e LL de indivíduos com e sem a DP. Doze indivíduos com DP (Hoehn & Yahr=II, III) e 12 sem a doença realizaram o TLE nos sentidos anterior, posterior, direito e esquerdo. A velocidade de movimento (VM), excursão máxima (EM) e o controle direcional (CD) do CMC foram avaliados em cada sentido. Os indivíduos com DP foram significativamente mais lentos em todos os sentidos de deslocamento do CMC (p<0,05). Não houve diferença significativa na EM e CD no sentido anterior entre os grupos (p>0,05). Por outro lado, a EM e CD do CMC foram menores para o grupo DP no sentido posterior (P) e na direção LL (p<0,05). Indivíduos leves a moderadamente afetados pela DP apresentaram redução do LE no sentido P e na direção LL quando comparados ao grupo controle. Os resultados sugerem que tal direção e sentido devam ser treinados em ortostatismo, com movimentos que deslocam voluntariamente o CMC, desde fases iniciais da DP.

Postural control in women with multiple sclerosis: effects of task, vision and symptomatic fatigue

People with multiple sclerosis (MS) often report problems with balance, which may be most apparent during challenging postural tasks such as leaning or reaching, and when relying on non-visual sensory systems. An additional obstacle facing people with MS is a high incidence of symptomatic fatigue (>70%). The purpose of this study was to investigate the changes in balance during upright stance in individuals with mild-to-moderate disability due to MS under normal and restricted vision and different levels of self-reported fatigue. Limb loading asymmetry, sway and magnitude of postural shift in center of pressure, and time-to-contact the stability boundary of the center of mass and center of pressure were assessed during quiet standing and maximal lean and reach tasks. Compared to controls, people with MS displayed greater postural sway, greater loading asymmetry, and shorter time-to-contact during quiet standing. In the postural perturbation tasks the MS group had smaller postural shifts and reduced stability compared to controls in the direction perpendicular to the lean and reach. Limiting vision increased loading asymmetry during quiet standing and postural instability during backward lean in the MS group. Inducing additional fatigue in the MS group did affect postural control in the more challenging balance conditions but had no impact during quiet upright standing. The results of this study indicate subtle changes in postural control during standing in people with mild-to-moderate impairments due to MS.

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.

Obstacle crossing in people with Parkinson's disease: foot clearance and spatiotemporal deficits

This study investigates the effects of Parkinson's disease (PD) on foot trajectories and spatiotemporal gait adaptations when approaching and stepping over a ground-based obstacle. Twenty people with mild-moderate PD and 20 age and sex matched controls walked 10 steps at their preferred speed along a walkway and stepped over an obstacle (height 10% of leg length × 600 mm × 10 mm). Control participants also performed trials at the same speed and step length as their matched PD participant. People with PD approached and stepped over the obstacle slower and with smaller steps, but had a similar foot clearance. Those with PD were also more likely to step on the obstacle because they did not place their foot close enough to the front of the obstacle before crossing it to accommodate for their reduced step length. During the lead limb crossing step, people with PD increased their step width, whereas controls maintained a narrow step width. These findings indicate that people with PD have difficulty lengthening their step over the obstacle rather than increasing foot height. Increasing step width is a possible compensation strategy used to overcome postural instability during obstacle crossing in those with PD.

Alternate rhythmic vibratory stimulation of trunk muscles affects walking cadence and velocity in Parkinson's disease

OBJECTIVE

During the administration of timed bilateral alternate vibration to homonymous leg or trunk muscles during quiet upright stance, Parkinsonian (PD) patients undergo cyclic antero-posterior and medio-lateral transfers of the centre of foot pressure. This event might be potentially exploited for improving gait in these patients. Here, we tested this hypothesis by applying alternate muscle vibration during walking in PD.

METHODS

Fifteen patients and 15 healthy subjects walked on an instrumented walkway under four conditions: no vibration (no-Vib), and vibration of tibialis anterior (TA-Vib), soleus (Sol-Vib) and erector spinae (ES-Vib) muscles of both sides. Trains of vibration (internal frequency 100 Hz) were delivered to right and left side at alternating frequency of 10% above preferred step cadence.

RESULTS

During vibration, stride length, cadence and velocity increased in both patients and healthy subjects, significantly so for ES-Vib. Stance and swing time tended to decrease. Width of support base increased with Sol-Vib or TA-Vib, but was unaffected by ES-Vib.

CONCLUSIONS

Alternate ES vibration enhances gait velocity in PD. The stronger effect of ES over leg muscle vibration might depend on the relevance of the proprioceptive inflow from the trunk muscles and on the absence of adverse effects on the support base width.

SIGNIFICANCE

Trunk control is defective in PD. The effect of timed vibratory stimulation on gait suggests the potential use of trunk proprioceptive stimulation for tuning the central pattern generators for locomotion in PD.

Neural basis of postural instability identified by VTC and EEG

In this study, we investigated the neural basis of virtual time to contact (VTC) and the hypothesis that VTC provides predictive information for future postural instability. A novel approach to differentiate stable pre-falling and transition-to-instability stages within a single postural trial while a subject was performing a challenging single leg stance with eyes closed was developed. Specifically, we utilized wavelet transform and stage segmentation algorithms using VTC time series data set as an input. The VTC time series was time-locked with multichannel (n = 64) EEG signals to examine its underlying neural substrates. To identify the focal sources of neural substrates of VTC, a two-step approach was designed combining the independent component analysis (ICA) and low-resolution tomography (LORETA) of multichannel EEG. There were two major findings: (1) a significant increase of VTC minimal values (along with enhanced variability of VTC) was observed during the transition-to-instability stage with progression to ultimate loss of balance and falling; and (2) this VTC dynamics was associated with pronounced modulation of EEG predominantly within theta, alpha and gamma frequency bands. The sources of this EEG modulation were identified at the cingulate cortex (ACC) and the junction of precuneus and parietal lobe, as well as at the occipital cortex. The findings support the hypothesis that the systematic increase of minimal values of VTC concomitant with modulation of EEG signals at the frontal-central and parietal-occipital areas serve collectively to predict the future instability in posture.

Walking stability using harmonic ratios in Parkinson's disease

Kinematic changes in Parkinson's disease (PD) gait are well documented; however, upper body dynamics are less understood. Harmonic ratios (HRs) measure the rhythm of trunk accelerations and can be examined in the vertical, anterior-posterior, and mediolateral planes, providing an indication of global walking stability (lower HR indicates poorer stability). We examined differences in HRs between persons with PD and healthy older adults and relationships between HRs and stride parameters. Eleven people with PD and 11 older adults walked over ground at their preferred pace. A triaxial accelerometer measured trunk accelerations. HRs and spatiotemporal parameters were calculated and standardized to remove the influence of gait velocity. The PD group exhibited lower HRs in all three planes, with the most pronounced differences in the mediolateral and anterior-posterior planes. Greater stride time variability was most closely associated with a lower anterior-posterior HR in PD and the presence of disease with the mediolateral HR. By demonstrating decreased walking stability in medial-lateral and anterior-posterior planes, we conclude that HRs offer unique information beyond that of typical stride parameters, and stride time variability is most closely associated with these direct measures of global walking stability.

Acceleration patterns of the head and pelvis during gait in older people with Parkinson's disease: a comparison of fallers and nonfallers

BACKGROUND

Falls are common in older people with Parkinson's disease (PD) and are likely to be related to gait disturbances associated with the condition. Although several studies have evaluated differences in basic gait parameters in people with PD, none have directly evaluated the stability of the upper body during gait.

METHODS

Temporospatial gait parameters and acceleration patterns at the head and pelvis were measured in three groups of older people: 33 controls without PD (mean age 67 +/- 4 years), 33 older people with PD and no history of falls (mean age 63 +/- 4 years), and 33 older people with PD and a history of falls (mean age 67 +/- 2 years). Harmonic ratios of head and pelvis accelerations in each plane were calculated to provide an indicator of upper body stability.

RESULTS

Compared with the control group, older people with PD exhibited significantly reduced walking speed and step length and increased step timing variability. Acceleration patterns were also significantly less rhythmic at the head and pelvis in all three planes. After adjusting for differences in walking speed and step timing variability, PD fallers exhibited significantly less rhythmic accelerations at the pelvis in the vertical and anteroposterior planes than PD nonfallers.

CONCLUSIONS

Acceleration patterns during gait differ between older people with and without PD and between older people with PD who do and do not fall. These findings suggest that an inability to control displacements of the torso when walking may predispose older people with PD to falls.

Leg power asymmetry and postural control in women with multiple sclerosis

The extent of and the interactions between muscle strength, walking speed, postural control, and symptomatic fatigue in multiple sclerosis (MS) are not known, nor are the effects of bilateral strength asymmetries on these variables.

PURPOSE

To quantify the magnitude of and the associations between bilateral strength and limb-loading asymmetries, postural control, and symptomatic fatigue in women with MS.

METHODS

Peak knee extensor (KE) and dorsiflexor (DF) isometric torque and isotonic power were assessed bilaterally in 12 women with MS (Expanded Disability Status Scale = 4 +/- 1) and 12 age-matched female controls using a Biodex dynamometer (Biodex Medical, Shirley, NY). Center of pressure (CoP) variability during 20 s of quiet stance was measured in the anteroposterior (AP) and the mediolateral (ML) directions using adjacent force plates. Bilateral asymmetry scores were calculated for power and torque. Normal and brisk walk times (25 ft) and symptomatic fatigue (Visual Analog Fatigue Scale and Fatigue Severity Scale) were measured before strength and balance testing.

RESULTS

Fatigue was greater and walk times (normal and brisk) were longer in MS (P < or = 0.01). Dorsiflexor (DF) isometric torque and power and knee extensor (KE) isometric strength were similar between groups. KE power was lower (mean +/- SD = 21.5 +/- 16.2%; P < or = 0.05) and KE power asymmetry was greater in MS than in controls (9.2 +/- 6.9%; P = 0.02). Postural variability of the CoP was greater in the AP direction in MS than in controls (7.52 +/- 3.02 and 4.33 +/- 1.79 mm, respectively; P = 0.005). KE power asymmetry was associated with fatigue and walk times (P < or = 0.02), and AP CoP variability was correlated with fatigue, walk times, and power asymmetries (P < or = 0.05).

CONCLUSIONS

These data provide new evidence of a potential role for KE strength asymmetries in the symptomatic fatigue and physical dysfunction of persons with MS, possibly through an effect on postural stability.

Systematic review of postural control and lateral ankle instability, part I: can deficits be detected with instrumented testing

OBJECTIVE

To answer the following clinical questions: (1) Is poor postural control associated with increased risk of a lateral ankle sprain? (2) Is postural control adversely affected after acute lateral ankle sprain? (3) Is postural control adversely affected in those with chronic ankle instability?

DATA SOURCES

PubMed and CINAHL entries from 1966 through October 2006 were searched using the terms ankle sprain, ankle instability, balance, chronic ankle instability, functional ankle instability, postural control, and postural sway.

STUDY SELECTION

Only studies assessing postural control measures in participants on a stable force plate performing the modified Romberg test were included. To be included, a study had to address at least 1 of the 3 clinical questions stated above and provide adequate results for calculation of effect sizes or odds ratios where applicable.

DATA EXTRACTION

We calculated odds ratios with 95% confidence intervals for studies assessing postural control as a risk factor for lateral ankle sprains. Effect sizes were estimated with the Cohen d and associated 95% confidence intervals for comparisons of postural control performance between healthy and injured groups, or healthy and injured limbs, respectively.

DATA SYNTHESIS

Poor postural control is most likely associated with an increased risk of sustaining an acute ankle sprain. Postural control is impaired after acute lateral ankle sprain, with deficits identified in both the injured and uninjured sides compared with controls. Although chronic ankle instability has been purported to be associated with altered postural control, these impairments have not been detected consistently with the use of traditional instrumented measures.

CONCLUSIONS

Instrumented postural control testing on stable force plates is better at identifying deficits that are associated with an increased risk of ankle sprain and that occur after acute ankle sprains than at detecting deficits related to chronic ankle instability.

Spatiotemporal postural control deficits are present in those with chronic ankle instability

Background

Postural control deficits have been purported to be a potential contributing factor in chronic ankle instability (CAI). Summary forceplate measures such as center of pressure velocity and area have not consistently detected postural control deficits associated with CAI. A novel measurement technique derived from the dynamical systems theory of motor control known as Time-to-boundary (TTB) has shown promise in detecting deficits in postural control related to chronic ankle instability (CAI). In a previous study, TTB deficits were detected in a sample of females with CAI. The purpose of this study was to examine postural control in sample of males and females with and without CAI using TTB measures.

Methods

This case-control study was performed in a research laboratory. Thirty-two subjects (18 males, 14 females) with self-reported CAI were recruited and matched to healthy controls. All subjects performed three, ten-second trials of single-limb stance on a forceplate with eyes open and eyes closed. Main outcome measures included the TTB absolute minimum (s), mean of TTB minima (s), and standard deviation of TTB minima (s) in the anteroposterior and mediolateral directions. A series of group by gender analyses of variance were conducted to evaluate the differences in postural control for all TTB variables separately with eyes open and eyes closed.

Results

There were no significant group by gender interactions or gender main effects for any of the measures. There, however, significant group main effects for 4 of the 6 measures with eyes closed as the CAI group demonstrated significant deficits in comparison to the control group. There were no significant differences between groups in any of the TTB measures with eyes open.

Conclusion

TTB deficits were present in the CAI group compared to the control group. These deficits were detected with concurrent removal of visual input. CAI may place significantly greater constraints on the sensorimotor system during single limb stance, resulting in a reorganization of postural control strategies. These deficits may be indicative of a diminished ability to respond effectively to changes in postural control demands in those with CAI.

Effects of Parkinson's disease and levodopa on functional limits of stability

BACKGROUND

The voluntary, maximum inclined posture reflects the self-perceived limits of stability. Parkinson's disease is associated with small, bradykinetic postural weight shifts while standing but it is unclear whether this is due to reduced limits of stability and/or to the selection of abnormal strategies for leaning. The aim of this study was to investigate the effects of Parkinson's disease and levodopa medication on voluntary limits of stability and strategies used to reach these limits.

METHODS

Fourteen subjects with Parkinson's disease (OFF and ON levodopa) and 10 age-matched controls participated in the study. Functional limits of stability were quantified as the maximum center of pressure excursion during voluntary forward and backward leaning. Postural strategies to achieve functional limits of stability were assessed by (i) body segments alignment, (ii) the difference between center of pressure and center of mass in preparation for a lean, (iii) the timing and the velocity of the preparation phase.

FINDINGS

Functional limits of stability were significantly smaller in subjects with Parkinson's disease compared to control subjects. Subjects with Parkinson's disease maintained their stooped posture while leaning, initiated leaning with a smaller difference between center of pressure and center of mass and had a slower leaning velocity compared to control subjects. Levodopa enlarged the limits of stability in subjects with Parkinson's disease because of an increase in maximum forward, but not backward leans, but did not significantly improve postural alignment, preparation for a leaning movement, or velocity of leaning.

INTERPRETATION

Parkinson's disease reduces functional limits of stability as well as the magnitude and velocity of postural preparation during voluntary, forward and backward leaning while standing. Levodopa improves the limits of stability but not the postural strategies used to achieve the leaning.

Assessing control of postural stability in community-living older adults using performance-based limits of stability

Background

Balance disability measurements routinely used to identify fall risks in frail populations have limited value in the early detection of postural stability deficits in community-living older adults. The objectives of the study were to 1) measure performance-based limits of stability (LOS) in community-living older adults and compare them to theoretical LOS computed from data proposed by the Balance Master^® system, 2) explore the feasibility of a new measurement approach based on the assessment of postural stability during weight-shifting tasks at performance-based LOS, 3) quantify intra-session performance variability during multiple trials using the performance-based LOS paradigm.

Methods

Twenty-four healthy community-living older adults (10 men, 14 women) aged between 62 to 85 (mean age ± sd, 71.5 ± 6 yrs) participated in the study. Subjects' performance-based LOS were established by asking them to transfer their body weight as far as possible in three directions (forward, right and left) without changing their base of support. LOS were computed as the maximal excursion of the COP in each direction among three trials. Participants then performed two experimental tasks that consisted in controlling, with the assistance of visual feedback, their centre of pressure (COP) within two predefined targets set at 100% of their performance-based LOS. For each tasks 8 trials were performed. Ground reaction forces and torques during performance-based LOS evaluation and experimental tasks were recorded with a force plate. Sway area and medio-lateral mean COP displacement speed variables were extracted from force plate recordings.

Results

Significant differences between theoretical LOS computed from maximum leaning angles derived from anthropometric characteristics and performance-based LOS were observed. Results showed that a motor learning effect was present as the participants optimized their weight-shifting strategy through the first three trials of each task using the visual biofeedback provided on their COP. Reliable measures of control of postural stability at performance-based LOS can be obtained after two additional trials after the learning phase (0.69 > ICC > 1.0).

Conclusion

Establishing performance-based LOS instead of relying on estimations of theoretical LOS offers a more individualized and realistic insight on the true LOS of an individual. Performance-based LOS can be used as targets during weight-shifting postural tasks with real time visual feedback of the COP displacement to assess postural stability of community-living older adults. In order to obtain reliable results, a learning phase allowing subjects to learn how to control their COP displacement is needed.

Lateral stepping for postural correction in Parkinson's disease

OBJECTIVE

To characterize the lateral stepping strategies for postural correction in patients with Parkinson's disease (PD) and the effect of their anti-parkinson medication.

DESIGN

Observational study.

SETTING

Outpatient neuroscience laboratory.

PARTICIPANTS

Thirteen participants with idiopathic PD in their on (PD on) and off (PD off) levodopa state and 14 healthy elderly controls.

INTERVENTIONS

Movable platform with lateral translations of 12 cm at 14.6 cm/s ramp velocity.

MAIN OUTCOME MEASURES

The incidence and characteristics of 3 postural strategies were observed: lateral side-step, crossover step, or no step. Corrective stepping was characterized by latency to step after perturbation onset, step velocity, and step length and presence of an anticipatory postural adjustment (APA). Additionally, percentages of trials resulting in falls were identified for each group.

RESULTS

Whereas elderly control participants never fell, PD participants fell in 24% and 35% of trials in the on and off medication states, respectively. Both PD and control participants most often used a lateral side-step strategy; 70% (control), 67% (PD off), and 73% (PD on) of all trials, respectively. PD participants fell most often when using a crossover strategy (75% of all crossover trials) or no-step strategy (100% of all no-step trials). In the off medication state, PD participants' lateral stepping strategies were initiated later than controls (370+/-37 ms vs 280+/-10 ms, P<.01), and steps were smaller (254+/-20 mm vs 357+/-17 mm, P<.01) and slower (0.99+/-0.08 m/s vs 1.20+/-0.07 m/s, P<.05). No differences were found between the PD off versus PD on state in the corrective stepping characteristics. Unlike control participants, PD participants often (56% of side-step strategy trials) failed to activate an APA before stepping, although their APAs, when present, were of similar latency and magnitude as for control participants. Levodopa on or off state did not significantly affect falls, APAs, or lateral step latency, velocity, or amplitude (P>.05).

CONCLUSIONS

PD participants showed significantly more postural instability and falls than age-matched controls when stepping was required for postural correction in response to lateral disequilibrium. Although PD participants usually used a similar lateral stepping strategy as controls in response to lateral translations, lack of an anticipatory lateral weight shift, and bradykinetic characteristics of the stepping responses help explain the greater rate of falls in participants with PD. Differences were not found between the levodopa on and off states. The results suggest that rehabilitation aimed at improving lateral stability in PD should include facilitating APAs before a lateral side-stepping strategy with faster and larger steps to recover equilibrium.

Assessment of postural instability in patients with Parkinson’s disease

Postural instability is one of the most disabling features of idiopathic Parkinson's disease (PD). In this study, we focused on postural instability as the main factor predisposing parkinsonians to falls. For this purpose, changes in sway characteristics during quiet stance due to visual feedback exclusion were studied. We searched for postural sway measures that could be potential discriminators for an increased fall risk. A group of 110 subjects: 55 parkinsonians (Hoehn and Yahr: 1-3), and 55 age-matched healthy volunteers participated in the experiment. Their spontaneous sway characteristics while standing quiet with eyes open and eyes closed were analyzed. We found that an increased mediolateral sway and sway area while standing with eyes closed are characteristic of parkinsonian postural instability and may serve to quantify well a tendency to fall. These sway indices significantly correlated with disease severity rated both by the Hoehn and Yahr scale as well as by the Motor Section of the UPDRS. A forward shift of a mean COP position in parkinsonians which reflects their flexed posture was also significantly greater to compare with the elderly subjects and exhibited a high sensitivity to visual conditions. Both groups of postural sway abnormalities identified here may be used as accessible and reliable measures which allow for quantitative assessment of postural instability in Parkinson's disease.