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Other Publications (71)

Articles by Fay B. Horak in JoVE

 JoVE Clinical and Translational Medicine

Method to Measure Tone of Axial and Proximal Muscle

1Department of Biomedical Engineering, Oregon Health and Science University, 2UCL Institute of Neurology, Queen Square, 3Department of Neurology, Oregon Health and Science University


JoVE 3677

We have developed a device (Twister) to study the regulation of tonic muscle activity during active postural maintenance. Twister measures torsional resistance and muscular responses in standing subjects during twisting of the body axis. The device can be flexibly configured to study various aspects of tonic control across the neck, trunk, and/or hips.

Other articles by Fay B. Horak on PubMed

Diabetic Neuropathy and Surface Sway-referencing Disrupt Somatosensory Information for Postural Stability in Stance

In order to determine the type of somatosensory information for postural control that is most affected by neuropathy, we compared the relative effects of three methods of sway-referencing the surface in a group of subjects with profound loss of somatosensory function associated with sensory polyneuropathy from diabetes with age-matched control subjects. Sway-referencing disrupted somatosensory feedback for postural control by servo-controlling the dorsi- and plantar-flexion rotation of the support surface in proportion to anterior-posterior excursion of (1) ankle angle, (2) center of body mass (CoM) angle or (3) filtered center of pressure (CoP). Postural sway in subjects with somatosensory loss was significantly larger than normal on a firm surface but not on the sway-referenced surfaces, suggesting that sway-referencing disrupts somatosensory information for postural control already disrupted by neuropathy. Control subjects standing on any sway-referenced surface swayed significantly more than neuropathy subjects who stood on a firm surface, suggesting that sway-referencing disrupts more somatosensory information than disrupted by severe neuropathy. CoP sway-referencing was less sensitive than ankle or CoM sway-referencing for distinguishing postural sway in subjects with somatosensory loss from age-matched control subjects. Given that filtered CoP sway-referencing disrupts the ability to utilize somatosensory information related to surface reactive force to a greater extent than the other two methods of sway-referencing, then these results support the hypothesis that subjects with diabetic peripheral neuropathy have lost more CoP information, than ankle or CoM angle information, for controlling postural sway in stance.

Does the Cerebellum Play a Role in Podokinetic Adaptation?

After sustained stepping in-place on a rotating disc, healthy subjects will inadvertently turn in circles when asked to step in-place on a stationary surface with eyes closed. We asked whether the cerebellum is important for this adaptive phenomenon, called podokinetic after-rotation (PKAR). Subjects with cerebellar degeneration and age-matched control subjects performed 15 min of stepping in-place with eyes open on a rotating disc, then 30 min of attempting to step in-place with eyes closed on a stationary surface. Rotational velocity of PKAR was measured during this 30-min period. All control subjects demonstrated PKAR; average initial rotational velocity for control subjects was 16.4+/-3.5 degrees /s. Five of the eight cerebellar subjects demonstrated impaired PK adaptation, defined as PKAR with an initial velocity more than two standard deviations below the control mean initial velocity. Average initial rotational velocity for cerebellar subjects was 7.8+/-0.2 degrees /s. Impaired PK adaptation was not associated with impaired time constants of decay and was not correlated with variability of PKAR velocity. Our results suggest that the cerebellum is important for regulation of the amplitude of PK adaptation and that reduced PKAR amplitude is not likely the result of dyscoordination or variability of movement in the subjects tested.

Vestibulospinal Control of Posture

To better understand the role of the vestibular system in postural coordination, we compared the ability of subjects with complete, bilateral loss of vestibular function and age-matched control subjects to maintain equilibrium and postural orientation during sinusoidal displacements of the support surface at a variety of frequencies. We also examined the ability of visual or somatosensory-light touch information to substitute for missing vestibular information in dynamic postural coordination. The results suggest that vestibular information is used as a gravitational reference frame to prevent slow drift of the trunk in space during complex postural tasks. Furthermore, visual information or somatosensory information from light touch of a finger on a stable reference can significantly substitute for loss of vestibular function.

Vestibular Stimulation Affects Medium Latency Postural Muscle Responses

This study explores whether galvanic vestibular stimulation can alter automatic postural muscle responses triggered 100 ms after surface translations. Our previous study concluded that a step of bipolar, galvanic vestibular stimulation delivered 500 ms prior to a platform translation tilted the internal representation of vertical because subjects' final center of foot pressure and center of mass equilibrium position shifted toward the anode and this tilt was larger than the sum of effects for platform translations and galvanic alone. In the current study, we show that 0.2-0.4 mA of galvanic vestibular stimulation produced significant changes in the tilt of the trunk in space that was realized by changes in the magnitude of the medium latency postural muscle responses at the ankle. The galvanic-induced changes in latency and magnitude of the first 50 ms of gastrocnemius muscle burst in response to the backward surface translation were consistent with changes in background muscle tone induced by the direct vestibulospinal effects of the galvanic current. However, the galvanic-induced changes in the second 50 ms of the gastrocnemius response were in the opposite direction and consistent with the forward- or backward-tilted, final postural equilibrium goal. Kinematic analysis showed that galvanic-induced tilt was first initiated in the trunk and that the shank and thigh segment angles were not altered by galvanic stimulation prior to platform translation such that changes in gastrocnemius and soleus postural responses to translations were not due to an effect of galvanic stimulation on initial ankle angle. More proximal muscles and antagonist muscles involved in the postural response were not altered by the galvanic stimulation. These results suggest that galvanic vestibulospinal stimulation can alter medium latency, automatic postural responses in prime movers by changing the postural equilibrium goal.

Voluntary Control of Postural Equilibrium Patterns

The ability to voluntarily transit from one whole-body movement to another is based on the multisensory integration of visual, vestibular, and somatosensory information. The role of functional sensory ranges and mechanical constraints on the ability to voluntarily transit between whole-body movements was studied by requiring subjects to switch from a head-fixed-to-surface to head-fixed-in-space postural pattern (and vice versa). The head-fixed-to-surface pattern required an erect stance characterized by an in-phase relationship between center of pressure (CoP) and platform motion. The head-fixed-in-space pattern required subjects to fix trunk-head position in-space while producing an anti-phase relationship between CoP and platform motion. The voluntary transition was performed with and without vision while standing on a surface oscillating in the anterior-posterior (A/P) direction. The support surface oscillated at five frequencies (0.2-1Hz) with amplitude fixed at 15cm. The voluntary transition was initiated with an auditory cue. The appropriate CoP-platform phase relationship for the two postural patterns was produced for all frequencies with and without vision. Upper-trunk kinematics revealed that subjects often failed to produce the head-fixed-to-surface pattern for frequencies >/=0.6Hz, while producing the head-fixed-in-space pattern at all frequencies with vision. Without vision, neither pattern was produced consistently based on upper-trunk kinematics. These findings demonstrate separate control processes for upper- and lower-body motion and that functional sensory ranges and mechanical constraints can facilitate or inhibit voluntary production of whole-body movements based on these control processes. The results are discussed in reference to neurological substrates that may be involved in the planning and execution of motor set-switching. The experimental protocol we employ may also have application as a diagnostic tool for the evaluation of postural deficits.

Gaining Insight by Going in Circles

Head and Body Center of Gravity Control Strategies: Adaptations Following Vestibular Rehabilitation

We present for the first time evidence that vestibulopathy impairs coordination of the head with the body center of gravity (CG) during free speed gait over ground. Vestibulopathic individuals demonstrate uncoordinated movement and gait due, at least in part, to impaired head stability and visual fixation. Vestibular rehabilitation increases speed and stability during gait and stair climbing, although the underlying mechanisms are poorly understood.

Effects of Cadence on the Acquisition and Expression of Podokinetic After-rotation

Podokinetic after-rotation (PKAR) occurs as blindfolded subjects inadvertently rotate when asked to step in-place following stepping in-place on a rotating surface. We examined the effects of using different cadences on PKAR to test the following hypothesis: the position signal indicating the amount of rotation between the trunk and the feet during each stance period of treadmill stimulation is used to determine the amount of rotation between the trunk and the feet that is expressed during each stance period of PKAR. Based on this hypothesis, we predicted that use of different cadences during treadmill stimulation would alter PKAR velocity because use of different cadences alters stance duration, thus changing the amount of limb rotation under the trunk during each stance phase of stimulation. We also predicted that use of different cadences during PKAR would alter PKAR velocity because the more steps that are taken in a given time the higher the velocity of PKAR given that the same rotation between trunk and feet occurs during each stance period. Use of different cadences during treadmill stimulation did not alter PKAR velocity, suggesting that PKAR velocity is not determined based upon a position signal regarding the relative rotation between the trunk and feet during stimulation. Use of different cadences during PKAR resulted in lower and higher velocities, respectively, than using a medium cadence. Based on these results, we now hypothesize that the PK system likely uses a velocity or acceleration signal present during stimulation to recalibrate the amount of relative rotation between the trunk and limbs that is expressed with each step during PKAR.

Comparison Between Subthalamic Nucleus and Globus Pallidus Internus Stimulation for Postural Performance in Parkinson's Disease

Nine subjects with Parkinson's disease, five subjects with electrodes implanted in the subthalamic nucleus (STN) and four with electrodes in the globus pallidus internus (GPi), were compared with a population of ten age-matched control subjects. The measures studied include a set of summary statistic scores, two stochastic parameters, the distribution of the center of pressure (CoP) displacement angles under each foot, and the distribution of bilateral patterns of CoP displacement angles. A Bayes classifier was designed to monitor the trend of postural performance in patients, with different treatments. Results suggested that the selected measures were sensitive to Parkinsonian postural sway abnormalities and highlight differences in response to treatments. Deep brain stimulation restored a more normal postural sway and levodopa increased sway abnormalities. Furthermore, the selected measures appear to detect different responses to levodopa between the STN and GPi groups: the negative side effects of levodopa on posture were less severe for STN than for GPi patients, perhaps due to the decreased need for levodopa intake in STN subjects. The measures proposed in this preliminary study may be useful adjuncts to evaluate balance and postural control strategies in patients with Parkinson's disease and may allow the comparison of DBS electrode sites, on stance posture.

Abnormal Force Patterns for Multidirectional Postural Responses in Patients with Parkinson's Disease

This study tests the hypothesis that the basal ganglia are involved in optimizing postural responses for changes in perturbation direction and stance width. We compared the patterns of horizontal and vertical ground reactive forces under each leg in response to eight directions of surface translation in Parkinson's disease (PD) subjects and age-matched control subjects standing with both narrow and wide stance. Although passive reactive forces were larger, active forces were weaker and in abnormal directions for subjects with PD. Unlike the control subjects, who corrected their postural equilibrium in response to lateral and diagonal-lateral perturbations primarily with their loaded limbs, the PD subjects used both legs more symmetrically to recover equilibrium. PD subjects also did not change the magnitude or direction of reactive forces when initial stance width changed. These results support the hypothesis that the basal ganglia are important for optimizing automatic postural response patterns for changes in perturbation direction and for initial stance conditions.

Effects of Bilateral Vestibular Loss on Podokinetic After-rotation

We asked what the role of the vestibular system is in adaptive control of locomotor trajectory in response to walking on a rotating disc. Subjects with bilateral vestibular loss (BVL) were compared to age- and gender-matched controls (CTRL). Subjects walked in place on the surface of a rotating disc for 15 min and then attempted to step in place without vision on a stationary surface for 30 min. CTRL subjects demonstrated podokinetic after-rotation (PKAR), involuntarily and unknowingly turning themselves in circles while attempting to step in place. PKAR in CTRLs was characterized by a rapid rise in turning velocity over the first 1-2 min, followed by a gradual decay over the remaining 28 min. Subjects with BVL also demonstrated PKAR and had no knowledge of their turning. However, PKAR in BVLs was characterized by an extremely rapid, essentially instantaneous rise. Subjects with BVL immediately turned at maximum velocity and exhibited a gradual decay throughout the entire 30 min period. Despite this difference in the initial portion of PKAR in BVLs, their responses were not significantly different from CTRLs during minutes 2 to 30 of the response. These results suggest that vestibular inputs normally suppress PKAR velocity over the first 1-2 min of the response, but do not greatly influence PKAR decay. PKAR is therefore a process mediated primarily by somatosensory information and vestibular inputs are not required for its expression. Additionally, the absence of vestibular inputs does not result in increased somatosensory sensitivity that alters podokinetic intensity or decay time constants.

Postural Feedback Responses Scale with Biomechanical Constraints in Human Standing

We tested whether human postural responses can be described in terms of feedback control gains, and whether these gains are scaled by the central nervous system to accommodate biomechanical constraints. A feedback control model can describe postural responses for a wide range of perturbations, but biomechanical constraints-such as on the torque that can be exerted on the ground-make a single set of feedback gains inappropriate for all perturbations. To observe how postural responses change with perturbation magnitude, we applied fast, backward perturbations of magnitudes 3-15 cm to 13 healthy young volunteers (4 men, 9 women, aged 20-32 years). We used a 3-segment, sagittal-plane biomechanical model and a linear state feedback controller to reproduce the observed postural responses. Optimization was used to identify the best-fit feedback control gains for each trial. Results showed that trajectories of joint angles and joint torques were scaled with perturbation magnitude. This scaling occurred gradually, rather than abruptly changing at magnitudes where biomechanical constraints became active. Feedback gains were found to fit reasonably well with data ( R(2)=0.92) and to be multivariate and heterogenic in character, meaning that the torque produced at any joint is generally a function of motions not only at the same joint, but other joints as well. Hip gains increased and ankle gains decreased nearly linearly with perturbation magnitude, in accordance with biomechanical limitations on ground reaction torque. These results indicate that postural adjustments can be described as a single feedback control scheme, with scalable heterogenic gains that are adjusted according to biomechanical constraints.

Postural Muscle Responses to Multidirectional Translations in Patients with Parkinson's Disease

The postural adaptation impairments of patients with Parkinson's disease (PD) suggest that the basal ganglia may be important for quickly modifying muscle activation patterns when the direction of perturbation or stance conditions suddenly change. It is unknown whether their particular instability to backward postural perturbations is due to specific abnormalities of parkinsonian postural muscle synergies in that direction and not present in other directions. In the present study, we test this hypothesis by comparing the patterns of leg and trunk muscle activation in 13 subjects with PD and 13 control subjects in response to eight randomly presented directions of horizontal surface translations while standing with either narrow or wide stance. The direction of maximum activation for each muscle was similar for PD and control subjects, suggesting that the basal ganglia is not critical for programming externally triggered postural synergies. However, antagonist muscle activation was earlier and larger in PD than in control subjects, resulting in coactivation. PD subjects also did not increase the magnitude of muscle activation as much as did control subjects when changing from wide to narrow stance. These results are consistent with the hypothesis that PD results in an inability to shape the pattern and magnitude of postural muscle responses for changes in perturbation direction and in stance position.

Audio-biofeedback for Balance Improvement: an Accelerometry-based System

This paper introduces a prototype audio-biofeedback system for balance improvement through the sonification using trunk kinematic information. In tests of this system, normal healthy subjects performed several trials in which they stood quietly in three sensory conditions while wearing an accelerometric sensory unit and headphones. The audio-biofeedback system converted in real-time the two-dimensional horizontal trunk accelerations into a stereo sound by modulating its frequency, level, and left/right balance. Preliminary results showed that subjects improved balance using this audio-biofeedback system and that this improvement was greater the more that balance was challenged by absent or unreliable sensory cues. In addition, high correlations were found between the center of pressure displacement and trunk acceleration, suggesting accelerometers may be useful for quantifying standing balance.

Can Stooped Posture Explain Multidirectional Postural Instability in Patients with Parkinson's Disease?

To determine the effects of the stooped posture of patients with Parkinson's disease (PD) on postural stability, we compared the kinetic, kinematic, and electromyographic responses of seven subjects with PD and 11 control subjects to eight directions of surface translations. Control subjects were studied in an upright posture and in a stooped posture that mimicked the posture of the PD subjects. When control subjects adopted a stooped posture, peak center of pressure displacements slowed and decreased, reducing stability margins toward values observed in PD subjects. Stooped control subjects, however, responded to translations with large joint angle displacements, whereas PD subjects exhibited small joint angle displacements. Stooping in control subjects did not lead to abnormally directed horizontal forces under each foot or antagonistic muscle co-activation at the hip and trunk, as seen in PD subjects. Upright and stooped control subjects never fell during the trials, whereas PD subjects fell in 16% of the trials. We conclude that stooped posture is a destabilizing posture, but it does not account for abnormal postural responses in PD.

Audio-biofeedback Improves Balance in Patients with Bilateral Vestibular Loss

To evaluate the effectiveness of an audio-biofeedback (ABF) system for improving balance in patients with bilateral vestibular loss (BVL).

Influence of a Portable Audio-biofeedback Device on Structural Properties of Postural Sway

Good balance depends on accurate and adequate information from the senses. One way to substitute missing sensory information for balance is with biofeedback technology. We previously reported that audio-biofeedback (ABF) has beneficial effects in subjects with profound vestibular loss, since it significantly reduces body sway in quiet standing tasks.

Improvement in Automatic Postural Coordination Following Alexander Technique Lessons in a Person with Low Back Pain

The relationship between abnormal postural coordination and back pain is unclear. The Alexander Technique (AT) aims to improve postural coordination by using conscious processes to alter automatic postural coordination and ongoing muscular activity, and it has been reported to reduce low back pain. This case report describes the use of the AT with a client with low back pain and the observed changes in automatic postural responses and back pain.

Direction-specific Postural Instability in Subjects with Parkinson's Disease

The purpose of this study was to determine whether and why subjects with Parkinson's disease (PD) have greater instability in response to specific directions of perturbations than do age-matched control subjects and how instability is affected by stance width. This study compared postural responses to 8 directions of surface translations in PD subjects and age-matched control subjects while standing in a narrow and wide stance. PD subjects were tested in their practical OFF state. A postural stability margin was quantified as the difference between peak center of pressure (CoP) and peak center of mass (CoM) displacement in response to surface translations. The control subjects maintained a consistent stability margin across directions of translations and for both narrow and wide stance by modifying rate of rise of CoP responses. PD subjects had smaller than normal postural stability margins in all directions, but, especially for backwards sway in both stance widths and for lateral sway in narrow stance width. The reduced stability margin in PD subjects was due to a slower rise and smaller peak of CoP in the PD subjects than in control subjects. Lateral postural stability was compromised in PD subjects by lack of trunk flexibility and backwards postural stability was compromised by lack of knee flexion, resulting in excessive displacements of the body CoM. Stability margins in PD subjects were related to their response on the pull test in the Unified Parkinson's Disease Rating Scale. Thus, PD patients have directionally specific postural instability due to an ineffective stiffening response and inability to modify their postural responses for changing postural demands related to direction of perturbation and initial stance posture. These results suggest that the basal ganglia, in addition to regulating muscle tone and energizing postural muscle activation, also are critical for adapting postural response patterns for specific biomechanical conditions.

Differences in Preferred Reference Frames for Postural Orientation Shown by After-effects of Stance on an Inclined Surface

This study reports a postural after-effect of leaning that follows a period of stance on an inclined surface with eyes closed. This leaning after-effect maintained the body-to-surface relationship as if subjects still stood on the incline. We examined the incidence and robustness of the leaning after-effect in 51 healthy subjects. The location of the center of pressure (CoP) under the feet and the alignment of the trunk and legs were measured before, during and after blindfolded subjects stood on a 5 degrees toes-up inclined surface for 2.5 min. When the surface was inclined, all subjects stood with their trunk and legs aligned near to gravity-vertical, similar to the alignment adopted in the pre-incline period. When the surface returned to horizontal in the post-incline period, there was a continuum of postural alignment strategies across subjects. At one extreme, subjects leaned forward, with an average trunk lean near 5 degrees . The leaned posture decayed exponentially toward baseline postural alignment across a period of up to 5 min. At the other extreme, subjects did not lean in the post-incline period, but instead, stayed aligned near upright with respect to gravity. Subjects were highly consistent in their post-incline postural behaviors upon repeated testing over days to months and across different directions of surface inclination. Our results suggest that individuals have well-established, preferred, sensory strategies for controlling postural orientation when vision is not available. Subjects who leaned in the post-incline period appear to depend more on the geometry of the support surface as a reference frame and to rely more on proprioceptive information to extract kinematic relationships, whereas subjects who did not lean appear to depend more on gravity as a reference frame and to rely more on sensory information related to forces and load.

Effects of Linear Versus Sigmoid Coding of Visual or Audio Biofeedback for the Control of Upright Stance

Although both visual and audio biofeedback (BF) systems for postural control can reduce sway during stance, a direct comparison between the two systems has never been done. Further, comparing different coding designs of audio and visual BF may help in elucidating how BF information is integrated in the control of posture, and may improve knowledge for the design of innovative BF systems for postural control. The purpose of this paper is to compare the effects of linear versus sigmoid coding of trunk acceleration for audio and visual BF on postural sway in a group of eight, healthy subjects while standing on a foam surface. Results showed that sigmoid-coded audio BF reduced sway acceleration more than did a linear-coded audio BF, whereas a linear-coded visual BF reduced sway acceleration more than a sigmoid-coded visual BF. In addition, audio BF had larger effects on reducing center of pressure (COP) displacement whereas visual BF had larger effects on reducing trunk sway. These results suggest that audio and visual BF for postural control benefit from different types of sensory coding and each type of BF may encourage a different type of postural sway strategy.

Postural Orientation and Equilibrium: What Do We Need to Know About Neural Control of Balance to Prevent Falls?

Postural control is no longer considered simply a summation of static reflexes but, rather, a complex skill based on the interaction of dynamic sensorimotor processes. The two main functional goals of postural behaviour are postural orientation and postural equilibrium. Postural orientation involves the active alignment of the trunk and head with respect to gravity, support surfaces, the visual surround and internal references. Sensory information from somatosensory, vestibular and visual systems is integrated, and the relative weights placed on each of these inputs are dependent on the goals of the movement task and the environmental context. Postural equilibrium involves the coordination of movement strategies to stabilise the centre of body mass during both self-initiated and externally triggered disturbances of stability. The specific response strategy selected depends not only on the characteristics of the external postural displacement but also on the individual's expectations, goals and prior experience. Anticipatory postural adjustments, prior to voluntary limb movement, serve to maintain postural stability by compensating for destabilising forces associated with moving a limb. The amount of cognitive processing required for postural control depends both on the complexity of the postural task and on the capability of the subject's postural control system. The control of posture involves many different underlying physiological systems that can be affected by pathology or sub-clinical constraints. Damage to any of the underlying systems will result in different, context-specific instabilities. The effective rehabilitation of balance to improve mobility and to prevent falls requires a better understanding of the multiple mechanisms underlying postural control.

Step Initiation in Parkinson's Disease: Influence of Initial Stance Conditions

In this study, we investigated how the size of preparatory postural adjustments prior to step initiation, and step length and velocity depend on initial stance width in patients with Parkinson's disease (PD) both in the ON and OFF levodopa states and in healthy elderly subjects. Twenty-one subjects with idiopathic PD and 24 age-matched healthy control subjects took two steps starting with feet on a two-plate force-platform, from either narrow or wide stance width. We measured how the magnitude of anticipatory postural adjustments (APA) and step characteristics scaled with stance width. Results showed that preparation for step initiation from wide stance was associated with a larger lateral and backward center of pressure (CoP) displacement than from narrow stance. Velocity and length of the first step were also sensitive to initial stance conditions, probably in relation with the differences in the corresponding APA. On the contrary, the duration of APA was not significantly affected by initial stance width, but it was longer in PD compared to healthy subjects, and speeded up by levodopa. Although subjects with PD did scale up the size of their APA with stance width, they had much more difficulty initiating a step from a wide stance than from a narrow stance, as shown by the greater differences from control subjects in the magnitude of the APA. Our results support the hypothesis that PD subjects maintain a narrow stance as a compensation for their inability to sufficiently increase the size of their lateral APA to allow fast step initiation in wide stance.

An Alternative Clinical Postural Stability Test for Patients with Parkinson's Disease

We compared the sensitivity and consistency of a new Push and Release Test versus the Pull Test (item 30 of the Unified Parkinson's Disease Rating Scale; UPDRS) as clinical measures of postural stability. Subjects with Parkinson's disease and age-matched control subjects participated in 3 protocols investigating: (1) the sensitivity and specificity of the two tests related to the subjects' balance confidence, as measured by the Activities-specific Balance Confidence (ABC) scale, (2) the inter-rater reliability of the two tests, and (3) the consistency of the perturbation forces applied to the subjects by each balance test. As a test for concurrent validity, the balance tests were also compared with the subjects' retrospective reports of fall frequency. Compared with the Pull Test, the Push and Release Test was more sensitive to subjects with low balance confidence, but less specific for subjects with high balance confidence. The inter-rater correlations were higher with the Push and Release Test. Examiners applied more consistent perturbation forces to the subjects with the Push and Release Test than with the Pull Test. The Push and Release Test correlated better with self-reported falls. Therefore, the Push and Release Test provided a more sensitive and consistent test of postural stability than the Pull Test.

Identification of Distinct Characteristics of Postural Sway in Parkinson's Disease: a Feature Selection Procedure Based on Principal Component Analysis

We selected descriptive measures of the centre of pressure (CoP) displacement in quiet standing, by means of a procedure based on principal component analysis, in two groups particularly different in terms of postural behaviours, such as subjects with Parkinson's disease (PD) in the levodopa off and on states. We computed 14 measures of the CoP: 5 measures of CoP trajectory over the support surface, 3 measures that estimated the area covered by the CoP, 1 measure that estimated the principal CoP sway direction, 1 measure that quantified the CoP total power, 1 measure that estimated the variability of CoP frequency content and 3 measures of characteristic CoP frequencies [L. Rocchi, L. Chiari, A. Cappello, Feature selection of stabilometric parameters based on principal component analysis, Med. Biol. Eng. Comput. 42 (2004) 71-79; L. Rocchi, L. Chiari, F.B. Horak, Effects of deep brain stimulation and levodopa on postural sway in Parkinson's disease, J. Neurol. Neurosurg. Psychiatry, 73 (2002) 267-274]. The feature selection, independently applied to the measures obtained in the two groups, resulted in different principal component (PC) subspaces of the 14-dimension original data set (4 PCs in the off and 3 PCs in the on state to account for over 90% of the original variance), but in the same 5 CoP measures (selected features) needed to describe the different postural behaviours: root mean square distance; mean velocity; principal sway direction; centroidal frequency of the power spectrum; frequency dispersion. The five selected features were found to provide insight into the postural control mechanisms and to describe changes in postural strategies in the two groups of PD subjects, off and on levodopa. Thus, the five selected features may be recommended for use in clinical practice and in research, in the direction toward the definition of a standard protocol in quantitative posturography.

Effects of Practicing Tandem Gait with and Without Vibrotactile Biofeedback in Subjects with Unilateral Vestibular Loss

Subjects with unilateral vestibular loss exhibit motor control impairments as shown by body and limb deviation during gait. Biofeedback devices have been shown to improve stance postural control, especially when sensory information is limited by environmental conditions or pathologies such as unilateral vestibular loss. However, the extent to which biofeedback could improve motor performance or learning while practicing a dynamic task such as narrow gait is still unknown. In this cross-over design study, 9 unilateral vestibular loss subjects practiced narrow gait with and without wearing a trunk-tilt, biofeedback device in 2 practice sessions. The biofeedback device informed the subjects of their medial-lateral angular tilt and tilt velocity during gait via vibration of the trunk. From motion analysis and tilt data, the performance of the subjects practicing tandem gait were compared over time with and without biofeedback. By practicing tandem gait, subjects reduced their trunk-tilt, center of mass displacement, medial-lateral feet distance, and frequency of stepping error. In both groups, use of tactile biofeedback consistently increased postural stability during tandem gait, beyond the effects of practice alone. However, one single session of practice with biofeedback did not result in conclusive short-term after-effects consistent with short-term retention of motor performance without this additional biofeedback. Results from this study support the hypothesis that tactile biofeedback acts similar to natural sensory feedback to improve dynamic motor performance but does not facilitate a recalibration of motor performance to improve function after short-term use.

Postural Control Adaptability to Floor Oscillation in the Elderly

We established a method to evaluate postural control adaptability, applying it to 341 subjects, aged 18-29 years (young subjects) and 50-79 years, in order to investigate the influences of age and gender on adaptability. Subjects stood with eyes closed on a force plate fixed to a floor oscillator, which was sinusoidally oscillated in the anteroposterior direction with 0.5 Hz frequency and 2.5 cm amplitude. Five trials of 1-minute oscillation were conducted, with a short rest between trials. The mean speed of fluctuation of the center of foot pressure (CFP), as detected by the force plate, was calculated as an index of postural steadiness. Mean CFP speed decreased significantly in all age groups with trial repetition. The adaptability capability of elderly subjects was categorized as "good," "moderate," or "poor," as evaluated against a standard value, based on the variation of the regression of mean CFP speed between the 1st and 5th trials in young subjects. Results showed that the magnitude of reduction in the mean speed, with practice, was linearly related to the initial mean speed. We found a general decline in adaptability, and increase in initial mean speed, in subjects aged 60 years and older, with no gender difference detected in any age group. The proportion of subjects exhibiting moderate and poor adaptability increased gradually with age. In conclusion, age, but not gender, appears to affect adaptation of postural sway with short-term practice, although some elderly subjects maintain postural sway velocity and adaptability capabilities similar to those of young subjects.

Interaction of Posture and Conscious Perception of Gravitational Vertical and Surface Horizontal

The extent to which postural orientation and perception of orientation share common central processes is unknown. If central resources are shared, automatic postural orientation in space and conscious perception of gravitational vertical (GV) or surface horizontal (SH) orientation should vary similarly with changing frequencies of surface tilts. Furthermore, shared central processing may result in altered postural orientation depending on the focus of conscious perception. Blindfolded, healthy subjects indicated perception of gravitational vertical and surface horizontal while standing on a tilting surface or sitting in an earth-fixed chair with only the feet on the tilting surface. Subjects oriented a hand-held rod to GV or perpendicular to the tilting support surface to indicate SH during anterior-posterior (A/P) sinusoidal surface tilt oscillation (+/-5 degrees /0.025-0.4 Hz). The gain and phase relationships of rod and postural tilt with respect to surface tilt were analyzed in all conditions. Average rod-tilt during the GV task was below 1.5 degrees and decreased with increasing frequency of surface oscillation, similar to the decrease in trunk tilt. Average rod-tilt when indicating SH was three to four times that of actual surface tilt during both sitting and standing postures and did not change across frequencies, unlike trunk tilt. Measures of postural stability were less variable during the GV than the SH task. This increased postural stability during the GV task and similar gain/phase patterns of postural and GV perceptual orientation across frequencies is consistent with shared central processing of automatic postural and conscious perception of orientation to gravity. In contrast, different gain/phase patterns of posture and conscious perception of SH orientation are consistent with separate processing of postural responses of changing surface orientation and perception of it. Conscious perception of sensory inputs related to verticality share central processing with postural control, which when accessed can have a stabilizing effect, whereas consciously perceiving a moving support surface can be destabilizing.

Postural After-effects of Stepping on an Inclined Surface

In previous studies, blindfolded, healthy subjects exhibited an after-effect of leaning while standing on a horizontal surface after a period of standing on an inclined surface. We investigated whether this kinesthetic after-effect would transfer from one task to another by asking blindfolded subjects to stand on a horizontal surface after stepping-in-place on an incline. Results showed that all subjects demonstrated a forward trunk leaning after-effect lasting from half a minute to over 6 min after stepping on a 10 degrees -toes-up incline for 2.5 min. For 5/7 subjects, the amplitude of the leaning after-effect was very similar following stepping or standing on the inclined surface. The similarity of the post-incline lean between the standing and stepping conditions suggests a common underlying mechanism for the after-effect following standing and walking on a gradient and suggests that prolonged maintenance of a constant ankle or leg posture is not a prerequisite condition for the after-effect. The transfer of a postural effect built-up during a locomotor task to a postural after-effect during a standing task is consistent with a central adaptive mechanism that adjusts the surface-referenced set point for whole body postural orientation for both gait and posture.

External Postural Perturbations Induce Multiple Anticipatory Postural Adjustments when Subjects Cannot Pre-select Their Stepping Foot

Previous research on human balance recovery suggests that, prior to an externally triggered postural perturbation, healthy subjects can pre-select their postural response based on the environmental context, but it is unclear whether this pre-selection includes the selection of a stepping leg when performing compensatory steps. We sought to determine how pre-selecting a stepping limb affects the compensatory steps and stability of young, healthy subjects when responding to postural perturbations. Nine healthy subjects (24-37 years of age) stepped in response to backward translations of a platform under their feet when, prior to the perturbations, the subjects either knew whether they were to step with their left or right leg to a visual target (the Predictable condition) or did not know whether to step with their left or right leg until one of two targets appeared at perturbation onset (the Unpredictable condition). The Unpredictable condition also included randomly inserted trials of toes-up rotations and backward translations without targets (catch trials). The results showed that, in the Predictable condition, the subjects consistently exhibited one anticipatory postural adjustment (APA; a lateral weight shift toward the stance limb) before stepping accurately to the target with the correct leg. In the Unpredictable condition, the subjects either (1) exhibited multiple APAs, late step onsets, and forward center-of-mass (CoM) displacements that were farther beyond their base of support, or (2) exhibited an early step with only one APA and kept their CoM closer to the base of support, but also stepped more often with the incorrect leg. Thus, when the subjects had to select a stepping leg at perturbation onset, they either became more unstable and used multiple APAs to delay stepping in order to provide enough time to select the correct stepping leg, or they stepped earlier to remain stable but often stepped with the incorrect leg. In addition, responses to catch trials in the Unpredictable condition included distorted step placements that resembled steps to anticipated targets, despite allowing the subjects to step with a leg of their choice and to a location of their choice. Lastly, the subjects' voluntary stepping latencies to visual targets presented without perturbations were twice as long as their stepping latencies to the backward platform translations. Therefore, healthy subjects appear to pre-select their stepping limb, even when the perturbation characteristics are unpredictable, because relying on visual input provided at perturbation onset requires a delayed response that leads to greater instability.

Adaptation of Postural Orientation to Changes in Surface Inclination

We previously showed that standing on an inclined surface resulted in an after-effect of leaning in many healthy, blindfolded subjects when they returned to standing on a horizontal surface (Kluzik et al. in Exp Brain Res 162:474-489, 2005). The direction of leaning depended on the direction of prior surface inclination, always in a direction that preserved the relative alignment between the body and the support surface. For example, subjects leaned forward after they stood on a toes-up-inclined surface. In the present study, we investigated how the amplitude of surface inclination affected postural muscle activity, joint position, body segment orientation, and body center of mass (CoM) and foot center of pressure (CoP) locations before, during, and after subjects stood on an inclined surface. We asked whether the mechanism that underlies the lean after-effect involves regulation of local postural variables, such as the position of the ankle joint or the level of muscle activity, or whether instead, the mechanism involves regulation of global, whole-body postural variables that can only be determined by multisensory processing, such as orientation of the trunk or the body's CoM. In one experiment, we found that varying the amplitude of a toes-up surface inclination between 2.5 degrees and 10 degrees had a systematic, linear, effect on the post-incline orientation of the trunk and head, but did not systematically affect the post-incline orientation of the legs, position of the ankle joint, the level of EMG activity, or the location of the CoP. In a second experiment, we found that preventing the legs from leaning in the post-incline period did not abolish leaning of the upper body. These findings suggest that (1) the body-to-support-surface relationship is an important reference for the CNS internal representation of postural orientation which is subject to adaptive modification and (2) the adaptive mechanism underlying the post-incline after-effect of leaning acts at the level of global, whole-body postural variables.

Auditory Biofeedback Substitutes for Loss of Sensory Information in Maintaining Stance

The importance of sensory feedback for postural control in stance is evident from the balance improvements occurring when sensory information from the vestibular, somatosensory, and visual systems is available. However, the extent to which also audio-biofeedback (ABF) information can improve balance has not been determined. It is also unknown why additional artificial sensory feedback is more effective for some subjects than others and in some environmental contexts than others. The aim of this study was to determine the relative effectiveness of an ABF system to reduce postural sway in stance in healthy control subjects and in subjects with bilateral vestibular loss, under conditions of reduced vestibular, visual, and somatosensory inputs. This ABF system used a threshold region and non-linear scaling parameters customized for each individual, to provide subjects with pitch and volume coding of their body sway. ABF had the largest effect on reducing the body sway of the subjects with bilateral vestibular loss when the environment provided limited visual and somatosensory information; it had the smallest effect on reducing the sway of subjects with bilateral vestibular loss, when the environment provided full somatosensory information. The extent that all subjects substituted ABF information for their loss of sensory information was related to the extent that each subject was visually dependent or somatosensory-dependent for their postural control. Comparison of postural sway under a variety of sensory conditions suggests that patients with profound bilateral loss of vestibular function show larger than normal information redundancy among the remaining senses and ABF of trunk sway. The results support the hypothesis that the nervous system uses augmented sensory information differently depending both on the environment and on individual proclivities to rely on vestibular, somatosensory or visual information to control sway.

Podokinetic After-rotation in a Simulated Reduced Gravity Environment

Stepping in place on a rotating platform for a period of 15 minutes induces an adaptive response, podokinetic after-rotation (PKAR), which causes a blindfolded individual to inadvertently rotate when attempting to step in place on the floor. The purpose of this investigation was to determine whether lower extremity load receptors were involved in this adaptation. As load receptor input is critical for locomotion, we hypothesized that manipulating load via body weight support (BWS) would influence PKAR. Eleven healthy female volunteers performed 15 minutes of stepping in place on a rotating treadmill (stimulation), followed by 10 minutes of stepping in place (response) without vision on a stationary surface. Response and stimulation periods were with 50% body weight support (BWS) and without body weight support (NoBWS) in all four possible combinations (BWS-BWS, NoBWS-NoBWS, BWS-NoBWS, and NoBWS-BWS). Conditions were randomly assigned to all subjects and performed on four separate occasions at least 48 hr apart. During the 10-min PKAR response period, trunk angular velocity was calculated and plotted versus time, and exponential models were applied to the data to obtain curve-fit parameters for each condition. Despite the manipulations of BWS, no significant differences were found for any parameter value (p = 0.13-0.98). BWS applied during stimulation only, response only, or during both stimulation and response does not appear to influence PKAR. This suggests that load receptors may not play a critical role in mediating adaptive changes in locomotor trajectory control in response to walking on a rotating surface.

Control of Dynamic Stability During Gait Termination on a Slippery Surface in Parkinson's Disease

This study investigated how Parkinson's disease (PD) affects the ability to switch from locomotion to gait termination (GT) during planned and cued GT and examined the effect of PD on the integration of a reactive, balance maintenance strategy into voluntary GT. After a series of stops on a stable surface, eight participants with and 10 without PD stopped on a surface, which slid quickly and unexpectedly forward mimicking a slippery surface. PD caused instability during the completely voluntary nonslippery stops (P = 0.012) but not during the slippery stops, which required a reactive movement. The PD group walked slower [0.9-1.0 m/s vs. 1.3 m/s, respectively (P < 0.001)] with shorter steps during the first step of nonslippery GT (P = 0.016) and with wider steps during all steps of nonslippery GT (P

Imbalance in Multiple Sclerosis: a Result of Slowed Spinal Somatosensory Conduction

Balance problems and falls are common in people with multiple sclerosis (MS) but their cause and nature are not well understood. It is known that MS affects many areas of the central nervous system that can impact postural responses to maintain balance, including the cerebellum and the spinal cord. Cerebellar balance disorders are associated with normal latencies but reduced scaling of postural responses. We therefore examined the latency and scaling of automatic postural responses, and their relationship to somatosensory evoked potentials (SSEPs), in ten people with MS and imbalance and ten age-, sex-matched, healthy controls. The latency and scaling of postural responses to backward surface translations of five different velocities and amplitudes, and the latency of spinal and supraspinal somatosensory conduction, were examined. Subjects with MS had large, but very delayed automatic postural response latencies compared to controls (161 +/- 31 ms vs. 102 +/- 21 ms, p < 0.01) and these postural response latencies correlated with the latencies of their spinal SSEPs (r = 0.73, p < 0.01). Subjects with MS also had normal or excessive scaling of postural response amplitude to perturbation velocity and amplitude. Longer latency postural responses were associated with less velocity scaling and more amplitude scaling. Balance deficits in people with MS appear to be caused by slowed spinal somatosensory conduction and not by cerebellar involvement. People with MS appear to compensate for their slowed spinal somatosensory conduction by increasing the amplitude scaling and the magnitude of their postural responses.

Changes in the Activity of the Cerebral Cortex Relate to Postural Response Modification when Warned of a Perturbation

To determine whether the cerebral cortex contributes to modifying upcoming postural responses to external perturbations when provided with prior warning of the perturbation.

Lateral Stepping for Postural Correction in Parkinson's Disease

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

Effects of Parkinson's Disease and Levodopa on Functional Limits of Stability

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.

Analyzing 180 Degrees Turns Using an Inertial System Reveals Early Signs of Progression of Parkinson's Disease

Changes in turning are one of the early motor deficiencies in Parkinson's Disease (PD). We have proposed a system based on wearable, inertial sensors and a novel automatic analysis algorithm that can assess 180 degrees turns. Twelve patients in early stages of PD and 14 age-matched healthy subjects were enrolled in this study. Inertial sensors were attached on shanks and sternum. Measurement protocol included walking on a straight pathway, turning 180 degrees and returning back. Subjects were measured 4 times, once every 6 months during an 18 months period. At the baseline, 9 subjects from each group repeated the test twice to assess test-retest reliability. Patients with mild PD had a very low Postural Instability Gait Difficulty (PIGD subscore of UPDRS III) score (average 0.67, min 0, max 3). The analysis showed that the patients had a significantly longer turning duration (2.18+/-0.43 vs. 1.79+/-0.27 seconds, p<0.02) and longer delay in their last step before initiating a turn (0.56+/-0.04 vs. 0.52+/-0.04 seconds, p<0.03). Estimated turning duration and other metrics had a high test-retest reliability (rho>0.85). Turning duration also showed a significant Group *Time interaction (p<0.03) during the longitudinal study highlighting early signs of the progression of the disease.

Influence of Visual and Haptic Cues on Podokinetic After-rotation

Active countercircling on a rotating platform for 15 min causes individuals to involuntarily circle in the same direction when they step in place on firm ground. This is referred to as podokinetic after-rotation (PKAR). It is unclear how interjecting brief periods of visual or haptic inputs for a stable orientation reference affects PKAR. The authors studied this issue in 16 healthy individuals who participated in three sessions each. Following active countercircling, participants attempted to step in place for 30 min on firm ground. In two of three sessions, participants received full visual input or made fingertip contact with a stationary object for 30 s during 30 min of ongoing PKAR. All participants slowed or stopped rotating during the presence of visual or haptic inputs and resumed PKAR after removal of these inputs. Exponential functions fitted to angular trunk velocity versus time plots revealed no significant differences across conditions (p > .05). The preservation of PKAR after brief exposure to a visual or haptic reference is consistent with a slowly decaying velocity storage that is not reset or dumped after exposure to conflicting visual or haptic cues.

Practice-related Improvements in Posture Control Differ Between Young and Older Adults Exposed to Continuous, Variable Amplitude Oscillations of the Support Surface

Healthy older adults were repeatedly exposed to continuous, variable amplitude oscillations of the support surface to determine (1) whether age affects the capacity for postural motor learning under continuous perturbation conditions with limited predictability and (2) whether practice leads to modifications in the control strategy used to maintain balance in older adults. During training, a translating platform underwent 45-s trials of constant frequency (0.5 Hz) and seemingly random amplitude oscillations (range +\- 2 to 15 cm). The middle 15 s of each trial contained the same sequence of oscillation amplitudes. This repeated middle segment was the same as the repeated segment used in Van Ooteghem et al. (Exp Brain Res 187(4): 603-611, 2008) and was therefore used for analyses. To examine learning, participants performed a retention test following a 24-h delay. Kinematic data were used to derive spatial and temporal measures of whole body centre of mass (COM), trunk, thigh, and shank segment orientation, and ankle and knee angle from performance during the repeated middle segment. Results showed that with training, older adults maintained the capacity to learn adaptive postural responses in the form of improved temporal control of the COM and minimization of trunk instability a a rate comparable to young adults. With practice, however older adults maintained a more rigid, 'platform-fixed' control strategy which differed from young adults who shifted towards 'gravity-fixed' control and decreased COM motion. This study provides important insight into the ability of older adults to demonstrate longer-term improvements in postural regulation.

Postural Feedback Scaling Deficits in Parkinson's Disease

Many differences in postural responses have been associated with age and Parkinson's disease (PD), but until now there has been no quantitative model to explain these differences. We developed a feedback control model of body dynamics that could reproduce the postural responses of young subjects, elderly subjects, and subjects with PD, and we investigated whether the postural impairments of subjects with PD can be described as an abnormal scaling of postural feedback gain. Feedback gains quantify how the nervous system generates compensatory joint torques based on kinematic responses. Seven subjects in each group experienced forward postural perturbations to seven different backward support surface translations ranging from 3- to 15-cm amplitudes and with a constant duration of 275 ms. Ground reaction forces and joint kinematics were measured to obtain joint torques from inverse dynamics. A full-state feedback controller with a two-segment body dynamic model was used to simulate joint kinematics and kinetics in response to perturbations. Results showed that all three subject groups gradually scaled postural feedback gains as a function of perturbation amplitudes, and the scaling started even before the maximum allowable ankle torque was reached. This result implies that the nervous system takes body dynamics into account and adjusts postural feedback gains to accommodate biomechanical constraints. PD subjects showed significantly smaller than normal ankle feedback gain with low scaling and larger hip feedback gain, which led to an early violation of the flat-foot constraint and unusually small (bradykinetic) postural responses. Our postural feedback control model quantitatively described the postural abnormality of the patients with PD as abnormal feedback gains and reduced ability to modify postural feedback gain with changes in postural challenge.

Vibrotactile Biofeedback Improves Tandem Gait in Patients with Unilateral Vestibular Loss

In a crossover design, subjects with unilateral vestibular loss (UVL) practiced tandem gait with eyes closed on two days, two weeks apart, with and without vibrotactile biofeedback (BF) applied to the lateral trunk. Results showed an immediate improvement in postural stability (reduction of lateral center-of-mass displacement, trunk tilt, and medial-lateral step width) that was significantly larger than effects of practice alone. However, BF did not increase the rate of improvement or retention of improved stability during gait.

Postural Compensation for Vestibular Loss

To what extent can remaining sensory information and/or sensory biofeedback (BF) compensate for loss of vestibular information in controlling postural equilibrium? The primary role of the vestibulospinal system is as a vertical reference for control of the trunk in space, with increasing importance as the surface becomes increasingly unstable. Our studies with patients with bilateral loss of vestibular function show that vision or light touch from a fingertip can substitute as a reference for earth vertical to decrease variability of trunk sway when standing on an unstable surface. However, some patients with bilateral loss compensate better than others, and found that those with more complete loss of bilateral vestibular function compensate better than those with measurable vestibulo-ocular reflexes. In contrast, patients with unilateral vestibular loss (UVL) who reweight sensory dependence to rely on their remaining unilateral vestibular function show better functional performance than those who do not increase vestibular weighting on an unstable surface. Light touch of <100 grams or auditory biofeedback can be added as a vestibular vertical reference to stabilize trunk sway during stance. Postural ataxia during tandem gait in patients with UVL is also significantly improved with vibrotactile BF to the trunk, beyond improvements due to practice. Vestibular rehabilitation should focus on decreasing hypermetria, decreasing an overdependence on surface somatosensory inputs, increasing use of any remaining vestibular function, substituting or adding alternative sensory feedback related to trunk sway, and practicing challenging balance tasks on unstable surfaces.

Reduced Performance in Balance, Walking and Turning Tasks is Associated with Increased Neck Tone in Parkinson's Disease

Rigidity or hypertonicity is a cardinal symptom of Parkinson's disease (PD). We hypothesized that hypertonicity of the body axis affects functional performance of tasks involving balance, walking and turning. The magnitude of axial postural tone in the neck, trunk and hip segments of 15 subjects with PD (both ON and OFF levodopa) and 15 control subjects was quantified during unsupported standing in an axial twisting device in our laboratory as resistance to torsional rotation. Subjects also performed six functional tests (walking in a figure of eight [Figure of Eight], Timed Up and Go, Berg Balance Scale, supine rolling task [rollover], Functional Reach, and standing 360-deg turn-in-place) in the ON and OFF state. Results showed that PD subjects had increased tone throughout the axis compared to control subjects (p=0.008) and that this increase was most prominent in the neck. In PD subjects, axial tone was related to functional performance, but most strongly for tone at the neck and accounted for an especially large portion of the variability in the performance of the Figure of Eight test (r(OFF)=0.68 and r(ON)=0.74, p<0.05) and the Rollover test (r(OFF)=0.67 and r(ON)=0.55, p<0.05). Our results suggest that neck tone plays a significant role in functional mobility and that abnormally high postural tone may be an important contributor to balance and mobility disorders in individuals with PD.

The Balance Evaluation Systems Test (BESTest) to Differentiate Balance Deficits

Current clinical balance assessment tools do not aim to help therapists identify the underlying postural control systems responsible for poor functional balance. By identifying the disordered systems underlying balance control, therapists can direct specific types of intervention for different types of balance problems.

Delaying Mobility Disability in People with Parkinson Disease Using a Sensorimotor Agility Exercise Program

This article introduces a new framework for therapists to develop an exercise program to delay mobility disability in people with Parkinson disease (PD). Mobility, or the ability to efficiently navigate and function in a variety of environments, requires balance, agility, and flexibility, all of which are affected by PD. This article summarizes recent research identifying how constraints on mobility specific to PD, such as rigidity, bradykinesia, freezing, poor sensory integration, inflexible program selection, and impaired cognitive processing, limit mobility in people with PD. Based on these constraints, a conceptual framework for exercises to maintain and improve mobility is presented. An example of a constraint-focused agility exercise program, incorporating movement principles from tai chi, kayaking, boxing, lunges, agility training, and Pilates exercises, is presented. This new constraint-focused agility exercise program is based on a strong scientific framework and includes progressive levels of sensorimotor, resistance, and coordination challenges that can be customized for each patient while maintaining fidelity. Principles for improving mobility presented here can be incorporated into an ongoing or long-term exercise program for people with PD.

Fibromyalgia is Associated with Impaired Balance and Falls

The purpose of this study was to determine whether fibromyalgia (FM) patients differ from matched healthy controls in clinical tests of balance ability and fall frequency.

Knee Trembling During Freezing of Gait Represents Multiple Anticipatory Postural Adjustments

Freezing of gait (FoG) is an episodic, brief inability to step that delays gait initiation or interrupts ongoing gait. FoG is often associated with an alternating shaking of the knees, clinically referred to as knee trembling or trembling in place. The pathophysiology of FoG and of the concomitant trembling knees is unknown; impaired postural adjustment in preparation for stepping is one hypothesis. We examined anticipatory postural adjustments (APAs) prior to protective steps induced by a forward loss of balance in 10 Parkinson's disease (PD) subjects with marked FoG and in 10 control subjects. The amplitude and timing of the APAs were determined from changes in the vertical ground-reaction forces recorded by a force plate under each foot and were confirmed by electromyographic recordings of bilateral medial gastrocnemius, tibialis anterior and tensor fascia latae muscles. Protective steps were accomplished with a single APA followed by a step for control subjects, whereas PD subjects frequently exhibited multiple, alternating APAs coexistent with the knee trembling commonly observed during FoG as well as delayed, inadequate or no stepping. These multiple APAs were not delayed in onset and were of similar or larger amplitude than the single APAs exhibited by the control subjects. These observations suggest that multiple APAs produce the knee trembling commonly associated with FoG and that FoG associated with a forward loss of balance is caused by an inability to couple a normal APA to the stepping motor pattern.

Effects of a Central Cholinesterase Inhibitor on Reducing Falls in Parkinson Disease

To investigate if a central cholinesterase inhibitor will reduce falling frequency in subjects with Parkinson disease (PD) with advanced postural instability.

Preparation for Compensatory Forward Stepping in Parkinson's Disease

To characterize preparation for compensatory stepping in people with Parkinson's disease (PD) compared with healthy control subjects, and to determine whether levodopa medication improves preparation or the execution phases of the step.

Aging Does Not Affect Generalized Postural Motor Learning in Response to Variable Amplitude Oscillations of the Support Surface

Postural motor learning for dynamic balance tasks has been demonstrated in healthy older adults (Van Ooteghem et al. in Exp Brain Res 199(2):185-193, 2009). The purpose of this study was to investigate the type of knowledge (general or specific) obtained with balance training in this age group and to examine whether embedding perturbation regularities within a balance task masks specific learning. Two groups of older adults maintained balance on a translating platform that oscillated with variable amplitude and constant frequency. One group was trained using an embedded-sequence (ES) protocol which contained the same 15-s sequence of variable amplitude oscillations in the middle of each trial. A second group was trained using a looped-sequence (LS) protocol which contained a 15-s sequence repeated three times to form each trial. All trials were 45 s. Participants were not informed of any repetition. To examine learning, participants performed a retention test following a 24-h delay. LS participants also completed a transfer task. Specificity of learning was examined by comparing performance for repeated versus random sequences (ES) and training versus transfer sequences (LS). Performance was measured by deriving spatial and temporal measures of whole body center of mass (COM) and trunk orientation. Both groups improved performance with practice as characterized by reduced COM displacement, improved COM-platform phase relationships, and decreased angular trunk motion. Furthermore, improvements reflected general rather than specific postural motor learning regardless of training protocol (ES or LS). This finding is similar to young adults (Van Ooteghem et al. in Exp Brain Res 187(4):603-611, 2008) and indicates that age does not influence the type of learning which occurs for balance control.

ITUG, a Sensitive and Reliable Measure of Mobility

Timed Up and Go (TUG) test is a widely used clinical paradigm to evaluate balance and mobility. Although TUG includes several complex subcomponents, namely: sit-to-stand, gait, 180 degree turn, and turn-to-sit; the only outcome is the total time to perform the task. We have proposed an instrumented TUG, called iTUG, using portable inertial sensors to improve TUG in several ways: automatic detection and separation of subcomponents, detailed analysis of each one of them and a higher sensitivity than TUG. Twelve subjects in early stages of Parkinson's disease (PD) and 12 age matched control subjects were enrolled. Stopwatch measurements did not show a significant difference between the two groups. The iTUG, however, showed a significant difference in cadence between early PD and control subjects (111.1 +/- 6.2 versus 120.4 +/- 7.6 step/min, p < 0.006) as well as in angular velocity of arm-swing (123 +/- 32.0 versus 174.0+/-50.4 degrees/s, p < 0.005), turning duration (2.18 +/- 0.43 versus 1.79 +/- 0.27 s, p < 0.023), and time to perform turn-to-sits (2.96 +/- 0.68 versus 2.40 +/- 0.33 s, p < 0.023). By repeating the tests for a second time, the test-retest reliability of iTUG was also evaluated. Among the subcomponents of iTUG, gait, turning, and turn-to-sit were the most reliable and sit-to-stand was the least reliable.

Invited Commentary

Influence of Vision on Adaptive Postural Responses Following Standing on an Incline

Previous studies demonstrated a leaning after-effect (LAE) following standing or walking on an inclined surface consistent with a long-lasting, somatosensory memory for body orientation relative to the surface. Here, we asked whether providing a brief visual reference during LAE resets postural orientation to the new visual reference. The results showed that subjects immediately return to upright when eyes were opened briefly during the post-incline period. However, the subjects also immediately resumed leaning after closing their eyes again following 20 s of eyes open. The duration of LAE was not influenced by 1 or 2 brief periods of vision. Also, the amplitude of the lean following the brief vision period was often larger than when subjects had their eyes closed for the entire post-incline period. These results suggest a powerful somatosensory memory contribution to postural orientation in space that is not eliminated or recalibrated with brief exposure to a visual reference.

Postural Compensation for Vestibular Loss and Implications for Rehabilitation

This chapter summarizes the role of the vestibular system in postural control so that specific and effective rehabilitation can be designed that facilitates compensation for loss of vestibular function.

The Instrumented Timed Up and Go Test: Potential Outcome Measure for Disease Modifying Therapies in Parkinson's Disease

The Timed Up and Go (TUG) test has been used to assess balance and mobility in Parkinson's Disease (PD). However, it is not known if this test is sensitive to subtle abnormalities present in early stages of the disease, when balance and gait problems are not clinically evident but may be detected with instrumented analysis of movement. We hypothesise that postural transitions and arm swing during gait will be the most sensitive characteristics of the TUG for early PD. In the present study, we instrumented the TUG test (iTUG) using portable inertial sensors, and extended the walking distance from 3 m (traditional TUG) to 7 m. Twelve subjects with early-to-moderate, untreated PD and 12 healthy individuals participated. Our findings show that although the stopwatch measure of TUG duration did not detect any abnormalities in early-to-mid-stage PD, the peak arm swing velocity on the more affected side, average turning velocity, cadence and peak trunk rotation velocity were significantly slower. These iTUG parameters were also correlated with the Unified Parkinson's Disease Rating Motor Scale. Thus, the iTUG test is sensitive to untreated PD and could potentially detect progression of PD and response to symptomatic and disease-modifying treatments.

Freezing of Gait is Associated with a Mismatch Between Motor Imagery and Motor Execution in Narrow Doorways, Not with Failure to Judge Doorway Passability

Many patients with Parkinson's disease (PD) develop freezing of gait (FoG), which may manifest as a hesitation or "getting stuck" when they attempt to pass through a doorway. In two experiments, we asked whether FoG is associated with (1) a deficit in internal representation of one's body size with respect to a doorway and (2) a mismatch between imagined and actual walking times when passing through a doorway.

Postural Compensation for Unilateral Vestibular Loss

Postural control of upright stance was investigated in well-compensated, unilateral vestibular loss (UVL) subjects compared to age-matched control subjects. The goal was to determine how sensory weighting for postural control in UVL subjects differed from control subjects, and how sensory weighting related to UVL subjects' functional compensation, as assessed by standardized balance and dizziness questionnaires. Postural control mechanisms were identified using a model-based interpretation of medial-lateral center-of-mass body-sway evoked by support-surface rotational stimuli during eyes-closed stance. The surface-tilt stimuli consisted of continuous pseudorandom rotations presented at four different amplitudes. Parameters of a feedback control model were obtained that accounted for each subject's sway response to the surface-tilt stimuli. Sensory weighting factors quantified the relative contributions to stance control of vestibular sensory information, signaling body-sway relative to earth-vertical, and proprioceptive information, signaling body-sway relative to the surface. Results showed that UVL subjects made significantly greater use of proprioceptive, and therefore less use of vestibular, orientation information on all tests. There was relatively little overlap in the distributions of sensory weights measured in UVL and control subjects, although UVL subjects varied widely in the amount they could use their remaining vestibular function. Increased reliance on proprioceptive information by UVL subjects was associated with their balance being more disturbed by the surface-tilt perturbations than control subjects, thus indicating a deficiency of balance control even in well-compensated UVL subjects. Furthermore, there was some tendency for UVL subjects who were less able to utilize remaining vestibular information to also indicate worse functional compensation on questionnaires.

Site of Deep Brain Stimulation and Jaw Velocity in Parkinson Disease

While deep brain stimulation (DBS) has proven to be an effective treatment for many symptoms of Parkinson disease (PD), a deterioration of axial symptoms frequently occurs, particularly for speech and swallowing. These unfavorable effects of DBS may depend on the site of stimulation. The authors made quantitative measures of jaw velocity to compare the relative effectiveness of DBS in the globus pallidus internus (GPi) or the subthalamic nucleus (STN). This was a randomized, double-blind, and longitudinal study, with matched healthy controls.

Postural Control Deficits in People with Fibromyalgia: a Pilot Study

Postural instability and falls are increasingly recognized problems in patients with fibromyalgia (FM). The purpose of this study was to determine whether FM patients, compared to age-matched healthy controls (HCs), have differences in dynamic posturography, including sensory, motor, and limits of stability. We further sought to determine whether postural instability is associated with strength, proprioception and lower-extremity myofascial trigger points (MTPs); FM symptoms and physical function; dyscognition; balance confidence; and medication use. Last, we evaluated self-reported of falls over the past six months.

Prolonged Weight-shift and Altered Spinal Coordination During Sit-to-stand in Practitioners of the Alexander Technique

The Alexander Technique (AT) is used to improve postural and movement coordination and has been reported to be clinically beneficial, however its effect on movement coordination is not well-characterized. In this study we examined the sit-to-stand (STS) movement by comparing coordination (phasing, weight-shift and spinal movement) between AT teachers (n=15) and matched control subjects (n=14). We found AT teachers had a longer weight-shift (p<0.001) and shorter momentum transfer phase (p=0.01), than control subjects. AT teachers also increased vertical foot force monotonically, rather than unweighting the feet prior to seat-off, suggesting they generate less forward momentum with hip flexors. The prolonged weight-shift of AT teachers occurred over a greater range of trunk inclination, such that their weight shifted continuously onto the feet while bringing the body mass forward. Finally, AT teachers had greatly reduced spinal bending during STS (cervical, p<0.001; thoracic, p<0.001; lumbar, p<0.05). We hypothesize that the low hip joint stiffness and adaptive axial postural tone previously reported in AT teachers underlies this novel "continuous" STS strategy by facilitating eccentric contractions during weight-shift.

Freezing of Gait: Moving Forward on a Mysterious Clinical Phenomenon

Freezing of gait (FoG) is a unique and disabling clinical phenomenon characterised by brief episodes of inability to step or by extremely short steps that typically occur on initiating gait or on turning while walking. Patients with FoG, which is a feature of parkinsonian syndromes, show variability in gait metrics between FoG episodes and a substantial reduction in step length with frequent trembling of the legs during FoG episodes. Physiological, functional imaging, and clinical-pathological studies point to disturbances in frontal cortical regions, the basal ganglia, and the midbrain locomotor region as the probable origins of FoG. Medications, deep brain stimulation, and rehabilitation techniques can alleviate symptoms of FoG in some patients, but these treatments lack efficacy in patients with advanced FoG. A better understanding of the phenomenon is needed to aid the development of effective therapeutic strategies.

What is the Most Effective Type of Audio-biofeedback for Postural Motor Learning?

Biofeedback is known to improve postural control and reduce postural sway. However, the effects that different biofeedback modes (coding for more or less complex movement information) may have on postural control improvement are still poorly investigated. In addition, most studies do not take into account the effects of spontaneous motor learning from repetition of a task when investigating biofeedback-induced improvement in postural control. In this study, we compared the effects of four different modes of audio-biofeedback (ABF), including direction and/or magnitude of sway information or just a non-specific-direction alarm, on the postural sway of 13 young healthy adults standing on a continuously rotating surface. Compared to the non-specific-direction alarm, ABF of continuous postural sway direction and/or amplitude resulted in larger postural sway reduction in the beginning of the experiment. However, over time, spontaneous postural motor learning flattened the effects of the different modes of ABF so that the alarm was as effective as more complex information about body sway. Nevertheless, motor learning did not make ABF useless, since all modes of ABF further reduced postural sway, even after subjects learned the task. All modes of ABF resulted in improved multi-segmental control of posture and stabilized the trunk-in-space. Spontaneous motor learning also improved multi-segmental control of posture but not trunk-in-space stabilization as much as ABF. In conclusion, although practice standing on a perturbing surface improved postural stability, the more body sway information provided to subjects using ABF, the greater the additional improvement in postural stability.

Trunk Accelerometry Reveals Postural Instability in Untreated Parkinson's Disease

While several studies have shown that subjects with advanced Parkinson's disease (PD) exhibit abnormalities in sway parameters during quiet standing, abnormalities of postural sway associated with untreated PD have not been reported. Although not clinically apparent, we hypothesized that spontaneous sway in quiet stance is abnormal in people with untreated PD. We examined 13 subjects, recently diagnosed with PD, who were not yet taking any anti-parkinsonian medications and 12 healthy, age-matched control subjects. Postural sway was measured with a linear accelerometer on the posterior trunk (L5 level) and compared with traditional force plate measures of sway. Subjects stood for 2 min under two conditions: eyes open (EO) and eyes closed (EC). One of the most discriminative measures of postural changes in subjects with untreated PD was the increased 'JERK' of lower trunk in the EO condition, measured with the accelerometer. Root mean square and the frequency dispersion of postural sway in the EO condition also discriminated sway in untreated PD subjects compared to control subjects. We conclude that accelerometer-based sway metrics could be used as objective measures of postural instability in untreated PD. Accelerometer-based analysis of spontaneous sway may provide a powerful tool for early clinical trials and for monitoring the effects of treatment of balance disorders in subjects with PD.

Milestones in Gait, Balance, and Falling

Gait, balance, and falls have become increasingly common topics of published articles in the Movement Disorders journal since its launch in 1986. This growth represents an increasing awareness of the importance of mobility to patients' quality of life. New methods have become available that allow for accurate measurement of many aspects for gait and balance. This has led to new concepts of understanding gait and balance disorders. Neuroimaging has begun to reveal the neural circuitry underlying gait and balance. The physiology and pathophysiology of balance and gait are beginning to tease out the many processes involved in mobility and how they may be disrupted by disease processes. With these advances, the old therapeutic nihilism that characterized the clinician's approach to falls and gait disorders is disappearing, as innovative physiotherapy, exercise, drugs, and deep brain stimulation are being employed for gait and balance disorders.

Errors in Postural Preparation Lead to Increased Choice Reaction Times for Step Initiation in Older Adults

This study asked whether older adults were more likely than younger adults to err in the initial direction of their anticipatory postural adjustment (APA) prior to a step (indicating a motor program error), whether initial motor program errors accounted for reaction time differences for step initiation, and whether initial motor program errors were linked to inhibitory failure.

Podokinetic Stimulation Causes Shifts in Perception of Straight Ahead

Podokinetic after-rotation (PKAR) is a phenomenon in which subjects inadvertently rotate when instructed to step in place after a period of walking on a rotating treadmill. PKAR has been shown to transfer between different forms of locomotion, but has not been tested in a non-locomotor task. We conducted two experiments to assess effects of PKAR on perception of subjective straight ahead and on quiet standing posture. Twenty-one healthy young right-handed subjects pointed to what they perceived as their subjective straight ahead with a laser pointer while they were recorded by a motion capture system both before and after a training period on the rotating treadmill. Subjects performed the pointing task while standing, sitting on a chair without a back, and a chair with a back. After the training period, subjects demonstrated a significant shift in subjective straight ahead, pointing an average of 29.1 ± 10.6° off of center. The effect was direction-specific, depending on whether subjects had trained in the clockwise or counter-clockwise direction. Postures that limited subjects' ability to rotate the body in space resulted in reduction, but not elimination, of the effect. The effect was present in quiet standing and even in sitting postures where locomotion was not possible. The robust transfer of PKAR to non-locomotor tasks, and across locomotor forms as demonstrated previously, is in contrast to split-belt adaptations that show limited transfer. We propose that, unlike split-belt adaptations, podokinetic adaptations are mediated at supraspinal, spatial orientation areas that influences spinal-level circuits for locomotion.

Deep Brain Stimulation for Parkinson Disease: an Expert Consensus and Review of Key Issues

To provide recommendations to patients, physicians, and other health care providers on several issues involving deep brain stimulation (DBS) for Parkinson disease (PD).

Assessing Mobility at Home in People with Early Parkinson's Disease Using an Instrumented Timed Up and Go Test

Gait and mobility problems are prominent features of Parkinson's Disease (PD), and are difficult to observe clinically in early stages of PD. We previously reported that gait changes were measurable in early to mid-stage PD subjects, when we used inertial sensors during an instrumented Timed Up and Go test (iTUG). With the advent of wearable inertial sensors, home assessment of mobility has become possible. We tested six people with early PD and eight control subjects using the iTUG in the home and laboratory. Our objectives were to 1) investigate the feasibility of testing subjects at home, and 2) compare performance at home versus laboratory. We found that home iTUG testing is feasible and the patients with PD were more affected than the healthy control subjects when tested at home.

Comparing the Mini-BESTest with the Berg Balance Scale to Evaluate Balance Disorders in Parkinson's Disease

Objective. The purpose of this study was to explore the usefulness of the Mini-BESTest compared to the Berg Balance Scale in evaluating balance in people with PD of varying severity. We evaluated (1) the distribution of patients scores to look for ceiling effects, (2) concurrent validity with severity of disease, and (3) the sensitivity/specificity of separating people with or without postural response deficits. Subjects. Ninety-seven people with PD were tested for balance deficits using the Berg, Mini-BESTest, Unified Parkinson's Disease Rating Scale (UPDRS) III and the Hoehn & Yahr (H&Y) disease severity classification. Setting. Clinical research facility at Oregon Health & Science University. Results. The Mini-BESTest is highly correlated with the Berg (r = 0.79, P < 0.001), but avoids the ceiling compression effect of the Berg for mild PD (skewness -2.30 Berg, -0.93 Mini-BESTest). Consequently, the Mini-BESTest is more effective than the Berg for predicting UPDRS Motor score (P < 0.001 Mini-BESTest versus P = 0.86 Berg), and for discriminating between those with and without postural response deficits as measured by the H&Y (ROC differential P = 0.06). Conclusion. The Mini-BESTest is a promising tool for discerning balance deficits in patients with PD, most importantly those with more subtle deficits.

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