Biomedical subjects

Fay B Horak

Publications and source records attributed to Fay B Horak.

At least 19 recordsLinked to original sources

PubMed2006

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.

Adaptation, Physiological↗

PubMed2006

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, Physiological↗

PubMed2006

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.

Adaptation, Physiological↗

PubMed2006

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.

Adaptation, Physiological↗

PubMed2006

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.

Aged↗

PubMed2006

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.

Adult↗

PubMed2006

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.

Accidental Falls↗

PubMed2006

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.

Auditory Perception↗

PubMed2005

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.

Aged↗

PubMed2005

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.

Aged↗

PubMed2005

Influence of a portable audio-biofeedback device on structural properties of postural sway.

BACKGROUND: 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. METHODS: In this paper, we present the effects of a portable prototype of an ABF system on healthy subjects' upright stance postural stability, in conditions of limited and unreliable sensory information. Stabilogram diffusion analysis, combined with traditional center of pressure analysis and surface electromyography, were applied to the analysis of quiet standing tasks on a Temper foam surface with eyes closed. RESULTS: These analyses provided new evidence that ABF may be used to treat postural instability. In fact, the results of the stabilogram diffusion analysis suggest that ABF increased the amount of feedback control exerted by the brain for maintaining balance. The resulting increase in postural stability was not at the expense of leg muscular activity, which remained almost unchanged. CONCLUSION: Examination of the SDA and the EMG activity supported the hypothesis that ABF does not induce an increased stiffness (and hence more co-activation) in leg muscles, but rather helps the brain to actively change to a more feedback-based control activity over standing posture.

Journal Article↗

PubMed2005

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.

Adaptation, Physiological↗

PubMed2005

Audio-biofeedback improves balance in patients with bilateral vestibular loss.

OBJECTIVE: To evaluate the effectiveness of an audio-biofeedback (ABF) system for improving balance in patients with bilateral vestibular loss (BVL). DESIGN: Before-after trial. SETTING: University balance disorders laboratory. PARTICIPANTS: Nine subjects with BVL and 9 unaffected subjects as controls. INTERVENTION: Trunk acceleration ABF while standing on foam with eyes closed. MAIN OUTCOME MEASURE: Balance stability was evaluated according to the following parameters: the root mean square of (1) the center of pressure (COP) displacements and of (2) the trunk accelerations; the COP bandwidth; the time spent by the participant within +/-1 degrees threshold from their baseline COP position; and the mean accelerations of the trunk while the participant was swaying outside this +/-1 degrees threshold. RESULTS: Participants with BVL had significantly larger postural sway than did unaffected participants. Those with BVL, while using ABF, decreased sway area by 23%+/-4.9%, decreased trunk accelerations by 46%+/-9.9%, and increased time spent within +/-1 degrees sway threshold by 195%+/-34.6%. CONCLUSIONS: ABF improved stance stability of participants with BVL by increasing the amount of postural corrections.

Adult↗

PubMed2005

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.

Aged↗

PubMed2005

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.

Acceleration↗

PubMed2005

Improvement in automatic postural coordination following alexander technique lessons in a person with low back pain.

BACKGROUND AND PURPOSE: 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. CASE DESCRIPTION: The client was a 49-year-old woman with a 25-year history of left-sided, idiopathic, lumbrosacral back pain. Automatic postural coordination was measured using a force plate during horizontal platform translations and one-legged standing. OUTCOMES: The client was tested monthly for 4 months before AT lessons and for 3 months after lessons. Before lessons, she consistently had laterally asymmetric automatic postural responses to translations. After AT lessons, the magnitude and asymmetry of her responses and balance improved and her low back pain decreased. DISCUSSION: Further research is warranted to study whether AT lessons improve low back pain-associated abnormalities in automatic postural coordination and whether improving automatic postural coordination helps to reduce low back pain.

Biofeedback, Psychology↗

PubMed2004

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.

Aged↗

PubMed2004

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.

Adaptation, Physiological↗