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PubMed2001

Falls in the elderly: the development of a risk questionnaire and posturographic findings.

In elderly people, owing to a perturbation at several levels, including the motor, sensory and cognitive levels, a condition of dizziness and unsteadiness complicated by frequent falls often appears. In this article, we review the most recent information about clinical and instrumental tools available for preventing mobility-related accidents and report the results of a comparative study of postural control, carried out through tetraataxiometry (by Tetrax, Tel Aviv, Israel), in two samples of elderly women belonging to two different populations: 24 Italian women (11 reporting falls and 13 without falls) having a mean age of 73.1 years, and 37 Israeli women (12 with falls and 25 without falls) having a mean age of 72.5 years. The posturographic findings show that the falling subjects, to maintain postural control, are highly dependent on somatosensory inputs and have a weaker "systeme postural fin" (fine postural system), according to Gagey. They also show that an elderly subpopulation exists that, for unknown reasons, is immune to destabilization and falls.

Accidental Falls↗
PubMed2026

High-altitude hypoxia alters the visual control of standing balance in lowlanders and Tibetan highlanders.

High-altitude hypoxia affects both visual function and postural control, yet the influence of optic-flow perturbations on standing balance under hypoxic stress remains unclear. Tibetan highlanders (TH) exhibit adaptations to chronic hypoxia, but whether their visually driven postural responses differ from those of lowlanders (LL) has not been investigated. We examined how high-altitude exposure and acclimatization influence static and dynamic visual contributions to balance by delivering sinusoidal optic-flow perturbations in virtual reality at low altitude (1,400 m) and after incremental ascent to high altitude (4,300 m) in acclimatizing LL (n = 15) and TH (n = 14). Anteroposterior center of pressure (AP CoP) velocity and mean power frequency (MPF) were measured during three visual-field conditions (full-, central-, and peripheral-vision) and two optic-flow velocities (peak 1 m/s and 8 m/s at 0.25 Hz). At high altitude, both groups showed attenuated responses to optic flow compared with 1,400 m, reflected by reduced AP CoP velocity and lower MPF across visual-field conditions, consistent with reduced responsiveness to dynamic visual-motion cues under high altitude hypoxia. In contrast, during eyes-open quiet stance [no virtual reality (VR)], TH but not LL exhibited increased AP CoP velocity and MPF at 4,300 m, and no altitude effect was observed with eyes-closed in either group. This finding indicates that TH adopt a visually dependent postural strategy at altitude, whereas LL show minimal changes in static visual balance control but reduced responsiveness to fast dynamic motion. Together, these findings demonstrate that high-altitude hypoxia disrupts dynamic visual processing for balance control in both groups, while revealing group differences in the use of static visual cues during quiet stance.NEW & NOTEWORTHY This is the first study to investigate how high-altitude hypoxia alters visually driven postural control using virtual reality (VR) optic-flow perturbations. We show that hypoxia attenuates sway responses to optic-flow in both lowlanders and Tibetan highlanders, and that visual weighting differs between these groups. These findings reveal altitude- and population-related changes in sensory weighting during standing balance, advancing sensorimotor understanding of postural control in hypoxia.

Humans↗
PubMed1996

Postural sensitivity to visual flow in aging adults with and without balance problems.

BACKGROUND: This study tested balance behavior of young adults and aging adults with and without balance problems in response to visual flow from a moving visual surround. METHODS: Balance behavior was indexed by force plate measures of maximum anterior/posterior displacement of the center of foot pressure and horizontal shear forces. The sample included normal young adults (n = 13; mean age 23 years, +/- 7.5), normal aging adults (n = 13; mean age 76 years, +/- 6.5), and aging adults with balance problems not directly attributable to a diagnosable neurological disease or dysfunction (n = 13; mean age 79 years, +/- 5.8). RESULTS: The balance-affected aging group had statistically greater sway responses than the young group when the stimulus was unexpected (as in the first trial; p < .05). Some individuals in each group had large responses that were statistical outliers from the group median. The balance-affected group had significantly greater shear forces than the young group. CONCLUSIONS: Greater sway responses suggest over-reliance on visual cues for posture control in the balance-affected aging group, which may be related to underlying, borderline somatosensory deficits, as indicated by the patterns of subclinical indications for somatosensory impairments on neurological exams in this group. Visually sensitive postural control, however, may issue from several different underlying processes. Elevated shear forces during balance responses in the balance-affected group suggest a greater use of hip movements in addition to ankle movements for postural adjustments.

Adult↗
PubMed2001

Postural stability and fractal dynamics.

Methods of non-linear dynamics and deterministic chaos may provide us with effective quantitative descriptors of the dynamics of postural control. The goal of this study was to introduce a new measure, which would allow to determine the fractal structure of posturographic signals and to measure the effect of the loss of visual feedback information in postural control. The results of the study show that fractal dimension (Df) is a very useful, reliable and sensitive measure of the complexity of posturographic signals. Therefore Df can be used for the evaluation of postural stability and its changes due to pathology or an age-related decline.

Aged↗
PubMed1996

The reliability of postural sway measures using the 3SPACE Tracker.

OBJECTIVE: To investigate the reliability of three-dimensional postural control sway measures, in a normal population, using an electromagnetic device. DESIGN: A repeated measures design was used within and between sessions. BACKGROUND: Electromagnetic measurement has been proposed as a method of measuring postural sway; however, reliability within a normal population has not been shown. METHOD: An electromagnetic device, the 3SPACE Tracker, measured postural sway at the pelvis using sway path length and displacements in the anterior-posterior, medial-lateral and vertical direction. Ten subjects performed a postural control task involving two trials for six different sensory conditions, on two separate occasions, two weeks apart. RESULTS: The reliability of the technique was demonstrated by the repeatability of results for individual subjects within and between sessions. Individual subjects and sensory conditions were discriminated. Sway path length was the most consistent of the measures used. CONCLUSIONS: Three dimensional measures of postural sway are reliable and discriminatory in a normal population.

Journal Article↗
PubMed1998

The anatomy of the central control of posture: consistency and plasticity.

Posture is usually thought to be steered by brainstem and cortical structures that have access to the motoneurons and their premotor interneurons of the axial and neck muscles. The present paper describes these pathways and their relation with gaze control structures. All these systems belong to the medial component of the so-called voluntary motor system. On the other hand, in the cat there also exist several postures that are not steered by the somatic, but by the emotional motor system. Examples are arching of the back and mating postures. The pathways thought to be involved in these behaviours are described. They belong to the lateral component of the emotional motor system and, especially in case of mating postures, are extremely plastic. They have been demonstrated to be almost 10 times as strong in oestrus than in non-oestrus cats.

Animals↗
PubMed2006

Modeling the dynamics of sensory reweighting.

Reweighting sensory information adaptively is considered critical for flexible postural control, but little is known of the time scale of the reweighting process. We analyzed the transient dynamics of sensory reweighting in a previously published nonlinear adaptive model of sensory integration in the human postural control system. The model's dynamics of adaptation were tested in response to abrupt changes in the amplitude of the motion of the visual surround. In addition to qualitatively reproducing the correct asymptotic response to such changes in visual amplitude, as previously found, the model qualitatively reproduced the asymmetric transient response elucidated in recent experiments (Oie et al. in Gait Posture 2005). In particular, the model adapts at an initially rapid rate to a switch from low to high amplitude visual motion, but at an initially slower rate upon the return to low amplitude motion. The observed temporal asymmetry has potential functional value. Rapid downweighting of a visual stimulus that suddenly increases is necessary to prevent loss of upright equilibrium. A visual stimulus that decreases in amplitude does not pose a threat to upright balance, allowing for slower upweighting without functional consequence.

Adaptation, Physiological↗
PubMed2003

Differential effect of fetal, neonatal and treatment variables on neurodevelopment in infants with congenital hypothyroidism.

OBJECTIVES: To identify the influence of fetal, neonatal and treatment variables on neurodevelopment in children with congenital hypothyroidism. METHODS: A multiple regression analysis was performed in the neurodevelopmental scores of 26 children with CH who were assessed at 3, 6, 12 and 18 months for postural control, fine eye-hand coordination and language. The independent variables were: duration of fetal hypothyroidism, initial biochemical severity, days at starting treatment, initial dose, and efficiency of treatment. RESULTS: Postural control was predicted by duration of fetal hypothyroidism and language by the biochemical severity of the disease at birth. Fine eye-hand coordination was not predicted by any variable. CONCLUSION: This pattern of influences suggests that fetal and neonatal hypothyroidism have different effects on brain maturation sites during the first 2 years of life and that some subtle neurocognitive deficits are probably unavoidable.

Child Development↗
PubMed2004

Does hypnotizability affect human upright stance?

Subjects highly (Highs) and low susceptible to hypnosis (Lows) show different imagery and attentional capabilities and also peculiar somatomotor, vegetative and electroencephalographic differences in basal and task conditions. Since attention is one of the main component of hypnotic susceptibility and also a relevant factor for postural control, the aim of the experiment was to study actual differences between Highs and Lows at the eyes closure during upright stance. Visual and motor imagery as well as attentional/disattentional capabilities were evaluated through psychological tests. Posture was monitored though Elite systems during upright stance with open and closed eyes. At the eyes closure, Highs and Lows exhibited a different body sway modulation. Possible different compensation mechanisms are suggested for the two groups and interactions between attentional/arousal systems responsible of hypnotic phenomenology and postural control are underlined.

Adolescent↗
PubMed2003

Behavior of Medaka fish under distributed gravity.

The threshold value of gravity for Medaka fish (Oryzias latipes) was determined using parabolic flights of an airplane. Rotating a turntable during a 20 sec of microgravity, a gradient field of centrifugal force was realized in the aquarium. Fish of HO5 strain were used because from the previous studies, in microgravity they were known to exhibit looping behavior more easily than any other strains. Looping fish became stable (i.e., recovered their posture control) when fish swam from a lower-gravity area of the aquarium to an area of a certain gravity value or beyond. On the other hand, stable fish lost their posture control and started looping when fish swam into an area of a gravity lower than a certain value. Using these phenomena, we obtained the gravity value of 0.21 to 0.26 G as for the threshold value for Medaka fish to sense the gravity.

Adaptation, Physiological↗
PubMed1998

The sensorimotor and cognitive integration of gravity.

In order to demonstrate that gravity is not only a load acting locally and continuously on the body limbs, but is also used by higher levels of the nervous system as a dynamic orienting reference for the elaboration of the motor act, a review of several experiments conducted both in 1 g and 0 g are presented. During various locomotor tasks, the strategy that consists of stabilizing the head with respect to gravity illustrates one of the solutions used by the CNS to optimize the control of dynamic equilibrium. A question which remains to be solved when considering experimental results obtained in weightlessness concerns, however, the maintenance of motor schema that has evolved under normal gravity. Results have suggested that the concept of conservative processes, that would adapt postural control to weightlessness by using previously learned innate strategies, must be reconsidered during goal-oriented tasks. In fact, it is proposed that when conservative processes and existing solutions derived from a repertoire of terrestrial postural strategies do not provide efficient output, the CNS has to create novel strategies through a slow learning process. As with the study of postural control, three-dimensional arm reaching movements also illustrate the central representation of gravity. Indeed, gravity can be regarded as either initiating or braking arm movements and, consequently, may be represented in the motor command at the planning level. Finally, from a prospective point of view, there is a need to determine new experimental paradigms in order to study the specific motor control of man in space. It is suggested that the formulation of experimental paradigms should not consider man in space simply as a terrestrial biped.

Arm↗
PubMed2001

Automatic postural response systems in individuals with congenital total blindness.

This study examined the effects of the absence of vision from birth on automatic postural responses to platform displacements during stance. Postural responses were induced by producing randomly four types of perturbations which consisted of forward and backward translations, and toe up and down rotations. Nine congenitally totally blind and nine sighted adults served as subjects. EMG signals were recorded from four muscles in the right leg, and reaction time to somatosensory stimuli generated by platform displacements was measured by pushing a hand-held button. To assess the ability to control postural balance, the root mean square (RMS) values for lateral and antero-posterior sway before, during, and after perturbations were calculated. The EMG amplitude in the gastrocnemius muscle of a blind subject was smaller than that of a sighted subject with eyes closed. No significant differences were found between blind and sighted subjects in EMG latencies of the lower extremity muscles in response to perturbations. The blind subjects had significantly faster reaction times to somatosensory stimuli triggered by platform displacements, but in toe down rotations no significant difference was found between blind and sighted subjects. The difference in the EMG latencies and reaction times between the two groups suggests that blindness from birth may not affect the spinal stretch reflex, but may affect a volitional act mediated through the motor cortex. There were also no significant differences in the RMS values for postural sway between blind and sighted subjects with eyes open or closed, although blind subjects swayed more after backward translations than did sighted subjects with eyes open. Results suggest that the ability to control postural balance during perturbations was not affected by vision loss from birth. Our findings suggest that the automatic postural response systems of humans are unaffected by the absence of vision from birth and are rather hard wired.

Adaptation, Physiological↗
PubMed2005

Pronounced overestimation of support surface tilt during stance.

A veridical internal notion of the kinematic state of the foot support is essential for postural control. The means by which this is obtained is still a matter of debate. We therefore measured the conscious perception of support tilt during transient anterior-posterior rotations of a motion platform in six healthy subjects, using a psychophysical matching procedure. Furthermore, we evaluated subjects' postural responses (in terms of displacement of subjects' center of mass, COM, and their ankle torque, as represented by the center of foot pressure, COP). The platform tilts were applied in absence of visual and auditory orientation cues. The platform rotations consisted of smoothed position ramps with different dominant frequencies (0.025, 0.05, 0.1, 0.2, 0.4, and 0.8 Hz) and different amplitudes (0.125 degrees, 0.25 degrees, 0.5 degrees, 1 degree, 2 degrees, 4 degrees, and 8 degrees) for the forward and backward directions, which yielded a 6x14 stimulus matrix. The stimuli were repeated five times in a random order. For the matching procedure, subjects tried to maintain an upright body orientation, while trying to orient a light-weight rod, which was attached to a belt around their waists, parallel to the perceived platform surface. We measured the stimulus-evoked angular excursions of the rod and of the subjects' COM as well as the COP shift. We found that the subjects' rod indications overestimated the platform tilts, particularly with small stimulus amplitudes. To characterize the overestimation, we compared the rod indications obtained while subjects stood on the tilting platform, to rod indications in a situation in which they stood next to the platform and tried to match the rod angle to the now visually perceived platform angle. From this comparison, we inferred that the subjects' kinesthetically derived notion of platform tilt overestimates the actual tilt by a factor of approximately 4. The estimates were linearly related to the angle between body (COM) and platform, i.e., to approximately the angle of the ankle joint, a finding which suggests a proprioceptive source of the overestimation. Further analyses supported this view; they showed that the onset latencies of the rod indications could be approximated by a theoretical indication mechanism with a reaction time of about 0.31 s, a velocity threshold of 0.099 degrees/s, and a displacement threshold of 0.12 degrees. These threshold values are well in line with previous work on the leg proprioceptive detection threshold of conscious perception of body sway. We therefore assume that the phenomenon of support tilt overestimation reflects a still unknown mechanism of leg proprioception in postural control.

Adaptation, Physiological↗
PubMed1997

Postural sway and stepping response among working population: reproducibility, long-term stability, and associations with symptoms of the low back.

OBJECTIVE: To investigate the day-to-day reproducibility and long-term (9 months) stability of variables of postural control, and the associations of these variables with low-back and lower-extremity pain in a working population. DESIGN: Test-retest measurements of 18 healthy subjects. Cross-sectional study of 165 working women and 343 men. BACKGROUND: Sudden loss of postural balance may result in microtraumas of back structures. Therefore workers with decreased postural control may run an increased risk of low-back pain. There are few reports describing the reproducibility of force-plate-based posturography. METHODS: Amplitude and velocity of postural sway were measured with a force-plate in a two-feet stance with eyes open and closed, and in a one-foot stance with eyes open. A stepping response test was developed to measure the dynamic components of balance. RESULTS: The mean differences between the repeated measurements were generally 5-10% and the standard deviations of these differences were up to a quarter or one third of the mean measurement values. Sway velocity showed the best overall reproducibility. Men had a larger sway than women independent of age and anthropometry. The group of non-symptomatic subjects showed wide variation and a slightly lower sway than the groups with low-back or lower-extremity pain. CONCLUSIONS: Postural sway has moderate stability, wide variation, and slight associations with low-back symptoms in a working population.

Journal Article↗
PubMed1994

Cerebellar control of postural scaling and central set in stance.

1. The effects of cerebellar deficits in humans on scaling the magnitude of automatic postural responses based on sensory feedback and on predictive central set was investigated. Electromyographic (EMG) and surface reactive torques were compared in patients with anterior lobe cerebellar disorders and in normal healthy adults exposed to blocks of four velocities and five amplitudes of surface translations during stance. Correlations between the earliest postural responses (integrated EMG and initial rate of change of torque) and translation velocity provided a measure of postural magnitude scaling using sensory information from the current displacement. Correlations of responses with translation amplitude provided a measure of scaling dependent on predictive central set based on sequential experience with previous like displacements because the earliest postural responses occurred before completion of the displacements and because scaling to displacement amplitude disappeared when amplitudes were randomized in normal subjects. 2. Responses of cerebellar patients to forward body sway induced by backward surface displacements were hypermetric, that is, surface-reactive torque responses were two to three times larger than normal with longer muscle bursts resulting in overshooting of initial posture. Despite this postural hypermetria, the absolute and relative latencies of agonist muscle bursts at the ankle, knee, and hip were normal in cerebellar patients. 3. Although they were hypermetric, the earliest postural responses of cerebellar patients were scaled normally to platform displacement velocities using somatosensory feedback. Cerebellar patients, however, were unable to scale initial postural response magnitude to expected displacement amplitudes based on prior experience using central set. Randomization of displacement amplitudes eliminated the set effect of amplitude on initial responses in normal subjects, but responses to randomized and blocked trials were not different in cerebellar patients. 4. Cerebellar patients compensated for hypermetric responses and lack of anticipatory scaling of earliest gastrocnemius activity by scaling large, reciprocally activated tibialis and quadriceps antagonist activity with the displacement velocity and amplitude. Correlations between these antagonist EMG integrals and displacement amplitudes were preserved when amplitudes were randomized, suggesting that feedback-dependent and not set-dependent mechanisms were responsible for scaling of antagonists by cerebellar patients. Antagonist compensation for initial hypermetric responses also could be induced in normals when they overresponded to unexpectedly small amplitudes of surface displacements. 5. The major effects of anterior lobe cerebellar damage on human postural responses involves impairment of response magnitude based on predictive central set and not on use of velocity feedback or on the temporal synergic organization of multijoint postural coordination.(ABSTRACT TRUNCATED AT 400 WORDS)

Adult↗
PubMed2000

Encoding of forces by cockroach tibial campaniform sensilla: implications in dynamic control of posture and locomotion.

Forces exerted by a leg in support and propulsion can vary considerably when animals stand upon or traverse irregular terrains. We characterized the responses of the cockroach tibial campaniform sensilla, mechanoreceptors which encode force via strains produced in the exoskeleton, by applying forces to the leg at controlled magnitudes and rates. We also examined how sensory responses are altered in the presence of different levels of static load. All receptors exhibit phasico-tonic discharges that reflect the level and rate of force application. Our studies show that: (1) tonic discharges of sensilla can signal the level of force, but accurate encoding of static loads may be affected by substantial receptor adaptation and hysteresis; (2) the absolute tonic sensitivities of receptors decrease when incremental forces are applied at different initial load levels; (3) phasic discharges of sensilla accurately encode the rate of force application; and (4) sensitivities to changing rates of force are strictly preserved in the presence of static loads. These findings imply that discharges of the sensilla are particularly tuned to the rate of change of force at all levels of leg loading. This information could be utilized to adapt posture and walking to varying terrains and unexpected perturbations.

Adaptation, Physiological↗
PubMed2003

Activity of different classes of neurons of the motor cortex during postural corrections.

The dorsal side-up body orientation in quadrupeds is maintained by a postural system that is driven by sensory feedback signals. The spinal cord, brainstem, and cerebellum play essential roles in postural control, whereas the role of the forebrain is unclear. In the present study we investigated whether the motor cortex is involved in maintenance of the dorsal side-up body orientation. We recorded activity of neurons in the motor cortex in awake rabbits while animals maintained balance on a platform periodically tilting in the frontal plane. The tilts evoked postural corrections, i.e., extension of the limbs on the side moving down and flexion on the opposite side. Because of these limb movements, rabbits maintained body orientation close to the dorsal side up. Four classes of efferent neurons were studied: descending corticofugal neurons of layer V (CF5s), those of layer VI (CF6s), corticocortical neurons with ipsilateral projection (CCIs), and those with contralateral projection (CCCs). One class of inhibitory interneurons [suspected inhibitory neurons (SINs)] was also investigated. CF5 neurons and SINs were strongly active during postural corrections. In most of these neurons, a clear-cut modulation of discharge in the rhythm of tilting was observed. This finding suggests that the motor cortex is involved in postural control. In contrast to CF5 neurons, other classes of efferent neurons (CCI, CCC, CF6) were much less active during postural corrections. This suggests that corticocortical interactions, both within a hemisphere (mediated by CCIs) and between hemispheres (mediated by CCCs), as well as corticothalamic interactions via CF6 neurons are not essential for motor coordination during postural corrections.

Action Potentials↗