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PubMed1995

Effects of the spatio-temporal structure of optical flow on postural readjustments in man.

How does the spatio-temporal structure of an oscillating radial optical flow affect postural stability? In order to investigate this problem, two different types of stimulus pattern were presented to human subjects. These stimuli were generated either with a constant spatial frequency or with a spatial frequency gradient providing monocular depth cues. When the stimulation was set in motion, the gain response of the antero-posterior postural changes depended upon the oscillation frequency of the visual scene. The amplitude of the postural response did not change with the amplitude of the visual scene motion. The spatial orientation of the postural sway (major axis of sway) depended strictly and solely on the structure of the visual scene. In static conditions, depth information resulting from the presence of a spatial frequency gradient enhanced postural stability. When set in motion, a visual scene with a spatial frequency gradient induced an organization of postural sway in the direction of the visual motion. Considering visual dynamic cues, postural instability depended linearly both on the logarithm of the velocity and on the logarithm of the temporal frequency. A nonlinear relationship existed between the amplitude of the fore-aft postural sway at the driving frequency and the temporal frequency, with a peak around 2-4 Hz. These results are discussed in terms of their implications for the separation of visual and biomechanical factors influencing visuo-postural control.

Adult↗

PubMed1995

Trunk and head stabilization during the first months of independent walking.

This study measured the rate of acquisition of head and trunk postural control during the two early developmental periods of independent walking, as defined by global gait parameters. Gait parameters were observed longitudinally in four children. The maximum angular deviations of the trunk and head oscillations were computed in the frontal and sagittal planes. These decreased most dramatically during the first 10-15 weeks of independent walking, during the same period when global gait parameters changed rapidly. This head and trunk stabilization may be a fundamental process that help to maintain equilibrium during walking, and may be a necessary step prior to the development of fine posturo-motor control.

Analysis of Variance↗

PubMed1974

Modification of the postural response of the normal dog by blindfolding.

1. Normal dogs were trained to stand quietly upon a movable platform. The animals' postural adjustments in response to sinusoidal motion of the platform were assessed in both the normal and the blindfolded condition. The induced postural adjustments were described by the Fourier coefficients for the fundamental of the input together with the coefficients which were integral multiples of the fundamental.2. All of the output variables, including motion at the hind limb joints and of the body, showed modifications when the dogs were blindfolded. The body motion increased in amplitude, as measured by the fundamental Fourier coefficient, and further lagged the input motion. Motion at the knee and phalangeal joints also increased in amplitude and the timing of these movements shifted from the normal in characteristic patterns for different frequencies of input motion.3. The modifications of the postural response due to blindfolding were most pronounced at the lower input frequencies: 0.2-1.0 Hz. At 1.4 Hz or above the fundamental coefficients of most of the output measures resembled those of the normal condition. Even at the higher input frequencies, however, the postural response of the blindfolded dog was more distorted than that of the normal dog.4. It is hypothesized that relatively slow movement of the visual surround provides the dog with information which is essential to normal operations of the postural control system.

Animals↗

PubMed2005

Development of postural muscles and their innervation.

Control of posture is a prerequisite for efficient motor performance. Posture depends on muscles capable of enduring contractions, whereas movements often require quick, forceful muscle actions. To serve these different goals, muscles contain fibers that meet these different tasks. Muscles with strong postural functions mainly consist of slow muscle fibers with a great resistance against fatigue. Flexor muscles in the leg and arm muscles are mainly composed of fast muscle fibers producing relatively large forces that are rapidly fatigable. Development of the neuromuscular system continues after birth. We discuss in the human baby and in animal experiments changes in muscle fiber properties, regression from polyneural into mononeural innervation, and developmental changes in the motoneurons of postural muscles during that period. The regression of poly-neural innervation in postural muscles and the development of dendrite bundles of their motoneurons seem to be linked to the transition from the immature into the adult-like patterns of moving and postural control.

Animals↗

PubMed2001

The effect of seat tilting on pelvic position, balance control, and compensatory postural muscle use in paraplegic subjects.

OBJECTIVE: To study the effect of seat tilting on pelvic tilt, balance control, and postural muscle use in persons with a thoracic spinal cord injury (SCI). DESIGN: Cross-sectional group study. SETTING: Rehabilitation centers and rehabilitation hospital departments. PATIENTS: Ten complete high thoracic SCI (level T2-8) patients, 10 complete low thoracic SCI (level T9-12) patients, and 10 matched able-bodied controls. INTERVENTION: A 10 degrees forward inclination of the seat. MAIN OUTCOME MEASURES: Pelvic tilt, center of pressure displacement, and muscle activity. RESULTS: Anterior tilting of the pelvis as a result of forward inclination of the seat could not be shown, either in persons with or without SCI. Balance control was not influenced by forward inclination of the seat. Participants' overall muscle activity decreased while they were seated in the chair with the forwardly inclined seat. CONCLUSIONS: Evidence provided by the kinematic and electromyographic data is not sufficient to develop a protocol for wheelchair prescription on the basis of pelvic positioning.

Adult↗

PubMed1995

Control of locomotion in marine mollusk Clione Limacina. IX. Neuronal mechanisms of spatial orientation.

1. When swimming freely, the pteropod mollusk Clione limacina actively maintains a vertical orientation, with its head up. Any deflection from the vertical position causes a correcting motor response, i.e., bending of the tail in the opposite direction, and an additional activation of the locomotor system. Clione can stabilize not only the vertical orientation with its head up, but also the posture with its head down. The latter is observed at higher water temperature, as well as at a certain stage of hunting behavior. The postural control is absent in some forms of behavior (vertical migrations, defensive reactions, "looping" when hunting). The postural reflexes are driven by input from the statocysts. After removal of the statocysts, Clione was unable to maintain any definite spatial orientation. 2. Activity of the neuronal mechanisms controlling spatial orientation of Clione was studied in in vitro experiments, with the use of a preparation consisting of the CNS and statocysts. Natural stimulation (tilt of the preparation up to 90 degrees) was used to characterize responses in the statocyst receptor cells (SRCs). It was found that the SRCs depolarized and fired (10-20 Hz) when, during a tilt, they were in a position on the bottom part of the statocyst, under the statolith. Intracellular staining has shown that the SRC axons terminate in the medial area of the cerebral ganglia. Electrical connections have been found between some of the symmetrical SRCs of the left and right statocysts. 3. Gravistatic reflexes were studied by using both natural stimulation (tilt of the preparation) and electrical stimulation of SRCs. The reflex consisted of three components: 1) activation of the locomotor rhythm generator located in the pedal ganglia; this effect of SRCs is mediated by previously identified CPA1 and CPB1 interneurons that are located in the cerebral ganglia and send axons to the pedal ganglia; 2) bending the tail evoked by differential excitation and inhibition of different groups of tail muscle motor neurons; this effect is mediated by CPB3 interneurons; and 3) modification of wing movements by differential excitation and inhibition of different groups of wing motor neurons; this effect is mediated by CPB2 interneurons. 4. Gravistatic reflexes in the tail motor neurons were inhibited or reversed at a higher water temperature. 5. The SRCs are not "pure" gravitation sensory organs because they are subjected to strong influences from the CNS. In particular, CPC1 interneurons, participating in coordination of different aspects of the hunting behavior, exert an excitatory action on some of the SRCs, and inhibitory actions on others.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed2006

Evaluation of time-to-contact measures for assessing postural stability.

Postural stability has traditionally been examined through spatial measures of the center of mass (CoM) or center of pressure (CoP), where larger amounts of CoM or CoP movements are considered signs of postural instability. However, for stabilization, the postural control system may utilize additional information about the CoM or CoP such as velocity, acceleration, and the temporal margin to a stability boundary. Postural time-to-contact (TtC) is a variable that can take into account this additional information about the CoM or CoP. Postural TtC is the time it would take the CoM or CoP, given its instantaneous trajectory, to contact a stability boundary. This is essentially the time the system has to reverse any perturbation before stance is threatened. Although this measure shows promise in assessing postural stability, the TtC values derived between studies are highly ambiguous due to major differences in how they are calculated. In this study, various methodologies used to assess postural TtC were compared during quiet stance and induced-sway conditions. The effects of the different methodologies on TtC values will be assessed, and issues regarding the interpretation of TtC data will also be discussed.

Adult↗

PubMed1991

Clinical assessment of postural stability following vestibular nerve section.

Little is known about the recovery of postural control in patients following acute vestibular loss. This paper reports on the results of moving-platform posturography to assess the recovery of postural stability in 24 patients following vestibular nerve section. Posturography was abnormal prior to surgery in 17% of the patients. Seven days following surgery, 44% of the patients demonstrated vestibular deficit patterns, while 56% of the patients had normal posturography. Posturography was normal in all patients 1 month following surgery. Abnormal posturography was subsequently noted in 21% of the patients between 3 and 20 months following surgery. Each of the patients with abnormal preoperative posturography subsequently demonstrated abnormal late postoperative posturography. These results suggest that perioperative posturography may be useful in the evaluation and counseling of patients considering ablative vestibular surgery.

Adult↗

PubMed2003

Vestibular input to brain monoamine neurons--a review.

Yates et al. reported that serotonergic RN neurons are associated with vestibulo-sympathetic responses and may control BP changes during body repositioning (Yates et al., 1992; 1993). Pompeiano et al. demonstrated that LC-NA neurons participate in the postural control and modify the vestibulo-spinal reflex (Pompeiano et al., 1990; 1991a; 2001). Nishiike et al. (1996a) examined the effects of caloric vestibular stimulation on the neuronal activity of LC-NA neurons in rats. The predominant effect of CA with both hot- and cold-water on the electrical activity of LC neurons is inhibitory and persists for several minutes. GABAA receptors located on the postsynaptic membrane of LC neurons are responsible for these inhibitory responses. The VLM may inhibit LC neuronal activity in response to the CA via GABAA receptors (Nishiike et al., 1997). It is suggested that LC-NA inhibition is involved in the development of motion sickness (Nishiike et al., 2001).

Animals↗

PubMed1985

Lateral orientation and stabilization of human stance: static versus dynamic visual cues.

The differential contributions of static versus dynamic visual cues to postural control were studied in human subjects. Lateral body oscillations were measured with accelerometers located at head, hips and ankle levels, while subjects righted their balance under various mechanical conditions: on either a soft (foam rubber) support or a hard one, and in either the classical or the sharpened Romberg stance. The visual pattern (horizontal or vertical rectangular grating) was illuminated with either a stroboscopic bulb or a normal one, and control measurements were also taken in darkness for each mechanical condition. Acceleration signals were processed into their frequency power spectra, the mean area and shape of which were taken to characterize the postural skills involved and the effects of either the visual suppressions or the mechanical destabilizations. Although dynamic visual cues have already been found to play a major role in the control of lateral body sway (Amblard and Crémieux 1976), we demonstrate here that static visual cues, the only ones available under stroboscopic illumination, also make a clear though minor contribution. Hence we suggest the existence of two modes of visual control of lateral balance in man, which are well separated in terms of the frequency range of body sway: the first mechanism, which operates below 2 Hz and is strobe-resistant, seems to control the orientation of the upper part of the body; the second mechanism, which operates above 4 Hz, centers on about 7 Hz and is strobe-vulnerable, seems to immobilize the body working upwards from the feet. Thus static visual cues may slowly control re-orientation or displacement, whereas dynamic visual cues may contribute to fast stabilization of the body. In between the frequency ranges at which these two visuomotor mechanisms come into play, at about 3 Hz, there is what we call a "blind frequency", a visually neutral sway frequency which may arise from the incompatibility of visual reorientation with visual stabilization, and where vision appears unable to reduce postural sway to any marked extent. Transmission of the destabilization produced by suppression of visual cues or by mechanical methods from one anatomical level to another is also briefly discussed in terms of bio-mechanical constraints, and the correlations between various pairs of levels are considered.

Adult↗

PubMed2004

Anxiety affects vestibulospinal function of labyrinthine-defective patients during horizontal optokinetic stimulation.

Comorbidity of vestibular and anxiety disorders was suggested by epidemiological studies and, recently, new insights into potential neural circuits which subserve both balance control and emotions, appear to support this hypothesis. In particular, disorienting visual surroundings, such as those generated by full-field moving scenes, equally disrupt postural control of patients with vestibular or panic disorders. In the present study, behaviour of body sway was assessed in response to an optokinetic stimulation by means of static posturography in 20 patients with vestibular neuritis (10 patients with normal affect and 10 with generalized anxiety disorders, as diagnosed according to the American Psychiatric Association criteria), and 20 normal subjects who served as controls. Optokinetic responses and vestibulo-spinal function during a full-field, bi-directional horizontal optokinetic stimulation, were recorded simultaneously. Labyrinthine-defective patients with low and high level of anxiety showed a common pattern of asymmetric optokinetic reflexes. On the contrary, body sway was found to be increased more by eye closure and optokinetic stimulation towards the defective labyrinth in patients affected by high level of anxiety as compared to those with normal affect and controls. These data confirm the combined effect of anxiety and labyrinthine dysfunction on vestibulo-spinal function which is disclosed by both visual suppression and disorienting visual contexts.

Adult↗

PubMed2000

Central control components of a 'simple' stretch reflex.

The monosynaptic stretch reflex is a fundamental feature of sensory-motor organization in most animal groups. In isolation, it serves largely as a negative feedback devoted to postural controls; however, when it is involved in diverse movements, it can be modified by central command circuits. In order to understand the implications of such modifications, a model system has been chosen that has been studied at many different levels: the crayfish walking system. Recent studies have revealed several levels of control and modulation (for example, at the levels of the sensory afferent and the output synapse from the sensory afferent, and via changes in the membrane properties of the postsynaptic neuron) that operate complex and highly adaptive sensory-motor processing. During a given motor task, such mechanisms reshape the sensory message completely, such that the stretch reflex becomes a part of the central motor command.

Animals↗

PubMed1993

Exercise to improve gait velocity in older persons.

The effect of 12 weeks of resistance and balance training on strength and gait measures in older persons was tested in a randomized control trial. Thirty-one residents of two life-care communities volunteered (mean age, 82.1 years). The exercise group (E) trained three times per week, performing resistance training to fatigue for knee extension, hip abduction, ankle dorsiflexion, hip extension, and knee flexion, and postural control exercises. The control group (C) met weekly for flexibility exercises performed sitting in a chair. The exercise group achieved significant improvements in muscle strength and gait velocity (p < .05). Knee extension one maximum repetition increased 32% (24% to 40%, 95% confidence interval [CI]), from 55 +/- to 72 +/- 4N.m. Isokinetic knee extension torque increased 25% (3% to 47%) from 40 +/- 5 to 50 +/- 5N.m at 120 degrees s-1. Usual gait velocity increased 8% (3% to 13%) in E from 1.04 +/- .07 to 1.12 +/- .06m/s. There was a trend of improvement in maximal gait speed 4% (0 to 8%) from 1.43 +/- 0.1 to 1.49 +/- 0.1m/s, p = 0.054. Gait and strength measures in C were unchanged. A short-term exercise program that trained strength and balance achieved a clinically significant improvement in gait velocity.

Aged↗

PubMed1999

Maximal and explosive force production capacity and balance performance in men of different ages.

A group of 32 healthy men (M) divided into three different age groups, i.e. M20 years [mean 21 (SD 1); n = 12], M40 [mean 40 (SD 2); n = 10] and M70 [mean 71 (SD 5); n = 10] volunteered as subjects for examination of maximal and explosive force production of leg extensor muscles in both isometric and dynamic actions (squat jump, SJ and counter movement jump, CMJ, and standing long-jump, SLJ). The balance test was performed on a force platform in both isometric and dynamic actions. Maximal bilateral isometric force value in M70 was lower (P < 0.001) than in M40 and as much as 46% lower (P < 0.001) than that recorded in M20 (P < 0.001). The maximal rate of force development (RFD) on the force-time curve was in M70 lower (P < 0.001) than in M40 and as much as 64% lower than in M20. The heights in SJ and CMJ and the distance in SLJ in M70 were lower (P < 0.001) than in M40 and M20 (P < 0.001). In response to modifications of the visual surroundings the older subjects were 24%-47% (P < 0.05 and P < 0.001) slower in their response time in reaching the lit centre (TT) and remained 20%-34% (P < 0.001) less time inside the centre (TC) from the overall time of lighting than M40 and M20, respectively. In both older groups the individual values of isometric RFD correlated significantly (P < 0.05) with the individual balance values of TT and TC. The present results would suggest that the capacity for explosive force production declines drastically with increasing age, even more than maximal muscle strength. Aging may also lead to impaired balance with a decrease in event detection and speed of postural adjustments. The decreased ability to develop force rapidly in older people seems to be associated with a lower capacity for neuromuscular response in controlling postural sway.

Adult↗

PubMedDate not supplied

Use of a computerized postural sway measurement system for neurobehavioral toxicology.

The study of standing posture has been associated with nervous system functioning for over a hundred years. Measures of human standing ability have attracted some attention as indicators of neurotoxic insult. The use of postural sway measures as subclinical indicators of toxicity has not been regularly incorporated into most neurobehavioral test batteries, but the development of microcomputer-controlled systems offers new possibilities. The mechanisms involved in controlling postural sway are discussed, as well as the various measurement techniques. In addition, studies involving the effects of some neurotoxic agents are cited. A postural sway measurement system that is noninvasive, has 1-2-minute test periods, provides immediate test results, and is relatively free of practice and motivation effects is described. Results present the normative characteristics of the sway parameters, a comparison of three data transformation techniques, and the effects of height and weight on the sway parameters. Power calculations were also performed to estimate the number of subjects needed to detect effects at both the 80% and 90% power levels.

Adolescent↗

PubMed2000

Frontal plane dynamic stability and coordination in subjects with cerebellar degeneration.

Objective attempts to characterize postural control in subjects with cerebellar (Cb) pathology have focused primarily on sagittal plane responses to static standing, semi-dynamic standing, and platform perturbation. Repeated, dynamic, functional movement may provide a better opportunity to study the effects of ataxia on frontal plane postural stability. The purpose of this investigation was to quantify lateral stability using center of gravity (CG) and center of pressure (CP) movement analysis, and to examine motor responses necessary to complete a repetitive stepping task. Whole body kinematic and kinetic data were collected as patients with Cb degeneration and non-disabled volunteers repeatedly ascended forward and descended backward from a 7.6-cm step to the beat of a metronome. Cb subjects demonstrated significantly greater lateral CG instability, consistent with increased CG velocity and displacement variability in the frontal plane compared to non-disabled subjects. Significantly greater lateral CP displacement was found in subjects with Cb degeneration. Phase plot patterns from patients with Cb degeneration showed poor movement quality, indicated by abruptly changing CG phase plot patterns and abnormal CG acceleration and deceleration phases. Ataxic postural reactions were observed consistently during lateral weight shifting phases toward the stance limb. We hypothesized that lateral postural instability in Cb subjects stems from altered lower extremity intersegmental coordination and inadequate lateral CG velocity control. Frontal plane CG instability may necessitate a wide-based gait. Despite the varied neuroanatomic sites of degenerative Cb pathology, subjects with Cb pathology demonstrated similar CG movement patterns and altered motor strategies to avoid destabilization. Dynamic assessment provides important information regarding frontal plane instability not revealed by static assessment methods. These findings suggest that a dynamic, constrained stepping task effectively differentiates Cb and non-disabled volunteers, contributing to our knowledge of the effect of ataxic movement disturbances on frontal plane postural stability and lower limb coordination.

Adult↗

PubMed2004

Nicergoline in the treatment of dizziness in elderly patients. A review.

In elderly patients, dizziness occurs very frequently with significant effects on the patient's life. Its frequency increases with age, and may arise from a variety of causes. Chronic dysequilibrium in elderly patients is most probably related to disturbances within the central nervous system, due either to altered neuronal functions or to an underlying vascular disease. Nicergoline, a drug used in the treatment of cognitive disturbances in geriatric patients, improves dizziness in elderly demented and non-demented patients. In a double blind,placebo controlled trial the drug improved (i) the severity of symptoms, measured by the dizziness assessment rating scale (DARS), (ii) the overall clinical conditions revealed by global impression scale, and (iii) the perceived quality of life estimated by the dizziness handicap inventory (DHI). These results indicate a possible positive effect also on posturographic measures. Moreover, the improvement occurred at no expense of the established strategy of postural control suggesting that the effect is mediated by a substitute compensatory mechanism allowing the patient to preserve consolidated postural strategies. The results of previous open clinical studies in about 3000 patients are in agreement with those findings. Overall, severity of symptoms decreased by 68 % (57 % in the control study). Globally, the results indicate a beneficial effect of nicergoline on symptoms related to balance disorders of central origin. Animal studies show that the drug displays a broad spectrum of actions on cellular and molecular mechanisms. Moreover, animal research specifically aimed at vestibular pathophysiology has revealed that nicergoline improves vestibular compensation in models of vestibular lesion. Chronic treatment with nicergoline improved the time-course of behavioral recovery in old rats after hemi-labyrinthectomy and counteracted the regulation of cholinergic receptors observed after lesion in old rats. Nicergoline interacts at several levels by various mechanisms, from the molecular level to cognitive function, probably enhancing spontaneous plasticity phenomena underlying the central vestibular compensation. This effect is not dependent from the interaction with a single-transmitter-identified neural pathway, but from anatomical, functional and neurochemical synergistic adjustments in several brain areas.

Aged↗

PubMedDate not supplied

Postural instability on one foot in patients with loss of unilateral peripheral vestibular function.

The vestibular system is a major contributor to postural control and the vestibular nuclei are closely connected to the limb spinal motor neurons. Acute loss of unilateral peripheral vestibular function results in, among others, postural instability which improves with time due to central compensatory processes. It has been shown, however, that a number of processes, such as sensorimotor restriction, drugs, and so forth, may affect the speed and the quality of compensation. Due to the difference of ipsilateral vs contralateral connectivity between the vestibular nuclei and the limb spinal motor neurons, and the peculiarities of compensatory processes, the ipsilateral may not compensate as well as the contralateral foot to lesion in patients who lost unilateral peripheral vestibular function. To explore this possibility, the sway, while standing on each foot, was recorded in normal subjects (N = 30) and compensated (except three) patients (N = 37) with loss of unilateral peripheral vestibular function and the percentage difference between the two sways was calculated. No difference was found between patients swaying more on the ipsilateral vs patients swaying more on the contralateral foot to lesion (p > .80). There was, however, a statistical difference between patients as a group vs normal subjects as a group (p = .005). These data suggest, although not evident in all patients of the group, that sway on the contralateral foot was less than the sway on the ipsilateral foot to lesion, as initially speculated. They may, however, signify that one foot compensates better than the other. This is a preliminary report and more testing is on-going to confirm these results.

Adolescent↗