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At least 253 records · Page 14Linked to original sources
PubMed1993

Open-loop and closed-loop control of posture: a random-walk analysis of center-of-pressure trajectories.

A new conceptual and theoretical framework for studying the human postural control system is introduced. Mathematical techniques from statistical mechanics are developed and applied to the analysis and interpretation of stabilograms. This work was based on the assumption that the act of maintaining an erect posture could be viewed, in part, as a stochastic process. Twenty-five healthy young subjects were studied under quite-standing conditions. Center-of-pressure (COP) trajectories were analyzed as one-dimensional and two-dimensional random walks. This novel approach led to the extraction of repeatable, physiologically meaningful parameters from stabilograms. It is shown that although individual stabilograms for a single subject were highly variable and random in appearance, a consistent, subject-specific pattern emerged with the generation of averaged stabilogram-diffusion plots (mean square COP displacement vs time interval). In addition, significant inter-subject differences were found in the calculated results. This suggests that the steady-state behavior of the control mechanisms involved in maintaining erect posture can be quite variable even amongst a population of age-matched, anthropometrically similar, healthy individuals. These posturographic analyses also demonstrated that COP trajectories could be modelled as fractional Brownian motion and that at least two control systems-a short-term mechanism and a long-term mechanism-were operating during quit standing. More specifically, the present results suggest that over short-term intervals open-loop control schemes are utilized by the postural control system, whereas over long-term intervals closed-loop control mechanisms are called into play. This work strongly supports the position that much can be learned about the functional organization of the postural control system by studying the steady-state behavior of the human body during periods of undisturbed stance.

Adult↗
PubMed1995

Age-related changes in open-loop and closed-loop postural control mechanisms.

In an earlier posturographic investigation (Collins and De Luca 1993) it was proposed that open-loop and closed-loop control mechanisms are involved in the regulation of undisturbed, upright stance. In this study, stabilogram-diffusion analysis was used to examine how the natural aging process affects the operational characteristics of these control mechanisms. Stabilogram-diffusion analysis leads to the extraction of repeatable center-of-pressure (COP) parameters that can be directly related to the steady-state behavior and functional interaction of the neuromuscular mechanisms underlying the maintenance of erect posture. Twenty-five healthy young males (aged 19-30 years) and twenty-five elderly males (aged 71-80 years) who were free of major gait and postural disorders were included in the study. An instrumented force platform was used to measure the time-varying displacements of the COP under each subject's feet during quiet standing. The COP trajectories were analyzed as one-dimensional and two-dimensional random walks, according to stabilogram-diffusion analysis. Using this technique, it was demonstrated cross-sectionally that healthy aging is associated with significant changes in the 'quasi-static' dynamics of the postural control system. (It was also shown that more traditional posturographic analyses, i.e., summary statistics, were not sensitive enough to detect these age-related differences.) It was found that the steady-state behavior of the open-loop postural control mechanisms in the elderly is more positively correlated and therefore perhaps more unstable, i.e., the output of the overall system has a greater tendency to move or drift away from a relative equilibrium point over the short term. In contrast with this result, it was also found that the steady-state behavior of the closed-loop postural control mechanisms in the elderly is more negatively correlated and therefore perhaps more stable, i.e., over the longer term, there is an increased probability that movements away from a relative equilibrium point will be offset by corrective adjustments back towards the equilibrium position. In addition, it was demonstrated that the elderly utilize open-loop control schemes for longer time intervals and correspondingly larger COP displacements during periods of undisturbed stance. This result suggests that in the elderly there is a greater delay, on average, before closed-loop feedback mechanisms are called into play. Finally, it was shown that there is an increased heterogeneity of postural control abilities in healthy older adults.

Adult↗
PubMed1979

Compensation of postural reactions to fall in the vestibular neurectomized monkey. Role of the reamining labyrinthine afferences.

The electromyographic (EMG) responses from soleus and tibialis anterior muscles and the monosynaptic H- and T-reflex responses from soleus muscles were recorded bilaterally from conscious baboon while unexpectedly dropping it with unrestricted vision. These responses were recorded either after unilateral vestibular neurectomy (U.N. Baboons) or after bilateral neurectomy performed in one stage (B.N. 1 baboons) and in two stages (B.N. 2 baboons). A positive correlation was found between modifications and development of EMG responses and reflex data. In the U.N. baboons, some differences were observed when comparing data from the H- and T-reflex methods, suggesting that recovery of normal responses to fall is achieved both by means of direct influences on alpha-motoneurons and via the gamma-loop. In the U.N. baboons postural reactions to fall developed in three distinct periods. The first or critical stage showed asymmetrical EMG and reflex responses with increased responses from contralateral soleus muscle and decreased responses from ipsilateral soleus. Opposite effects were recorded from tibialis anterior flexor muscles. The second or acute stage which began around 4 to 7 days after surgery exhibited symmetrical, but very reduced, responses when compared to the control in soleus muscles, and symmetrical, but increased, responses from tibialis anterior muscles. This stage lasted until about the end of the second postoperative week and was followed by the third or compensatory stage during which EMG as well as reflex responses developed towards the control pattern in all tested muscles. Almost normal responses were recorded on both sides 3 weeks after surgery. Only a partial recovery was found in the B.N. 1 baboons, indicating that the contralateral remaining labyrinthine afferences constitute a necessary condition for the full compensation of postural reactions to fall in the case of unilateral vestibular neurectomy. The Bechterew's compensation was obtained in the B.N. 2 baboons. These results are discussed in relation with the general organization of the vestibulospinal pathways and with those concerning development of the postoperative activity at the vestibular nuclei level. A model of vestibular compensation achieved by means of a multisensory substitution process is suggested.

Afferent Pathways↗
PubMed1980

Characteristics of postural instability induced by ischemic blocking of leg afferents.

After minimizing proprioceptive input from the legs by ischemia without degradation of muscle force and excluding visual stabilization by eye closure, a characteristic anterior-posterior postural sway around 1 Hz was observed in three normal subjects. This is similar to the instability seen in two tabes dorsalis patients. From the spectral analysis of head and hip movements, displacements of the center of force and of ankle angle as well as from EMG recordings of the anterior tibial and gastrocnemius muscle it is concluded that the oscillations around 1 Hz are due to the long latency and high threshold of vestibularly induced leg muscle discharges (200-300 ms) arriving in the counterbalancing phase of the trunk, which causes an overshoot in body sway.

Adult↗
PubMed1983

Variability of postural "reflexes" in humans.

The functional role of spinal and supraspinal EMG-responses for the maintenance of upright human posture was investigated in ten healthy subjects standing on a force measuring platform, which could be rotated in pitch around an axis aligned with the subject's ankle joint. Voluntary changes of body posture prior to the platform movement by leaning forward or backward led to a change in the amplitude and temporal organization of EMG-responses as compared to platform movements starting from a neutral position. Tilting the platform toe-up while leaning backward led to an increase of the latency of the short- and medium-latency responses in the triceps surae muscle and to a decrease of the latency of the stabilizing response in the anterior tibial muscle. Functionally, a cocontraction of both antagonistic muscles could be observed which partly compensated for the destabilizing action of the "reflex" response in the stretched triceps surae muscle. In analogy, leaning forward and tilting the platform toe-down led to a cocontraction of the two antagonistic muscles. The observed changes of latencies of short-, medium-, and long-latency response show the functional variability of segmental and suprasegmental "reflex" mechanisms. EMG-activities, which are functionally destabilizing posture, can be suppressed or compensated by reflexive cocontractions of antagonists.

Adult↗
PubMed1983

Visual influence on postural control in the cat.

The influence of horizontal optokinetic stimulation on posture and postural readjustments, induced by horizontal rotation of a turntable, were studied in the cat. Optokinetic stimulations at constant velocities lead to modifications of posture depending on the velocity of the stimulus. However, habituation as well as interindividual variability of these postural responses make difficult a systematic and quantitative approach to the phenomenon. Optokinetic stimulations at sinusoidal velocities induced reproductible postural responses, whose phase and gain were studied at different velocities and frequencies of stimulation. At low frequencies (from 0.05 Hz to 0.2 Hz) the postural responses tend to lead the position of the optokinetic stimulus while a lag appears at higher frequencies (up to 1 Hz). In these conditions the gain increases and seems to depend on the performances of the simultaneously elicited oculomotor response. Induced postural readjustments are also modulated by the relative velocity of the visual stimulation with respect to the cat. A given visual input is more effective on an induced postural readjustment than on the posture of a static animal, especially at low frequencies (from 0.05 Hz to 0.3 Hz). These data extend to the cat the strong visual component of the postural control system previously described in other species.

Animals↗
PubMed1981

Relation of automatic postural responses and reaction-time voluntary movements of human leg muscles.

This study contrasts the properties of compensatory postural adjustments in response to movements of the support surface with those of reaction-time voluntary movements in human subjects. Subjects stood upon a six degrees-of-freedom movable platform and performed tone and movement-triggered voluntary sways about the ankle joints both under conditions of postural stability and instability. These triggered movements could be executed as rapidly as postural adjustments to support surface perturbations (80-120 ms), but only when the former were well practiced, single-choice (direction) and were performed under conditions of postural stability. Evaluation of the properties of postural adjustments and reaction-time voluntary movements revealed a number of clear organizational differences between the two categories of movement, but most interesting was the finding that, when reaction-time movements were triggered by or at the onset of platform movement, the postural adjustments always occurred first. Only when subjects were given a tone trigger 50 ms in advance of platform movement were they able to execute the reaction-time movement first. We found that the dichotomous voluntary/reflexive classification of movements was not consistent with all of the identified properties of postural adjustments and reaction-time movements. Instead, we find a system which classifies movements by function, as either stabilizating or orientational adjustments, to be more useful. In the context of whole-body movement then, intentional focal components would be closely associated with others directed towards postural stabilization.

Electromyography↗
PubMed1985

Visual and vestibular contributions to pitch sway stabilization in the ankle muscles of normals and patients with bilateral peripheral vestibular deficits.

Vestibular, visual, and proprioceptive influences on muscle activity correcting for backwards body tilt were investigated in normals and patients with bilateral peripheral vestibular deficits. Body tilt was induced by a dorsi-flexion rotation of the feet about the ankle joints while the subject stood on a force measuring platform. Ankle muscle activity and torque were monitored as upright stance was reestablished, and correlated with head angular accelerations and neck muscle activity. In normals with eyes closed, soleus stretch reflex activity at 50-80 ms was followed by two bursts of tibialis anterior (TA) EMG activity at ca 80 and 125 ms from the onset of 36 deg/s, 3 deg amplitude platform rotations. Neck muscle activity rotated the head backwards at the same time as TA activity rotated the body forwards about the ankle joints. Under the influence of vision, i.e. eyes open, slight increases in the second burst of TA activity, and ankle torque were observed. When the subjects sat, and were instructed to activate TA rapidly on onset of the platform movement, TA EMG activity increased gradually at ca. 150 ms and not as a burst. In patients with long-lasting bilateral vestibular deficits, both bursts of TA activity were significantly less than normal with eyes closed. Consequently sway correcting torques were abnormally low and all but one of the patients fell over backwards. With eyes open, TA activity was slightly less than, and ankle torques were approximately equal to normal values. In contrast to normals, TA responses obtained in standing and sitting positions were not significantly different. Neck EMG activity varied from normal, consisting of a long burst 100 ms in duration. The present data indicate that a coordinated pattern of ankle, and neck muscle activity occurs during the first 150 ms following induced backward tilt. Ankle muscle activity corrects for the body sway, and neck muscle activity attempts to stabilise the head with respect to earth fixed coordinates. It is proposed that the vestibulo-spinal reflex system predominantly underlies the genesis and coordination of this muscle activity.

Adult↗
PubMed1995

Head and trunk movements in the frontal plane during complex dynamic equilibrium tasks in humans.

Eight normal human subjects were asked to maintain monopodal equilibrium on a narrow beam (task 1) or bipodal equilibrium on an unstable rocking platform (task 2) for 5 s. Each task was performed under four experimental conditions: (1) in light, (2) in darkness, (3) in light while subject had to hold a full cup of water, and (4) as in 3, but with additional instructions to fix the gaze on the cup. The movements of the trunk and head in the frontal plane were recorded by means of a 50-Hz TV image analyzer that computed the coordinates of small reflective markers glued on the skin of the subjects. On the beam the trunk was inclined on the side of the supporting foot (13 +/- 9 degrees), on the rocking platform the mean trunk orientation during the tests was nearly vertical (2 +/- 7 degrees). Nevertheless, in both tasks the mean head position was the same and close to vertical: 1.5 +/- 4 degrees on the rocking platform and 1.5 +/- 5 degrees on the beam. For both tasks and all experimental conditions the head remained stabilized relative to vertical, despite large translations in the frontal plane. Standard deviations of head orientation from its mean value were 2.8 +/- 2 degrees for task 1 and 2 +/- 1.5 degrees for task 2. The changes of trunk orientation were significantly higher: 6.2 +/- 4.8 degrees and 4.5 +/- 4 degrees, respectively. The differences in angular stability of head and trunk, measured through the standard deviations of angular displacements, were especially pronounced in trials with large trunk movements. It was concluded that head angular stabilization, providing the central nervous system with necessary visual and vestibular references, is essential for effective dynamic postural control in the frontal plane during complex equilibrium tasks.

Adult↗
PubMed1979

Vertigo and drop attacks caused by acute transient monocular disequilibrium (Halpern's syndrome).

Among the causes of acute vertigo the syndrome of sensorimotor induction in unilateral disequilibrium (Halpern's syndrome) should be considered. This syndrome was found in a patient and described in detail. The major features of this syndrome are the displacement of vertical and horizontal axes induced by looking with the "affected" eye only, which are further aggravated by applying red filters to the eyes only, which are further aggravated by applying red filters to the eyes and corrected by blue filters. The symptomatology is described and discussed in detail. Theories causing this syndrome are discussed.

Acute Disease↗
PubMed1986

Posture testing (posturography) in the diagnosis of peripheral vestibular pathology.

In patients with peripheral vestibular deficiencies, the testing of posture or "posturography" can give specific information about any compensation obtained in the vestibulospinal reflex (VSR). We have used the statokinesimetric parameter of length in this study. Nearly 50% of the patients with unilateral vestibular hypofunction as well as those patients with paroxysmal positional vertigo (PPV) show abnormal results. These findings indicate deficient compensation at the vestibulospinal level, which is independent of any compensation already achieved at the vestibulo-ocular level. The tests used for the latter, such as positioning and rotational tests, are unable to provide information about the degree of compensation reached in the VSR. The examinations used in the different modalities of sensory interaction can show the presence of influences of ocular fixation and changes of head position. We have observed three types of deviant interaction. Our posturographic data have allowed us to assess functional situations in a more precise way. Any rehabilitation exercises used should be adapted according to these data.

Humans↗
PubMed1994

Use of the visual suppression test using post-rotatory nystagmus to determine skill in ballet dancers.

Twelve ballet dancers with various levels of dancing experience and skill were examined with the visual suppression test using post-rotatory nystagmus (PRVST) and caloric stimulation (CVST). The PRVST results showed a suppression rate that was higher than in untrained subjects. The CVST results showed a suppression rate similar to that in untrained subjects. A correlation between the PRVST and CVST suppression rates and the length of dancing experience showed that the suppression rate increased as the level of experience and skill rose. These results indicate that the PRVST and CVST can aid in the clinical and quantitative assessment of the function of the central nervous system in visual-vestibular interactions in ballet dancers. Additionally, testing may have determined function of vestibulo-cerebellar pathways through habituation of visual-vestibular interactions. Findings indicate that it may be possible to use suppression rates of PRVST and CVST to determine the approximate level of a dancer's experience and skill.

Adult↗