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At least 865 records · Page 48Linked to original sources

PubMed2002

Stability analysis for postural control in a two-joint limb system.

The stability behavior of a multi-joint limb with electrically activated muscles provides important clues for postural control of motor tasks. The stability property of the musculoskeletal system can be characterized with its eigenvalues evaluated at operating postures in the workspace. A planar arm model with shoulder and elbow joints and three pairs of antagonistic muscles was constructed in ADAMS. Stability behavior of shoulder and elbow joints was analyzed using the loci of eigenvalues in the s-plane. In the analysis of open-loop cocontraction of antagonist muscles with increasing activation from 5% to 100%, the eigenvalues of the shoulder and elbow joints were confined within the left half of the s-plane in a stripe of +/- j 0.5, and moved toward left onto the real axis. The shoulder eigenvalues were generally nearer to the imaginary axis than the elbow ones, indicating a more oscillatory behavior at the shoulder joint than that at the elbow joint. The effects of joint configuration evaluated within the workspace from 40 degrees to 110 degrees for the elbow and from 40 degrees to 120 degrees for the shoulder showed that the elbow eigenvalues were more prone to configuration changes, particularly elbow angles. We also developed a simulation paradigm for sampled data FES control systems that contain a mixture of continuous time components and sampling and hold effects. This simulation paradigm is useful for realistic simulation of local feedback controller performance.

Computer Simulation↗

PubMed2003

Estimation of the 3-D center of mass excursion from force-plate data during standing.

Biomechanical models are used with force-plate information to determine the center of mass (COM) trajectory during standing. They are usually based on simplifying assumptions and are often limited to a single-plane analysis. The objectives of this study were to present a three-dimensional (3-D) model to calculate the excursion of the COM of the human body, validate it and compare its performance to a video-based system during quiet standing and antero-posterior (AP) and medio-lateral (ML) self-imposed oscillations. In addition to the vertical displacements of the COM, the originality of the method lies in eliminating the accelerations terms in the model and their related assumptions. The model was able to estimate closely the COM displacements in quiet standing [a root mean square (RMS) of 0.9 mm or less]. For the self-imposed oscillations, the RMS differences were 6.6 mm in the AP and ML directions and 1.6 mm along the vertical axis. For all three testing conditions, the coefficients of correlation of the COM displacements between the model and the video methods were above 0.8 with the exception of the vertical direction, where the values were more variable.

Acceleration↗

PubMed2003

Foot placement alters the mechanisms of postural control while standing and reaching.

This study investigated the effects of altering foot placement on the strategies used by able-bodied subjects to perform reaching tasks while standing. The motivation for this study was to consider the results in the context of a person with a spinal cord injury using a functional neuromuscular stimulation (FNS) system to stand while reaching. Three foot placement conditions were compared as subjects reached to the left, right, and center. Centers of pressure (COP), joint angles, and joint moments were calculated as postural parameters using force platform and video marker data. Side-by-side and wide foot placements resulted in similar postural parameters. In contrast, the modified tandem stance (feet spaced at pelvic width with one foot shifted forward) resulted in anterior/posterior COP excursions that were larger in magnitude and more consistent across reach directions when compared to the other foot placement conditions. Furthermore, the movement patterns used during the tandem stance were more consistent and may be more readily achievable with FNS than the movement patterns utilized with the side-by-side and wide stances. These results suggest that the modified tandem stance may enhance the functionality of FNS standing systems and may also be useful in other standing rehabilitation programs.

Adaptation, Physiological↗

PubMed2003

A model-based approach to stabilizing crutch supported paraplegic standing by artificial hip joint stiffness.

The prerequisites for stable crutch supported standing were analyzed in this paper. For this purpose, a biomechanical model of crutch supported paraplegic stance was developed assuming the patient was standing with extended knees. When using crutches during stance, the crutches will put a position constraint on the shoulder, thus reducing the number of degrees of freedom. Additional hip-joint stiffness was applied to stabilize the hip joint and, therefore, to stabilize stance. The required hip-joint stiffness for changing crutch placement and hip-joint offset angle was studied under static and dynamic conditions. Modeling results indicate that, by using additional hip-joint stiffness, stable crutch supported paraplegic standing can be achieved, both under static as well as dynamic situations. The static equilibrium postures and the stability under perturbations were calculated to be dependent on crutch placement and stiffness applied. However, postures in which the hip joint was in extension (C postures) appeared to the most stable postures. Applying at least 60 N x m/rad hip-joint stiffness gave stable equilibrium postures in all cases. Choosing appropriate hip-joint offset angles, the static equilibrium postures changed to more erect postures, without causing instability or excessive arm forces to occur.

Computer Simulation↗

PubMed2003

Static and dynamic evaluation of the influence of supplementary hip-joint stiffness on crutch-supported paraplegic stance.

Paraplegic persons can stand with hip-knee-ankle-foot orthoses (HKAFO) and crutches. However, current HKAFOs restrict body movement extensively, which may impede functional upper-body movements. A more compliant body support using a more compliant orthosis or well-controlled functional electrical stimulation system may increase freedom of movement to the user, but should not impede stability and required arm support. In the current study, we investigated the consequences of varying stiffness applied at the hip to postural stability and required crutch force during paraplegic stance. Experiments were performed on five paraplegic persons with spinal cord lesions varying from T1 to T12. Static postures and dynamic responses to perturbations were tested for varying hip stiffness and crutch placements. The minimal hip-joint stiffness for stable stance appeared to depend on lesion level. In contrast to the predictions of a previous modeling study, no statistically significant influences of hip-joint stiffness or crutch-to-foot distance on posture and applied crutch forces were found. It is hypothesized that the main reasons of this discrepancy are the active upper-body efforts the paraplegic HKAFO users are still able to exert and the remaining flexibility of the upper trunk and shoulder region, which is present despite the restrictions of the orthosis.

Computer Simulation↗

PubMed2004

New results in feedback control of unsupported standing in paraplegia.

The aim of this study was to implement a new approach to feedback control of unsupported standing and to evaluate it in tests with an intact and a paraplegic subject. In our setup, all joints above the ankles are braced and stabilizing torque at the ankle is generated by electrical stimulation of the plantarflexor muscles. A previous study showed that short periods of unsupported standing with a paraplegic subject could be achieved. In order to improve consistency and reliability and to prolong the duration of standing, we have implemented several modifications to the control strategy. These include a simplified control structure and a different controller design method. While the reliability of standing is mainly limited by the muscle characteristics such as reduced strength and progressive fatigue, the results presented here show that the new strategy allows much longer periods (up to several minutes) of unsupported standing in paraplegia.

Adult↗

PubMed2005

Nonlinear modeling of FES-supported standing-up in paraplegia for selection of feedback sensors.

This paper presents analysis of the standing-up manoeuvre in paraplegia considering the body supportive forces as a potential feedback source in functional electrical stimulation (FES)-assisted standing-up. The analysis investigates the significance of arm, feet, and seat reaction signals to the human body center-of-mass (COM) trajectory reconstruction. The standing-up behavior of eight paraplegic subjects was analyzed, measuring the motion kinematics and reaction forces to provide the data for modeling. Two nonlinear empirical modeling methods are implemented--Gaussian process (GP) priors and multilayer perceptron artificial neural networks (ANN)--and their performance in vertical and horizontal COM component reconstruction is compared. As the input, ten sensory configurations that incorporated different number of sensors were evaluated trading off the modeling performance for variables chosen and ease-of-use in everyday application. For the purpose of evaluation, the root-mean-square difference was calculated between the model output and the kinematics-based COM trajectory. Results show that the force feedback in COM assessment in FES assisted standing-up is comparable alternative to the kinematics measurement systems. It was demonstrated that the GP provided better modeling performance, at higher computational cost. Moreover, on the basis of averaged results, the use of a sensory system incorporating a six-dimensional handle force sensor and an instrumented foot insole is recommended. The configuration is practical for realization and with the GP model achieves an average accuracy of COM estimation 16+/-1.8 mm in horizontal and 39+/-3.7 mm in vertical direction. Some other configurations analyzed in the study exhibit better modeling accuracy, but are less practical for everyday usage.

Adolescent↗

PubMed2006

Quantitative analysis of the ankle strategy under translational platform disturbance.

The ankle strategy is one of the postural adjustment maneuvers humans utilize when the support platform is disturbed. This paper presents a quantitative analysis of the ankle strategy. A three-link sagittal biped model is considered. The first link represents the two legs locked together. The second link represents the two thighs locked together. The third link represents the hip, the torso, the upper limbs, the neck, and the head. The dynamics, control, and stability of the three-link biped, under platform translation, are considered. The disturbance of the platform is represented as an input and the effect of the muscular system is reduced to a set of torques applied to the joints and across the joints. Two digital computer simulations are presented to demonstrate the behavior of the biped under backward or forward platform disturbance. The simulations are compared with experimental measurements of humans subjected to postural disturbances. It is shown that the effect of a horizontal disturbance at the ankle appears to be about 40 times that of the effect of the disturbance at the knees and at least a few hundred times larger than the effect of a disturbance at the hip. This means that, under translational platform disturbance, the ankle angle is subjected to the largest excursion. The knee and the hip angle excursions are relatively minor. Consequently, the biped, as a whole, appears to move as a single inverted pendulum. Major postural corrections are initiated by the ankle excursion. Further, when the available ankle torque is limited or nonexistent, the stability requires resorting to the knee or hip strategies.

Ankle Joint↗

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↗

PubMed1985

Effect on postural sway of various benzodiazepine tranquillizers.

The effects of various benzodiazepine tranquillizers (clobazam 20 mg, bromazepam 6 mg and lorazepam 2 mg) were investigated by posturography in 16 subjects in a controlled trial. Twelve received each of the three anxiolytics for 1 week in a cross-over design, four received placebo for 1 week during the three successive treatment periods. A pharmacodynamic study was carried out after the first administration, and another assessment was done after 1 week of treatment. The first administration of lorazepam caused the most marked disturbances of body sway (increase of spectral energies, length and amplitude of the stabilogram). The first administration of bromazepam was also accompanied by an increase of the posturographic parameters, although less marked. Administration of clobazam did not produce any impairment of equilibrium, indicating that it is devoid of any sedative effect measurable by posturography. No changes of the postural sway can be detected on the measurement recorded 10 h after the last dose of 1 week's treatment.

Adult↗

PubMed1985

Patients with an acoustic neurinoma examined with a tilting room.

Patients with acoustic neurinomas do not usually show vertigo or spontaneous nystagmus, suggesting a gradual decrease of vestibular function with adequate compensation. Seventeen patients with acoustic neurinoma were examined to test their postural control. Examination in the tilting room showed a visual dominance in postural control with these patients irrespective of involvement of the central vestibular system. The subjective horizon was determined in addition to routine audiovestibular examination, to be informed about the compensation process. When routine audiovestibular examination points to an acoustic neurinoma, a pathological subjective horizon favours the diagnosis, although this is not mandatory. The combination of tilting room and stabilometry is superior in detecting visual dominance to the standard Romberg test.

Humans↗

PubMed1987

Vestibulospinal function in two syndromes with spontaneous attacks of vertigo: evaluation by posturography.

Two vestibular syndromes comprising attacks of spontaneous vertigo were studied and disturbance of the vestibulospinal reflex was evaluated. Patients with Menière's disease were examined between attacks and patients with a sudden unilateral vestibular loss syndrome at least 3-4 weeks after the initial attack. Posturography, (recording and analysing the postural sway) was used to evaluate the influence of these disorders upon the vestibulospinal reflex. In 50% of the patients, the postural sway was disturbed compared with the performance of a normal control group. Menière's disease had a more pronounced effect on the posturographic results, whereas for the unilateral vestibular loss syndrome the rotation tests showed more asymmetry. It is obvious that the effect upon the vestibulospinal reflex cannot be deduced from the rotational test results. Indeed in a large number of patients discordance between rotational test results and posturography was seen. The presence or absence of attacks during the previous year in patients with Menière's disease had no influence on the presence of abnormal posturographic results. If other complaints of dizziness were mentioned by the patient, posturography was nearly always abnormal. It is evident that a complete evaluation of such patients must include information obtained by posturography.

Adult↗

PubMed1996

Head stabilization in peripheral vestibular syndromes.

In classical posturography the standing position has been evaluated by recording the postural sway on a force-plate and measuring some parameters such as surface and velocity (Platform or P-recording). The postural sway not only results in forces exerted on the support surface by the feet, but the head also participates in the movements of this postural sway. For recording of the head movements (H-recording), a lightbulb is placed on top of the head and the described light-path is recorded by a TV camera. Such an H-recording can be achieved simultaneously with the platform-recording. A comparative study indicates that, for some patients differences could be found between H- and P-recordings, for others not. The H-recording, in fact, gives information about the stabilization of the head, compared with the stability on the platform (P-recording). The importance of the information furnished by the double recording in Static Posturography type IV (SPGIV) can be deduced from the number of patients showing one or more differences between the types of recording. Nearly 70% showed a difference in at least one test. In fact, 55% showed it in two or more tests. In half of the tests, considered separately, a difference was noted. The differences observed provide complementary information in this combined recording. If the H-recording were to be used as a separate technique, it has to be born in mind that the results are certainly not always the same as when using P-recording, as in classical posturography.

Adolescent↗

PubMed1992

Postural stability, tardive dyskinesia and developmental disability.

The postural stability of four adult populations was examined through force platform methods. The four groups were classified as: (1) developmentally disabled (severely and profoundly mentally retarded) with tardive dyskinesia; (2) developmentally disabled but with no history of neuroleptic medication; (3) tardive dyskinetic but of normal intelligence; and (4) a normal and healthy control group. Postural conditions included standing still with arms at side, standing still with one arm or both arms parallel to the ground, and standing still while swinging both arms in the sagittal plane. The findings showed that both tardive dyskinetic and/or developmentally disabled groups exhibited greater sway and variability in centre of pressure motion in contrast to the control group. The developmentally disabled with tardive dyskinesia group also exhibited a strong tendency to produce a different form to the postural sway strategy in that they produced rhythmical centre of pressure motions during stance that were, to some degree, task dependent. The findings show that the combined effects of developmental disability and tardive dyskinesia produce qualitatively and quantitatively different features in postural stability patterns. The data suggest that postural stability measures may be a useful index to assess tardive dyskinesia.

Antipsychotic Agents↗

PubMed2006

Effects of anterior cruciate ligament (ACL) injury on postural control and muscle activity of head, neck and trunk muscles.

The aim of the study was to evaluate the effects that an anterior cruciate ligament injury of the knee has on postural control and activity of neck, head and trunk muscles to investigate the existence of connections between the masticatory system and body posture. Surface electromyographic activity of the muscles at mandibular rest position, and during maximal voluntary clenching and posturometric and stabilometric measurements of 25 adult patients having pathology on the left knee were compared with a control non-pathological group. At rest, the patients showed a higher muscular activity of anterior temporalis, masseter, sternocleidomastoid and lower trapezius, compared with the control subjects (P < 0.05). At maximal voluntary clenching, the patients showed a lower muscular activity of the right anterior temporalis and masseter and a higher muscular activity of the lower trapezius, compared with the control subjects. For the stabilometric measurements, all the subjects showed a significant reduction in the postural centre of pressure path length during the test with eyes open and cotton rolls, compared with the test with eyes closed and mandibular rest position (P < 0.05). In addition, the patients showed a significant displacement of the postural centre of pressure in a forward direction (P < 0.05) and into the right side (P < 0.05), compared with the control subjects. Anterior cruciate ligament injury appears to be associated to a change in the activity of head, neck and trunk muscles and to a change in the position of the postural centre of pressure. Cotton rolls seem to improve the stability of the subject.

Adult↗

PubMed1984

Impairment of postural stability following perivascular axillary block with mepivacaine.

Perivascular axillary block was performed on 10 patients using a catheter technique. All patients received a standard dose of 50 ml mepivacaine 1% with adrenaline 1:200 000. Using a quantitative Romberg-test performed on a computerized force-plate system, postural stability was tested before and 40 min after the local anesthesia. There was a significant increase in body-sway following axillary block (P less than 0.01). Nine out of 10 patients had an increased sway-tendency, an average of 31%. A proportionate increase in postural imbalance has been found in normal persons after alcohol consumption, when blood levels were 0.8 mg/ml.

Adult↗

PubMed1993

Evaluation of postural stability by computerised posturography following outpatient paediatric anaesthesia. Comparison of propofol/alfentanil/N2O anaesthesia with thiopentone/halothane/N2O anaesthesia.

Simple clinical tests, like Romberg's test or a walking test, have proved to be inadequate guidelines for safe discharge after outpatient anaesthesia. A randomised study was therefore planned to compare postural stability measured by computerised posturography in 31 oral midazolam-atropine premedicated children aged 6.9 (s.e. 0.4) years who had been anaesthetised with either propofol/alfentanil/N2O or thiopentone/halothane/N2O. The sway velocity of the children was measured before premedication and 1, 2 and 3 h after the end of anaesthesia. Results show that sway velocity had returned to baseline values 3 h after the end of anaesthesia in all children who had received propofol/alfentanil/N2O and in 12 of the 15 children who had received thiopentone/halothane/N2O. The quantified version of the Romberg test performed with eyes open or closed was not impaired after anaesthesia, compared with the control values, indicating that in children poor equilibrium is not compensated by vision. The clinical recovery with respect to the times to eye opening, to responding to command or to being fully awake did not differ between the two anaesthesia methods. On the basis of recovery assessed by postural stability, propofol/alfentanil/N2O anaesthesia was not preferable to thiopentone/halothane/N2O anaesthesia after minor paediatric otolaryngological surgery.

Alfentanil↗

PubMed2006

Brief posturographic test as an indicator of fatigue.

The purpose of the present paper was to analyze the efficiency of an abbreviated, albeit objective posturographic test as an indicator of fatigue. Posturography was measured in 10 healthy adults (age 18-33 years, male/female 7/3). Baseline posturographic measurements were taken for each subject. Later, a shorter (3-min) posturographic test was administered 12 times during 25 h of sleep deprivation. This was correlated with subjective assessment of fatigue using a questionnaire and cognitive performance assessed with a reaction time test (Psychomotor Vigilance Test). Although showing significant individual differences, the score of the abbreviated posturographic examination, normalized to each subject's baseline performance ('fatigue index') had a pronounced circadian pattern with a peak of instability in the early morning hours. Fatigue index was highly correlated with the cognitive test (r=0.80-0.90). A substantial, albeit weaker correlation was found between the fatigue index and the subjective fatigue scores (r=0.59-0.75). Fourier spectral analysis showed that low median sway (0.10-0.50 Hz), considered to be an expression of vestibular control, was most affected by fatigue. The study demonstrates that cognitive deterioration caused by fatigue can be objectively predicted by an abbreviated postural test of <or=3 min. This test is promising to be valid, reliable, and practicable, while being significantly correlated with physiological markers and validated cognitive measures of fatigue obtained by substantially more time-consuming and less convenient methods.

Adolescent↗