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At least 217 records · Page 12Linked 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↗

PubMed1992

Postural control, muscle function and psychological factors in rheumatoid arthritis. Are there any relations?

The aim of the present study was to relate postural control, as measured on a quantitative test battery as standing balance on an AMTI force platform, to results of muscle function of the lower extremities, aerobic capacity, disease characteristics, attitudes revealing anxiety, and demographic variables in patients with rheumatoid arthritis (RA). A group of 61 patients with RA was investigated. Multiple regression analyses, using both the stepwise and the backward elimination method, were employed for length of sway path in two-leg standing looking straight ahead and blindfolded as dependent variables. The results revealed postural control in two-leg standing looking straight ahead to be highly related to age and sex but also to isokinetic endurance and to anxiety. In the model, 55% of the variance was explained by these variables at a significance level of p = 0.05. Characteristics of the RA disease, such as c-reactive protein and joint-mobility did not play any major role in explaining postural control.

Adult↗

PubMed1996

Anthropometric, demographic, and cardiovascular predictors of left ventricular mass in young children.

Left ventricular (LV) mass is a strong independent predictor of cardiovascular morbidity and mortality. Few longitudinal studies have examined predictors of LV mass in children. This study assessed the contributions of anthropometric, demographic, and cardiovascular parameters (at rest and after exposure to laboratory stressors) as predictors of LV mass 3.6 years after the initial examination in a sample of 68 Caucasian and African-American children 7.9 +/- 0.7 years old. At the initial examination, all subjects had standard anthropometrics measured and hemodynamics assessed at rest and during 3 stressors: postural change, forehead cold stimulation, and treadmill exercise. On the follow-up examination 3 to 4 years later, echocardiographic evaluations were conducted to estimate LV mass and related LV geometry. LV mass and LV internal diameter in diastole were adjusted for linear growth (LV mass/height2.7 and LV internal dimension during diastole/height0.80, respectively). Hierarchical stepwise multiple regression analyses were conducted using parameters significant in univariate comparisons (p < 0.05). Initial weight (R2 = 0.38), height (R2 = 0.42), and cardiac output reactivity to standing and treadmill exercise (final model R2 = 0.55) were significant predictors of LV mass, whereas LV mass/height2.7 was predicted by initial adiposity (R2 = 0.07) and cardiac output and systolic pressure reactivity to postural change (final model R2 = 0.25). Follow-up relative wall thickness was significantly predicted by ethnicity (African-Americans greater than Caucasians, R2 = 0.15), adiposity (R2 = 0.20), and systolic pressure reactivity to postural change (final model R2 = 0.28). These findings suggest the potential benefit of weight control in childhood as a primary prevention for later onset of cardiovascular disease.

Anthropometry↗

PubMed2005

On modeling center of foot pressure distortion through a medium.

The center of foot pressure (COP) is a commonly used output measure of the postural control system as it is indicative of the systems stability. A dense piece of foam, i.e., a sponge, can be used to emulate random environmental conditions that distort the ground reaction forces received and interpreted by the cutaneous sensors in the feet; thus introducing uncertainty into the control system. In this paper, the density and size of the sponge was selected such that a subject's weight did not cause full compression. In general, the COP is measured from the bottom of the sponge. As the sponge is used to distort ground reaction forces, it is reasonable then to assume that the COP signal would also be distorted. The use of other sensory information to identify state of balance, and compute necessary balance adjustments, is therefore required. In addition to a sponge, many different types of specialized footwear and inserts are used for people with peripheral neuropathy, such as diabetics. However, it is difficult to design diabetic footwear without a better understanding of the mechanical and physiological effects that different surfaces typical of outdoor terrains, such as a sponge, which cannot be predicted without the sense of the foot, have on balance. Therefore, the goal of this study was to investigate the change of the COP signal from the top and bottom of the sponge. Portable force sensing mats from Vista Medical were used to obtain the COP from the top and bottom of the sponge. The COP measured on the bottom of the sponge is not the same as the COP measured on the top, particularly in the medial-lateral direction. Several linear and nonlinear models were used to identify the unknown plant; i.e., the sponge. Overall, the nonlinear neural network method had superior performance when compared with the linear models. Thus, the results indicate that the signals from the top and bottom of the sponge are in fact different, and furthermore, they are nonlinearly related. A nonlinear mathematical model is proposed which describes COP distortion through a medium such as a sponge. Although the values for the model parameters determined were for a particular sponge, this study suggests that a neural network plant identification model may be applied to any medium other than the sponge; the information can then be used to determine how the balance control model is affected given the sensory information received.

Adult↗

PubMed1997

Ankle trauma significantly impairs posture control--a study in basketball players and controls.

Both ankle and hip movements are used to maintain balance in strategies defined as bottom up and top down models. This suggests that pathological impairment of either of these articulations could modify the parameters of balance control. In order to appreciate the bearing of ankle damage on the proper control of equilibrium, posturographic recordings were compared, in a static and two dynamic tests, between 15 professional national basketball players, with histories of 10 to 15 ankle sprains, and 50 controls. In statokinesigrams obtained with eyes open or closed, center of foot pressure displacements were of similar length (way) between controls and players, while the area covered was increased for the latter and related to the history of ankle trauma. In a toes-up dynamic test (4 degrees, 50 degrees/s) coupled with electromyographic recordings, the short latency response (myotatic reflex in the triceps surae muscle) normalized to one meter body height, was shorter in players without relationship to histories of ankle damage. In a prolonged sinusoidal dynamic test, players with the largest number of ankle trauma showed greater difficulties in maintaining posture control. These data pinpoint the role of ankles in the control of both static and dynamic balance and demonstrate that the role of the hip joints becomes more important (top down strategy) in the case of damaged ankles.

Adolescent↗

PubMed2003

Predicting the dynamic postural control response from quiet-stance behavior in elderly adults.

Human postural sway, as measured by fluctuations of the center of pressure (COP) under the feet of a quietly standing individual, can be characterized as a stochastic process. The fluctuation-dissipation theorem (FDT) provides a linear relationship between the fluctuations of a quasi-static, stochastic system to the same system's relaxation to equilibrium following a perturbation. We applied a similar linear relationship, based on the FDT, to the human postural control system to explore whether anterior-posterior (AP) fluctuations of the COP during quiet stance can be used to predict the AP response of the postural control system to a weak posteriorly directed mechanical perturbation (tug or pull at the waist). We tested 10 healthy elderly (mean age of 69yr) and 10 healthy young (mean age of 25yr) adult subjects. We found that this linear relationship was applicable to the postural control system of all 10 young and eight of the 10 elderly adult subjects. These results suggest that it is possible to predict an individual's dynamic response to a mild perturbation using quiet-stance data, regardless of age. The existence of this FDT-based linear relationship with respect to the human postural control system suggests that, for a given individual, the postural control system may use the same control mechanisms during quiet stance and mild-perturbation conditions, regardless of age.

Adult↗

PubMed1998

Stiffness control of balance in quiet standing.

Our goal was to provide some insights into how the CNS controls and maintains an upright standing posture, which is an integral part of activities of daily living. Although researchers have used simple performance measures of maintenance of this posture quite effectively in clinical decision making, the mechanisms and control principles involved have not been clear. We propose a relatively simple control scheme for regulation of upright posture that provides almost instantaneous corrective response and reduces the operating demands on the CNS. The analytic model is derived and experimentally validated. A stiffness model was developed for quiet standing. The model assumes that muscles act as springs to cause the center-of-pressure (COP) to move in phase with the center-of-mass (COM) as the body sways about some desired position. In the sagittal plane this stiffness control exists at the ankle plantarflexors, in the frontal plane by the hip abductors/adductors. On the basis of observations that the COP-COM error signal continuously oscillates, it is evident that the inverted pendulum model is severely underdamped, approaching the undamped condition. The spectrum of this error signal is seen to match that of a tuned mass, spring, damper system, and a curve fit of this "tuned circuit" yields omega n the undamped natural frequency of the system. The effective stiffness of the system, Ke, is then estimated from Ke = I omega n2, and the damping B is estimated from B = BW X I, where BW is the bandwidth of the tuned response (in rad/s), and I is the moment of inertia of the body about the ankle joint. Ten adult subjects were assessed while standing quietly at three stance widths: 50% hip-to-hip distance, 100 and 150%. Subjects stood for 2 min in each position with eyes open; the 100% stance width was repeated with eyes closed. In all trials and in both planes, the COP oscillated virtually in phase (within 6 ms) with COM, which was predicted by a simple 0th order spring model. Sway amplitude decreased as stance width increased, and Ke increased with stance width. A stiffness model would predict sway to vary as Ke-0.5. The experimental results were close to this prediction: sway was proportional to Ke(-0.55). Reactive control of balance was not evident for several reasons. The visual system does not appear to contribute because no significant difference between eyes open and eyes closed results was found at 100% stance width. Vestibular (otolith) and joint proprioceptive reactive control were discounted because the necessary head accelerations, joint displacements, and velocities were well below reported thresholds. Besides, any reactive control would predict that COP would considerably lag (150-250 ms) behind the COM. Because the average COP was only 4 ms delayed behind the COM, reactive control was not evident; this small delay was accounted for by the damping in the tuned mechanical system.

Adult↗

PubMed2006

Pathophysiological model for chronic low back pain integrating connective tissue and nervous system mechanisms.

Although chronic low back pain (cLBP) is increasingly recognized as a complex syndrome with multifactorial etiology, the pathogenic mechanisms leading to the development of chronic pain in this condition remain poorly understood. This article presents a new, testable pathophysiological model integrating connective tissue plasticity mechanisms with several well-developed areas of research on cLBP (pain psychology, postural control, neuroplasticity). We hypothesize that pain-related fear leads to a cycle of decreased movement, connective tissue remodeling, inflammation, nervous system sensitization and further decreased mobility. In addition to providing a new, testable framework for future mechanistic studies of cLBP, the integration of connective tissue and nervous system plasticity into the model will potentially illuminate the mechanisms of a variety of treatments that may reverse these abnormalities by applying mechanical forces to soft tissues (e.g. physical therapy, massage, chiropractic manipulation, acupuncture), by changing specific movement patterns (e.g. movement therapies, yoga) or more generally by increasing activity levels (e.g. recreational exercise). Non-invasive measures of connective tissue remodeling may eventually become important tools to evaluate and follow patients with cLBP in research and clinical practice. An integrative mechanistic model incorporating behavioral and structural aspects of cLBP will strengthen the rationale for a multidisciplinary treatment approach including direct mechanical tissue stimulation, movement reeducation, psychosocial intervention and pharmacological treatment to address this common and debilitating condition.

Adaptation, Physiological↗

PubMedDate not supplied

Closed-loop, estimator-based model of human posture following reduced gravity exposure.

A computational and experimental method is employed to provide an understanding of a critical human space flight problem, posture control following reduced gravity exposure. In the case of an emergency egress, astronauts' postural stability could be life saving. It is hypothesized that muscular gains are lowered during reduced gravity exposure, causing a feeling of heavy legs, or a perceived feeling of muscular weakness, upon return to Earth's 1 g environment. We developed an estimator-based model that is verified by replicating spatial and temporal characteristics of human posture and incorporates an inverted pendulum plant in series with a Hill-type muscle model, two feedback pathways, a central nervous system estimator, and variable gains. Results obtained by lowering the variable muscle gain in the model support the hypothesis. Experimentally, subjects were exposed to partial gravity (3/8 g) simulation on a suspension apparatus, then performed exercises postulated to expedite recovery and alleviate the heavy legs phenomenon. Results show that the rms position of the center of pressure increases significantly after reduced gravity exposure. Closed-loop system behavior is revealed, and posture is divided into a short-term period that exhibits higher stochastic activity and persistent trends and a long-term period that shows relatively low stochastic activity and antipersistent trends.

Adult↗

PubMedDate not supplied

Comparison of head- and body-velocity trajectories during locomotion among healthy and vestibulopathic subjects.

The optimal strategies for improving locomotor stability in people with vestibulopathy remain unclear. To help identify likely targets for intervention, we sought to determine whether vestibulopathic postural control impairment during locomotor activity was more localized to either the head or the whole body. We used high curvature analysis (HCA) to quantify the smoothness of head- and body-velocity trajectories during repeated stepping in 18 vestibulopathic and 17 healthy subjects. We employed a mixed-model repeated measures analysis of variance to compare differences in head- and body-trajectory HCA scores. Pearson coefficients were used to describe relationships between head- and body-trajectory HCA scores within each group. The results revealed that neither head- nor body-velocity trajectories were relatively more impaired in subjects with vestibulopathy. Importantly, however, the smoothness of head and body trajectories was more strongly related in subjects with vestibulopathy compared with healthy subjects, suggesting that the fundamental motor control impairment produced by vestibulopathy may be related to an abnormal coupling of head and body motion. We discuss implications for locomotor training in patients with vestibulopathy.

Adult↗

PubMed2002

A new look at posturographic analysis in the clinical context: sway-density versus other parameterization techniques.

In order to identify useful guidelines for the clinical practitioner as regards the use of static posturographic analysis, we collected a set of posturograms from 3 groups of participants (normal participants, Parkinsonian patients, and osteo-porotic patients), according to the Romberg test. From each posturogram, we extracted global parameters (in the time domain and frequency domain) and structural parameters (based on diffusion plots and sway-density plots), with a total of 38 parameters. The discriminative power of each parameter was evaluated by means of statistical analysis in relation to the condition effect (open vs. closed eyes) and the pathology effect (normal participants vs. patients). The initial set of 38 parameters was reduced to 24 by identifying clear redundancies, and then to 18 by eliminating the parameters that did not pass the condition effect with normal participants. These parameters were analyzed for reliability and discriminative power in the general framework of a biomechanic model of postural stabilization. At the end of this analysis, we suggested that a set of 4 parameters is particularly valuable in the clinical practice: 2 global parameters (sway-path and frequency band of the posturogram) and 2 structural parameters (mean value of peaks and mean inter-peak distance in the sway-density plots).

Aged↗

PubMed1997

[Errors in positioning the patient during transcutaneous radiotherapy of the pelvis].

INTRODUCTION: Quality controls in radiotherapy allow to check the correct running of treatment units and to test our procedures. Portal films taken during the first treatment session are used in quality assurance programs to compare scheduled to administered doses. MATERIAL AND METHODS: To analyze the accuracy of patient positioning in pelvic cancer irradiation, a retrospective study was carried out on 50 treatment schedules carried out at the Radiotherapy Department of S. Anna Hospital (Como, Italy) from June to December, 1996. We checked field mispositioning for correlations with patient or treatment variables, such as patient age, sex, weight, thickness, or unit type. We took one portal film at the first treatment session and checked field positioning by measuring the distance of the isocenter from fixed anatomical structures on the simulation film and on the portal film taken in anteroposterior and lateral projections. RESULTS: Four vectors were defined to evaluate field mispositioning along right-left (ADS), craniocaudal (ACC, LCC) and anteroposterior (AP) directions. The average values of these four vectors were respectively 2.94, 5.23, 5.54 and 3.20 mm. We found a major shift in field centering leftward and toward the patient's feet. To obtain more information about the total isocenter displacement, a vector T was calculated by summing the vectors ADS, ACC and LAP; a further evidence of field mispositioning is given by the vector T mean value (8.66 +/- 4.95 mm). No correlation was found between vector T values and any patient or treatment variable. DISCUSSION: The acquaintance with uncertainties requires adequate statistical tools. A single check at treatment beginning could show a systematic error, but not the random fluctuations which can be recognized only with periodic portal films. To correct a possible systematic error without likely worsening set-up conditions, an adequate threshold value must be chosen for field mispositioning, according to each center's historical data. CONCLUSION: One portal film at the beginning of treatment is the minimum requirement in a quality assurance program. We feel the need to change our protocol and acquire more than one portal film, because the higher the number of portal films the easier the distinction of systematic from random errors. Using serial portal films, all at the first session, we will be able to introduce quantitative criteria for various action levels.

Female↗

PubMed1982

A model for evaluation of postural hypotension induced by drugs in conscious restrained normotensive rats.

A rat model was developed for use in evaluating postural hypotensive activity of a compound. Conscious normotensive rats were restrained on a tilt board with special restrainers to avoid blocking limb circulation. Dose response curves were explored to delineate the potential postural hypotensive activity of a compound. In response to a 2-min 90 degree head-up tilt, only a negligible changes in blood pressure was observed in the control state. The conscious rat was therefore able instantaneously to compensate for postural change. This ability, however, was blocked by guanethidine. The compound induced a dose-related hypotension and a further, dose-dependent drop in blood pressure on tilt. Hydralazine, on the other hand, induced only dose-dependent hypotension, but no change in blood pressure response from control tilts. Since these findings appear to be in accord with human clinical data, this rat model may be suitable for use in evaluating potential postural hypotensive activity of compounds.

Anesthetics↗

PubMedDate not supplied

Motor dysfunction in a mouse model for Down syndrome.

Motor deficits are among the most frequently occurring features of Down syndrome (DS). Individuals with DS exhibit disturbances in the dynamics of movement production and postural control that are thought to have a significant impact in delaying their acquisition of motor skills. The origin of these deficits has been hypothesized to be cerebellar. The Ts65Dn mouse is the most robust and genetically sound animal model for DS currently available. Ts65Dn mice show many DS-like features, including significant learning deficits in different behavioral tasks and neurodegeneration of cholinergic neurons. In the present study, we investigate the motor function of these animals. We have analyzed hind paw print patterns during walking, running speeds, rotarod performance, grip force production, swim paths, and swimming speeds. Our results indicate that Ts65Dn mice present mild to severe dysfunction according to all of the above assessments. The most evident impairments presented by these mice were related to equilibrium and motor coordination, which agrees with reported clinical observations made on individuals with DS. Because none of these findings were readily apparent by simple inspection of these animals, these findings reiterate the need for a careful evaluation of any mutant mouse strain for which there is reason to suspect motor deficits. The identification of motor dysfunction in Ts65Dn mice may have important consequences for the interpretation of some previous assessments of learning and memory of these animals that assumed intact motor function, and further strengthens the use of this aneuploid mouse strain as a model for DS.

Animals↗

PubMedDate not supplied

Human energy - optimal control of disturbance rejection during constrained standing.

An optimal control system that enables a subject to stand without hand support in the sagittal plane was designed. The subject was considered as a double inverted pendulum structure with a voluntarily controlled degree of freedom in the upper trunk and artificially controlled degree of freedom in the ankle joints. The control system design was based on a minimization of cost function that estimated the effort of the ankle joint muscles through observation of the ground reaction force position relative to the ankle joint axis. By maintaining the centre of pressure close to the ankle joint axis the objective of the upright stance is fulfilled with minimal ankle muscle energy cost. The performance of the developed controller was evaluated in a simulation-based study. The results were compared with the responses of an unimpaired subject to different disturbances in the sagittal plane. The proposed cost function was shown to produce a reasonable approximation of human natural behaviour.

Algorithms↗

PubMed1997

Postural stability, neck proprioception and tension neck.

We examined whether tension neck (TN) may due to inadequate proprioceptive and vestibular activation of the cervico-collic reflex (CCR). CCR and vestibulospinal responses (VSRs) were recorded from 106 forest workers by stimulating the neck, lumbar or calf proprioceptors by vibration. The VSRs were recorded with posturography. TN occurred in 27 out of 106 subjects. The subjects with TN (48.5 years) were older than those without TN (43.1). The mean body sway during quiet stance was the same in both groups during the neck stimulation. In subjects with tension neck stimulation of neck or lumbar proprioceptors caused excessive, unpredictable body excursion in the lateral and anteroposterior direction that continued after stimulation. Results from stimulation of lower limb proprioceptors did not significantly differ between the 2 groups. In logistic regression analysis a model to predict TN consisting of perstimulatory postural stability (odds ratio 1.4) and poststimulatory postural stability (odds ratio 1.8) turned out to be statistically significant. The anatomical findings of CCR in the medulla oblongata suggest that neck muscle afferents control the posture and muscle activity of the neck. The erroneus facilitation of proprioception in TN subjects indicate that TN may be raised by inadequate facilitation of CCR.

Adult↗

PubMed2002

A prospective study of computer users: II. Postural risk factors for musculoskeletal symptoms and disorders.

BACKGROUND: Despite widespread recommendations regarding posture during computer use, associations between specific postures and musculoskeletal health are not well characterized. METHODS: Six hundred and thirty-two newly hired computer users were followed prospectively to evaluate associations between posture and neck or shoulder (N/S) and hand or arm (H/A) musculoskeletal symptoms and musculoskeletal disorders. Participants' postures were measured at entry and they reported symptoms on weekly diaries. Participants reporting symptoms were examined for specific disorders. Multivariate Cox regression models were used to estimate associations between postural variables and risk of symptoms and disorders, controlling for confounding variables. RESULTS: Keying with an inner elbow angle > 121 degrees, greater downward head tilt, and presence of armrests on the participants chair were associated with lower risk of N/S symptoms or N/S disorders. Keying with elbow height below the height of the "J" key and the presence of a telephone shoulder rest were associated with a greater risk of N/S symptoms or N/S disorders. Horizontal location of the "J" key > 12 cm from the edge of the desk was associated with a lower risk of H/A symptoms and H/A disorders. Use of a keyboard with the "J" key > 3.5 cm above the table surface, key activation force > 48 g, and radial wrist deviation of > 5 degrees while using a mouse was associated with a greater risk of H/A symptoms or H/A disorders. The number of hours keying/week was associated with H/A symptoms and disorders. CONCLUSIONS: The results suggest that the risk of musculoskeletal symptoms and musculoskeletal disorders may be reduced by encouraging specific seated postures.

Arm↗

PubMed2001

Low back joint loading and kinematics during standing and unsupported sitting.

The aim was to examine lumbar spine kinematics, spinal joint loads and trunk muscle activation patterns during a prolonged (2 h) period of sitting. This information is necessary to assist the ergonomist in designing work where posture variation is possible -- particularly between standing and various styles of sitting. Joint loads were predicted with a highly detailed anatomical biomechanical model (that incorporated 104 muscles, passive ligaments and intervertebral discs), which utilized biological signals of spine posture and muscle electromyograms (EMG) from each trial of each subject. Sitting resulted in significantly higher (p<0.001) low back compressive loads (mean +/- SD 1698 +/- 467 N) than those experienced by the lumbar spine during standing (1076 +/- 243 N). Subjects were equally divided into adopting one of two sitting strategies: a single 'static' or a 'dynamic' multiple posture approach. Within each individual, standing produced a distinctly different spine posture compared with sitting, and standing spine postures did not overlap with flexion postures adopted in sitting when spine postures were averaged across all eight subjects. A rest component (as noted in an amplitude probability distribution function from the EMG) was present for all muscles monitored in both sitting and standing tasks. The upper and lower erector spinae muscle groups exhibited a shifting to higher levels of activation during sitting. There were no clear muscle activation level differences in the individuals who adopted different sitting strategies. Standing appears to be a good rest from sitting given the reduction in passive tissue forces. However, the constant loading with little dynamic movement which characterizes both standing and sitting would provide little rest/change for muscular activation levels or low back loading.

Adult↗