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PubMed2002

Comparative neurobiology of postural control.

In recent years, studies of nervous mechanisms for the control of body posture have been performed on animal models of different complexity - cat, rabbit, lamprey and the mollusc Clione. These studies have greatly expanded our knowledge of how the control system operates, how the system can change the stabilized body orientation and how the postural functions are distributed within different parts of the CNS. For simpler animal models, the postural network has been analyzed in considerable detail and main cell types and their interactions have been identified.

Anatomy, Comparative↗
PubMed2000

Multisensory information for human postural control: integrating touch and vision.

Despite extensive research on the influence of visual, vestibular and somatosensory information on human postural control, it remains unclear how these sensory channels are fused for self-orientation. The focus of the present study was to test whether a linear additive model could account for the fusion of touch and vision for postural control. We simultaneously manipulated visual and somatosensory (touch) stimuli in five conditions of single- and multisensory stimulation. The visual stimulus was a display of random dots projected onto a screen in front of the standing subject. The somatosensory stimulus was a rigid plate which subjects contacted lightly (<1 N of force) with their right index fingertip. In each condition, one sensory stimulus oscillated (dynamic) in the medial-lateral direction while the other stimulus was either dynamic, static or absent. The results qualitatively supported five predictions of the linear additive model in that the patterns of gain and variability across conditions were consistent with model predictions. However, a strict quantitative comparison revealed significant deviations from model predictions, indicating that the sensory fusion process clearly has nonlinear aspects. We suggest that the sensory fusion process behaved in an approximately linear fashion because the experimental paradigm tested postural control very close to the equilibrium point of vertical upright.

Adult↗
PubMed1990

Postural maintenance during movement: simulations of a two joint model.

Voluntary movements of the upper body are accompanied by anticipatory postural adjustments to the lower body in a standing subject. The long-standing hypothesis is that these anticipatory adjustments serve to counteract the perturbation to the body's center of gravity caused by the voluntary arm movement. This paper presents model simulations investigating the possible roles of anticipatory postural activity that accompanies a rapid, upward arm swing. The model incorporates two (idealized) antagonistic muscle pairs controlling the movements of a double-joint system, with a "shoulder joint" between the arm and stiff body links, and an "ankle joint" between the stiff body-leg segment and the ground. Each muscle is represented by a nonlinear viscoelastic element and also includes proprioceptive feedback. Four inputs to the model define the motor control signals for muscle force generation in both the arm and the postural muscle pairs. The neurological component of the model describes consequences of alternate strategies for cocontractions, stretch reflex activity, and anticipatory and synchronous postural activities (or combinations thereof). Simulations with this model show that: (1) none of the postural maintenance schemes considered in these simulations (including varying anticipation) could suppress the initial backward thrust on the body link; (2) the more important destabilizing perturbation is a subsequent forward sway that, left uncountered by postural activity, would eventually leave the body to fall flat on its face; and (3) anticipatory silencing of the postural extensor followed by a brief period of extensor activation (descending control) and synchronous reflex activity (feedback control) appears to be the most likely postural stabilizing strategy that inhibits the continuous forward sway and is consistent with the experimental evidence.

Arm↗
PubMed1993

Effects of increased inertial load in dynamic and randomized perturbed posturography.

Muscular weakness is present in many diseases. The present study attempted to model a relative weakness of postural control muscles by loading the subjects with extra weight, and the effects on EquiTest dynamic posturography and randomized perturbed posturography were assessed. Ten healthy subjects aged 15-39 years (mean 26 years) were used. Their weights ranged 53-82 kg. Equilibrium was measured with and without an extra 20% of body weight attached to the upper part of the trunk by means of pieces of lead metal placed in a specially sewn shirt. Dynamic posturography (EquiTest) comprises a sensory organization test in which the support surface and visual surround are either stable or referenced to the patient's sway, his or her eyes open or closed. In a movement coordination part the platform makes active movements. Antero-posterior sway on a stable support surface with absent or distorted visual cues was marred, and the correction pattern was altered on a movable support surface when the vision was absent. Furthermore, the EquiTest apparatus was driven by special software to produce randomized antero-posterior movements during 45 s. In addition to stable conditions, small and large amplitude perturbations were used. The subject's ability to remain in equilibrium was estimated by the sway velocity and a confidence ellipse sway area during 45 s. During weightbearing, sway areas were larger on the stable support surface both with and without vision. Postural sway velocity was lower when vision was absent both using small and large amplitudes.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed2004

How postural behaviour in undisturbed upright stance can be used to assess the physical characteristics of various models of ankle orthoses.

OBJECTIVES: To assess various ankle orthoses presenting different physical characteristics. DESIGN: The postural performance in bi-pedal stance of 14 healthy adults was assessed during four different ankle orthoses conditions. BACKGROUND: Resultant ankle stiffness is an important component in undisturbed upright postural regulation. METHODS: The centre of pressure displacements issued from the force platform were used to compute the horizontal motions of the centre of gravity, the controlled variable, and those of the difference between the centre of pressure and the vertical projection of the centre of gravity from which ankle stiffness can be estimated. Their respective contribution was assessed through a frequency analysis and a fractional Brownian motion modelling to determine their spatio-temporal linkage and their degree of control. RESULTS: Ligacast, the more rigid model, induces the larger decrease, principally in the medio-lateral direction, in both centre of pressure minus centre of gravity and centre of gravity amplitudes. Dynastab reduces only the centre of pressure minus centre of gravity motions in both medio-lateral and antero-posterior directions. Lastly, no specific effect was observed for the less rigid Ligaflex for each motion and each direction. These effects are mainly explained by a reduction in the distance covered until the corrective process is initiated, the corresponding time interval being unchanged. In addition, depending on the model, the corrective processes appear more or less controlled. CONCLUSIONS: Modulating the ankle joint stiffness by using various models of orthoses induces some contrasted behaviours, hence suggesting that postural control does not result solely from resultant ankle stiffness. RELEVANCE: This data, contributing to the understanding of orthotic devices by emphasising their benefits, allows the practitioners and physical therapists to take into account the specific effects generated by each model.

Adult↗
PubMed1999

Synergistic influences of the striatum and the globus pallidus on postural and locomotor control.

We have investigated the role of the globus pallidus in locomotor and postural control in a previously established animal model of striatal dysfunction. Striatal efferent activity was suppressed by intracerebral infusions of antisense oligodeoxynucleotides targeted to the messenger RNA of the proto-oncogene, c-fos. This suppression produced robust circling behavior and an atypical expression of c-fos in the ipsilateral globus pallidus following psychostimulant challenge. Simultaneous infusions of antisense oligodeoxynucleotides into both the caudate-putamen and the ipsilateral globus pallidus produced an approximate threefold increase in the intensity of rotation elicited by D-amphetamine. Excitotoxic lesioning of the globus pallidus produced marked postural asymmetry and circling behavior upon stimulation. The intensity of this rotational behavior was similar to that produced by dual infusions of antisense oligodeoxynucleotides into the caudate-putamen and the globus pallidus, and was not further potentiated by suppression of striatal c-fos expression. These results demonstrate the importance of the globus pallidus in postural and motor control, and suggest that activation of this nucleus through a reduction in striatopallidal inhibition may function to balance the output activity of the basal ganglia.

Animals↗
PubMed2000

The polymodal sensory cortex is crucial for controlling lateral postural stability: evidence from stroke patients.

In modern literature, internal models are considered as a general neural process for resolving sensory ambiguities, synthesising information from disparate sensory modalities, and combining efferent and afferent information. The polymodal sensory cortex, especially the temporoparietal junction (TPJ), is thought to be a nodal point of the network underlying these properties. According to this view, a pronounced disruption of the TPJ functioning should dramatically impair body balance. Surprisingly, little attention has been paid to this possible relationship, which was the subject of investigation in this study. Twenty-two brain-damaged patients and 14 healthy subjects were subjected to a self-regulated lateral balance task, performed while sitting for 8 s on a rocking platform. Their lateral body balance was analysed both with and without vision (darkness). Support displacements in the frontal plane were recorded by means of an accelerometer. Two criteria were taken into account to evaluate body stability in each trial: the number of aborted trials due to balance loss and the angular dispersion of the supporting surface. Lesions involving the temporoparietal junction were found to markedly increase body instability, both with and without vision. Therefore, the temporoparietal junction plays a pivotal role in lateral body stabilisation, irrespective of the sensory condition in which the task is performed. This suggests that body stability is controlled throughout internal model(s).

Adult↗
PubMed2001

Backward and forward leaning postures modelled by an fBm framework.

Body leaning effects on postural control have been assessed by recording the centre of pressure (CP) displacements in healthy subjects under three experimental conditions (REF, BWD and FWD corresponding to upright, backward leaning and forward leaning of the body, respectively). The CP displacements were used to compute the motions of the vertical projections of the centre of gravity (CG(v)) and those of the difference CP-CG(v). A frequential analysis shows that the main effect takes place on CP-CG(v) motions, suggesting increased muscular activity in these leaning postures. In addition, changes also occur on CG(v) motions, especially in the antero-posterior (AP) direction. Modelling these motions as fractional Brownian motion (fBm) indicates that leaning the body induces, in the AP direction, a shift in the time interval Deltat at which the corrective process takes over the initial one operating in open-loop. In FWD and BWD conditions, the Deltat is diminished whilst the mean distance covered at this Deltat is increased for both CG(v) and CP-CG(v) motions. Moreover, more determinism in the overall upright stance control is observed in the corrective (closed-loop) process involving CG(v) motions. These facts emphasize the inability for the CP displacements to express properly the overall body sway in upright stance control.

Adult↗
PubMed2001

First-order statistics of human stabilogram.

We investigated the statistical properties of the centre of pressure (COP) vector during quiet standing, as measured on a force platform. To derive a statistical model for the data, the COP vector, locally averaged COP, and deviations from the average COP were analysed. Whereas indefinite multimodal distributions for the components of the COP were observed, the density functions of deviation from the averaged COP had reproducible properties. The anterior-posterior and mediolateral components of the deviations had in common that they were correlated variables with a non-circular statistic. The main axes of the equiprobability density curves were rotated with respect to the natural anterior-posterior and mediolateral directions. By means of more detailed statistical analysis of the data, two distinct populations of subjects were identified. In some cases we have found Gaussian distributions of the components of deviation from the average COP. In most cases the statistics was non-Gaussian. The average contrast, a new dimensionless characteristic quantity, independent of platform calibration, was introduced as a useful indicator of non-Gaussian effects in the postural control system.

Humans↗
PubMed2000

Direct cortical control of muscle activation in voluntary arm movements: a model.

What neural activity in motor cortex represents and how it controls ongoing movement remain unclear. Suggestions that cortex generates low-level muscle control are discredited by correlations with higher-level parameters of hand movement, but no coherent alternative exists. I argue that the view of low-level control is in principle correct, and that seeming contradictions result from overlooking known properties of the motor periphery. Assuming direct motor cortical activation of muscle groups and taking into account the state dependence of muscle-force production and multijoint mechanics, I show that cortical population output must correlate with hand kinematics in quantitative agreement with experimental observations. The model reinterprets the 'neural population vector' to afford unified control of posture, movement and force production.

Arm↗
PubMed2004

Integration of multiple motor segments for the elaboration of locomotion: role of the fastigial nucleus of the cerebellum.

This chapter provides a conceptual overview of the role and operation of higher structures of the central nervous system (CNS) in the control of posture and locomotion in the mammal, including the nonhuman primate and the human. Both quadrupedal and bipedal locomotion require the integrated neural control of multiple body segments against gravity. During development, and in selected instances in the adult, motor learning is required, particularly for merging anticipatory and reactive CNS processes, the latter being necessary after tripping and stumbling. We have recently found that the fastigial nucleus (FN) of the cerebellum in the cat plays a particularly important role in the control of locomotion, by virtue of its critical position in uniting the cerebro-cerebellar and the spino-cerebellar loops of neural activity that participate in the integrated control of multiple body segments. Further understanding of the CNS structures that achieve this integration has come from our recent study of an intact nonhuman primate, the Japanese monkey, Macaca fuscata, as it learns to elaborate bipedal locomotion rather than its normal quadrupedal fashion. Based on findings from these two animal species, we now present a model of the overall integrated control of posture and locomotion that features the combined operation of parallel and distributed neural circuitry throughout the CNS.

Animals↗
PubMed1998

Position and velocity coupling of postural sway to somatosensory drive.

Light touch contact of a fingertip to a stationary surface provides orientation information that enhances control of upright stance. Slight changes in contact force at the fingertip lead to sensory cues about the direction of body sway, allowing attenuation of sway. In the present study, the coupling of postural sway to a moving contact surface was investigated in detail. Head, center of mass, and center of pressure displacement were measured as the contact surface moved rhythmically at 0.1, 0.2, 0.4, 0.6, and 0.8 Hz. Stimulus amplitude decreased with frequency to maintain peak velocity constant across frequency. Head and body sway were highly coherent with contact surface motion at all frequencies except 0.8 Hz, where a drop-off in coherence was observed. Mean frequency of head and body sway matched the driving frequency </=0.4 Hz. At higher frequencies, non-1:1 coupling was evident. The phase of body sway relative to the touch plate averaged 20-30 degrees at 0.1-Hz drive and decreased approximately linearly to -130 degrees at 0.8-Hz drive. System gain was approximately 1 across frequency. The large phase lags observed cannot be accounted for with velocity coupling alone but indicate that body sway also was coupled to the position of the touch plate. Fitting of a linear second-order model to the data suggests that postural control parameters are not fixed but adapt to the moving frame of reference. Moreover, coupling to both position and velocity suggest that a spatial reference frame is defined by the somatosensory system.

Adaptation, Physiological↗
PubMed2001

Stabilization of human standing posture using functional neuromuscular stimulation.

Functional neuromuscular stimulation (FNS)/functional electrical stimulation (FES) is a potential way to restore some functionality to the limbs of patients with spinal cord injury through direct/indirect stimulation of the motoneuron. One of the constraints for wider use of FNS on paraplegic patients is the lack of efficient control algorithm. Most of the published works on FNS/FES control are based on oversimplified models of human body dynamics. An innovative control strategy for stabilizing the standing posture of paraplegic patients is proposed here which is a combination of a proportional-plus-derivative controller for motions of the skeletal system and a control action prediction mechanism to produce musculotendon activation. The goal is to produce musculotendon torque which can approximate those demanded by the controller for the skeletal system. In computer simulations, using a detailed skeletal-musculotendon-muscle activation dynamics model of human body, this FNS/FES control approach can stabilize a paraplegic patient's standing posture with the minimum number of musculotendon groups. Also, it is found that this control strategy can maintain stability even in the presence of reasonable variations in the controller's musculotendon parameters.

Computer Simulation↗
PubMed1995

Is there a reasonable differential indication for different hydrocephalus shunt systems?

We tested various shunt systems for pressure/flow characteristics and long-term reliability. In addition, we used a model to simulate activities of daily life postural changes, blood and airway pressure changes and their impact on CSF pressure and flow through various ventriculo-peritoneal shunt systems. In the recumbent position, the changes in flow rate and CSF pressure depended on the valve resistance. Various valves showed deviations from the pressure/flow characteristics claimed for them and proved to be unreliable during long-term perfusion. The flow rate increased in the head-up position. Negative intracranial CSF pressure was due to the continued flow through the shunt system afforded by the siphon effect. The siphon effect was so marked in the upright position that valves of various kinds and with various resistances did not make any significant difference in the resulting intracranial pressure (ICP). The shunt systems, however, differed in the maximum flow rate in the upright position, leading to a different steep fall in ICP following elevation of the body. Ball-and-spring valves had the highest flow rates (> 500 ml/h), leading to negative ICP within seconds. Diaphragm valves, and especially the self-adjusting diaphragm valve, demonstrated a slower drop in ICP, taking several minutes to reach negative ICP. ASD and SCD, however, did prevent any siphoning effects, leading to an ICP within the corresponding valve opening/closing pressure range. Our results demonstrate that in most patients there is no significant difference in various different shunting systems as long as the patient is mobile.(ABSTRACT TRUNCATED AT 250 WORDS)

Activities of Daily Living↗
PubMed1994

Acceptance testing prone stereotactic breast biopsy units.

When the Mammography Quality Standards Act becomes law in October, 1994, stereotactic breast biopsy units may require yearly physicist calibration. Upright stereotactic units can be easily tested using conventional mammography procedures and a gelatin phantom containing simulated calcifications, but prone units are difficult to assess because of the under-table tube configuration. The two current manufacturers of these units have made different design decisions which affect each unit's calibration. There are a number of important distinctions between screening and prone biopsy units. For the two currently available prone units, a pronounced heel effect makes ion chamber position critical. Focal spot measurements are particularly difficult on one unit because there is no light field. The fixed grid on the other unit must be tested with a flood film. Physicists who inspect these units before their clinical use should be aware of variations needed by this equipment for specific acceptance tests.

Biophysical Phenomena↗
PubMed1998

Agreement between a frequency-weighted filter for continuous biomechanical measurements of repetitive wrist flexion against a load and published psychophysical data.

A previous pilot study demonstrated that a force and frequency-weighted filter network could be developed for processing continuous biomechanical measures of repetitive wrist motions and exertions. The current study achieves the objective by modelling subjective discomfort for repetitive wrist flexion using controlled posture, pace and force. A three-level fractional factorial experiment was conducted involving repetitive wrist flexion (2 s/motion, 6 s/motion, 10 s/motion) from a neutral posture to a given angle (10 degrees, 28 degrees, 45 degrees) against a controlled resistance (5 N, 25 N, 50 N) using a Box Behnken design. Ten subjects participated. Discomfort was reported on a 10 cm visual analogue scale. Results of response surface regression analysis revealed that main effects of force, wrist flexion angle, and repetition were all significant (p < 0.05) and that no second-order effects were observed. Linear regression analysis on these factors established a discomfort model on which the filter characteristics were based. The pure error test model revealed no significant lack of fit (p > 0.05). The continuous model was compared and agreed with discrete psychophysical data from other published studies. The model was used for generating parameters for a force and frequency-weighted digital filter that weighs continuous wrist postural signals with corresponding force in proportion to the equal discomfort function as a function of frequency of repetition. These filters will enable integration of large quantities of biomechanical data in field studies.

Adult↗
PubMed1999

A behavior-based inverse kinematics algorithm to predict arm prehension postures for computer-aided ergonomic evaluation.

In this paper, the computational problem of inverse kinematics of arm prehension movements was investigated. How motions of each joint involved in arm movements can be used to control the end-effector (hand) position and orientation was first examined. It is shown that the inverse kinematics problem due to the kinematic redundancy in joint space is ill-posed only at the control of hand orientation but not at the control of hand position. Based upon this analysis, a previously proposed inverse kinematics algorithm (Wang et Verriest, 1998a) to predict arm reach postures was extended to a seven-DOF arm model to predict arm prehension postures using a separate control of hand position and orientation. The algorithm can be either in rule-based form or by optimization through appropriate choice of weight coefficients. Compared to the algebraic inverse kinematics algorithm, the proposed algorithm can handle the non-linearity of joint limits in a straightforward way. In addition, no matrix inverse calculation is needed, thus avoiding the stability and convergence problems often occurring near a singularity of the Jacobian. Since an end-effector motion-oriented method is used to describe joint movements, observed behaviors of arm movements can be easily implemented in the algorithm. The proposed algorithm provides a general frame for arm postural control and can be used as an efficient postural manipulation tool for computer-aided ergonomic evaluation.

Adult↗
PubMed2002

A mathematical model of the stability control of human thorax and pelvis movements during walking.

A mathematical model is developed to study the human thorax and pelvis movements in the frontal plane during normal walking. The model comprises of two-link base-excited inverted pendulums with one-degree of rotational freedom for each link. Since the linear motion of the pelvis has a significant effect on the upper body stability, this effect is included in the model by having a base point moving in the frontal plane in a general way. Furthermore, because the postural stability is the primary requirement of normal human walking, the control law is developed based on Lyapunov's stability theory, which guarantees the stability of the pendulum system around the up-right position. To evaluate the model, the simulation results, including the angular displacement of each link and the torque applied on each link, are compared with those from gait measurements. It is shown that the simulation results match those from gait measurements closely. These results suggest that the proposed model can provide a useful framework for analysis of postural control mechanisms.

Algorithms↗