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PubMed2004

A new dynamic model of the cortico-basal ganglia loop.

An important issue in the neural control of posture and movement is how motor-related areas of the cerebral cortex modulate the activity of the output nuclei of the basal ganglia. In this chapter, the functional significance of the 'hyperdirect' cortico-subthalamo-pallidal pathway is emphasized, and further a new dynamic model of basal ganglia function is presented. When a voluntary movement is about to be initiated by cortical mechanisms, a corollary signal is conveyed through the 'hyperdirect' pathway to first inhibit large areas of the thalamus and cerebral cortex that are related to both the selected motor program and other competing programs. Next, another corollary signal is sent through the 'direct' cortico-striato-pallidal pathway to disinhibit this second pathway's targets, and ensure activation of only the selected motor program. Finally, a third corollary signal is sent through the 'indirect' cortico-striato-external pallido-subthalamo-internal pallidal pathway to strongly inhibit this third pathway's targets. This sequential information processing ensures that only the selected motor program is initiated, executed and terminated at the appropriate times, whereas other competing programs are canceled.

Animals↗
PubMed1990

Postural dynamics of walking in humans.

The dynamics of postural control in human biped locomotion were studied using (1) a model, and (2) experimentally applied impulsive force disturbances. The model was planar, and contained five rigid segments, articulating at frictionless pin joints. The model was used to identify joint torque combinations which would successfully correct for an impulsive force disturbance applied at different points in the walking cycle. The simulation results suggested that (1) early responses (within 80 ms) can be effective in compensating for impulsive disturbances, (2) the same strategies which successfully counteract similar disturbances during quiet standing are also effective in certain phases of the walking cycle, (3) modifications in the response strategies are needed to accommodate differences in the dynamics over the stride cycle, and (4) the swing leg is ineffective in compensating for disturbances in the short term. These model predictions were tested experimentally. Subject responses to an impulsive force disturbance applied during walking were studied. The electromyographic results generally support the model predictions.

Computer Simulation↗
PubMed2002

Capturing the frame of reference of shoulder muscle forces.

We have adjusted and validated models of the lines of action of six human shoulder muscles. Compared to other models of the shoulder mechanism (e.g., 9, 11, 17, 24) ours is greatly simplified in that the scapula and its many muscles are largely neglected, and the action of each of six broad muscles is summarized by a single line segment. The close correspondence between measured and predicted moment direction over a wide range of postures suggests that this latter simplification was reasonable. We found that assignment of two via points (one fixed to the origin and one fixed to the insertion) was adequate to represent the measured actions, and our sensitivity analysis highlights the importance of via point placement. Because our model predicts the action of each muscle over a very large range of arm postures, it should be useful for future investigations of the control of muscle forces.

Adult↗
PubMed2004

Influence of an asymmetrical body weight distribution on the control of undisturbed upright stance.

Postural asymmetry in humans is generally associated with different pathologies. However, its specific influence on undisturbed upright stance is poorly understood. To evaluate its separate effects on each support, the centre of pressure (CP) displacements were recorded through two force platforms. In a second step, the complex resultant centre of pressure trajectories (CP(Res)) were computed and decomposed into two elementary components: the horizontal displacements of the centre of gravity (CG(h)) and the difference in the plane of support between the vertical projection of CG(h) and CP(Res) (CP-CG(v)). These motions were then processed through a frequency analysis and modelled as fractional Brownian motion to gain some additional insight into their spatio-temporal organisation. Ten healthy adults were tested in three conditions consisting of various weight distributions. The quality of the mechanism involved in the control of the unloaded support CP motions appears to decrease as the asymmetry becomes more pronounced. To be precise, larger increases of the CP displacements are observed for the unloaded support compared to the loaded one. As a result, the CP(Res) motions are themselves augmented in the ML direction, inducing in turn larger CG(h) and CP-CG(v) motions. Postural asymmetry thus constitutes an important constraint on the control of upright undisturbed stance by generating changes in the control of both supports and by reducing the efficiency of the hip load/unload mechanisms. On the other hand, by inducing larger body sways, postural asymmetry necessitates higher energy expenditure and the setting of particular control mechanisms.

Adult↗
PubMed1991

Human postural dynamics.

The study of posture dynamics is important not only to understand disorders of impaired equilibrium and protective reactions to unexpected displacements of the human body, but also to the design of prosthesis and functional neuromuscular stimulation as aids to patients with impaired postural stability and locomotion. Coordinated control of the body segments is a complex aspect of motor behavior, owing to the multiple degrees of freedom of the controlled system. Several interacting subsystems are involved in the dynamics of human posture and locomotion, including the skeletal, neuromuscular, and sensory systems. Human posture control is maintained by somatosensory, vestibular, and visual feedback, integrated within the locomotor and central nervous systems. Studies of posture dynamics and stability therefore entail the study of mechanical aspects of the human body, its sensory systems, and the principles governing coordination in motion control. In this paper is reviewed some of the research done in the field of human posture dynamics, including such topics as biomechanics, equilibrium, stability, motion coordination, neural feedback, neural control systems modeling, motion strategies, optimality of motion, and adaptation. We consider experimental approaches and theoretical models, as well as the gap between them. Principles for the experimental investigation of control systems are considered.

Animals↗
PubMed2001

Delayed stochastic differential model for quiet standing.

A physiological quiet standing model, described by a delayed differential equation, subject to a white noise perturbation, is proposed to study the postural control system of human beings. It has been found that the white noise destabilizes the equilibrium state, and inertia accelerates the destabilizing process, and that the position of a person is detected and processed by the person's nervous system with a delay. This paper focuses on the analysis of Hopf bifurcation and its stability in this context. Based on the analytical predictions confirmed by numerical simulations, it has been shown that the posture of a person is controlled in such a way that possible amplitude oscillations are minimized.

Computer Simulation↗
PubMed1993

An integrated EMG/biomechanical model of upper body balance and posture during human gait.

Full scale biomechanical and EMG analyses of the balance during human gait are required to understand the neural control of locomotion. The purpose of this paper was to develop an inverted pendulum model of upper body balance in both the plane of progression and the frontal plane, and a medical/lateral balance model of the total body. EMG evidence was also recorded to reinforce the conclusions from the moment of force analyses. The kinematics and kinetics for up to ten natural walking trials on each of four subjects and EMG records from walking trials on eleven subjects were investigated. The results support the following conclusions. (1) The hip extensors/flexors have an over-powering role in maintaining dynamic balance of the head, arms and trunk (HAT) in the plane of progression. Because of the lack of suitable neurological and biomechanical delays between the small head acceleration, presumably exciting vestibular afferents, and the hip moment patterns, the vestibular system appears not to be involved as a feedback sensor in the balance control during gait. (2) In the frontal plane, the hip abductors are dominant in countering the large medial-lateral (M/L) imbalance of HAT during single support but are assisted by the medial acceleration of the hip joint. (3) the total body M/L balance is achieved by the M/L placement of the foot with some opposition and some assistance by the M/L acceleration of the subtalar joint. The subtalar invertors/evertors play an insignificant role during single stance.(ABSTRACT TRUNCATED AT 250 WORDS)

Arm↗
PubMed2006

The Physical Performance Test as a predictor of frequent fallers: a prospective community-based cohort study.

OBJECTIVE: To construct a risk model in order to identify elderly individuals at risk of frequent falling. DESIGN: Prospective community-based cohort study over 12 months. SETTING: Baseline measures were performed at a local community centre. SUBJECTS: Two hundred and sixty-three community-dwelling elderly people (mean age 72 years). MEASUREMENTS: A variety of variables were evaluated, including medical, psychological, sensory, physical and postural control measurements. Fall incidence was monitored retrospectively and during one-year follow-up. RESULTS: Logistic regression analysis showed that polypharmacia was the most prominent medical fall predictor with an odds ratio (OR) of 1.29 (P= 0.005), poor visual acuity the best sensory predictor (OR = 0.84; P= 0.009) and general fear of falling the most crucial psychological predictor (OR = 3.25; P< 0.001). Increased postural sway in near-tandem stance with eyes open was selected as the best balance predictor for falls (OR = 5.60; P= 0.010), followed by delayed anteroposterior movement velocity during rhythmic weight shifts (OR = 0.42; P= 0.004). The best physical predictor was a low score on the Physical Performance Test (OR =4.16; P< 0.001), followed by decreased maximal handgrip strength (OR = 0.87; P< 0.001) and increased timed chair-stands (OR = 1.13; P= 0.003). Step-by-step regression analysis revealed a risk model for the prediction of future falls, as a combination of the Physical Performance Test and maximal handgrip strength. CONCLUSION: This study confirms the multicausality of falls, since medical, psychological, sensory, postural control as well as physical variables provides a predictive value. The composed fall risk model was mainly physically oriented.

Accidental Falls↗
PubMed1985

Sensorimotor deficits related to postural stability. Implications for falling in the elderly.

The effects of age-related sensorimotor and central processing deficits on postural control are reviewed, and the paucity of knowledge about proprioceptive changes with age is noted. A model of processing stages in the production of responses to postural instability is outlined. Even slight response slowing produces disproportionate increases in clinically slow responses. Many aspects of postural responses are "cognitively penetrable". Research on falls should examine learned as well as automated behaviors.

Accidents↗
PubMed1993

The Cascade Neural Network Model and a Speed-Accuracy Trade-Off of Arm Movement.

We propose a hybrid neural network model of aimed arm movements that consists of a feedforward controller and a postural controller. The cascade neural network of Kawato, Maeda, Uno, and Suzuki (1990) was employed as a computational implementation of the feedforward controller. This network computes feedforward motor commands based on a minimum torque-change criterion. If the weighting parameter of the smoothness criterion is fixed and the number of relaxation iterations is rather small, the cascade model cannot calculate the exact torque, and the hand does not reach the desired target by using the feedforward control alone. Thus, one observes an error between the final position and the desired target location. By using a fixed weighting parameter value and a limited iteration number to simulate target-directed arm movements, we found that the cascade model generated a planning time-accuracy trade-off, and a quasi-power-law type of speed-accuracy trade-off. The model provides a candidate neural mechanism to explain the stochastic variability of the time course of the feedforward motor command. Our approach also accounts for several invariant features of multijoint arm trajectories, such as roughly straight hand paths and bell-shaped speed profiles.

Journal Article↗
PubMed1999

Automatic determination of the transition between successive control mechanisms in upright stance assessed by modelling of the centre of pressure.

A recently introduced concept models the trajectory of the centre of pressure as a fractional Brownian motion and reveals that two successive scaling regimes, acting hypothetically as open and closed loop mechanisms, are implicated in posture control. Objectivity is obviously required in the determination of the transition point, i.e. the point at which an open-loop control mechanism would switch to a closed-loop one, in order to provide reproducibility and automatism in the processing of data. In the method proposed herein, the transition point corresponds to the maximal distance separating a diffusion curve in a double logarithmic plot (mean square distances MSD calculated on each axis versus increasing time intervals Deltat) from a straight line characterising a pure stochastic behaviour. In closed eye conditions, the switch appears medio-laterally in a 0. 26-0.52 s range for Deltat, the corresponding MSD being in the range of 1.86-10.50 mm(2). In the forward-backward direction, the transition is in a 0.28-0.42 s range and the corresponding MSD is between 3.60 and 15.17 mm(2). Finally, these co-ordinates induce scaling exponents over 0.50 for the shortest Deltat, thus suggesting open-loop control, whereas those of longest Deltat, ranged between 0 and 0.20, give evidence of close-loop control. This data is compared to previous data based upon empirical methods.

Algorithms↗
PubMed2007

The expression pattern of nuclear receptors during cerebellar development.

The cerebellum is essential for fine control of movement and posture, and it has been a useful model for studying many aspects of neural development because of its relatively simple anatomy and developmental program. However, the roles of nuclear receptors (NRs) underlying formation of the cerebellum and maintenance of cerebellar functions are still poorly characterized. As a contribution to the Nuclear Receptor Signaling Atlas (NURSA), we employed immunohistochemistry to investigate the expression pattern of 18 NRs in the cerebellum. Ten receptors were demonstrated to be expressed in the postnatal day 21 (P21) cerebellum. Among them, five receptors (COUP-TFI, COUP-TFII, RORalpha, ERbeta, and ERRgamma) were expressed at all stages (embryonic stage, P0, P7, and P21) examined. Interestingly, COUP-TFI and COUP-TFII show differential anterior-posterior expression patterns during cerebellar development. Taken together, our results suggest that members of the nuclear receptor superfamily might play importantly physiological roles in the cerebellum.

Animals↗
PubMed2006

A neural network approach to motor-sensory relations during postural disturbance.

This study explored whether artificial neural networks (ANN) can be used to quantify the motor-sensory relationship during postural disturbance. An ANN model was constructed with seven mechanical stimuli to the visual, vestibular and somatosensory systems (i.e., head angular and linear accelerations, eye-target distance, ankle joint rotation and velocity, as well as normal and shear ground contact forces under the feet) as inputs, and electromyographic activities of tibialis anterior and gastrocnemius muscles as outputs. These inputs and outputs were directly measured during a sudden toes-up-down rotation of the supporting base in two groups of elderly subjects: people with peripheral neuropathy (NP) who have severe loss of mechanoreception in the sole of their feet and people without NP. The products of ANN weights were used in a summary statistic called the Q-value to estimate the contribution of each mechanical stimulus to sensory systems in determining each leg muscle activity. It was found that: (1) the stimuli to the vestibular system and/or ankle proprioceptors have greater contributions to leg muscle activities, especially the TA muscle, in people with NP than people without NP; (2) the stimuli to somatosensory receptors have the greatest contribution, and the stimuli to the vestibular system have the least contribution to both muscle activities in both groups. These findings are supported by previous studies and have demonstrated the potential of the Q-value concept in the ANN model in studying the motor-sensory relationship in human postural control.

Aged↗
PubMed2002

A new approach to off-line setup corrections: combining safety with minimum workload.

Off-line patient setup correction protocols based on electronic portal images are an effective tool to reduce systematic patient setup errors. Recently, we have introduced the no action level (NAL) protocol which establishes a significant error reduction at a very small workload. However, this protocol did not include an explicit verification of the applied setup corrections. Systematic mistakes in the execution of setup corrections (e.g., a setup correction is always executed in the +X direction whereas a correction in the -X direction was prescribed) may introduce large systematic setup errors (irrespective of the setup protocol) and may seriously impair treatment outcome. We have therefore extended the NAL protocol with a correction verification (COVER) stage, solely aimed at detecting such mistakes. In short, COVER tests the magnitude of the postcorrection setup error in each relevant direction. If these residue errors are below the acceptance threshold T, no more electronic portal images are required and the protocol has finished. If not, the origin of this result should be investigated; if no obvious mistakes are present, the procedure is repeated for one more treatment fraction. If the residue setup errors are confirmed to be larger than T, the entire protocol is restarted. Using both Monte Carlo simulations and analytical calculations, we performed a risk analysis and evaluated the workload for various choices of T. A threshold T = 3 x sigma(r), where sigma(r) is the mean standard deviation of the random setup errors, ensured that (1) COVER introduces only a small additional workload (1.05 measurement per patient, while the absolute minimum is 1.0) and (2) serious correction mistakes are detected with high probability. Even if setup corrections are wrongly applied in each patient (worst case scenario), COVER ensures that the final distribution of systematic errors is not wider than the precorrection distribution of systematic errors; for realistic frequencies of correction mistakes (<< 1 per patient) this distribution becomes much more narrow. The combination of NAL and COVER thus provides a highly efficient as well as safe method to reduce systematic setup errors.

Algorithms↗
PubMed1986

Varying gradient angles and offsets to optimize imaging planes in MR.

A simple technique for determining the appropriate viewing angle rotations necessary to obtain images in any desired plane is presented, along with an explanation of the imaging coordinates used in magnetic resonance imaging. Viewing angle rotations caused a minimal loss in image quality but no loss in signal-to-noise ratio, and were found to be compatible with both multisection and multiecho acquisitions. Several examples that show the practical applications of these techniques are provided.

Audiovisual Aids↗
PubMed1982

Heat balance of ox steers (Bos taurus) in steady-temperature environments.

Work was undertaken to investigate body heat storage in cattle subjected to steady temperature environments. Heat production (M) of and heat losses (H) from two steers were studied by simultaneous direct and indirect calorimetry over periods of up to 11 days. M and H followed similar trends except after feeding and watering. When the animals stood both M and H leveled approximately 20% higher than when they were lying down. Over 24-h periods total M and H agreed within the limits of experimental error. Heat storage of the calorimeter contents (MH) was calculated from the accumulated difference between M and H. Day-night cycles observed in MH were partly due to the heat of warming food and water and of cooling excreta (FF). The difference (TB = MH - FF) represented calorimetrically determined changes in heat storage in the animal's body. Day-night cycles in TB were small (less than 1 degree C range in equivalent change in mean body temperature) but agreed well with observed changes in body core (carotid artery) temperatures. It is concluded that day-night changes in heat storage in fixed-temperature environments are small but measurable by calorimetry and that the methods used are applicable for future studies with cyclic environmental temperatures.

Animals↗
PubMed2006

Running over rough terrain reveals limb control for intrinsic stability.

Legged animals routinely negotiate rough, unpredictable terrain with agility and stability that outmatches any human-built machine. Yet, we know surprisingly little about how animals accomplish this. Current knowledge is largely limited to studies of steady movement. These studies have revealed fundamental mechanisms used by terrestrial animals for steady locomotion. However, it is unclear whether these models provide an appropriate framework for the neuromuscular and mechanical strategies used to achieve dynamic stability over rough terrain. Perturbation experiments shed light on this issue, revealing the interplay between mechanics and neuromuscular control. We measured limb mechanics of helmeted guinea fowl (Numida meleagris) running over an unexpected drop in terrain, comparing their response to predictions of the mass-spring running model. Adjustment of limb contact angle explains 80% of the variation in stance-phase limb loading following the perturbation. Surprisingly, although limb stiffness varies dramatically, it does not influence the response. This result agrees with a mass-spring model, although it differs from previous findings on humans running over surfaces of varying compliance. However, guinea fowl sometimes deviate from mass-spring dynamics through posture-dependent work performance of the limb, leading to substantial energy absorption following the perturbation. This posture-dependent actuation allows the animal to absorb energy and maintain desired velocity on a sudden substrate drop. Thus, posture-dependent work performance of the limb provides inherent velocity control over rough terrain. These findings highlight how simple mechanical models extend to unsteady conditions, providing fundamental insights into neuromuscular control of movement and the design of dynamically stable legged robots and prosthetic devices.

Animals↗