Biomedical subjects

Z Matjacić

Publications and source records attributed to Z Matjacić.

13 recordsLinked to original sources

PubMed2003

Control of posture with FES systems.

One of the major obstacles in restoration of functional FES supported standing in paraplegia is the lack of knowledge of a suitable control strategy. The main issue is how to integrate the purposeful actions of the non-paralysed upper body when interacting with the environment while standing, and the actions of the artificial FES control system supporting the paralyzed lower extremities. In this paper we provide a review of our approach to solving this question, which focuses on three inter-related areas: investigations of the basic mechanisms of functional postural responses in neurologically intact subjects; re-training of the residual sensory-motor activities of the upper body in paralyzed individuals; and development of closed-loop FES control systems for support of the paralyzed joints.

Algorithms↗

PubMed2001

Functional postural responses after perturbations in multiple directions in a standing man: a principle of decoupled control.

The objective of this study was to assess functional postural responses by analyzing the net joint torques (NJT) in the ankles and the hips resulting from perturbations delivered in multiple directions to subjects standing quietly. A total of eight subjects were standing on two force platforms while an apparatus randomly delivered controlled perturbations at the level of the pelvis in eight directions: anterio-posterior (AP), medio-lateral (ML), and four combinations of these principal directions. Perturbations were repeated five times in each direction for six conditions (i.e., three different perturbation strengths and three different feet orientations). The comparison of the averaged ankle sum NJT (AP) responses showed that the time courses of the responses elicited by a perturbation acting only in the AP direction were identical to those elicited by a combination of two corresponding AP and ML perturbations. In contrast the observed averaged ankle NJT (ML) responses did not follow the same similarity. The comparison of the averaged ankle and hip sum NJT (ML) responses revealed that the time courses of the responses elicited by a perturbation acting only in the ML direction were identical to those elicited by a combination of two corresponding AP and ML perturbations. These findings were invariable of the experimental conditions and were consistent among all the eight subjects. Thereby, we conclude that the ankle sum NJT (AP) and the ankle and hip sum NJT (ML) are the global variables being controlled. This shows that CNS controls the recovery from the multiple direction perturbations of moderate strength by decoupling the AP-ML postural space into two orthogonal directions (AP and ML).

Adult↗

PubMed2000

Postural activity of constrained subject in response to disturbance in sagittal plane.

This study examines the postural activity of partially constrained subjects for three different initial standing postures in response to disturbances in the sagittal plane. When the ankle strategy suffices for disturbance rejection in response to anterior disturbances, a mostly linear relationship between the ankle torque and ankle angle was observed, resulting in a constant stiffness at the ankles. However, when the ankle torque saturation was reached, a combined ankle-trunk postural strategy was utilized mainly in response to posterior perturbations due to the properties of the base-of-support. This caused a nonlinear scaling of ankle responses, thereby increasing variability of ankle stiffness. Distinctions in the ankle responses were also observed for different initial standing postures. The anterior initial stance generally increases the overall postural stability and renders the utilization of ankle strategy feasible, even for the rejection of posterior disturbances. Therefore, a linear torque-angle relationship at the ankles was observed for the anterior initial stance, regardless of the perturbation parameters.

Adult↗

PubMed1999

Postural activity of constrained subject in response to disturbance in sagittal plane.

The study examines postural activity of a constrained subject in response to various disturbances in the sagittal plane. Three different initial standing postures were examined. Each response to a perturbation was divided into three consecutive phases according to the intensity of the muscular activity. The relation of the ankle joint torque versus the ankle joint angle was studied. A linear relationship, resulting in constant ankle stiffness, was observed in each phase of the response. Only negligible differences in the stiffness values were observed among different phases. The results indicate an ankle stiffness value of 10 N m/o for the majority of initial stance postures and perturbation intensities.

Adult↗

PubMed1998

Arm-free paraplegic standing--Part I: Control model synthesis and simulation.

The following paper is the first part of our investigation into the feasibility of arm-free paraplegic standing. A novel control strategy for unsupported paraplegic standing which utilizes the residual sensory and motor abilities of the thoracic spinal cord injured subjects is proposed. The strategy is based on voluntary and reflex activity of the paraplegic person's upper body and artificially controlled stiffness in the ankles. The knees and hips are maintained in an extended position by functional electrical stimulation (FES). The analysis of a linearized double inverted pendulum model revealed that with properly selected ankle stiffness the system can be easily stabilized. We developed a closed-loop double inverted pendulum model including a neural system delay, trunk muscles dynamics, body segmental dynamics and linear quadratic regulator (LQR) optimal controller. Through simulations of the closed-loop model two different strategies for disturbance rejection were explained. We investigated the capability of the closed-loop model to reject disturbances, imposed at the ankle joint (in anterior and posterior directions) for various stiffness levels and neural system delays in the presence of biomechanical constraints. By limiting permissible excursions of the center of pressure, we found out that the length of the foot is the most important constraint, while the strength of the trunk muscles is not of major importance for successful balancing. An ankle stiffness of approximately 10 Nm/degree suffices for arm-free standing of paraplegic subjects.

Ankle Joint↗

PubMed1998

Arm-free paraplegic standing--Part II: Experimental results.

In Part I, we proposed an approach for restoring unsupported standing to thoracic-level paraplegics. The theoretical analysis and simulation of an underactuated double inverted pendulum, representing the standing subject, showed that arm-free standing might be achieved. Here in Part II, we present the mechanical apparatus which we used in our experiments and experimental results from tests of the balance-control strategy. We demonstrate that an intact and a paraplegic subject could perform quiet standing with the ankle stiffness set to 8 Nm/degree or even less (the intact subject). Both were also able to recover from disturbances, imposed by the artificial ankle joint of the apparatus. Introducing cognitive auditory feedback greatly improved the standing abilities of both subjects.

Adult↗

PubMed1997

Improvement in step clearance via calf muscle stimulation.

The aim is to study the influence of electrically stimulated calf muscles on the effectiveness of the swinging leg movement. The study is carried out with a group of patients with incomplete spinal cord injuries both under stationary conditions and during crutch-assisted walking. Before stimulation is applied to the ankle plantar flexors, the knee extensors are inactivated. In each cycle, after ankle plantar flexor stimulation, peroneal stimulation is started, triggering the flexion reflex. From a biomechanical point of view, functional electrical stimulation (FES) of the ankle plantar flexors results in increased ground clearance of the lower extremity. Additionally, the FES-assisted lifting of the heel results in the elimination of extensor tone and thus shortens the swing time.

Adolescent↗

PubMed1997

Wireless control of functional electrical stimulation systems.

With the assistance of crutches and functional electrical stimulation (FES), we are able to restore standing and simple gait in some spinal cord injured (SCI) patients. In present rehabilitative systems, the patient divides the gait cycle into stance and swing phases via pushbuttons mounted on the handles of the crutches, which are hardwired to the functional electrical stimulator. The surface-mount technology based telemetry system, which makes use of the radiofrequency medium at 40 MHz, was developed to provide wireless control of the FES system. Signals from crutch pushbuttons were coded and transferred from the transmitter to the receiver. The receiver was firmly attached to the patient's waist and was connected to the stimulator.

Crutches↗

PubMedDate not supplied

A multi-purpose rehabilitation frame: an apparatus for experimental investigations of human balance and postural control.

Moving and rotating platforms are often used in experimental investigations of human balance and postural control. These devices are not well suited for testing elderly and neurologically impaired individuals, because of inherent risk of injury to the experimental subject due to a potential fall. This paper describes a novel mechanical apparatus that generates perturbations to a standing subject by applying pushing forces at the level of the pelvis and can also provide restoring forces in the case of destabilization. The device has two degrees of freedom, which are actuated by hydraulic servo systems, and can deliver a perturbation in any direction comprised within anterio-posterior and medio-lateral postural space. The accuracy and repeatability of the perturbations elicited in eight different directions was evaluated. The results, showing a high degree of correlation between the trajectories in both degrees of freedom of the apparatus, demonstrate that accurate and repeatable perturbations can be imposed on the subjects tested.

Biomechanical Phenomena↗

PubMedDate not supplied

Development and evaluation of a two-dimensional electrocutaneous cognitive feedback system for use in paraplegic standing.

Fatigue of electrically activated paralysed muscles is a major factor limiting the duration of functional electrical stimulation (FES) supported paraplegic standing. Fatigue can be significantly delayed by changing the posture. Since paralysed individuals are deprived of proprio- and exteroception from the lower limbs they are not aware of the posture and loading of their paralysed legs. If suitable cognitive feedback (CF) information about posture in the sagittal and frontal planes is provided, they might be able to successfully exercise posture switching. A two-dimensional electrocutaneous CF system was developed. Relative limb loading and the location of the weighted centre of pressure were selected as informational variables. Discrete encoding schemes in the form of spatial and frequency codes were employed and the informational signals were divided into three sub-regions. The ability to correctly interpret the CF was investigated using one- and two-dimension tracking tests in three paralysed subjects, each of whom were studied over five consecutive days. All three subjects were able to use the CF in one-dimension tests. Two subjects could do the same also in two-dimension tests. The encoding scheme which was developed to communicate the selected biomechanical variables proved to be easily understood and thus appropriate for use in paraplegic standing.

Adult↗

PubMedDate not supplied

A quantitative gait assessment method based on energy exchange analysis during walking: a normal gait study.

In this paper a gait efficiency assessment method, Gait Energy Efficiency Index (GEEI), which can be used in evaluation of the progress of the rehabilitation process in disabled persons, is proposed and described. The method is based on calculation of cross correlation between normalized time courses of kinetic and potential energy of the body's centre of mass (COM). We hypothesized that GEEI in energetically optimal normal walking should be high and invariable of gait speed. The method was tested on twelve healthy subjects walking at three different speeds and contrasted to five established gait energy consumption assessment methods. The results showed that GEEI in normal walking is close to 1 regardless of walking speed.

Adult↗

PubMedDate not supplied

Voluntary telemetry control of functional electrical stimulators.

With the assistance of crutches and functional electrical stimulation (FES) we are able to restore standing and simple gait in some spinal cord injured (SCI) patients. In the present rehabilitative systems the patient divides the gait cycle into 'stance' and 'swing' phase by using pushbuttons mounted in the handles of the crutches. These are then hard wired to the functional electrical stimulator. We present the development and evaluation of a surface mount technology based telemetry system that provides reliable and interference resistant wireless control of FES assisted walking. The system makes use of radio frequency carriers operating at a frequency of 40 MHz. Crutch pushbutton signals are coded and transferred from the transmitter placed in the crutch to the receiver which is firmly attached to the patient's waist and connected to the stimulator. The telemetry system was found to be of special importance for both complete and incomplete SCI subjects and is currently in use at the Rehabilitation Institute of the Republic of Slovenia.

Activities of Daily Living↗

PubMedDate not supplied

A multi-purpose rehabilitation frame: a novel apparatus for balance training during standing of neurologically impaired individuals.

We present a novel mechanical apparatus, named Multi-purpose Rehabilitation Frame (MRF), and methods for balance training during standing of neurologically impaired individuals. The device has two degrees of freedom (DOF), which allow for constrained movement of both lower limbs and pelvis in the sagittal and frontal planes. The MRF aims at improving balancing in impaired individuals by providing a stiffness support and action of perturbations, which facilitate development of alternative balancing strategies. The level of stiffness support and strength of perturbations, which are generated by means of two hydraulic servo-controlled actuators, can be selected according to current balancing abilities of an impaired individual. We further present preliminary results of nine days of balance training in two paraplegic and two incomplete tetraplegic subjects standing in the MRF. All subjects improved their balancing abilities as measured from the level of needed supporting stiffness provided by the MRF.

Adult↗