Biomedical subjects

D Popović

Publications and source records attributed to D Popović.

At least 19 recordsLinked to original sources

PubMed2001

Novel metallic behavior in two dimensions.

Experiments on a sufficiently disordered two-dimensional (2D) electron system in silicon reveal a new and unexpected kind of metallic behavior, where the conductivity decreases as sigma(n(s),T) = sigma(n(s),T = 0)+A(n(s))T(2) (where n(s) is carrier density) to a nonzero value as temperature T-->0. In 2D, the existence of a metal with dsigma/dT>0 is very surprising. In addition, a novel type of a metal-insulator transition obtains, which is unlike any known quantum phase transition in 2D.

Journal Article↗

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↗

PubMed1999

Clinical evaluation of the bionic glove.

OBJECTIVE: Clinical evaluation of the Bionic Glove, a prototype of a new functional electrical stimulation device designed to improve the function of the paralyzed hand after spinal cord injury. PATIENTS: Twelve people with spinal cord injury at C5-C7 who had used the device 6 months or more. SETTING: Measurements were made at the Institute "Dr Miroslav Zotović" in Belgrade as a part of a multicenter clinical trial. METHODS: Measures include Upper Extremity Function Test, Functional Independence Measure, and Quadriplegia Index of Function. RESULTS: The daily use of a Bionic Glove had two major effects: (1) increasing the power grasp; and (2) increasing the range of movements. Active force was significantly greater than passive tenodesis force, as shown in other studies. Most manual tasks improved significantly with the use of the assistive system, as judged by the time needed to complete a task or the subject's qualitative ratings of a task difficulty. Most subjects who retained some dexterity without the assistive system hesitated to use the assistive system to manipulate small objects. CONCLUSION: The Bionic Glove can significantly improve independence in people with C5-C7 spinal cord injury if their initial Functional Independence Measure and Quadriplegia Index of Function scores are 20% to 50% of the maximum values.

Activities of Daily Living↗

PubMed1999

Automatic synthesis of synergies for control of reaching--hierarchical clustering.

In this paper we describe a novel method for determining synergies between joint motions in reaching movements by hierarchical clustering. A set of recorded elbow and shoulder trajectories is used in a learning algorithm to determine the relationships between angular velocities at elbow and shoulder joints. The learning algorithm is based on optimal criteria for obtaining the hierarchy of descriptions of movement trajectories. We show that this method finds complex synergism between optimal joint trajectories for a given set of data and angular velocities at the shoulder and elbow joints. Three other machine learning techniques (ML) are used for comparison with our method of hierarchical clustering of trajectories. These MLs are: (1) radial basis functions (RBF), (2) inductive learning (IL), and (3) adaptive-network-based fuzzy inference system (ANFIS). Better error characteristics were obtained using the method of hierarchical clustering in comparison with the other techniques. The advantage of the method of hierarchical clustering with respect to the other MLs is in integrating the spatial and temporal elements of reaching movements. Determination and analysis of spatio-temporal events of movement trajectories is a useful tool in designing control systems for functional electrical stimulation (FES) assisted manipulation.

Algorithms↗

PubMed1999

Three machine learning techniques for automatic determination of rules to control locomotion.

Automatic prediction of gait events (e.g., heel contact, flat foot, initiation of the swing, etc.) and corresponding profiles of the activations of muscles is important for real-time control of locomotion. This paper presents three supervised machine learning (ML) techniques for prediction of the activation patterns of muscles and sensory data, based on the history of sensory data, for walking assisted by a functional electrical stimulation (FES). Those ML's are: 1) a multilayer perceptron with Levenberg-Marquardt modification of backpropagation learning algorithm; 2) an adaptive-network-based fuzzy inference system (ANFIS); and 3) a combination of an entropy minimization type of inductive learning (IL) technique and a radial basis function (RBF) type of artificial neural network with orthogonal least squares learning algorithm. Here we show the prediction of the activation of the knee flexor muscles and the knee joint angle for seven consecutive strides based on the history of the knee joint angle and the ground reaction forces. The data used for training and testing of ML's was obtained from a simulation of walking assisted with an FES system [39]. The ability of generating rules for an FES controller was selected as the most important criterion when comparing the ML's. Other criteria such as generalization of results, computational complexity, and learning rate were also considered. The minimal number of rules and the most explicit and comprehensible rules were obtained by ANFIS. The best generalization was obtained by the IL and RBF network.

Algorithms↗

PubMed1999

Optimal control of walking with functional electrical stimulation: a computer simulation study.

Bipedal locomotion was simulated to generate a pattern of activating muscles for walking using electrical stimulation in persons with spinal cord injury (SCI) or stroke. The simulation presented in this study starts from a model of the body determined with user-specific parameters, individualized with respect to the lengths, masses, inertia, muscle and joint properties. The trajectory used for simulation was recorded from an able-bodied subject while walking with ankle-foot orthoses. A discrete mathematical model and dynamic programming were used to determine the optimal control. A cost function was selected as the sum of the squares of the tracking errors from the desired trajectories, and the weighted sum of the squares of agonist and antagonist activations of the muscle groups acting around the hip and knee joints. The aim of the simulation was to study plausible trajectories keeping in mind the limitations imposed by the spinal cord injury or stroke (e.g., spasticity, decreased range of movements in some joints, limited strength of paralyzed, externally activated muscles). If the muscles were capable of generating the movements required and the trajectory was achieved, then the simulation provided two kinds of information: 1) timing of the onset and offset of muscle activations with respect to the various gait events and 2) patterns of activation with respect to the maximum activation. These results are important for synthesizing a rule-based controller.

Algorithms↗

PubMed1999

Fatal strongyloidosis following corticosteroid therapy in a patient with chronic idiopathic thrombocytopenia.

A patient with chronic idiopathic thrombocytopenia and fatal strongyloides hyperinfection syndrome following prolonged corticosteroid therapy is briefly described. Diagnosis was difficult to perform due to absence of eosinophilia and diarrhea at presentation, as well as to the negativity of multiple stool specimens examined by direct microscopy of saline smear, formol-ether concentration techniques, and Baermann's test. The striking hypoalbuminemia in the setting of the normal results of liver function tests and prothrombin time was assumed to be due to enteropathy. Therefore, an upper endoscopy was undertaken, revealing Strongyloides stercoralis (SS) larvae in the biopsy specimens of the gastric and duodenal mucosa. The SS larvae were also demonstrated in the multiple specimens of the concentrated sputum. Despite thiabendazol treatment, death ensued. On autopsy, SS larvae were recovered in the gastrointestinal tract and lungs. The importance of early diagnosis and of ruling out strongyloidosis prior to administration of corticosteroids are discussed, as well as the pathogenetic aspects of strongyloidosis in the patient under corticosteroids.

Adrenal Cortex Hormones↗

PubMed1998

Tuning of a nonanalytical hierarchical control system for reaching with FES.

Point-to-point functional movements involve simultaneous shoulder and elbow joint rotations. In able-bodied subjects these movements are fully automatic, and feed-forward control ensures the synergistic activity of many muscles. Synergy between joint rotations was defined and described as a scaling between joint angular velocities [19]. Similarly, subjects who can control their shoulder movements may be assisted in reaching tasks by functional electrical stimulation (FES) of elbow extensor muscles. The synergistic control paradigm can be implemented in real-time by employing a hierarchically structured production-rules method. The use of production-rules necessitates the acquisition of knowledge and the assembly of a rule-base. A nonparametric technique was designed for the identification of the rules. The identification process was divided into two phases: determination of the scaling parameters, and determination of the stimulation parameters. The scaling parameters, needed for the coordination of movements, were determined in able-bodied subjects. Those depend exclusively on the initial and target positions of the hand. The number of scalings could be reduced by dividing the workspace into 12 zones. The stimulation parameters, needed for the execution of movements, were determined in subjects with paralyzed elbow extensor muscles by identifying triplets: elbow angular velocity, elbow angular acceleration (velocity increments), and the corresponding pulse durations for various classes of movements and loads attached to the hand.

Activities of Daily Living↗

PubMed1997

[Retroperitoneal marsupialization of renal cysts].

The use of minimal invasive surgery in urology continue to increase. Retroperitoneoscopic approach in performing minimal invasive surgery of retroperitoneum shortens the duration of operation in comparison with transabdominal approach, with minimal risk of intraabdominal complications. We described the use of the retroperitoneoscopic approach to the upper pole of a kidney for marsupialization of a symptomatic renal cyst. The procedure was minimally traumatic, morbidity was negligible and the patient was discharged from the hospital the third day after the operation. We believe that retroperitoneoscopic management of giant symptomatic renal cysts will be applicable, together with other existing methods.

Aged↗

PubMed1995

Optimal control for an above-knee prosthesis with two degrees of freedom.

Our previous research and clinical tests of a self-contained powered above-knee prosthesis (AKP) showed that a knee joint with one degree of freedom (DOF) increases the energy cost of walking with respect to able-bodied subjects. Better symmetry of the gait can improve performance, so we suggest here the integration of a second powered DOF into the knee joint mechanism to control the internal-external rotation of the shank-foot complex. The control for the AKP with two DOFs is based on a method of optimal tracking. The data used for analysis were collected in able-bodied subjects braced with an ankle splint to experimentally duplicate a gait like that of amputees using a two-DOF prosthesis. The simulation showed the following: (1) the technique of optimal programming can be used for simulation of the artificial leg during locomotion; (2) the optimal tracking method is an efficient tool for selection of actuators for the above-knee prosthesis, ensuring that the tracking remains within limits. Limitation of joint torque is desirable in order to reduce the size of the motor, but beyond a certain point limiting maximal torques lead to tracking errors that are associated with higher energy costs and hence the need for a larger power source. The errors are also associated with higher forces at the interface between the socket and the prosthesis. The optimal tracking method allows the optimization of tracking with constraints on the size of the motor used and its energy cost.

Adult↗

PubMed1995

Perspectives on the role of afferent signals in control of motor neuroprostheses.

K.O. Johnson reviews the architecture and low level neural mechanisms by which the external environment is transduced and encoded into the neural system, summarizing work that correlates neurophysiological and psychophysical testing with isolation of sensory components. The slowly adapting Type I afferent system is responsible for form and texture perception; the rapidly adapting afferent system is responsible for motion perception; and the Pacinian corpuscle system is responsible for vibratory sensation. R.R. Riso reviews the current level of understanding of the major factors to be considered in the design of a functional neuromuscular stimulation (FNS) grasp controller that uses cutaneous sensory feedback to detect slip. The elegant natural control scheme that matches the ratio of grip and lift forces to frictional conditions provides a model for implementing a slip-based control algorithm. D. Popović discusses the possible use of recordings from more proximal peripheral nerves to determine needed information for synthesis of locomotion. The discussion is illustrated with an animal model where rule-based closed-loop control is used for the ankle joint during treadmill locomotion. Neural signals from the tibial and superficial peroneal nerves were employed to substitute for missing afferent input from cutaneous and proprioceptive sensors. The feasibility of more invasive intraneural electrodes for distinguishing sensory from motor information in mixed nerves is considered. M. Koris raises surgical and functional issues relevant to developing clinical FNS systems. C. Van Doren suggests alternative neurophysiological and engineering approaches.

Afferent Pathways↗

PubMed1995

An EMG-controlled grasping system for tetraplegics.

A two-channel, portable, battery operated, functional electrical stimulation (FES) system with surface electrodes to enhance grasping in tetraplegics was developed and tested. This system is meant for tetraplegics capable of grasping by tenodesis. Candidates for this system must retain some wrist extension, and have paralyzed but innervated finger flexors, and nearly normal shoulder and elbow coordination within the working space. The control signal that turns the stimulation of forearm finger and thumb flexors on and off is based on the detection of the threshold of the amplified, rectified, and integrated electromyographic recordings using surface electrodes positioned over the wrist extensors. The voluntary contraction of wrist extensors is suitable for triggering the stimulation, and it is reproducible enough for daily home use. The new device was tested on subjects with tetraplegia, and the general conclusions are: 1) the system increases the strength of the grasp: 2) no side effects or related problems were noticed: 3) the training period is short: 4) the reliability of the operation is good; and, 5) the design of the analog part of the system allows its easy integration into a computerized device. Functional tests of the system showed that some of the study subjects did not benefit from this approach due to disuse and denervation types of muscle atrophy of their finger flexors, lack of controllable wrist extension, curled resting position of distal and proximal interphalangeal (IP) joints, and/or inability to bring the thumb in the opposition of fingers.

Activities of Daily Living↗

PubMed1993

[Attitudes of spouses toward examinations for the early detection of cervical and breast carcinoma].

Our research program included 3474 women from Novi Sad (1957) and villages (1517) in the territory of Vojvodina. The poll investigated especially the attitudes of husbands toward the participation of their wives in check-ups for the detection of cervical and breast neoplasms. Positive attitudes toward cervical check-ups were found in 750 males from the city (54.6%) and 697 (45.9%) from the villages which represents the majority compared to those who either don't care or have no opinion of their own. More authentic relations were obtained when "negative attitudes" included also males who didn't give any answer, in that case 1206 males from the city (61.6%) and 843 from the villages (55.6%) don't encourage their wives for the detection check-ups. Similar answers were obtained to the question about breast check-ups. Positive attitude was found in 747 (54.3%) males from the city and 602 (54.1) from the villages while the number of those who either don't care or have no attitudes is smaller (628--45.7% from the city and 510--45.9% from the villages). If "negative attitudes" include those who didn't answer the question (1217--62.2% from the city and 936--61.7% from the villages) husbands who don't encourage the check-ups represent the majority. We are sure that wives' attitudes toward detection check-ups are influenced by their husbands' attitudes. Our data show that the majority of husbands' do not encourage detection check-ups negatively influencing their wives' attitudes.(ABSTRACT TRUNCATED AT 250 WORDS)

Attitude↗

PubMed1992

Clinical characteristics of haemorrhagic fever with renal syndrome in children.

From January 1988 to September 1989, seven patients (4 girls and 3 boys, aged 3-12 years) with haemorrhagic fever with renal syndrome (HFRS) were hospitalised at the University Children's Hospital in Belgrade. In four patients the disease appeared as a family outbreak, the others were sporadic cases. In six patients the clinical presentation was suggestive of HFRS, as they had fever with headache, myalgia, sore throat and gastrointestinal illness followed by renal abnormalities. However, severe haemorrhagic syndrome with petechia, haematoma, haematemesis and melaena was present in one patient only. Renal disease presented as nephritic syndrome and/or acute renal failure. Five patients recovered after 2-3 weeks without sequellae, one patient had decreased renal function 17 months after the start of the disease and the remaining patient died. In six patients the diagnosis of HFRS was confirmed serologically by a significant rise in antibody titres against hantaviruses, while in the patient with the fetal and fulminant course of the disease, the diagnosis was established on the basis of epidemiological and autopsy findings. We suggest that children living in endemic areas who develop an ill-defined, febrile and gastrointestinal disease with renal dysfunction should be evaluated for HFRS.

Animals↗

PubMed1992

Microcomputer-based portable long-term spasticity recording system.

A device for long-term monitoring of muscle activity (EMG) with surface electrodes and method of its application are described in this paper. This device is called a microcomputer two-channel EMG monitor. The device can be used for up to 24 h monitoring of EMG activity, followed by data transfer to a host computer for signal analysis. This device records amplified, rectified, and integrated EMG activity. Shorter recording time allows shorter sampling periods suitable for different other EMG analysis. Recording of spontaneous EMG in complete spinal cord injured subjects was the original reason for the design of the long-term monitor. These recordings were used for estimation of spasticity in complete spinal cord patients.

Electromyography↗

PubMed1991

Optimal control for the active above-knee prosthesis.

Control of an active above-knee prosthesis has been simulated for a selected gait activity using a hierarchical closed-loop method. An extension of finite-state control, referred to as artificial reflex control, was adopted at the strategic level of control. At the actuator level of control an optimal tracking method, based on dynamic programming, is applied. This deals mainly with the actuator level of control, but considers the interaction of the leg dynamics and the switching effects of artificial reflex control. Optimal tracking at the actuator level of the above-knee prosthesis reduces the on-off effects of finite-state methods, such as artificial reflex control. The proposed method can also be used for the design of prosthetic elements. Specific attention is paid to the limited torque and power in the prosthetic joint actuator, which are imposed by the principle of self-containment in the artificial leg. The hierarchical structure, integrating artificial reflex control and optimal tracking, can be used in real time, as estimated from the number of computer operations required for the suggested method.

Artificial Limbs↗

PubMed1991

Medium- and long-term prognosis of patients with acute poststreptococcal glomerulonephritis.

The prognosis of acute poststreptococcal glomerulonephritis (APSGN) is still a matter of considerable debate. In an attempt to elucidate this controversy, the medium-term prognosis was evaluated in 40 patients 5-9 years after the onset of the disease, and the long-term prognosis in 88 patients 10-17 years after the onset of the disease. All were sporadic cases. In the medium-term follow-up study, abnormalities were revealed in 5.0% (2/40) of the patients. Hypertension and proteinuria were the only abnormalities detected. In the long-term follow-up study, abnormalities were revealed in 6.8% (6/88) of the patients. Hypertension was found in 3.4, proteinuria in 2.3, and microhaematuria in 2.3% of the patients. In both studies, all patients had normal creatinine clearance. We conclude that the medium- and long-term outcome of patients with APSGN is excellent.

Acute Disease↗

PubMed1991

[Attitude of an urban female population to detection of breast carcinoma].

An anonymous poll was conducted among Novi Sad women (957 employed and 1000 unemployed) in order to investigate the attitude of women towards breast cancer (BC). Preliminary research established the fact that the majority of women did not have their breasts examined in the last 3 years (64% employed and 60.7% unemployed). The consideration of the possibility of becoming affected by BC is rare or doesn't exist at all in 86.3% of women, therefore the very important emotional stimulus is absent. The general knowledge about BC is insufficient and 1395 women (69.4%) have no knowledge about the methods of breast examination or are very poorly informed, and that also regards the method of breast self-examination which is regularly used by only 9.9% of employed, and 13.3% of unemployed women. A part of the polled women does not want to see a physician out of various reasons: ignorance, indifference, cancerophobia or distrust of used methods, but a portion of the women wants to be examined but cannot make this happen because of the small number of institutions in which such examinations are conducted. Seeing a physician for breast examination is usually provoked by personal motivation, and the influence of the press, radio and TV is very small (1.7% with the employed and 3.1% with the unemployed), as well as the advise of girlfriends or the presence of BC in the family. Under all of these circumstances about 50% of the women are ready to pay all costs of breast examination regardless of the existing health insurance, and the rest of them do not accept this because of various reasons.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗