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Biomedical subjects

R E Kearney

Publications and source records attributed to R E Kearney.

At least 37 records · Page 2Linked to original sources
PubMed1995

Nonlinear behavior of muscle reflexes at the human ankle joint.

1. Pulse inputs (similar to tendon jerks) were applied to the human ankle joint with the use of a hydraulic actuator. Inputs of only 1-2 degrees could elicit large responses (> 20% of maximum voluntary contraction). The magnitude of the response depended nonlinearly on a number of factors: the amplitude, direction, and duration of the pulse; the angle of the ankle; and the level of voluntary activation of the ankle muscles. 2. Pulses that flexed or extended the ankle could both produce reflex torques in the same direction (extensor torque). Although an extension of the ankle did not itself produce a response, it could affect the response to a subsequent flexion for up to 1 s. 3. The influence of random perturbations on the stretch reflex at the ankle was assessed. Responses to pulse displacements alone and to pulses superimposed on random perturbations were compared at the same level of voluntary activity. Reflex responses decreased in a graded manner with increasing amplitude or bandwidth of the random perturbations. 4. These results demonstrate that stretch reflexes can generate substantial torques, but in a highly nonlinear manner. In particular, passive joint movements markedly alter stretch reflex gain, and these changes must be considered in interpreting the functional significance of reflex actions.

Ankle↗
PubMed1993

Identification of time-varying dynamics of the human triceps surae stretch reflex. I. Rapid isometric contraction.

We have examined the time variations of stretch reflex dynamics throughout rapid voluntary changes in the isometric contraction level of the human triceps surae muscles. This was achieved by superimposing a small stochastic displacement upon many such changing contractions and then identifying the time-varying relationship between the perturbation and the evoked electromyograms (EMGs). An "ensemble" time-varying system identification technique was used to estimate these input-output dynamics as a set of impulse response functions, one for each time before, during, and after the change in contraction level, with a temporal resolution equal to the data acquisition rate. Three main findings resulted. First, stretch reflex gain (relating joint velocity to EMG) was significantly modulated during changes in voluntary contraction level, increasing as the subject contracted the muscles and decreasing as the subject relaxed. Second, stretch reflex dynamics did not change with contraction level, even when its gain varied substantially. Third, the time course of the gain changes closely followed the level of the EMG, even though the subjects used rather different activation and deactivation patterns. These results suggest that, for the behavior studied (i.e., rapid changes in isometric contraction level), stretch reflex gain and motoneuron pool activation level were controlled by a common descending command rather than being independently specified.

Adult↗
PubMed1993

Identification of time-varying dynamics of the human triceps surae stretch reflex. II. Rapid imposed movement.

We examined the time-varying dynamics of the human triceps surae stretch reflex before, during, and after a large stretch was imposed upon the ankle joint, during a constant voluntary contraction of 15% of maximum voluntary contraction. Stretch reflex dynamics were estimated by superimposing a small stochastic displacement on many such stretches and using an "ensemble-based" time-varying identification procedure to compute impulse response functions relating the perturbation to the evoked electromyogram (EMG) at each point throughout the task. We found that stretch reflex magnitude (relating joint velocity to EMG) varied directly with baseline EMG activity during steady-state conditions before and after the large imposed stretch. Following the large stretch and the reflex activity it evoked, both background EMG and stretch reflex magnitude declined for up to 100 ms; changes in the stretch reflex were substantially greater in magnitude and followed a different time course from the corresponding changes in background EMG, however, indicating that stretch reflex properties were modulated independently of motoneuron pool activation level. Based on timing and the invariance of stretch reflex dynamics across time, it is argued that this behavior is largely mediated via peripheral neural mechanisms. This peripheral modulation of the stretch reflex presumably supplements various descending influences to adjust reflex properties.

Adult↗
PubMed1993

Nonparametric two-dimensional point spread function estimation for biomedical imaging.

The problem of identifying optical system point spread functions (PSFs) arises frequently in the area of image processing and restoration. The paper presents a method for determining two-dimensional PSFs from input/output image signals. The PSF of the system is determined from a set of linear equations involving elements of the input autocorrelation function and the input/output cross-correlation function. The resulting PSF is the one that minimises the sum of squares difference between the actual output image and the predicted one.

Diagnostic Imaging↗
PubMed1993

Quantitative analysis of four EMG amplifiers.

Four typical EMG amplifiers were tested quantitatively to observe the diversity and specificity of available equipment. Gain, phase, common mode rejection ratio (CMRR) and noise characteristics were measured for each device. Various gain and phase responses were observed, each best suited to specific application areas. For all amplifiers, the CMRR was shown to decrease dramatically in the presence of input impedance mismatches of more than 10 k omega between the two electrodes. Because such impedance mismatches are common on the skin surface, these results indicate that proper skin preparation is required to maximize the noise rejection capabilities of the tested amplifiers.

Amplifiers, Electronic↗
PubMed1992

A new algorithm for the identification of multiple input Wiener systems.

Multiple-input Wiener systems consist of two or more linear dynamic elements, whose outputs are transformed by a multiple-input static non-linearity. Korenberg (1985) demonstrated that the linear elements of these systems can be estimated using either a first order input-output cross-covariance or a slice of the second, or higher, order input-output cross-covariance function. Korenberg's work used a multiple input LNL structure, in which the output of the static nonlinearity was then filtered by a linear dynamic system. In this paper we show that by restricting our study to the slightly simpler Wiener structure, it is possible to improve the linear subsystem estimates obtained from the measured cross-covariance functions. Three algorithms, which taken together can identify any multiple-input Wiener system, have been developed. We present the theory underlying these algorithms and detail their implementation. Simulation results are then presented which demonstrate that the algorithms are robust in the presence of output noise, and provide good estimates of the system dynamics under a wide set of conditions.

Algorithms↗
PubMed1992

Identification of time-varying biological systems from ensemble data.

The theory underlying a new method for the identification of time-varying systems is described. The method uses singular value decomposition to obtain least-squares estimates of time-varying impulse response functions from an ensemble of input-output realizations. No a priori assumptions regarding the system structure or form of the time-variation are required and there are few restrictions on the input signal. Simulation studies, using a model of time-varying joint dynamics, show that the method can track rapid changes in system dynamics accurately and is robust in the presence of output noise. An application of the method is demonstrated by using it to track dynamic ankle stiffness during a rapid, voluntary, isometric contraction. During the transient phase of the contraction, low-frequency ankle stiffness gain decreased in a manner which could not be described with the second-order model of joint dynamics often used under stationary conditions.

Adult↗
PubMed1991

Prospective study of a prosthetic H-graft portacaval shunt.

This study was undertaken to prospectively evaluate the 8-mm Gore-Tex interposition H-graft portacaval shunt. Thirty-six high-risk patients at the University of South Florida-affiliated hospitals received small-diameter shunts because of bleeding esophagogastric varices over a recent 2-year period. Portal vein and portal vein-inferior vena cava gradients were significantly reduced after shunting. These pressure changes were manifested clinically by the absence of variceal rebleeding and improvement of ascites; in addition, the incidence of encephalopathy was low. The 8-mm graft maintained hepatopedal flow in 67% of the patients, but reversal of flow did not result in complications commonly associated with poor portal perfusion. Graft thrombosis occurred in four (11%) patients. All grafts were successfully revised, three by operative revision and one by an interventional radiologist. Operative mortality was low (11%), and morbidity was unusual. The small-diameter H-graft portacaval shunt is a safe and effective method of treatment for bleeding esophagogastric varices.

Adult↗
PubMed1991

Diagnostic peritoneal lavage. Limited indications due to evolving concepts in trauma care.

This study was undertaken to determine the appropriateness of celiotomy in 100 consecutive patients who underwent celiotomy solely because of positive diagnostic peritoneal lavage (DPL) following blunt (B) or stab (S) abdominal trauma. A total of 32 (32%) patients had positive DPL by laboratory criteria: blunt trauma: greater than 100K RBC/mm3, greater than 500 WBC/mm3; stab trauma: greater than 50K RBC/mm3, greater than 250 WBC/mm3. DPL in 68 patients was positive by gross inspection; 18 of these 68 patients' DPL laboratory results returned after surgery and did not satisfy the laboratory definition of positive DPL. In all 61 per cent underwent therapeutic celiotomy (TC) and 39 per cent underwent nontherapeutic celiotomy (NTC). Grade I and II spleen and/or liver injuries led to 79 per cent of NTCs. Positive DPL, determined by gross inspection or by laboratory testing, has a very poor accuracy rate when evaluated in light of evolving beliefs that promote nonoperative therapy for grade I and II liver and spleen injuries. When positive DPL is the sole indication for celiotomy in patients with blunt or stab abdominal trauma, an unacceptably large number of NTCs will be performed. DPL should have a limited role in the evaluation of patients with abdominal trauma.

Abdominal Injuries↗
PubMed1990

Treatment of candidosis in severely injured adults with short-course, low-dose amphotericin B.

Thirty-three (0.7%) of 4,818 trauma patients admitted between January 1, 1987, and July 1, 1989, developed invasive candidosis requiring IV antifungal therapy. All patients were seriously traumatized. Before developing candidosis, all patients had documented bacterial infections. These infections were generally polymicrobial and were treated with multiple broad-spectrum antibiotics (an average of 5.4 antibiotics for 17.2 days). Twenty-eight (85%) of 33 patients received enteral feedings for an average of 11 days +/- 1.5 (SEM) before developing candidosis and 24 (73%) received NG/oral nystatin for an average of 7.6 days +/- 0.9 before developing candidosis. All patients with candidosis were treated with intravenous amphotericin B: cumulative dose of 157.3 mg +/- 31.3 mg given over 10 days +/- 1.1. One patient developed recurrent candidosis despite NG/oral prophylaxis and enteral feedings. Six patients (18%) died due to sepsis and multiple organ failure. The patients who died did not objectively differ from the survivors. Candidosis is an infrequent infection in severely injured patients. Candidosis was invariably preceded by treatment with multiple broad-spectrum antibiotics for a variety of polymicrobial bacterial infections. NG/oral nystatin and enteral feedings did not prevent candidosis, in contrast to widely accepted beliefs. Amphotericin B therapy was safe. Recurrent candidosis was unusual. Candida infections had a high mortality rate associated with multiple blood transfusions and prolonged hospitalization. Candidosis represents a sign of severe injury and illness but can be amenable to prompt, aggressive treatment.

Adult↗
PubMed1990

System identification of human joint dynamics.

The dynamics of joint mechanics are a fundamental characteristic of the motor system. They determine the displacements evoked by perturbing forces during postural control and the forces that must be generated to perform a voluntary movement. This article reviews experimental studies of these dynamics, with an emphasis on the behavior of single joints in alert humans. Technical aspects of the experimental and analytic methods that have been used are summarized first. Major results obtained with the different methods are then reviewed, compared, and contrasted. The interpretation of these results in terms of the underlying physiological mechanisms is then considered, with an emphasis on the relative contributions of passive properties of tissue, the mechanical behavior of muscle, and stretch reflexes. Finally, important unanswered questions regarding the dynamics of joint mechanics are reviewed.

Biomechanical Phenomena↗
PubMed1989

Method for EMG conduction velocity estimation which accounts for input and output noise.

A new technique is derived for estimating the conduction velocity of muscle fibre action potentials from surface EMG recordings in humans. The method is based on system identification and extends previous such methods by explicitly accounting for additive noise at the output as well as at the input. Experimental and simulation results demonstrate that at moderate noise levels the new method yields estimates comparable to those obtained with previous methods. However, simulation studies show that the new method should be particularly valuable at high noise levels, where it should provide much better estimates than previous techniques provided that an adequate representation of the noise is available.

Action Potentials↗
PubMed1989

Outcome of treatment of combined orthopedic and arterial trauma to the lower extremity.

To ascertain morbidity and disability associated with vigorous attempts at limb salvage after lower extremity combined orthopedic and arterial injuries we reviewed more than 10,000 trauma admissions. Twenty-two patients (0.2%) suffered 14 blunt and eight penetrating lower extremity orthoarterial injuries. In all, more than 90 extremity operations were performed within 30 days of admission. The mechanism of injury, ISS, sequence of orthopedic and vascular procedures, use of temporary arterial shunts, the nature of the arterial reconstruction, length of ischemic time, and the presence of open fractures did not affect limb salvage or outcome. The presence of neurosensory/motor impairment and/or serious soft-tissue loss and injuries at or distal to the popliteal artery were associated with a high frequency of disability and amputation. Tremendous effort was expended to achieve limb salvage in the patients of this review. Patients who underwent amputation had the more uneventful hospitalizations and the lower rates of disability. The notion that limb salvage need to be obtained in all patients at all costs may often lead to the triumph of technique and technology over reason.

Adult↗
PubMed1988

Nonlinear identification of stretch reflex dynamics.

The objective of this study was to use nonlinear identification techniques to study the dynamics of stretch reflexes in the human calf muscles (gastrocnemius-soleus). Stochastic perturbations of ankle position were applied while subjects maintained a constant, tonic contraction of gastrocnemius-soleus. Linear models of the relation between ankle velocity and the electromyographic (EMG) activity under these conditions typically accounted for less than 40% of the observed EMG variance. Nonlinear system identification techniques were then applied. The first- and second-order Wiener kernels were computed as the initial stage of this analysis. These did not provide an adequate description of system behavior; subsequent simulation studies showed that the major problem with the Wiener analysis was that the input spectrum was not adequately white. Nevertheless, the shape of the second-order Wiener kernel suggested that a Hammerstein structure consisting of a static nonlinearity followed by a dynamic linear system would be appropriate. Consequently, we used an iterative procedure for Hammerstein system identification to determine the form of the static nonlinearity and the associated linear dynamics. The resulting nonlinear model provided a much better description of the system's behavior than did the linear models (variance accounted for greater than 60%). Furthermore, they confirmed our previous empirical findings; the static nonlinearity closely resembled a half-wave rectifier while the dynamics were typified by a pure delay and a velocity filter. The application of nonlinear identification techniques thus produced a much improved, physically meaningful model of stretch reflex behavior.

Electromyography↗
PubMed1988

Human ankle joint stiffness over the full range of muscle activation levels.

System identification techniques have been used to track changes in dynamic stiffness of the human ankle joint over a wide range of muscle contraction levels. Subjects lay supine on an experimental table with their left foot encased in a rigid, low-inertia cast which was fixed to an electro-hydraulic actuator operating as a position servo. Subjects generated tonic plantarflexor or dorsiflexor torques of different magnitudes ranging from rest to maximum voluntary contractions (MVC) during repeated presentations of a stochastic ankle angular position perturbation. Compliance impulse response functions (IRF) were determined from every 2.5 s perturbation sequence. The gain (G), natural frequency (omega n), and damping (zeta) parameters of the second-order model providing the best fit to each IRF were determined and used to compute the corresponding inertial (I), viscous (B) and elastic (K) stiffness parameters. The behaviour of these parameters with mean torque was found to follow two simple rules. First, the elastic parameter (K) increased in proportion to mean ankle torque as it was varied from rest to MVC; these changes were considerable involving increases of more than an order of magnitude. Second, the damping parameter (zeta) remained almost invariant over the entire range of contractions despite the dramatic changes in K.

Adult↗