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Biomedical subjects

L Stark

Publications and source records attributed to L Stark.

At least 37 records · Page 2Linked to original sources
PubMed1987

Muscle models: what is gained and what is lost by varying model complexity.

Three structurally different types of models have evolved over the years to describe muscle-joint systems. The first, based on an input-output analysis of a given task, results in a simple second-order differential equation description that is adequate over a certain movement operating range. The second, based on the classic structural model of Hill (1938), results in a higher-order nonlinear model described by ordinary differential equations. The third, based on an analysis of the biophysical contractile mechanism, results in a complex partial differential equation description. The advantages and disadvantages of each type of model are considered, based on the criteria of identifying the simplest model that can adequately simulate any fundamental type of human movement without modifying model parameters for different tasks. It is shown that an eighth-order Hill-based antagonistic muscle-joint model is able to satisfy these criteria for a given joint if each of the four basic mechanically-significant non-linearities of the system are included in the model. This same model structure has been used successfully for eight different muscle-joint systems, ranging in size from knee flexion-extension to eye rotation--the only difference between the models is in the parameter values. Second-order models are shown to be task-specific special cases of the input-output behavior of the eighth-order model, while the more complex biophysical models are hypothesized to have insignificant advantages and many disadvantages over the Hill-based model during normal human movement.

Animals↗
PubMed1987

Late agonist activation burst (PC) required for optimal head movement: a simulation study.

Fast as possible (time optimal) single joint movements throughout the body are characterized by the triphasic (3 pulse) pattern of activation in the agonist and antagonist muscles. Simulation studies using a sixth order, non-linear model were undertaken to determine the relationship between time optimal movement and three pulse control. Exhaustive exploration of the multidimensional space formed by descriptive parameters of the control signal yielded control signals which drove the model to produce optimal movements. The result of these one to two week computer simulation runs was that if the limb is required to stay close to the target immediately after the end of the control signal, the fastest movements are produced by a three pulse control signal.

Biomechanical Phenomena↗
PubMed1987

Eye-head coordination with laterally "modulated" gaze field.

Discrimination of peripheral targets is possible with progressive lenses if visual strategy is changed. However, there is a penalty in response time. This penalty is reduced after adaptation. This adaptation involves making earlier and more rapid head movements on the one hand, and recalibrating the saccadic and vestibulo-ocular systems on the other hand. These changes enable subjects to gaze directly through a zone of the lenses where discrimination is possible. This adaptation also consists of learning to use information from blurred images. In this way visual processing can begin earlier.

Adaptation, Physiological↗
PubMed1986

An intrinsic mechanism for the oscillatory contraction of muscle.

A new model based on the theory of dynamical systems is proposed for the intrinsic random or systems is proposed for the intrinsic random or pseudo-random mechanism underlying certain types of muscular tremor. The active length-tension curve of the individual sarcomere, in conjunction with the passive length-tension relation is a map from length to tension with an observed time delay between length change and resulting tension change. The passive length tension relation is assumed to instantaneously relate this tension change back to a change in length. The stability properties of this iterated interval map are investigated by means of computer simulation and computation of the Lyapunov exponent and the bifurcation tree. The resulting analysis is related to experimental tremor data in the literature in terms of period doubling, bifurcation points, and "chaotic" behavior. The model appears to have its most fruitful application in understanding the insect type and isometric mammalian types of tremor.

Animals↗
PubMed1986

Dynamics of accommodation: measurements for clinical application.

Accommodation dynamics have not been used in clinical diagnosis as have eye movement and pupillary dynamics; the difficulty of clinical observation is matched by limitations in measurement methods. An instrument suitable for clinical use is described that allows measurement of step response latencies and especially time constants. With computer analysis, phase plane trajectories, and noise spectra can be quickly obtained also. The utility of these dynamical parameters for clinical diagnosis is illustrated by a study of changes of time constants with age in prepresbyopia.

Accommodation, Ocular↗
PubMed1985

Systems model for pupil size effect. II. Feedback model.

The human pupillary control system has been the subject of interest to biologists and engineers as an example of a sensorimotor reflex which can be embedded in a control system paradigm. We present a nonlinear feedback model whose compact structure allows us to hypothesize possible physiological mechanisms which generate the proper behavior of the pupil system. The important pupil responses, including pupil size effect, asymmetry, and response the high-frequency stimuli, are defined. This model was simulated on a digital computer and comparisons to the paradigm experimental responses were performed, demonstrating a fit to each of the observed conditions. Improvements on previous models are discussed.

Feedback↗
PubMed1985

Roles of the elements of the triphasic control signal.

In fast (time-optimal) movements about many joint systems, the triphasic EMG pattern has been observed. Although the first agonist burst obviously initiates the movement, the roles of the second and third bursts, appearing in the antagonist and agonist respectively, have been less clear. In this study, the timing of experimentally measured EMG signals led to construction of a three-pulse control signal that produced an accurate simulation of experimentally measured time-optimal head rotations using a sixth-order nonlinear model in conjunction with an optimization algorithm. By ablating pulses from the model control signal and observing the resulting dynamics, the roles of the three pulses can be assessed. As a result, the pulses can be designated PA, the action pulse (for the first agonist burst), PB, the braking pulse (for the antagonist burst), and PC, the clamping pulse (for the second agonist burst). Comparison of dynamic parameters from the simulated movements revealed strategies used to generate control signals for movements of various speeds.

Electromyography↗
PubMed1985

Effects of applied vibration on triphasic electromyographic patterns in neurologically ballistic head movements.

Vibration of agonist or antagonist muscle tendon produced changes in the triphasic electromyographic pattern of neck muscles; EMG signals were rectified, averaged, and also integrated by planimetry. The triphasic EMG envelopes obtained during fast horizontal head rotation showed unmodified early agonist pulse, the action pulse (PA), under all conditions; increased antagonist pulse, the braking pulse (PB), only with antagonist muscle vibration; and increase of late agonist pulse, the clamping pulse (PC), only with agonist muscle vibration. Vibration experiments can be considered as a model for studying interactions between central and peripheral effects on control of normal movements.

Electromyography↗
PubMed1985

Eye movements during reading: further case reports.

Eye movements during reading and nonreading tasks were measured in four patients having either Friedreich's ataxia, congenital dyslexia, acquired dyslexia with quadrantanopia, or acquired dyslexia with hemianopia. Abnormalities included saccadic intrusions, abnormally large dynamic overshoots, increased fixation duration and number of fixations, and/or reduced reading rate. The results clearly demonstrate the importance of objective eye movement recording for the precise specification of the abnormal reading pattern in patients with neurological dysfunctions.

Adult↗
PubMed1985

Paradoxic pupillary constriction in a patient with congenital stationary night blindness.

Paradoxic pupillary constriction to an "off-step" of light was studied in a 35-year-old woman with congenital stationary night blindness. ERG was of the Schubert-Bornschein type (scotopic B-wave absent); fundi were normal for a high myope. Paradoxic constrictions were larger in response to full field and peripheral off-steps from low photopic levels than from higher photopic levels. The patient's "on response" was smaller in magnitude and slower in both latency and dynamics than the on-response of normal subjects. Pupillary hippus was larger in magnitude and more peaked in the patient than in normal subjects. Steady-state (tonic) pupil size increased paradoxically with increased light level over the range 1-2 log fL.

Adult↗