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R E Kearney

Publications and source records attributed to R E Kearney.

69 records · Page 4Linked to original sources
PubMed1982

Dynamics of human ankle stiffness: variation with mean ankle torque.

The left foot of five human subjects was rotated in a fixed stochastic pattern about a constant ankle angle and the forces opposing these perturbations were measured. The dynamic stiffness transfer functions relating ankle angular position to ankle torque were calculated. Stiffness gain was flat at low frequencies, had a resonant valley at intermediate frequencies and rose at about 40 dB/decade at high frequencies. The low frequency gain and resonant frequency increased progressively with increases in tonic muscular activity. The dynamic stiffness of the ankle was well described by a linear, under-damped, second-order transfer function having inertial, viscous and elastic terms. Estimates of the inertial parameter were independent of the level of muscle activity whereas the viscous and elastic parameters increased with increases in mean torque level.

Adult↗
PubMed1982

Dynamics of human ankle stiffness: variation with displacement amplitude.

The left foot of five normal human subjects was rotated in a fixed stochastic pattern about a constant ankle angle and the torques opposing these rotations were measured. The dynamic stiffness transfer functions relating ankle angular position to ankle torque were calculated. Stiffness gain was flat at low frequencies, had a resonant valley at intermediate frequencies, and rose to about 40 dB/decade at high frequencies. The mean ankle torque was held constant and the peak-to-peak amplitude of the displacement was varied. The low frequency gain and resonant frequency decreased progressively with increases in the peak-to-peak amplitude of the displacement. The dynamic stiffness was well described by a linear, second-order transfer function having inertial, viscous and elastic terms. Estimates of the inertial parameter were independent of the displacement amplitude but the viscous and elastic parameters decreased with increases in displacement amplitude.

Adult↗
PubMed1981

Respiratory components of human postural sway.

The effects of respiration on postural sway in normal human subjects was studied using averaging and spectral techniques. During voluntarily paced respiration a substantial proportion of sway was found to result from respiratory activity. The magnitude of the respiratory contribution to sway was approximately constant over the normal range of respiratory rates and linearly related to respiration amplitude. Respiration evoked similar changes in reaction torque in seated subjects. These results indicate that body movements associated with paced respiration are an important disturbance to the postural control system. Furthermore, these disturbances are not reduced by compensatory movements of the hip as has been suggested by others.

Humans↗
PubMed1981

Interlimb reflexes evoked in human arm muscles by ankle displacement.

Interlimb reflexes evoked by ankle displacements were studied in arm muscles of 6 normal subjects. EMGs from grastrocnemius (G), tibialis anterior (TA), biceps brachii (BB), and triceps brachii (TB) were amplified, rectified and low-pass filtered before recording. Averaging and Wiener filtering were used to detect changes in tonic EMG activity evoked by dorsiflexing or plantarflexing displacements of the ankle. A consistent pattern of response was observed in all subjects. In the leg muscles, the responses to stretch were consistent with previous reports. In the arm muscles, the response of TB was dominant. Dorsiflexing displacements of the ankle evoked a small excitation followed by a more marked decrease in TB activity but had no effect on BB. In contrast, plantarflexing displacements of the ankle resulted in a large, early period of excitation followed by a decreased level of activity in TB. A similar but smaller pattern of activity was observed in BB. It is notable that the TB responses to displacement were sizable, often modulating the tonic EMG activity by as much as 80%. Interlimb reflexes evoked by ankle displacement were larger and of shorter latency than those evoked by cutaneous electrical stimulation of the foot reported previously. This suggests that proprioceptive afferents may have stronger and more direct interlimb reflex connections than cutaneous afferents and may therefore play an important role in the coordination of movement.

Adult↗
PubMed1979

Influence of midbrain stimulation on the excitability of neurons in the medial hypothalamus of the rat.

Extracellular action potentials were recorded from 1098 neurons in the medial hypothalamus of pentobarbital anesthetized male rats. Their excitability was analyzed after single 1 Hz stimulation of the midbrain periaqueductal gray (PAG) or adjacent reticular formation. Cells were also examined for their response to median eminence (ME), amygdala, lateral septum (LS) or anterior hypothalamic/preoptic area (AHA/POA) stimulation. Antidromic invasion from midbrain stimulation was recorded from 110 neurons. Eight of these neurons showed features of axon branching and displayed antidromic invasion from both midbrain and amygdala (2 cells) or AHA/POA (6 cells). Many neurons with midbrain projections displayed orthodromic responses to stimulation in the amygdala, but few responded to AHA/POA or LS stimulation. Midbrain stimulation evoked orthodromic responses from 99 medial hypothalamic neurons. Many of these cells also displayed orthodromic responses to amygdala or AHA/POA stimulation, whereas a small number were activated antidromically by stimulation in these sites. None of 42 neurons activated antidromically from median eminence stimulation were responsive to midbrain stimulation. These results provide electrophysiological evidence of reciprocal connections between medial hypothalamic and medial midbrain areas, and indicate that medial hypothalamic neurons with midbrain connections are subject to influences from other extrahypothalamic areas.

Action Potentials↗
PubMed1979

Tibialis anterior response to sudden ankle displacements in normal and Parkinsonian subjects.

It is well known that in Parkinsonian subjects with akinesia, reaction times are increased but reflex latencies remain normal. We have attempted to use this knowledge to distinguish between 'reflex' and 'voluntary' components of the electromyographic (EMG) response to ankle displacement. The EMG and torque responses of tibialis anterior (TA) to randomly applied servo-controlled plantar-flexing displacements of the ankle with and without the subject's intentional opposition were examined in 9 Parkinsonian and 9 age-matched normal humans. To obtain a measure of akinesia, the response latency to a visual stimulus was subsequently measured in the same subjects. Three principal findings emerged. (1) The intermediate latency EMG component (PSR) of the response evoked by ankle displacement with the subject instructed to relax was more regularly evoked and of lower threshold in Parkinsonians than in normals. This finding corresponds to the enlarged M2 component in upper limb muscles. However, the facilitation of PSR was not found to be associated with an increase in torque. In fact, the patients did not exhibit more stiffness than normals under our experimental conditions. (2) Mean latency estimate of the PSR was indistinguishable between Parkinsonians and normals. This finding puts the PSR in the nature of a reflex. Indeed, in accordance with reflex behaviour which is proportional to input characteristics, its area increased linearly with increase in the magnitude of displacement velocity. (3) In contrast, the 'late' EMG response (FSR) evoked by opposing sudden ankle displacement exhibited a significantly longer latency in 6 out of 8 Parkinsonians than normals. In the same patients, the EMG response latency to a visual signal was similarly increased. The delay of FSR in akinesia patients thus argued against its being a stereotyped reflex. The result is discussed with reference to the recent finding that preprogrammed responses are delayed in Parkinsonians.

Adult↗
PubMed1979

Evaluation of the Wiener filter applied to evoked EMG potentials.

The application of the Wiener filter to the estimation of responses evoked in the tonic EMG activity of tibialis anterior by cutaneous electrical stimulation of the foot is described. The effectiveness of the filter is assessed using both experimental and simulated data by computing the mean square error between the actual evoked potential and ensemble average and Wiener filter estimates of it. The Wiener filter is shown to provide a better estimate of the final response than simple ensemble averaging. The improvement is most marked in cases where the signal-to-noise ratio is small, but the Wiener filter estimate is always better than the ensemble average estimate. It is concluded that Wiener filtering significantly reduces the number of responses required to obtain a good estimate of the evoked potential in this experimental situation.

Computers↗
PubMed1978

Conduction of nervous impulses in spinal roots and peripheral nerves of dystrophic mice.

Conduction was studied in the sacral ventral roots and ventral tail nerves of dystrophic mice (dy/dy) and phenotypically normal littermates. In myelinated ventral root fibers of normal mice, conduction velocity was uniform with internodal conduction time 45 +/- 5 musec (26 degrees C). In ventral root fibers of dystrophic mice, conduction velocity was decreased and strikingly non-uniform; both saltatory and continuous conduction were observed in different portions of the same nerve fiber. Continuous conduction with velocity less than 2 m/sec (26 degrees C) was characteristically observed in mid-root where the axons are bare; conduction was saltatory close to the exit from the spinal canal and near the spinal cord where the axons are myelinated. Maximum conduction velocity in ventral tail nerves was 21 +/- 3 m/sec for dystrophic mice and 31 +/- 4 m/sec for littermate controls (37 degrees C). Internodal lengths were somewhat decreased in the dystrophic peripheral nerves but there was no significant difference in maximum fiber diameters, myelin thickness or nodal morphology between dystrophic and normal nerves.

Animals↗
PubMedDate not supplied

Generalized eigenvector algorithm for nonlinear system identification with non-white inputs.

Traditional methods for nonlinear system identification require a white, Gaussian, test input, a restriction that has limited their usability in many fields. In this study, we address the problem of identifying the dynamics of a nonlinear system when the input is highly colored-a restriction commonly encountered in the study of physiological systems. An extension of the parallel cascade method is developed that is optimal in a constrained minimum mean squared error sense and exactly corrects for the distortion induced by the non-white input spectrum. However, this correction is a deconvolution, which may become extremely ill-conditioned if the input spectrum departs significantly from whiteness; to confront this, we develop a low-rank projection operation that stabilizes the deconvolution. The overall algorithm is robust and places few requirements on the nature of the test input. Practical application of this new method is demonstrated by using it to identify a known analog nonlinear system from experimental data.

Algorithms↗
PubMedDate not supplied

Nonparametric block-structured modeling of lung tissue strip mechanics.

Very large amplitude pseudorandom uniaxial perturbations containing frequencies between 0.125 and 12.5 Hz were applied to five dog lung tissue strips. Three different nonlinear block-structured models in nonparametric form were fit to the data. These models consisted of (1) a static nonlinear block followed by a dynamic linear block (Hammerstein model); (2) the same blocks in reverse order (Wiener model); and (3) the blocks in parallel (parallel model). Both the Hammerstein and Wiener models performed well for a given input perturbation, each accounting for greater than 99% of the measured stress signal variance. However, the Wiener and parallel model parameters showed some dependence on the strain amplitude and the mean stress. In contrast, a single Hammerstein model accounted for the data at all strain amplitudes and operating stresses. A Hammerstein model featuring a fifth-order polynomial static nonlinearity and a linear impulse response function of 1 s duration accounted for the most output variance (99.84%+/-0.13%, mean+/-standard deviations for perturbations of 50% strain at 1.5 kPa stress). The static nonlinear behavior of the Hammerstein model also matched the quasistatic stress-strain behavior obtained at the same strain amplitude and operating stress. These results show that the static nonlinear behavior of the dog lung tissue strip is separable from its linear dynamic behavior.

Animals↗
PubMedDate not supplied

An adaptive filter to reduce cardiogenic oscillations on esophageal pressure signals.

Measurements of pressure swings in the esophagus (Pes) can be used to estimate variables of clinical importance, e.g., intrinsic positive end-expiratory pressure (PEEPi). Unfortunately, cardiogenic oscillations frequently corrupt Pes and complicate further analysis. Due to significant band overlap with the respiratory component of Pes, cardiogenic oscillations cannot be suppressed adequately using standard filtering techniques. In this article, we present an adaptive filter that employs the electrocardiogram to identify and suppress the cardiogenic oscillations. This filter was tested using simulated data, where the variance accounted for relative to the simulated respiratory pressure swings increased from as low as 55% for the unfiltered Pes signal to over 95% when the adaptive filter was used. In patient data, the adaptive filter reduced the apparent cardiogenic oscillations without noticeably distorting the sharp deflections in Pes due to respiration. Furthermore, the filter suppressed peaks in the Fourier transform of Pes at integer multiples of the heart rate, while the remaining frequencies remained largely unchanged. During stable breathing, the standard deviation of PEEPi was reduced by between 44% and 71% in these four patients when the filter was used. We conclude that our filter removes a significant fraction of the cardiogenic oscillations that corrupt records of Pes.

Biomedical Engineering↗