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Biomedical subjects

I W Hunter

Publications and source records attributed to I W Hunter.

At least 55 records · Page 3Linked to original sources
PubMed1984

NEXUS: a computer language for physiological systems and signal analysis.

NEXUS is a general purpose interpretive computer language for the analysis of physiological systems and signals. Sophisticated analyses may be undertaken with minimal programming experience using a powerful set of system and signal analysis techniques embodied in the NEXUS operators. The language provides a syntax enabling various stages of an analysis to be performed interactively and/or automatically. NEXUS also provides graphics capabilities and extensive on-line help facilities.

Computers↗
PubMed1984

Induction of labor using high-dose or low-dose prostaglandin vaginal pessaries.

Two hundred women were studied in a randomized controlled trial of induction of labor using high-dose (3 mg) or low-dose (0.5 mg) prostaglandin E2 vaginal pessaries. Induction using 3-mg pessaries was more successful, with a shorter overall induction-delivery interval and less requirement for analgesia, although uterine hyperstimulation occurred in two patients. In contrast, the low-dose regimen did not cause hypertonus and, though less successful in inducing labor, the total dose of prostaglandin E2 used was considerably less than in the 3-mg group. The cesarean section rate when the cervix was initially unfavorable was high in both groups (high dose 18.2%, low dose 16.7%). The response to prostaglandin E2 did not relate closely to the initial cervical state.

Cesarean Section↗
PubMed1983

System identification of human triceps surae stretch reflex dynamics.

The interpretation of stretch-evoked reflex responses is complicated by the fact that the pattern of response will depend upon both the underlying reflex mechanisms and the time course of the stretch used to evoke the response. The objective of the present study was to use engineering systems analysis techniques to identify the dynamics of the human triceps surae (TS) stretch reflex in terms of its impulse response by deconvolving the position input from the observed response. Five normal subjects were instructed to maintain a tonic contraction of (TS) while subjected to repeated, computer-generated, stochastic perturbations of ankle position. Position, torque and smoothed, rectified surface EMGs were recorded and ensemble averaged over 25 stimulus presentations. Linear impulse response functions describing the dynamic relation between ankle velocity and TS EMG were found to account for a significant amount of the observed EMG variance (mean 60%). However, the impulse responses were noisy and the predicted EMG was systematically smaller than the observed EMG during the dorsiflexing phases of displacement. These findings suggested that a direction dependent nonlinearity might be present. Consequently, impulse responses relating half-wave rectified velocity to TS EMG were computed and found to be less noisy and to account for significantly more variance (mean 74%) than the purely linear model. The undirectional, velocity-sensitive impulse response functions were dominated by a large peak at about 40 ms followed by a smaller period of reduced activity. This is consistent with its mediation by primary spindle afferents. Although the shape of the impulse response remained unchanged, its amplitude, which provides a measure of relative gain, varied systematically with the level of contraction and the displacement amplitude. Multiple regression analysis demonstrated that most of the variation in the impulse response amplitude could be attributed to proportional increases with level of contraction (measured by average EMG) and proportional decreases with displacement amplitude.

Adult↗
PubMed1983

Generation of random sequences with jointly specified probability density and autocorrelation functions.

A new method is presented for the generation of stochastic (random) sequences with an arbitrarily specified first-order probability distribution function (PDF) and an arbitrarily specified first-order auto-correlation function (ACF). A set of numbers with the desired PDF are first generated. These are then given a white (independent) ACF by double stochastic interchange. The desired ACF is then obtained by stochastically shuffling the series to minimize a sum of squares criterion between desired and actual ACFs. The technique is particularly useful for generating experimental stimuli for system identification, sequences for numerical stimulation, and test series for evaluating signal processing algorithms when colored (non-white) non-Gaussian data are required.

Mathematics↗
PubMed1983

Force and EMG correlates of constant effort contractions.

The relation between the force exerted by the left arm and the electromyographic (EMG) activity of the brachial biceps and triceps muscles was examined during constant effort contractions maintained for 120 s. The initial force levels were set at 35%, 50% and 65% of each subject's maximal strength, but thereafter no feedback was provided. In contrast to previous results it was found that the direction of the change in force and EMG during constant effort contractions was dependent on the level of force initially exerted. During the lowest initial force contraction the force remained constant, while for the other two force levels there was an exponential decline in the force exerted. These changes in force during the three contractions were well described by an exponential equation with two free parameters. The EMG also varied as a function of initial force. For the higher two forces the amplitude of the EMG fluctuated during the first 40 s but thereafter remained constant, while it increased steadily during the lowest initial force contraction. These results suggest that depending upon the initial level of exertion either peripheral sensory cues relating to the actual force exerted, or centrally-generated signals reflecting the magnitude of the descending motor command may be used by subjects to maintain a constant level of muscular effort.

Adult↗
PubMed1983

Effect of fatigue on force sensation.

Recent experiments have suggested that a sense of effort can be separated from a sense of developed force or tension in muscular contractions. The evidence for this distinction was examined during submaximal fatiguing contractions. Subjects were required to maintain until maximal endurance a constant isometric force with their right, reference arm, and at 15-s intervals they estimated the magnitude of this force with a matching contraction of the contralateral arm. The matching force produced by the unfatigued limb was the measure of force sensation. Both force and the brachial biceps and triceps EMG were recorded from each arm. During the fatiguing contractions the matching force increased linearly as did the biceps EMG of the fatiguing muscle. The rate of increase was dependent on the level of force exerted. A linear relation between the reference arm EMG and the perceived force was observed, which suggested that the over-estimation of force was due to the increase in the excitatory input to the fatiguing muscle. These results provide support for a centrally mediated theory of force perception, and indicate that during fatigue subjects are unable to estimate accurately the force of contraction. Furthermore, they suggest that under those conditions a sense of tension is not distinguishable from a sense of effort.

Adult↗
PubMed1983

Invariance of ankle dynamic stiffness during fatiguing muscle contractions.

Our objective was to determine the effect of muscle fatigue on the dynamic stiffness of the human ankle. Four subjects were required to maintain constant-force contractions of tibialis anterior until the required force could no longer be maintained. Repeated pseudo-random displacements of ankle angular position were applied throughout each contraction. The dynamic relation between ankle angular position and ankle torque was identified by determining non-parametric compliance impulse response functions (CIRFs). The CIRFs were redetermined every 2.55S throughout the sustained contractions to provide a quantitative measure of changes in ankle stiffness dynamics. Inspection of these CIRFS revealed little change in shape or magnitude throughout the contractions, despite large increases in tibialis anterior EMG. The dynamics were further quantified by estimating the equivalent joint inertia, viscosity and elasticity associated with each CIRF. As each contraction progressed, the inertial and elastic terms remained constant whereas the viscous term decreased slightly. These findings demonstrate that fatigue of tibialis anterior during sustained constant mean force contractions results in little change in the mechanical dynamics of the human ankle.

Adult↗
PubMed1982

Force sensation in isometric contractions: a relative force effect.

A contralateral limb matching paradigm was employed to investigate the perception of isometric forces. Both force and brachial biceps and triceps EMG were recorded from each arm as subjects matched isometric contractions on the basis of equal sensation. It was found that smaller forces were consistently overestimated in magnitude and that the most accurate estimation of force occurred around the middle of the force domain. It is proposed that these results reflect the properties of the biceps muscle which is predominantly involved in gross motor activity.

Arm↗
PubMed1982

FILELB: a data file structure for the analysis of physiological systems.

FILELB is a comprehensive library of subroutines for the storage, retrieval, location, inspection and entry of sequences of experimental and analytical data. The general purpose, highly organised data file structure implemented by FILELB is the core of our interactive computer package for the analysis of physiological systems.

Data Display↗
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↗