Search PubMed⌕ Search

Biomedical subjects

I W Hunter

Publications and source records attributed to I W Hunter.

At least 37 records · Page 2Linked to original sources
PubMed1990

Influence of the mechanical properties of a manipulandum on human operator dynamics. 1. Elastic stiffness.

An active servo-system was used to change the stiffness of a manipulandum used in a position-control pursuit-tracking task. The elastic stiffness of the manipulandum connected to the forearm was set by a computer at one of five levels ranging from 0 N/m to 2000 N/m. Subjects were required to track, either by moving their forearm or by generating a force isometrically, a visually presented target whose position changed randomly every second for 100 s. Nonparametric and parametric impulse response functions were calculated between the input (target) and output (force or position) in each tracking condition, and revealed that for all subjects force control was faster than position control when the stiffness of the manipulandum was set at 0 N/m. Subjects were also consistently faster in reaching the target when the stiffness was greater than zero, and were more accurate (steady-state response) when the stiffness of the manipulandum was set at lower rather than higher amplitudes. The parametric impulse response functions revealed that the human operator system was underdamped (0.7) with a natural frequency of approximately 8 rad/s. These findings were interpreted in terms of the responses of the various subsystems (visual, cognitive, contractile, limb mechanics) that comprise the human operator's response.

Adult↗
PubMed1990

A perceptual analysis of stiffness.

The perception of stiffness was studied in ten human subject using two servo-controlled electromagnetic linear motors with computer-controlled stiffness, one motor coupled to each wrist of the subject. Using the contralateral limb-matching procedure in which subjects adjusted the stiffness of the motor connected to one (matching) arm until it was perceived to be the same as that connected to the other (reference) arm, a psychophysical function for stiffness was calculated. Eight different stiffness intensities were matched by subjects with five repetitions at each stimulus amplitude. The relation between the stiffness of the reference and matching motors was linear, and the accuracy with which subjects could match stiffness paralleled that reported previously for force and displacement. The Weber fraction for stiffness was 0.23 which is three times that reported for elbow flexion forces and forearm displacement. These findings were interpreted as indicating that subjects can perceive changes in the stiffness of mechanical devices used to effect action in the environment and that these perceptions are based on sensory signals conveying force and movement information.

Adult↗
PubMed1990

The identification of nonlinear biological systems: Wiener kernel approaches.

Detection, representation, and identification of nonlinearities in biological systems are considered. We begin by briefly but critically examining a well-known test of system nonlinearity, and point out that this test cannot be used to prove that a system is linear. We then concentrate on the representation of nonlinear systems by Wiener's orthogonal functional series, discussing its advantages, limitations, and biological applications. System identification through estimating the kernels in the functional series is considered in detail. An efficient time-domain method of correcting for coloring in inputs is examined and shown to result in significantly improved kernel estimates in a biologically realistic system.

Linear Models↗
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↗
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

A subtractive method for decomposing muscle tension into individual twitches.

When an isolated strip of skeletal muscle tissue is held at fixed length and stimulated electrically with a very brief pulse of electricity it responds with a transient increase in tension known as a twitch. If the same strip is stimulated repetitively with short pulses it produces a tension that can be considered to be composed of individual twitches arising from each stimulus. We present a subtractive method for decomposing a muscle tension curve into individual twitches of various heights. The locations of the twitches are known a priori from the times at which each stimulus was applied to the muscle. The shape of each twitch is assumed the same as that of a single twitch obtained just prior to the repetitive stimulation tension signal. We therefore find the heights of the individual twitches composing a tension curve. We find that most of a tension signal from isolated dog diaphragm strips can be accounted for as the sum of twitches of the same shape but different heights. We also derive a refractory factor, for stimulation at a given frequency, that describes how the heights of the twitches in a tension curve are attenuated as a function of stimulation frequency.

Animals↗
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↗
PubMed1986

Position dependence of stretch reflex dynamics at the human ankle.

The purpose of this study was to examine the effect of ankle position on the human ankle stretch reflexes during tonically-maintained contractions over most of the range of motion. The ankle was placed at randomly selected mean positions. Target levels of triceps surae (TS) or tibialis anterior (TA) tonic contractions were generated while the ankle was displaced by small amplitude, stochastic perturbations. System identification techniques were used to identify the stretch reflex dynamics at each combination of tonic level and ankle angle. As shown previously, the TS stretch reflex was characterized by an unidirectional, velocity-sensitive impulse response function whereas the TA stretch reflex was characterized by a linear impulse response function between ankle velocity and TA EMG. TS stretch reflexes showed a strong dependence on ankle position while TA stretch reflexes did not. Thus the TS stretch reflex magnitude increased greatly as the ankle was progressively dorsiflexed. In contrast, ankle mean position had only a minor effect on the TA stretch reflex magnitude. Our results indicate that the position-dependent facilitation of the TS stretch reflex is not due to changes in the level of skeletal motoneuron excitability. Rather, this effect may be accounted for by mechanisms that modulate the efficacy of the stochastic ankle perturbation. Such mechanisms could include position-induced: modulation of monosynaptic and polysynaptic afferent inputs to skeletal motoneurons, alterations in the extent of fusimotor drive and changes in the transmission of the joint perturbation to spindle receptors. Such mechanisms are discussed in terms of the differences between TS and TA stretch reflexes. Finally, the functional significance of position-dependent reflex responses are considered.

Adult↗
PubMed1986

The identification of nonlinear biological systems: LNL cascade models.

Systems that can be represented by a cascade of a dynamic linear (L), a static nonlinear (N) and a dynamic linear (L) subsystem are considered. Various identification schemes that have been proposed for these LNL systems are critically reviewed with reference to the special problems that arise in the identification of nonlinear biological systems. A simulated LNL system is identified from limited duration input-output data using an iterative identification scheme.

Kinetics↗
PubMed1986

The identification of nonlinear biological systems: Wiener and Hammerstein cascade models.

Systems that can be represented by a cascade of a dynamic linear subsystem preceded (Hammerstein cascade model) or followed (Wiener cascade model) by a static nonlinearity are considered. Various identification schemes that have been proposed for the Hammerstein and Wiener systems are critically reviewed with reference to the special problems that arise in the identification of nonlinear biological systems. Examples of Wiener and Hammerstein systems are identified from limited duration input-output data using an iterative identification scheme.

Kinetics↗
PubMed1986

Position dependence of ankle joint dynamics--I. Passive mechanics.

System identification techniques were used to examine the position dependence of passive ankle joint mechanics. The relaxed ankle was stochastically perturbed about different angles in the range of motion (ROM). The linear dynamic relation between ankle position and torque was identified and modelled as a second-order underdamped system, having inertial (I), viscous (B) and elastic (K) parameters. Mean joint torque changed as the ankle was rotated through the ROM; it was small at mid-range and became much larger toward either extreme. While I remained constant both B and K changed as a function of ankle angle. At the extremes of the ROM, K was much larger than previously assumed and the relation between stiffness and the passive torque generated when the ankle was placed at different mean positions was linear. These results show that large variations in joint mechanics are possible even in the absence of voluntary muscle contraction. Moreover, these changes appear to be related to the torque generated when passive joint structures are stretched.

Adult↗
PubMed1986

Position dependence of ankle joint dynamics--II. Active mechanics.

System identification techniques were used to examine the position dependence of active ankle joint mechanics. Subjects were required to maintain tonic contractions in either the tibialis anterior (TA) or triceps surae (TS) muscles while the ankle was stochastically displaced about different mean angular positions. The dynamic relation between ankle position and torque was determined for each mean position/tonic torque combination; a non-linear minimization technique was used to estimate the three parameters (inertial, viscous and elastic) of a second-order, underdamped system. Whereas the inertial parameter remained essentially invariant across all test conditions, the viscous and elastic (K) parameters became larger as the level of tonic activity increased and as the joint was rotated toward the extremes of the range of motion. The relation between K and torque was linear at all ankle angles. The slope of this relation remained constant at all mean positions during plantarflexor contractions; during dorsiflexor contractions the slope increased as the ankle was rotated from maximum plantarflexion to maximum dorsiflexion. These findings are discussed in terms of: the physiological correlates of ankle mean position, the relative significance of passive and active joint mechanics and contrasts in joint behaviour during active dorsiflexor and plantarflexor contractions.

Adult↗
PubMed1985

Effect of muscle tendon vibration on the perception of force.

The effect of vibrating the biceps muscle tendon on the perception of forces exerted by the elbow flexor muscles was examined during briefly maintained, submaximal contractions. Subjects were required to estimate the perceived magnitude of isometric forces exerted by the elbow flexor muscles under normal conditions and during vibration of the right biceps tendon. The matching forces produced by the unperturbed left arm provided an estimate of the perceived intensity of the reference arm contraction. Both force and the brachial biceps and triceps EMGs were recorded from each arm. In comparison with the matching forces produced under normal conditions, there was a significant overestimation of the forces exerted by the vibrated biceps muscle. This increase in the perceived intensity of the reference force was associated with an increase in the EMGs of the biceps and triceps muscles of the reference arm. It appeared that during vibration the triceps muscle was cocontracting presumably as a means of controlling the reference force. The overestimation of the forces exerted by the vibrated muscle could therefore reflect either the enhanced excitatory drive required to overcome the antagonist activity, or the concomitant increase in the force generated by the agonist muscle. Previous results suggest that the former is the more probable explanation.

Adult↗
PubMed1984

System identification of human stretch reflex dynamics: tibialis anterior.

System identification methods have shown that the stretch reflex in the human ankle extensor (triceps surae, TS) may be modeled as velocity feedback via a single, short latency pathway containing a uni-directional rate-sensitive nonlinearity. Evidence of differences in the reflex organization in flexors and extensors led us to use the same methods to examine stretch reflexes in the ankle flexor (tibialis anterior, TA). Five normal subjects maintained a tonic contraction of TA while subjected to repeated, stochastic perturbations of ankle position. Position, torque, and smoothed, rectified EMGs from TA and TS were recorded and ensemble averaged over 25 stimulus presentations. Linear impulse response functions relating TA EMG to ankle velocity were then determined. The results confirmed the existence of significant differences between stretch reflexes in TA and TS. In particular: 1. TA impulse response functions were characterized by two distinct peaks of excitation separated by a period of inhibition. The impulse response function amplitude increased with increasing mean torque and decreased with displacement amplitude although the sensitivity of the two peaks to these changes was different. This was interpreted as suggesting that TA stretch reflexes involve two separate pathways both involving muscle spindle information. 2. TA stretch reflexes modulate tonic activity much less than do those in TS. 3. The strong uni-directional rate-sensitive nonlinearity seen in TS was not present in TA. The functional significance of these differences remains to be determined.

Adult↗