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R E Kearney

Publications and source records attributed to R E Kearney.

At least 55 records · Page 3Linked to original sources
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

Visual, vestibular and voluntary contributions to human head stabilization.

We have investigated the ability of humans to stabilize their heads in space and assessed the influence of mental set and the relative importance of visual and vestibular cues. Ten normal subjects and 3 patients with bilateral vestibular loss were studied. Subjects were fixed firmly to the chair of a turntable facing a screen on which was projected a target spot. A 'gunsight' spot generated by a small projector fixed to the head provided feedback of head position. Four conditions were studied (1) Gunsight (GU): subjects were instructed to stabilize the head in space by superimposing the 'gunsight' spot on the fixed target spot while chair position was displaced according to a random pattern with a bandwidth from 0-1 Hz. (2) Imagined gunsight (IGU): identical to condition 1 except that the subject was blindfolded and so had to imagine the target position. (3) Mental arithmetic (MA): subjects did mental arithmetic while the chair was displaced. (4) Visual tracking (VT): subjects were instructed to track the target spot with the 'gunsight' spot while the chair was fixed and the target spot driven to follow the chair displacement trajectory used in conditions 1, 2 and 3. In GU normal subjects stabilized their head position extremely well (mean HEAD/CHAIR gain = 0.81). Significant stabilization was present in IGU although the gain (mean gain = 0.61) was reduced compared to GU. There was very little stabilization in MA (mean gain = 0.12). In VT, subjects tracked the target with about the same gain (mean gain = 0.68) as in IGU. By comparison, the vestibular patients could not perform IGU, for which their performance (mean gain = 0.08) was similar to MA (mean gain = 0.06). In GU (mean gain = 0.54), their performance was attributable to visual tracking (mean gain in VT = 0.50). For the frequency bandwidth in which subjects were tested, the results show that: When subjects were distracted by mental arithmetic, the contribution to head stability of the short latency cervico-collic (CCR) and vestibulo-collic (VCR) reflexes is negligible. As expected, vision plays an important role in stabilizing the head. Equally important are long latency stabilizing mechanisms whose onset times (140 ms) are shorter, but still comparable to that of vision. The latter mechanisms are of vestibular origin and their influence is under voluntary control so as to permit augmenting head stability compared to what it would be if vision acted alone.

Adult↗
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↗
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↗
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

The controversy over the influence of static tilt on soleus motoneuron excitability in man.

The influence of static fore-aft body tilts on the excitability of the human soleus motoneuronal pool had been studied by mapping the H reflex amplitude as a function of head positions by two groups of investigators with different results. In 11 of 13 subjects studied, we (C.W.Y. Chan and R.E. Kearney, 1982, Neurosci. Lett. 33: 333-338) found that the H reflex was a minimum when the head was near the vertical position, and often increased as a sinusoidal function of tilt to a maximum of 60 degrees forward or backward. In contrast, I.G. Aiello, G. Rosati, G. Serra, V. Jugnoli, and M. Manca (1983, Exp. Neurol. 79: 18-26) reported a linear decrease in the H reflex amplitude in all their subjects when the body was tilted from the vertical to the horizontal supine position. The differences in these findings were discussed from the point of view of methodology and in light of recent animal and human studies.

H-Reflex↗
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

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

Influence of static tilt on soleus motoneuron excitability in man.

The influence of whole head-body rotations with respect to gravity on the excitability of the human soleus motoneuron pool, was studied by mapping the H reflex amplitude as a function of static head displacements. In most subjects, the H reflex reached a minimum when the head was near the vertical position (90 degrees), and often increased as a sinusoidal function of tilt in the sagittal plane. These changes were attributed to the tonic labyrinthine reflexes which, unlike some findings in decerebrate cats, generally increase the extensor tone of the human lower limb when the head is displaced from the upright position.

H-Reflex↗
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

Is the functional stretch response servo controlled or preprogrammed?

Recent findings in humans and primates suggest that the long latency electromyographic (EMG) responses, evoked by opposing sudden limb displacements, might be mediated via a transcortical pathway. However, it is not clear whether these "late" responses are servo controlled or preprogrammed. We have addressed these questions in two interrelated experimental series. Firstly, the late EMG responses (termed the FSR) evoked in the ankle muscles of 8-12 normal human subjects were mapped as a function of displacement amplitude and velocity. Secondly, the FSR evoked by sustained ramp displacements (lasting 500 msec) were compared with those elicited by transient pulse displacements (lasting 60 msec) having entirely different amplitude and velocity profiles. The findings demonstrated a general lack of a systematic relationship between the characteristics of the initial component of the FSR and displacement parameters, although exceptions were noted in individual subjects. Furthermore, no marked difference was found in either the latency or the rise time of the FSRs evoked by ramp and pulse displacements. The similarity of the FSRs in spite of widely different input patterns is not consistent with the behaviour of a servo response. However, such a finding is in agreement with the concept of a preprogrammed pattern of activity which, once triggered, has to run its full course without the possibility of modification. It is therefore concluded that, at least the initial part of the FSR is generated by the triggered release of a preformulated pattern of intended movement relatively independent of the specific pattern of limb displacement, rather than acting in the manner of a servo mechanism.

Ankle↗
PubMed1982

Contrasts between the reflex responses to tibialis anterior and triceps surae to sudden ankle rotation in normal human subjects.

A detailed comparative study was made of the reflex responses of human ankle flexors (tibialis anterior, TA) and extensors (triceps surae, TS) to sudden ankle rotation. Subjects were instructed to maintain a tonic contraction of either TS or TA and subjected to sudden displacements of the ankle from its midposition in either the dorsiflexing or plantarflexing directions. Joint position, torque and smoothed rectified EMGs were recorded. The results revealed 5 principal differences in the behavior of the two muscles. (1) The TS response was typically a short, synchronous burst of activity starting at about 40 msec and lasting no more than 50 msec, termed the MSR. in contrast, the TA responses started at about the same latency but lasted much longer and frequently a second component (termed the PSR) could be distinguished. (2) The threshold of the TA reflex was much higher than that of TS. (3) Increasing displacement velocity increased the magnitude of the TS MSR as well as that of the TA MSR, but decreased the magnitude of the TA PSR. (4) Increasing the level of tonic activity augmented the magnitude of the TS activity initially but saturation soon occurred. In contrast, both components fo the TA response increased with increasing tonic TA activity. (5) Finally, shortening of the tonically active TS evoked no consistent response, where as shortening of TA evoked a significant pause in its activity. The contrasting behavior of the ankle flexors and extensors suggest that there are significant differences in their reflex organizations.

Ankle↗