Biomedical subjects

N O'Dwyer

Publications and source records attributed to N O'Dwyer.

7 recordsLinked to original sources

PubMed2004

Variation of magnitude and timing of wrist flexor stretch reflex across the full range of voluntary activation.

This paper reports an investigation of the magnitude and timing of the stretch reflex over the full range of activation of flexor carpi radialis. While it is well established that the magnitude of the reflex increases with the level of muscle activation, there have been few studies of reflex magnitude above 50% of maximum voluntary contraction (MVC) and virtually no study of the timing of the response in relation to activation level. Continuous small amplitude (approximately 2 degrees) perturbations were applied to the wrist of 12 normal subjects while they maintained contraction levels between 2.5-95% MVC, monitored via surface electromyography (EMG). Both narrow band (4-5 Hz) and broad band (0-10 Hz) stretch perturbations were employed. The gain (EMG output/stretch input) and phase advance of the reflex varied with the level of muscle activation in a similar manner for both types of stretch, but there were significant differences in the patterns of change due to stretch bandwidth. Consistent with previous studies, the group average reflex gain initially increased with muscle activation level and then saturated. Inspection of individual data, however, revealed that the gain reached a peak at about 60% MVC and then decreased at higher contraction levels, the pattern across the full range of activation being well described by quadratic functions (mean r2=0.82). This quadratic pattern has not been reported previously for the neural reflex response in any muscle but is consistent with the pattern that has been reliably observed in studies of the mechanical reflex response in lower limb muscles. In contrast to the pattern for reflex gain, the phase advance of the reflex (at a stretch frequency of 4.5 Hz) decreased linearly from approximately 130 degrees at the lowest contraction levels to approximately 50 degrees as maximum voluntary contraction was reached (mean r2=0.69). This decrease corresponds to a delay of 49 ms introduced centrally in reflex pathways. All subjects showed clearly defined quadratic functions relating reflex gain and linear functions relating reflex phase to activation level, but there were considerable individual differences in the slopes of these functions which point to systematic differences in synaptic behaviour of the motoneuron pool. Thus, there was wide inter-subject variation in both the contraction level at which the reflex gain reached a peak (31-69% MVC) and the highest target contraction level that could be sustained during reflex measurement (47-95% MVC). A high correlation between these variables (r2=0.78) suggests a linear relation between afferent support of contraction and muscle fatigability. The decline in reflex gain at high levels of muscle activation signals a failure of muscle afferent input and subjects in whom the gain reached a peak and declined early were unable to sustain higher target contraction levels. The results of the study show that both the timing and magnitude of the stretch reflex vary markedly over the full range of voluntary muscle activation. The pattern of variation may account for why the stretch reflex contributes most effectively to muscle mechanics over the lower half of the range of activation, while progressive reductions in both gain and phase advance at higher levels render the reflex mechanically less effective and make tremor more likely.

Action Potentials↗

PubMed2001

Coordination of the ankle joint complex during walking.

A simple model of the joint between the leg and the rearfoot is that of two hinges each with one degree of freedom. The superior hinge at the talocrural joint caters for most of the sagittal plane motion due to its mediolateral orientation. Motion in the transverse and coronal planes is largely taken up by the subtalar joint which has an inclination of 42 degrees to the transverse plane and deviates 23 degrees to the medial side. It could be expected from the orientation of this joint that rearfoot motion in the coronal plane (inversion/eversion) would be tightly coupled to motion in the transverse plane (adduction/abduction). A simple, non-viscous mechanical hinge joint would produce an in-phase, fixed gain relationship between inversion/eversion and adduction/abduction. This relation could be adequately quantified by Pearson's product moment correlation (r(P)). In contrast, muscular control of the joint, which introduces visco-elastic elements and time-delayed control lines and feedback loops, could be characterised by more complex phase and gain relations among the frequency components of the signals. These relations can be characterised by a transfer function relation (gain and phase angle) computed using a dynamic analysis. In the present study, both Pearson's correlation and linear systems analysis were employed. For 43 normal adults, five trials of one walking stride (stance and swing phases) were videographed at 30 Hz with three markers placed on each of the leg and rearfoot. Using inversion/eversion as the input and adduction/abduction as the output, the proportion of the total variance accounted for by a dynamic relation was measured by the coherence square function, which is analogous to the correlation squared (r(P)(2)). The mean r(P)(2) was 0.26, whereas the mean overall coherence was 0.62, thus accounting for more than twice the variance. This result suggests that while there may be a degree of simple mechanical coupling between inversion/eversion and adduction/abduction during walking, the major control is via muscular and/or visco-elastic passive elements.

Adult↗

PubMed1999

Dependence of stretch reflexes on amplitude and bandwidth of stretch in human wrist muscle.

The tonic stretch reflex was investigated using small-amplitude displacements (<4.2 degrees ) of the wrist while subjects maintained average contraction levels of 25% of maximum in flexor carpi radialis. The wrist displacements were designed to preclude voluntary following but at the same time were confined to the frequency range most relevant to voluntary movements. They included a broad-frequency band (0-12 Hz) signal as well as sets of narrow-band signals spanning the range from 0 to 10 Hz. The maximum frequency was set so as to remain within the linear encoding bandwidth of the reflex system and thereby minimize distortion. The effects of frequency bandwidth and amplitude of the displacement perturbations were tested in separate experiments. The coherence square, gain and phase between the EMG and angular displacement were calculated in order to characterize the stretch reflex under these conditions. It was found that the phase of the reflex response was dependent on both bandwidth and amplitude. For narrow-band displacements, the phase advance was about 30 degrees greater over the frequency range tested than for broad-band displacements, suggesting that the reflex response may be influenced by the predictability of the perturbation. At the smallest amplitude of 0.3 degrees, the peak phase advance was about 20 degrees greater than at the largest amplitude of 4.2 degrees. The gain was also higher and rose more steeply with frequency at smaller amplitudes. In the frequency range up to 12 Hz, the tonic stretch reflex responds most effectively to smaller-amplitude, more regular, higher-frequency inputs and this is consistent with a role for the reflex in counteracting small-amplitude oscillations, tremors and errors of voluntary movement.

Adult↗

PubMed1999

Slowness to develop force contributes to weakness after stroke.

OBJECTIVE: To examine weakness after stroke, in terms of both level and rate of torque generation. DESIGN: Descriptive. T tests for dependent and independent samples and Pearson's product moment correlation coefficients were performed. SETTING: A rehabilitation unit. PARTICIPANTS: Ten stroke subjects, aged 56 to 81 years, undergoing rehabilitation. Ten neurologically normal subjects aged 55 to 78 years were the controls. OUTCOME MEASURES: Peak isometric elbow flexor and extensor torque and time to 90% peak elbow flexor and extensor torque at 6 weeks and at 25 weeks after stroke. RESULTS: At 6 weeks after stroke, subjects were only half as strong and took two to three times longer to produce torque compared to controls (p < or = .05). By 25 weeks after stroke, significant improvements in peak torque (p < or = .02) and time to 90% peak flexor torque (p < or = .05) were seen so that values were within normal limits. CONCLUSION: Decreased rate of torque development compounds the problem of reduced peak torque, which may have significant implications for stroke patients, especially in situations where muscles are very weak or where force needs to be generated quickly.

Aged↗

PubMed1996

Tracking performance with sinusoidal and irregular targets under different conditions of peripheral feedback.

When studying muscle stretch reflexes with tonic stimuli or making a clinical assessment of muscle tone, it is imperative that the subject does not track the stretch stimulus either consciously or unconsciously. Such tracking contaminates reflex responses with voluntary ones and so invalidates any conclusions reached. Ideally, the stimuli used should be beyond the speed of a person's tracking ability. Both experiments on tonic stretch reflexes and clinical assessment of muscle tone of necessity involve the application of perturbations to the same limb from which a response is to be measured. These perturbations produce different peripheral feedback from the limb, including particularly cutaneous signals but also different Golgi tendon and muscle spindle afference than would occur for similar movements made voluntarily. This combination of peripheral signals resulting from perturbation of a limb is referred to here as perturbational feedback. There is evidence in the literature that subjects can generate voluntary responses to same-limb perturbations within latencies normally accepted for reflexes. Such fast responses might enable faster targets to be tracked voluntarily. In this study the tracking frequency response for the forearm was investigated using sinusoidal and irregular target signals. Perturbations were applied to a manipulandum and the subjects were required to voluntarily track these perturbations under two conditions: (1) where their arm was secured in the manipulandum and therefore they had perturbational feedback of tracking errors and (2) where their tracking arm was not in contact with the manipulandum and they had only visual or kinesthetic feedback of tracking errors. For sinusoidal target inputs, perturbational feedback allowed superior tracking performance. Many subjects could produce good tracking responses at 5 Hz and some as high as 7 Hz. This is a considerably higher frequency than was found when perturbational feedback was not present and greater than has been reported in the literature for all other types of tracking (typically about 2 Hz). In contrast, when irregular signals having power up to 4 Hz were used, perturbational feedback conferred only a marginal advantage on tracking performance. The enhancement of sinusoidal tracking performance by feedback may be due to the fact that cyclic movements can be internally generated by the subject and minimal reference to external cues provided by perturbational feedback can be used to synchronise such self-generated movements with a target. In contrast, this proposed mechanism cannot be used for tracking of irregular targets. Since perturbational feedback did not improve the maximum tracking frequency for irregular targets, there was no evidence for the operation of a shorter latency, same-limb displacement response.

Adult↗

PubMed1994

Reduction of spasticity in cerebral palsy using feedback of the tonic stretch reflex: a controlled study.

Subjects with cerebral palsy aged six to 19 years undertook a training programme to reduce spasticity and contracture of the triceps surae muscle using feedback of the gain of the tonic stretch reflex. Stretch reflex gain was significantly reduced (by 50 per cent on average) in all eight test subjects, whereas there was no significant change in control subjects, matched for age and level of spasticity, who had no reflex training. Contracture of the triceps surae muscle was not altered. Since the reduction in spasticity can be expected to slow the recurrence of muscle shortening, it is suggested that for correction of muscle contracture, muscle-lengthening procedures to promote muscle growth should be combined with reflex training to maintain the growth achieved.

Adolescent↗

PubMed1988

Onset sequencing of selected lip muscles in stutterers and nonstutterers.

The present study examined lip muscle activity during the speech production of stutterers and fluent speakers to provide information about the nature of stuttering blocks. Depressor Anguli Oris (DAO) and Depressor Labii Inferioris (DLI) were recorded, using hooked-wire electromyography (EMG), in 3 stutterers and 3 nonstutterers during productions of the words "peek", "puck", and "pack." EMG records indicated that nonstutterers activated DAO prior to DLI for production of the initial/p/. Stutterers frequently reversed this sequence of onset, particularly when they stuttered. Results are discussed in terms of mistiming versus anticipatory hypertension hypotheses about stuttering.

Electromyography↗