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PubMed1997

Spindle and motoneuronal contributions to the phase advance of the human stretch reflex and the reduction of tremor.

1. The human stretch reflex is known to produce a phase advance in the EMG reflexly evoked by sinusoidal stretching, after allowing for the phase lag introduced by simple conduction. Such phase advance counteracts the tendency to tremor introduced by the combined effect of the conduction delay and the slowness of muscle contraction. The present experiments confirm that the EMG advance cannot be attributed solely to the phase advance introduced by the muscle spindles, and show that a major additional contribution is provided by the dynamic properties of individual motoneurones. 2. The surface EMG was recorded from biceps brachii when two different types of sinusoidally varying mechanical stimuli were applied to its tendon at 2-40 Hz. The first was conventional sinusoidal displacement ('stretch'); the spindle discharge would then have been phase advanced. The second was a series of weak taps at 103 Hz, with their amplitude modulated sinusoidally ('modulated vibration'). The overall spindle discharge should then have been in phase with the modulating signal, since the probability of any individual 1 a fibre responding to a tap would increase with its amplitude. The findings with this new stimulus apply to motoneurone excitation by any rhythmic input, whether generated centrally or peripherally. 3. The sinusoidal variation of the EMG elicited by the modulated vibration still showed a delay-adjusted phase advance, but the value was less than that for simple stretching. At 10 Hz the difference was 70-80 deg. This was taken to be the phase advance introduced by the spindles, very slightly underestimated because of the lags produced by tendon compliance in transmitting sinusoidal stretch to the muscle proper. The adjusted phase advance with modulated vibration was taken to represent that introduced by the reflex centres, undistorted by tendon compliance. At 10 Hz the reflex centres produced about the same amount of phase advance as the muscle spindles. 4. At modulation frequencies above 10 Hz the adjusted central phase advance remained approximately constant. However, when the frequency was reduced to below 6 Hz the central phase advance decreased. The depth of EMG modulation (reflex gain) also fell rapidly, starting from a slightly higher frequency. Thus the central phase advance mechanisms behave like a high-pass filter. 5. A simple model of the motoneurone, incorporating synaptic noise and an after-hyperpolarization, was tested with sinusoidal inputs and gave a phase advance over a wide range of frequencies. The effect was tightly linked to two particular facets of the motor discharge; these were the ratio between the stimulus frequency and the mean firing rate (the 'carrier frequency' of the unit), and the coefficient of variation of the interspike interval distribution. The gain rose to a maximum at the carrier frequency, while the phase advance showed a maximum at 0.8 of the carrier. The more regular the discharge, the greater were these effects. The phase advance might increase to above 90 deg, showing that the motoneurone potentially provides a major contribution to the phase advance of the stretch reflex. Related effects have already been observed in other neuronal models and for the discharge of the muscle spindle, without their significance for the motoneurone being appreciated. In essence, a rhythmically firing neurone is particularly affected by a rhythmic stimulus when the two frequencies approximately coincide. 6. Recording from single human motor units confirmed the role of the 'carrier frequency' in determining the phase advance with sinusoidal inputs. In particular, for both stretching and modulated vibration, the phase advance of the response elicited by a fixed sinusoidal stimulus changed appropriately when the firing rate of the unit varied 'spontaneously' over a long recording period. 7. Thus a combination of modelling and experiment has shown that the motoneurones themselves produce a significant phase advance.

Adult↗
PubMed1997

Effect of manual therapy techniques on the stretch reflex in normal human quadriceps.

The effect of four manual therapy techniques on the quadriceps stretch reflex amplitude (elicited by mechanical vibration) was studied in a randomized, controlled trial in 120 (20 in each of six groups) healthy human subjects aged 18-64 years. Passive and active techniques were studied; each under static and dynamic conditions. The passive procedures were massage (static) and knee oscillation (dynamic) for 5 min. Active techniques involved eight repetitions of isometric quadriceps contractions (static) and leg extension (dynamic). The two static techniques had no effect on the stretch reflex amplitude. Both dynamic techniques caused a reduction (active by 25%, p < 0.00001 and passive 12%, p < 0.05). Only the active, dynamic technique caused a greater change than in the control group (p < 0.005). The effect lasted for less than 1 min. These data question the ability of such techniques to make clinically valid changes in motoneuron excitability. Further studies are required on those with neuromuscular pathology, who may respond differently to normal subjects.

Adult↗
PubMed2005

Phase-dependent and task-dependent modulation of stretch reflexes during rhythmical hand tasks in humans.

Phase-dependent and task-dependent modulation of reflexes has been extensively demonstrated in leg muscles during locomotory activity. In contrast, the modulation of reflex responses of hand muscles during rhythmic movement is poorly documented. The objective of this study was to determine whether comparable reflex modulation occurs in muscles controlling finger motions during rhythmic, fine-motor tasks akin to handwriting. Twelve healthy subjects performed two rhythmic tasks while reflexes were evoked by mechanical perturbations applied at various phases of each task. Electromyograms (EMGs) were recorded from four hand muscles, and reflexes were averaged during each task relative to the movement phase. Stretch reflexes in all four muscles were found to be modulated in amplitude with respect to the phase of the rhythmic tasks, and also to vary distinctly with the tasks being conducted. The extent and pattern of reflex modulation differed between muscles in the same task, and between tasks for the same muscle. Muscles with a primary role in each task showed a higher correlation between reflex response and background EMG than other muscles. The results suggest that the modulation patterns observed may reflect optimal strategies of central-peripheral interactions in controlling the performance of fine-motor tasks. As with comparable studies on locomotion, the phase-dependency of the stretch reflexes implies a dynamically fluctuating role of proprioceptive feedback in the control of the hand muscles. The clear task-dependency is also consistent with a dynamic interaction of sensory feedback and central programming, presumably adapted to facilitate the successful performance of the different fine-motor tasks.

Adult↗
PubMed1997

Influences of muscle stretch reflexes on voluntary, velocity-controlled movements in spastic paraparesis.

We studied voluntary, velocity-controlled knee movements in 22 patients with spastic paraparesis (11 male, 11 female) and 22 healthy controls (11 male, 11 female). Torque and EMG activity of the quadriceps and the hamstring muscles were determined in maximal voluntary concentric (shortening) and eccentric (lengthening) actions of knee extensor and flexor muscles at constant movement velocities of 30, 60, 120 and 180 degrees/s, using an active, isokinetic dynamometer. In the spastic patients, the voluntary strength and the agonist EMG activity were reduced in all movements. The reduction was largest in concentric actions at high velocity. The antagonist EMG activity was reduced in the same proportion as the agonist EMG activity in eccentric actions. In concentric actions when stretch is imposed upon antagonists, the antagonist EMG activity increased with the velocity of stretch, indicating stretch reflex activation. In parallel with the stretch reflex activation of antagonists, there was reduced activation of the agonists compatible with Ia reciprocal inhibition of agonist motoneurons. When agonists were stretched in eccentric actions, stretch reflexes appeared to support the voluntary, agonist activation of knee flexor muscles but not knee extensors.

Adult↗
PubMed1996

The stretch reflex response in the normal and spastic ankle: effect of ankle position.

OBJECTIVE: The influence of stretch of the gastrocnemiussoleus muscle on the stretch reflex activity was studied, by varying the ankle angle in steps from 10 degrees of plantarflexion (PF) to 5 degrees of dorsiflexion (DF). DESIGN: Nonrandomized control trial. SETTING: Department of Rehabilitation Medicine of a university medical center. PATIENTS: Sixteen subjects with and 16 subjects without spasticity. MAIN OUTCOME MEASURES: The passive elastic stiffness and active reflex response, expressed by the total and elastic path lengths, were determined at each ankle angle as a sinusoidal displacement of 5 degrees was applied to the joint at frequencies from 3 to 12 Hz. RESULTS: The elastic stiffness showed no difference between the spastic and normal subjects for all ankle angles (p > .05). The elastic stiffness increased linearly similarly in both groups when the ankle was dorsiflexed. The reflex response was significantly greater in the spastic group for all positions (p < or = .01). The total and elastic path lengths showed a linear increase in both groups when the ankle angle was varied from PF to DF. The spastic group, however, had a significantly faster increase (p < or = .005). Between-group comparison showed a significant quadratic trend in the elastic path length for the spastic group (p < or = .05), with a maximum at 2.5 degrees of DF. CONCLUSIONS: This study showed that the stretch reflex activity varies with the ankle position. This must be considered when performing spasticity tests subsequent to an intervention that has changed the available range of motion and when comparing subjects measured at different ankle positions.

Adult↗
PubMed2001

Influence of methodological parameters on human jaw-stretch reflexes.

The influence of methodological parameters and experimental conditions on the human jaw-stretch reflex was studied in healthy subjects in order to develop a reliable tool for investigation of the excitability of motoneuron pool. Short-latency excitatory reflex responses were evoked by a custom-made stretch device with the subjects biting on a jaw-bar with their front teeth. The displacement and ramp time of the stretches were accurately controlled and automatically triggered by a computer. The reflex responses were measured in the surface electromyogram (EMG) of the masseter and anterior temporalis muscles with online monitoring of the clenching level. The peak-to-peak amplitude of the jaw-stretch reflex was shown to be proportional to the level of EMG activity during isometric contractions, to increase proportionally with increasing stretch displacement at a given ramp time, and to decrease proportionally with increasing ramp time at a given stretch displacement. There were no significant differences in the reflex amplitude between repeated recordings within one session or between different sessions. Local anesthetic around the lower incisors as well as the upper incisors had no significant influence on the reflex amplitude. However, different biting positions on the bars of the stretch device significantly influenced the amplitude of the stretch reflex.

Adult↗
PubMed2005

Modulation of pre-programmed muscle activation and stretch reflex to changes of contact surface and visual input during movement to absorb impact.

The purpose of this study was to examine the extent of modification of the preactivation and stretch reflex response in ankle joint muscles to different contact surfaces and visual input during movement to absorb impact. Experimental movements like landing were performed using a special sliding apparatus. Seven subjects made landings on the hard surface (Hard-S) of a metal force platform or soft surface (Soft-S) of a foam cushion with eyes open or closed. The electromyographic activities from the medial gastrocnemius (MG), soleus (Sol), and tibialis anterior (TA) muscles, contact force, and ankle joint angle were recorded. The preactivation levels of MG and TA to Hard-S increased compared to Soft-S. After foot contact, dorsiflexion velocity, impulse, and responses of the stretch reflex in MG and Sol were significantly larger on Hard-S than Soft-S. With eyes closed, there were trends of decrease in the preactivation. Although the dorsiflexion velocity and impulse showed no significant differences between both visual conditions, the stretch reflex responses with eyes closed were larger than those with eyes open for both surfaces. These results suggest that the preactivation is modulated to different surface and the reflex gain is enlarged by visual suppression.

Adaptation, Physiological↗
PubMed2003

Evaluation of extensibility, passive torque and stretch reflex responses in triceps surae muscles following serial casting to correct spastic equinovarus deformity.

PRIMARY OBJECTIVE: Spastic equinovarus deformity of the ankle in adults with acquired brain injury can severely limit the achievement of rehabilitation goals. This study examined changes in triceps surae muscle extensibility, passive resistive torque and soleus stretch reflex responses in 10 adult brain injured subjects undergoing serial casting to correct ankle equinovarus deformity. METHOD: Goniometric measurement of maximal passive dorsiflexion was used to evaluate extensibility of the triceps surae muscles. Computer controlled ankle dynamometry and surface electromyography were used to identify passive resistive torque and soleus stretch reflex onset angle in response to stretches at two velocities. RESULTS: The mean casting period was 5 weeks. Casting was discontinued in one subject due to failure to achieve measurable gain in ankle range over three consecutive cast changes. Median improvements in maximal ankle dorsiflexion, with the knee flexed or extended, of 30 degrees and 15 degrees, respectively, were achieved in the remaining nine subjects (p < 0.0001). The median passive ankle range in response to a displacing torque of 10 Nm increased 4.3 degrees over the intervention period (p < 0.0001). Consistent soleus reflex activity in response to passive stretches at 25 degrees x s(-1) was elicited in only four subjects. A trend for the stretch reflex onset to move further into the available range was demonstrated in these subjects. CONCLUSION: In the present study, serial casting contributed to significant change in triceps surae extensibility and passive resistive torque, corresponding with improved maximal passive ankle dorsiflexion range and an increase in the angle achieved with a displacing torque of 10 N.m. Increased stretch reflex threshold was observed in some subjects. The use of pre-determined outcome criteria and careful measurement of responses to this intervention were important to prevent premature discontinuation of casting when gains were slower than expected.

Adolescent↗
PubMed2005

Influences of experimental factors on spinal stretch reflex latency and amplitude in the human triceps surae.

The spinal stretch reflex (SSR) is commonly assessed via electromyographic (EMG) analysis of joint perturbations inducing changes in muscle length. Previous literature indicates that when large experimental changes in magnitude of agonist background EMG, perturbation velocity, and perturbation amplitude are employed, SSR latency and amplitude are significantly altered. The purpose of this investigation was to evaluate the relative dependence of SSR latency and amplitude on inherent variability in these experimental variables. Soleus SSR latency and amplitude were assessed in 40 healthy subjects following dorsiflexion perturbation under an active state ( approximately 14% MVC). Experimental variables displayed limited variability (means +/- SD): soleus background EMG (13.47 +/- 7.08% MVC), perturbation velocity (96.1 +/- 30 degrees /s), and perturbation amplitude (4 +/- 1 degrees ). SSR latency was not significantly related to soleus background EMG (r = 0.189), perturbation velocity (r = 0.213), or perturbation amplitude (r = 0.202). Similarly, SSR amplitude was not significantly related to soleus background EMG (r = 0.306), perturbation velocity (r = 0.053), or perturbation amplitude (r = 0.056). Variability in experimental variables was much smaller than what has been reported in the literature to significantly impact SSR characteristics. These results suggest that SSR latency and amplitude are independent of agonist background EMG, perturbation velocity, and perturbation amplitude when experimental variability is relatively limited.

Adult↗
PubMed1999

Directional sensitivity of stretch reflexes and balance corrections for normal subjects in the roll and pitch planes.

A large body of evidence has been collected which describes the response parameters associated with automatic balance corrections in man to perturbations in the pitch plane. However, perturbations to human stance can be expected from multiple directions. The purpose of the present study was to describe the directional sensitivities of muscle responses re-establishing disturbed stance equilibrium in normal subjects. The contributions of stretch reflex and automatic balance-correcting responses to balance control, and concomitant biomechanical reactions, were examined for combinations of pitch and roll perturbations of the support surface. More specifically, muscle responses, initial head accelerations and trunk velocities were analyzed with the intention of identifying possible origins of directionally specific triggering signals and to examine how sensory information is used to modulate triggered balance corrections with respect to direction. Fourteen healthy adults were required to stand on a dual-axis rotating platform capable of delivering rotational perturbations with constant amplitude (7.5 degrees ) and velocity (50 degrees /s) through multiple directions in the pitch and roll planes. Each subject was randomly presented with 44 support surface rotations through 16 different directions separated by 22.5 degrees first under eyes-open, and then, for a second identical set of rotations, under eyes-closed conditions. Bilateral muscle activities from tibialis anterior, soleus, lateral quadriceps and paraspinals were recorded, averaged across direction, and areas calculated over intervals with significant bursts of activity. Trunk angular velocity and ankle torque data were averaged over intervals corresponding to significant biomechanical events. Stretch reflex (intervals of 40-100, 80-120 ms) and automatic balance-correcting responses (120-220, 240-340 ms) in the same muscle were sensitive to distinctly different directions. The directions of the maximum amplitude of balance-correcting activity in leg muscles were oriented along the pitch plane, approximately 180 degrees from the maximum amplitude of stretch responses. Ankle torques for almost all perturbation directions were also aligned along the pitch plane. Stretch reflexes in paraspinal muscles were tuned along the 45 degrees plane but at 90 degrees to automatic balance corrections and 180 degrees to unloading responses in the same muscle. Stretch reflex onsets in paraspinal muscles were observed at 60 ms, as early as those of soleus muscles. In contrast, unloading reflexes in released paraspinal muscles were observed at 40 ms for perturbations which caused roll of the trunk towards the recorded muscle. Onsets of trunk roll velocities were earlier and more rapid than those observed for pitch velocities. Trunk pitch occurred for pure roll directions but not vice versa. When considered together, early stretch and unloading of paraspinals, and concomitant roll and pitch velocities of the trunk requiring a roll-and-pitch-based hip torque strategy, bring into question previous hypotheses of an ankle-based trigger signal or ankle-based movement strategies for postural balance reactions. These findings are compatible with the hypothesis that stretch-, force- and joint-related proprioceptive receptors at the level of the trunk provide a directionally sensitive triggering mechanism underlying a, minimally, two-stage (pitch-based leg and pitch-and-roll-based trunk) balance-correcting strategy. Accelerometer recordings from the head identified large vertical linear accelerations only for pitch movements and angular roll accelerations only during roll perturbations with latencies as early as 15 ms. Thus, it appears that balance corrections in leg and trunk muscles may receive strong, receptor-dependent (otolith or vertical canal) and directionally sensitive amplitude-modulating input from vestibulospinal signals.

Adult↗
PubMed1986

Enhancement by serotonin of tonic vibration and stretch reflexes in the decerebrate cat.

The effects of pharmacological manipulation of serotonergic systems on spinal reflexes were determined in the unanesthetized decerebrate cat. The prolonged motor output that continues after cessation of high frequency longitudinal tendon vibration was strongly enhanced by the serotonin reuptake blocker fluoxetine and the serotonin precursor 5-hydroxytryptophan, and was decreased by the serotonin receptor antagonist methysergide. In addition, both dynamic and static stretch reflex stiffness was markedly increased by fluoxetine and 5-hydroxytryptophan, while methysergide produced a decrease in stretch reflex stiffness. These powerful effects on tonic vibration and stretch reflexes could not be explained by drug-induced alterations in muscle spindle primary afferent discharge. In light of other recent results on serotonin-mediated effects on motoneurons, we believe that the effects of these agents result from modification of an intrinsically mediated prolonged depolarization of spinal neurons. However, the possibility that these drugs modify longlasting discharge in associated interneuronal pathways cannot be ruled out.

Animals↗
PubMed2001

Effects of electrical stimulation of the sciatic nerve on background electromyography and static stretch reflex activity of the trunk muscles in rats: possible implications of neuronal mechanisms in the development of sciatic scoliosis.

STUDY DESIGN: The effects of electrical stimulation of the sciatic nerve on background electromyographic and static stretch reflex activity of the trunk muscles were studied. OBJECTIVES: To verify the hypotheses that sciatic scoliosis is induced reflexively by radiculopathic pain, and that scoliosis might be maintained by prolonged asymmetric alteration of the trunk muscle tonus caused by central sensitization of the spinal neurons that constitute the postural reflex pathways. SUMMARY OF BACKGROUND DATA: Sciatic scoliosis usually occurs with convexity to the side of the herniated disc. The neuronal mechanism of sciatic scoliosis has not been well clarified. Recently, prolonged alteration of motor function in the hindlimbs of animals caused by central sensitization has been reported. METHODS: In spinalized rats (transection of the spinal cord), the sciatic nerve was stimulated electrically as a conditioning stimulus. Muscle stretch elicited by bending of the lumbar spine was applied as a test stimulus. Background and stretch reflex activities of the bilateral oblique abdominal, psoas, and quadratus lumborum muscles were recorded. Rats in which MK-801, an N-methyl-d-aspartate antagonist, was preadministered also were used. RESULTS: The conditioning stimulus enhanced background electromyographic activity in bilateral oblique abdominal, contralateral psoas, and quadratus lumborum muscles. Furthermore, the conditioning stimulus induced prolonged facilitation and depression of stretch reflex activity of the contralateral psoas and quadratus lumborum, and ipsilateral psoas and quadratus lumborum muscles, respectively. Preadministration of MK-801 reduced these excitatory and inhibitory effects. CONCLUSION: It was found that the pattern of electromyographic activity of the trunk muscles evoked by sciatic nerve stimulation coincided with the typical direction of sciatic scoliosis in patients with lumbar disc herniation. It was supposed that the prolonged asymmetric alteration of the trunk muscle tonus was caused by central sensitization, and that central sensitization of spinal neurons may underlie the neuronal mechanism of sciatic scoliosis.

Animals↗
PubMed1990

Impaired habituation of long-latency stretch reflexes of the wrist muscles in Huntington's disease.

Electromyographic responses to sudden wrist extension were recorded from the forearm and finger flexor muscles in 10 patients with Huntington's disease (HD) and in 10 normal controls. Stretch reflexes were characterized by a short-latency (SL) and a long-latency (LL) component both in patients and controls. Latency, duration, and size of the SL component were not different in the two groups, whereas the LL component was delayed in latency and reduced in size in HD patients. Increasing the stretch repetition rate from 0.1 to 0.4 cycles/s did not affect the SL component of either group, whereas the LL stretch reflex was reduced in size and duration in normal controls, but not in HD patients. These findings suggest an impairment of the "gain" mechanisms of the sole LL component, responsible for a desaturation of this component. This study supports the hypothesis that LL stretch reflexes are mediated by a transcortical long loop, possibly damaged in HD.

Adult↗
PubMed1987

Stretch reflexes and parkinsonian tremor.

By coupling human limbs to a machine, which drives a single joint through sinusoidal flexion-extension movements, it is often possible to entrain the resting tremor of Parkinson's disease. Entrainment is most likely to occur when the imposed movement is large in amplitude and close in frequency to the spontaneous tremor.

Humans↗