Academic literature on the topic 'Tonic stretch reflex threshold'

Background Commonly used spasticity scales assess the resistance felt by the evaluator during passive stretching. These scales, however, have questionable validity and reliability. The tonic stretch reflex threshold (TSRT), or the angle at which motoneuronal recruitment begins in the resting state, is a promising alternative for spasticity measurement. Previous studies showed that spasticity and voluntary motor deficits after stroke may be characterized by a limitation in the ability of the central nervous system to regulate the range of the TSRT. Objective The study objective was to assess interevaluator reliability for TSRT plantar-flexor spasticity measurement. Design This was an interevaluator reliability study. Methods In 28 people after stroke, plantar-flexor spasticity was evaluated twice on the same day. Plantar-flexor muscles were stretched 20 times at different velocities assigned by a portable device. Plantar-flexor electromyographic signals and ankle angles were used to determine dynamic velocity-dependent thresholds. The TSRT was computed by extrapolating a regression line through dynamic velocity-dependent thresholds to the angular axis. Results Mean TSRTs in evaluations 1 and 2 were 66.0 degrees (SD=13.1°) and 65.8 degrees (SD=14.1°), respectively, with no significant difference between them. The intraclass correlation coefficient (2,1) was .851 (95% confidence interval=.703, .928). Limitations The notion of dynamic stretch reflex threshold does not exclude the possibility that spasticity is dependent on acceleration, as well as on velocity; future work will study both possibilities. Conclusions Tonic stretch reflex threshold interevaluator reliability for evaluating stroke-related plantar-flexor spasticity was very good. The TSRT is a reliable measure of spasticity. More information may be gained by combining the TSRT measurement with a measure of velocity-dependent resistance.

This study characterizes tonic and phasic stretch reflex and stiffness and viscosity changes associated with spastic hemiparesis. Perturbations were applied to the ankle of 27 hemiparetic and 36 healthy subjects under relaxed or active contracting conditions. A nonlinear delay differential equation model characterized phasic and tonic stretch reflex gains, elastic stiffness, and viscous damping. Tendon reflex was characterized with reflex gain and threshold. Reflexively, tonic reflex gain was increased in spastic ankles at rest ( P < 0.038) and was not regulated with muscle contraction, indicating impaired tonic stretch reflex. Phasic-reflex gain in spastic plantar flexors was higher and increased faster with plantar flexor contraction ( P < 0.012) than controls ( P < 0.023) and higher in dorsi-flexors at lower torques ( P < 0.038), primarily because of its increase at rest ( P = 0.045), indicating exaggerated phasic stretch reflex especially in more spastic plantar flexors, which showed higher phasic stretch reflex gain than dorsi-flexors ( P < 0.032). Spasticity was associated with increased tendon reflex gain ( P = 0.002) and decreased threshold ( P < 0.001). Mechanically, stiffness in spastic ankles was higher than that in controls across plantar flexion/dorsi-flexion torque levels ( P < 0.032), and the more spastic plantar flexors were stiffer than dorsi-flexors at comparable torques ( P < 0.031). Increased stiffness in spastic ankles was mainly due to passive stiffness increase ( P < 0.001), indicating increased connective tissues/shortened fascicles. Viscous damping in spastic ankles was increased across the plantar flexion torque levels and at lower dorsi-flexion torques, reflecting increased passive viscous damping ( P = 0.033). The more spastic plantar flexors showed higher viscous damping than dorsi-flexors at comparable torque levels ( P < 0.047). Simultaneous characterizations of reflex and nonreflex changes in spastic hemiparesis may help to evaluate and treat them more effectively.

The order of recruitment of flexor carpi radialis (FCR) motor units was studied during voluntary and reflexive activation of the motoneuron pool for two adult subjects. During slow "voluntary" activation, the recruitment threshold for tonic motoneuron firing was determined, and then the twitch profile of the motor unit was computed by the spike-triggered averaging technique. A positive correlation (r = 0.79 and 0.68 for the two subjects, respectively) between recruitment threshold and twitch amplitude implies that recruitment of FCR motoneurons during slow ramp isometric contractions proceeds in order of increasing size. The accompanying paper describes the behavior of single motor units during the short- and long-latency periods of the stretch reflex. When the effects of sufficient voluntary facilitation (preload) combined with a sufficiently large torque step were just adequate to cause a motor unit to fire during the stretch reflex, its response was virtually always confined to the long-latency period. In addition, the first unit to begin responding to muscle stretch always had the lowest voluntary recruitment threshold relative to other units "visible" at that recording site. By making this unit tonic, the reflex response to the same load increased substantially during the short-latency reflex period, while a second unit increased its reflex response probability during the long-latency period. Thus the voluntary recruitment order of two or more motor units is preserved during the stretch reflex, and is in fact maintained within first the long-latency and then short-latency components of this reflex.

We describe several infl uential hypotheses in the field of motor control including the equilibrium-point (referent confi guration) hypothesis, the uncontrolled manifold hypothesis, and the idea of synergies based on the principle of motor abundance. The equilibrium-point hypothesis is based on the idea of control with thresholds for activation of neuronal pools; it provides a framework for analysis of both voluntary and involuntary movements. In particular, control of a single muscle can be adequately described with changes in the threshold of motor unit recruitment during slow muscle stretch (threshold of the tonic stretch reflex). Unlike the ideas of internal models, the equilibrium-point hypothesis does not assume neural computations of mechanical variables. The uncontrolled manifold hypothesis is based on the dynamic system approach to movements; it offers a toolbox to analyze synergic changes within redundant sets of elements related to stabilization of potentially important performance variables. The referent confi guration hypothesis and the principle of abundance can be naturally combined into a single coherent scheme of control of multi-element systems. A body of experimental data on healthy persons and patients with movement disorders are reviewed in support of the mentioned hypotheses. In particular, movement disorders associated with spasticity are considered as consequences of an impaired ability to shift threshold of the tonic stretch reflex within the whole normal range. Technical details and applications of the mentioned hypo theses to studies of motor learning are described. We view the mentioned hypotheses as the most promising ones in the field of motor control, based on a solid physical and neurophysiological foundation.

A reduction in fingertip forces during a visually occluded isometric task is called unintentional drift. In this study, unintentional drift was studied for two conditions, with and without “epilogue.” We define epilogue as the posttrial visual feedback in which the outcome of the just-concluded trial is shown before the start of the next trial. For this study, 14 healthy participants were recruited and were instructed to produce fingertip forces to match a target line at 15% maximum voluntary contraction. The results showed a significant reduction in unintentional drift in the epilogue condition. This reduction is probably due to the difference in the shift in λ, the threshold of the tonic stretch reflex, the hypothetical control variable that the central controller can set.

Studies suggest that deficits in central regulation of stretch reflex thresholds (SRT) underlie both spasticity and other disorders of motor control. We investigated intra- and inter-evaluator reliability to quantify spasticity based on tonic SRT (TSRT) and the relationship between TSRT and Modified Ashworth Scale (MAS, clinical assessment of resistance to stretch). Spasticity was evaluated in 20 subjects with chronic stroke-related spasticity in two different days, by three evaluators. Twenty different velocity-dependent dynamic SRT (angle where biceps brachii EMG signal increased for a given velocity of stretch) were recorded. TSRT (excitability of motoneurons at 0°/sec) was then computed. Spasticity was also estimated with MAS. Reliability was moderately good for subjects with moderately high spasticity (intra--evaluator: 0.46 to 0.68, inter--evaluator: 0.53 to 0.68). There was no correlation between TSRT and MAS since they measure different phenomena. TSRT is a promising new measure of spasticity. Further improvements for its quantification are suggested.

When active skeletal muscle is stretched, it generally responds with a contraction which resists the stretch. This response is termed the muscle stretch reflex. The size (gain) and timing (phase) of the response has been found to depend on many factors including the characteristics of the applied stretch, the muscle contraction level and the subject's intention. Investigations of this stretch reflex have often involved stretches to muscle which contained frequencies either beyond the range of voluntary movement or else which could be consciously tracked. This study sought to characterise the frequency response of the stretch reflex, in terms of its gain and phase, under a variety of conditions while using stretches to the muscle which were relevant to voluntary movement, yet which were too irregular to be tracked. The types of stretch which satisfied these criteria had first to be determined by an investigation of tracking performance under different conditions of peripheral feedback. Having established the types of stretch which could be used to guarantee reflex rather than voluntary responses, the stretch reflex was investigated using stretches of different amplitude and bandwidth and spanning the full range of contraction level. Research was also undertaken to determine whether the gain and phase of the reflex response could be decoupled from the background contraction level of the muscle and to examine any associated effects on the mechanical properties of the limb. Explanatory models for some of these reflex responses were developed. An interaction between normal physiological tremor and the stretch reflex response was also investigated.

Conselho Nacional de Desenvolvimento Científico e Tecnológico
A espasticidade é observada na maioria dos pacientes após Acidente Vascular Encefálico (AVE), e exercem influência na presença de deficiências e incapacidades, comprometendo a função motora. As estratégias de neuroreabilitação, o biofeedback eletromiográfico (EMG), têm sido utilizado com aceitação na comunidade médica para reajustes nas habilidades sensório-motoras como retreinamento motor, redução da espasticidade e/ou treinamento de relaxamento. A intervenção ainda apresenta evidências delimitadas quanto a sua efetividade na reabilitação, principalmente, quanto a interferência sobre a atividade cortical e na redução dos sinais espásticos que oferece características negativas na execução do movimento. O objetivo dessa pesquisa foi analisar a possível interferência do treino com biofeedback eletromiográfico sobre a conscientização do controle motor no membro espástico e a caracterização da atividade das bandas de baixa frequência em diferentes regiões corticais orientada pela técnica de treinamento. Dezesseis voluntários acometidos por AVE isquêmicos foram selecionados e divididos em dois grupos (n=8). Grupo experimental (GE) submetidos ao treino com biofeedback associado a fisioterapia, e o grupo controle (GC) submetido apenas à fisioterapia convencional. Foi realizada a avaliação do grau de espasticidade pelo limiar de reflexo do estiramento tônico (LRET) e pela Escala Modificada de Ashworth (EMA) antes e três semanas após o término do treinamento com a técnica. Os sujeitos foram submetidos ao biofeedback durante 6 semanas, com 2 sessões semanalmente. O mesmo tempo para o tratamento fisioterápico foi padronizado para o GC. Os resultados mostram variação do percentual médio de melhora do grau de espasticidade, mensuradas pelo LRET, de 38,59% (dp=13,03%) no GE comparado com 18,58% (dp=11,90%) do GC. Essa variação apresentou diferença significativa (p=0,020; t=2,776; p<5%) entre os grupos (controle e experimental), e a diferença significativa do LRET antes e após do treinamento no GE (p=0,003; t=5,338; p<5%) quando comparado ao GC (p=0,015; t=3,657; p<5%). A medida semi-quantitativa da EMA antes e após o término das sessões mostraram variações apenas no GE. Com relação a atividade cortical, houve diferença de atividade das bandas (delta, teta, alfa e beta) quando a 3ª e 12ª sessão foram comparadas para cada sujeito do GE. Essa diferença foi encontrada, principalmente, em regiões frontal, central (vértex), parietal e occipital em ambos hemisférios (ipsilateral e contralateral a lesão) tanto na fase de planejamento cognitivo motor quanto na execução do movimento. Houve predominância da diferença de atividade para a banda delta, alfa e beta em diferentes sujeitos distribuída difusamente ao longo dos canais de registro de viii EEG. A atividade diferenciada das bandas foi devido ao aumento e/ou diminuição da energia espectral entre as sessões, notado apenas em alguns voluntários do GE. Observou ainda diferença de atividade em áreas motoras secundárias. As avaliações, principalmente do LRET mostram que o treino com biofeedback EMG foi efetivo na redução do grau de espasticidade. A diferença de atividade cortical das bandas de frequência entre as sessões sugere que o biofeedback modula a cognição por meio do esforço e atenção imposta pela tarefa na tentativa do movimento no membro acometido. Além de que a diferença de energia espectral entre as sessões é dependente do ajuste e complexidade da tarefa direcionado pelos sinais do biofeedback, auxiliando na aprendizagem motora.
Spasticity is observed in most patients after cerebrovascular accident (CVA), and exerts influence in the presence of disabilities, affecting motor function. For neurorehabilitation strategies, electromyographic biofeedback (EMG) has been used with acceptance in the medical community for adjustments in the sensory-motor skills as a motor retraining, reducing spasticity and/or relaxation training. The intervention still presents limited evidence regarding their effectiveness in rehabilitation, especially as the interference of cortical activity and the reduction of spastic signs that provides negative characteristics in movement execution. The aim of this study was to analyze the workout possible interference with EMG biofeedback on the motor control awareness in spastic member in the characterization of the activity of low-frequency bands in different cortical regions targeted by the training technique. Sixteen volunteers affected by ischemic stroke were selected and divided into two groups (n = 8). Experimental group (EG) underwent biofeedback training associated with physical therapy and control group (CG) only conventional physiotherapy. The assessment of the degree of spasticity by reflex threshold of the tonic stretch (TSRT) and Modified Ashworth Scale (MAS) was performed before and three weeks after the end of treatment with the technique. The subjects underwent biofeedback for 6 weeks, with two sessions weekly. The same time for physiotherapy treatment was standardized to the GC. The results show variation of the average percent improvement in the degree of spasticity measured at TRST, 38,59% (sd=13,03%) in GE compared to 18,58% (sd=11,90%) of GC. This variation showed a significant difference (p=0.020; t=2,776; p<5%) between groups (control and experimental), and the significant difference of TRST before and after training in EG (p = 0.003; t=5,338; p <5%) when compared to the CG (p=0.015; t=0,015; p<5%). The semi-quantitative measure of the MAS before and after the end of the sessions presented variations only in GE. Regarding the cortical activity, there were band activities differences when 3rd and 12th sessions were compared for each subject of GE. This difference was found primarily in the frontal, central (vertex), parietal and occipital lobe in both hemispheres (contralateral and ipsilateral to the lesion) in both the cognitive motor planning phase and in the movement execution. It was observed the predominance of activity difference for the delta band, alpha and beta in different subjects distributed diffusely over the EEG recording channels. The different activity of the bands was due to the increase and/or decrease the spectral energy between sessions, x noticed only in some GE volunteers. It was also pointed out distinct activity in secondary motor areas. Evaluations mainly from TSRT show that training with EMG biofeedback was effective in reducing the degree of spasticity. The difference in cortical activity of the frequency bands between sessions suggested that biofeedback modulates cognition through the effort and attention required by the task of movement attempt in the affected limb. Besides that, the spectral energy difference between the sessions depends on the tuning and task complexity driven by biofeedback signals, helping motor learning.
Tese (Doutorado)

The Muscle Shortening Maneuver (MSM), a physiotherapy approach, was introduced by Grimaldi et al. in the eighties and is derived from Feldman’s λ model of motor control (Melchiorre, 2014). In the λ model the regulation of the stretch reflex threshold, that is the lower muscle length or joint angle at which motoneuronal recruitment occurs, plays a pivotal role (Feldman, 1995; Latash, 2010). The dynamic stretch reflex threshold (DSRT) is influenced by stretch speed. The tonic stretch reflex threshold (TSRT) represents the specific value of the DSRT at zero velocity. DSRTs and TSRTs are expressed in relation to the configuration of the joints, within a body frame of reference (FR) [4]. MSM is non-invasive and free of side effects, and is locally used in clinical practice (Ferrarello, 2021); it consists of two essential simultaneously applied elements: a muscle shortening and a solicitation in traction. A physiotherapist applies a series of fast accelerations to a skeletal segment (e.g., the foot) in the presence of forces acting in the opposite direction (added mass), thus producing a tensile stress. MSM provokes a dynamic lengthening associated with sudden shortening of the agonist and antagonist muscles. Tissue deformation stimulates the muscle spindles, with an enrolment of motor units and an attempt to produce muscle tension. However, the development of tension is prevented by the sudden shortening of the muscle due to the therapeutic maneuver. It is widely used in clinical practice but with little scientific evidence. The aim of the thesis is to investigate the neurophysiological mechanisms underlying it. With these assumptions, an intervention study on patients with chronic stroke in which, through the use of a portable device, it has been possible to use the TSRTs as an objective outcome measure, has been designed. In order to test this protocol and before embarking on a real randomized clinical trial, a pilot study was performed. Despite the limitations of this type of study design do not allow to make assumptions on the real efficacy of the treatment, it was already possible to identify real physiological changes in patients. Moreover, the pilot study has been useful to detect the criticalities of the original research project and therefore to make changes to support a better execution of the future trial. In the meanwhile a retrospective analysis which led to a re-elaboration of data obtained in the recent past was conducted. In particular, in a case series of 9 subjects with infantile cerebral palsy, the evaluation of objective outcomes (muscle strength, range of motion) is found for the first time in relation to functional outcomes measured by scales (Selective Motor Control Scale, Physician Rating Scale). Despite the many limitations that a series of cases with such a small sample can have, it was considered important to proceed with a more in-depth analysis of the data obtained in order to reflect on the possible presence of strong outcomes that would really change the physiological and functional characteristics of the subjects. This would have allowed to lay the foundations for future work to better investigate the effects of MSM on movement control in individuals with central nervous system injury. The effects of the maneuver were also investigated in terms of pure electromyographic activity in a pediatric patient with post-surgical peroneal nerve resection. Also in this patient it was possible to find a clear modification of the outcomes in question with a strong reduction in clinical and functional findings and greater daily autonomy in the activities of life. Obviously, the limits of the observation of a single case are evident and the results cannot be considered as evidence of efficacy but it is considered important, even in this case, to detect the presence of hard outcomes of the therapeutic path. Furthermore, the literature on MSM shows the possible effects of this type of treatment also on orthopedic injuries. In particular, two articles by Melchiorre et al. showed that this type of intervention seems effective on patients with Shoulder Impingement Syndrome (SIS) in terms of morpho-structural changes in the musculotendinous compartment of the joint, detectable by ultrasound, and of increased strength and pain relief. This protocol was selected for the design and execution of a study on subjects with SIS from a population that practices an overhead sport, water polo. In this randomized and controlled study, it was possible to observe objective and immediate changes in the musculotendinous components in correspondence with a significant decrease of pain. In conclusion, the results of these studies seem to highlight a real effect of MSM on motor control understood as modulation of TSRT, in the perspective of Feldman's threshold referent control theory.