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Strokes cause spasticity via stretch reflex hyperexcitability in the spinal cord, and spastic paralysis due to involuntary muscle contraction in the hands and fingers can severely restrict skilled hand movements. However, the underlying neurological mechanisms remain unknown. Using a mouse model of spasticity after stroke, we demonstrate changes in neuronal activity with and without electrostimulation of the afferent nerve to induce the stretch reflex, measured using quantitative activation-induced manganese-enhanced magnetic resonance imaging. Neuronal activity increased within the ventral medullary reticular formation (MdV) in the contralesional brainstem during the acute post-stroke phase, and this increase was characterised by activation of circuits involved in spasticity. Interestingly, ascending electrostimulation inhibited the MdV activity on the stimulation side in normal conditions. Moreover, immunohistochemical staining showed that, in the acute phase, the density of GluA1, one of the α-amino-3 hydroxy‑5 methyl -4 isoxazolepropionic acid receptor (AMPAR) subunits, at the synapses of MdV neurons was significantly increased. In addition, the GluA1/GluA2 ratio in these receptors was altered at 2 weeks post-stroke, confirming homeostatic plasticity as the underlying mechanisms of spasticity. These results provide new insights into the relationship between impaired skilled movements and spasticity at the acute post-stroke phase., Competing Interests: Declaration of competing interest The authors declare no competing interests., (Copyright © 2024. Published by Elsevier Inc.)

Amyotrophic lateral sclerosis (ALS) diagnosis relies on signs of progressive damage to both lower motoneuron (LMN), given by clinical examination and electromyography (EMG), and upper motoneuron (UMN), given by clinical examination only. Recognition of UMN involvement, however, is still difficult, so that diagnostic delay often remains too long. Shortening the time to clinical and genetic diagnosis is essential in order to provide accurate information to patients and families, avoid time-consuming investigations and for appropriate care management. This study investigates whether combined patellar tendon reflex recording with motor-evoked potentials to the lower limbs (T-MEP-LL) is relevant to assess corticospinal function in ALS, so that it might serve as a tool improving diagnosis. T-MEP-LL were recorded in 135 patients with suspected motor neuron disease (MND) from February 2010 to March 2021. The sensitivity, specificity, and ability to improve diagnosis when added to Awaji and Gold Coast criteria were determined. The main finding of the study is that T-MEP-LL can detect UMN dysfunction with a 70% sensitivity and 63% specificity when UMN clinical signs are lacking. The sensitivity reaches 82% when considering all MND patients. Moreover, at first evaluation, using T-MEP-LL to quantify reflex briskness and to measure central conduction time, can improve the diagnostic accuracy. T-MEP-LL is easy to perform and does not need any electrical stimulation, making the test rapid, and painless. By the simultaneous quantification of both UMN and LMN system, it could also help to identify different phenotype with more accuracy than clinical examination in this broad-spectrum pathology. The question whether T-MEP-LL could further be a real biomarker need further prospective studies., (Copyright © 2024 The Authors. Published by Elsevier Masson SAS.. All rights reserved.)

The Tonic Stretch Reflex Threshold (TSRT) is the joint angle or muscle length (λ) at which muscle activation begins. In spasticity, the TSRT abnormally lies inside the biomechanical joint range. It is determined by measuring the Dynamic Stretch Reflex Thresholds (DSRTs) by stretching the resting muscle at different velocities. The metric μ, characterizes the velocity-sensitivity of the DSRTs and is expressed as the time required to lengthen the passive muscles from DSRT to TSRT at the respective stretch velocity. The original formulation of the TSRT, DSRT and μ is summarized. Then, a thorough search of literature prior to December 2023 was conducted that returned 25 papers that have used the technique. Eleven of these papers come from the research group of the authors, including 1 reporting on treatment effects. Of the remaining 14 papers, 11 report variations of the methodology with different populations and 3 report on the effects of an intervention. The review discusses how specific modifications to data collection and analysis procedures have either improved the methodology or, in some cases, led to uninterpretable results. The influence of modifications to the data collection and analysis procedures is discussed., (Copyright © 2024 International Federation of Clinical Neurophysiology. Published by Elsevier B.V. All rights reserved.)

The primary motor cortex does not uniquely or directly produce alpha motoneurone (α-MN) drive to muscles during voluntary movement. Rather, α-MN drive emerges from the synthesis and competition among excitatory and inhibitory inputs from multiple descending tracts, spinal interneurons, sensory inputs, and proprioceptive afferents. One such fundamental input is velocity-dependent stretch reflexes in lengthening muscles, which should be inhibited to enable voluntary movement. It remains an open question, however, the extent to which unmodulated stretch reflexes disrupt voluntary movement, and whether and how they are inhibited in limbs with numerous multiarticular muscles. We used a computational model of a Rhesus Macaque arm to simulate movements with feedforward α-MN commands only, and with added velocity-dependent stretch reflex feedback. We found that velocity-dependent stretch reflex caused movement-specific, typically large and variable disruptions to arm movements. These disruptions were greatly reduced when modulating velocity-dependent stretch reflex feedback (i) as per the commonly proposed (but yet to be clarified) idealized alpha-gamma (α-γ) coactivation or (ii) an alternative α-MN collateral projection to homonymous γ-MNs. We conclude that such α-MN collaterals are a physiologically tenable propriospinal circuit in the mammalian fusimotor system. These collaterals could still collaborate with α-γ coactivation, and the few skeletofusimotor fibers (β-MNs) in mammals, to create a flexible fusimotor ecosystem to enable voluntary movement. By locally and automatically regulating the highly nonlinear neuro-musculo-skeletal mechanics of the limb, these collaterals could be a critical low-level enabler of learning, adaptation, and performance via higher-level brainstem, cerebellar, and cortical mechanisms., Competing Interests: Competing interests statement:The authors declare no competing interests.

Reaching movements generally show smooth kinematic profiles that are invariant across varying movement speeds even as interaction torques and muscle properties vary nonlinearly with speed. How the brain brings about these invariant profiles is an open question. We developed an analytical inverse dynamics method to estimate descending activation patterns directly from observed joint angle trajectories based on a simple model of the stretch reflex, and of muscle and biomechanical dynamics. We estimated descending activation patterns for experimental data from eight different planar two-joint movements performed at two movement times (fast: 400 ms; slow: 800 ms). The temporal structure of descending activation differed qualitatively across speeds, consistent with the idea that the nervous system uses an internal model to generate anticipatory torques during fast movement. This temporal structure also depended on the cocontraction level of antagonistic muscle groups. Comparing estimated muscle activation and descending activation revealed the contribution of the stretch reflex to movement generation that was found to set in after about 20% of movement time. NEW & NOTEWORTHY By estimating descending activation patterns directly from observed movement kinematics based on a model of the dynamics of the stretch reflex, of muscle force generation, and of the biomechanics of the limb, we observed how brain signals must be temporally structured to enable fast movement.

Introduction: Diabetic peripheral neuropathy is the most common complication of diabetes producing metabolic disruptions in the peripheral nervous system. Alteration in the predictable nature of tendon reflexes is the most common indicator suggesting the possibility of diabetic neuropathy. Evaluation of tendon reflexes is a part of various clinical scoring systems that assess neuropathy. The conventional reflex grading scales are subjective, lack temporal data, and have high inter-rater variability. Hence, an indigenous quantification tool was developed to evaluate the tendon reflexes in order to assess diabetic peripheral neuropathy., Materials and Methods: A cross-sectional study was carried out in 140 healthy volunteers and 140 patients with type 2 diabetes. The mean age of controls and diabetics (49.1 ± 8.9, 50.7 ± 7.5) years, weight (66.9 ± 9.4, 69.8 ± 11.5) kilograms and BMI (24.5 ± 3.8, 26.1 ± 4.7), respectively. All of them are subjected to evaluation of tendon reflexes using the reflex quantification tool comprised of surface mechanomyography and electrogoniometry that can provide various static and dynamic variables of tendon reflex., Results: The dynamic variables such as reflex amplitude, muscle velocity and angular velocity were significantly low in diabetic patients (p: <0.001) whereas latency and duration (p: <0.001) were prolonged. Furthermore, no significant difference was observed in the application of tendon striking force (p: 0.934) among the participants., Conclusion: The current study demonstrates that the proposed reflex quantification tool provides several dynamic variables of patellar tendon reflex, which are significantly affected and altered in diabetic patients suggesting the involvement of peripheral neurons., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Research Trust of DiabetesIndia (DiabetesIndia) and National Diabetes Obesity and Cholesterol Foundation (N-DOC). Published by Elsevier Ltd. All rights reserved.)

Spasticity attributable to exaggerated stretch reflex pathways, particularly affecting the ankle plantar flexors, often impairs overground walking in persons with incomplete spinal cord injury. Compelling evidence from rodent models underscores how exposure to acute intermittent hypoxia (AIH) can provide a unique medium to induce spinal plasticity in key inhibitory pathways mediating stretch reflex excitability and potentially affect spasticity. In this study, we quantify the effects of a single exposure to AIH on the stretch reflex in able-bodied individuals. We hypothesized that a single sequence of AIH will increase the stretch reflex excitability of the soleus muscle during ramp-and-hold angular perturbations applied to the ankle joint while participants perform passive and volitionally matched contractions. Our results revealed that a single AIH exposure did not significantly change the stretch reflex excitability during both passive and active matching conditions. Furthermore, we found that able-bodied individuals increased their stretch reflex response from passive to active matching conditions after both sham and AIH exposures. Together, these findings suggest that a single AIH exposure might not engage inhibitory pathways sufficiently to alter stretch reflex responses in able-bodied persons. However, the generalizability of our present findings requires further examination during repetitive exposures to AIH along with potential reflex modulation during functional movements, such as overground walking., (© 2024 The Authors. Experimental Physiology published by John Wiley & Sons Ltd on behalf of The Physiological Society.)

Most individuals experience their dominant arm as being more dexterous than the non-dominant arm, but the neural mechanisms underlying this asymmetry in motor behaviour are unclear. Using a delayed-reach task, we have recently demonstrated strong goal-directed tuning of stretch reflex gains in the dominant upper limb of human participants. Here, we used an equivalent experimental paradigm to address the neural mechanisms that underlie the preparation for reaching movements with the non-dominant upper limb. There were consistent effects of load, preparatory delay duration and target direction on the long latency stretch reflex. However, by comparing stretch reflex responses in the non-dominant arm with those previously documented in the dominant arm, we demonstrate that goal-directed tuning of short and long latency stretch reflexes is markedly weaker in the non-dominant limb. The results indicate that the motor performance asymmetries across the two upper limbs are partly due to the more sophisticated control of reflexive stiffness in the dominant limb, likely facilitated by the superior goal-directed control of muscle spindle receptors. Our findings therefore suggest that fusimotor control may play a role in determining performance of complex motor behaviours and support existing proposals that the dominant arm is better supplied than the non-dominant arm for executing more complex tasks, such as trajectory control., (© 2023 The Authors. European Journal of Neuroscience published by Federation of European Neuroscience Societies and John Wiley & Sons Ltd.)

The dynamic nature of movement and muscle activation emphasizes the importance of a sound experimental design. To ensure that an experiment determines what we intend, the design must be carefully evaluated. Before analyzing data, it is imperative to limit the number of outliers, biases, and skewness. In the present study, a simple center-out experiment was performed by 16 healthy volunteers. The experiment included three load conditions, two preparatory delays, two perturbations, and four targets placed along a diagonal path on a 2D plane. While the participants performed the tasks, the activity of seven arm muscles were monitored using surface electromyography (EMG). Principal component analysis (PCA) was used to evaluate the study design, identify muscle synergies, and assess the effects of individual quirks. With PCA, we can identify the trials that trigger stretch reflexes and pinpoint muscle synergies. The posterior deltoid, triceps long head, and brachioradialis were engaged when targets were in the direction of muscle shortening and the perturbation was applied in the opposite direction. Similarly, the pectoralis and anterior deltoid were engaged when the targets were in the direction of muscle shortening and the perturbation was applied in the opposite direction. The stretch reflexes were not triggered when the perturbation brought the hand in the direction of, or into the target, except if the muscle was pre-loaded. The use of PCA was also proven valuable when evaluating participant performance. While individual quirks are to be expected, failure to perform trials as expected can adversely affect the study results., Competing Interests: The authors have declared that no competing interests exist., (Copyright: © 2023 Frida Torell. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.)

Although rigidity is a cardinal motor sign in patients with Parkinson's disease (PD), the instrumental measurement of this clinical phenomenon is largely lacking, and its pathophysiological underpinning remains still unclear. Further advances in the field would require innovative methodological approaches able to measure parkinsonian rigidity objectively, discriminate the different biomechanical sources of muscle tone (neural or visco-elastic components), and finally clarify the contribution to 'objective rigidity' exerted by neurophysiological responses, which have previously been associated with this clinical sign (i.e. the long-latency stretch-induced reflex). Twenty patients with PD (67.3 ± 6.9 years) and 25 age- and sex-matched controls (66.9 ± 7.4 years) were recruited. Rigidity was measured clinically and through a robotic device. Participants underwent robot-assisted wrist extensions at seven different angular velocities randomly applied, when ON therapy. For each value of angular velocity, several biomechanical (i.e. elastic, viscous and neural components) and neurophysiological measures (i.e. short and long-latency reflex and shortening reaction) were synchronously assessed and correlated with the clinical score of rigidity (i.e. Unified Parkinson's Disease Rating Scale-part III, subitems for the upper limb). The biomechanical investigation allowed us to measure 'objective rigidity' in PD and estimate the neuronal source of this phenomenon. In patients, 'objective rigidity' progressively increased along with the rise of angular velocities during robot-assisted wrist extensions. The neurophysiological examination disclosed increased long-latency reflexes, but not short-latency reflexes nor shortening reaction, in PD compared with control subjects. Long-latency reflexes progressively increased according to angular velocities only in patients with PD. Lastly, specific biomechanical and neurophysiological abnormalities correlated with the clinical score of rigidity. 'Objective rigidity' in PD correlates with velocity-dependent abnormal neuronal activity. The observations overall (i.e. the velocity-dependent feature of biomechanical and neurophysiological measures of objective rigidity) would point to a putative subcortical network responsible for 'objective rigidity' in PD, which requires further investigation., (© The Author(s) 2023. Published by Oxford University Press on behalf of the Guarantors of Brain. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.)

Decisions for action are accompanied by a continual processing of sensory information, sometimes resulting in a revision of the initial choice, called a change of mind (CoM). Although the motor system is tuned during the formation of a reach decision, it is unclear whether its preparatory state differs between CoM and non-CoM decisions. To test this, participants ( n = 14) viewed a random-dot motion (RDM) stimulus of various coherence levels for a random viewing duration. At the onset of a mechanical perturbation that rapidly stretched the pectoralis muscle, they indicated the perceived motion direction by making a reaching movement to one of two targets. Using electromyography (EMG), we quantified the reflex gains of the pectoralis and posterior deltoid muscles. Results show that reflex gains scaled with both the coherence level and the viewing duration of the stimulus. We fit a drift diffusion model (DDM) to the behavioral choices. The decision variable (DV), derived from the DDM, correlated well with the measured reflex gain at the single-trial level. However, when matched on DV magnitude, reflex gains were significantly lower in CoM than non-CoM trials. We conclude that the internal state of the motor system, as measured by the spinal reflexes, reflects the continual deliberation on sensory evidence for action selection, including the postdecisional evidence that can lead to a change of mind. NEW & NOTEWORTHY Using behavioral findings, EMG, and computational modeling, we show that not only the perceptual decision outcome but also the accumulating evidence for that outcome is continuously sent to the relevant muscles. Moreover, we show that an upcoming change of mind can be detected in the motor periphery, suggesting that a correlate of the internal decision making process is being sent along.

Spasticity is characterized by a velocity-dependent increase in the tonic stretch reflex. Evidence suggests that spasticity originates from hyperactivity in the descending tract or reflex loop. To pinpoint the source of hyperactivity, however, is difficult due to lack of human data in-vivo. Thus, we implemented a neuromorphic model to revive the neurodynamics with spiking neuronal activity. Two types of input were modeled: (1) the additive condition (ADD) to apply tonic synaptic inputs directly into the reflex loop; (2) the multiplicative (MUL) condition to adjust the loop gains within the reflex loop. Results show that both conditions produced antagonist EMG responses resembling patient data. The timing of spasticity is more sensitive to the ADD condition, whereas the amplitude of spastic EMG is more sensitive to the MUL condition. In conclusion, our model shows that both additive and multiplicative hyperactivities suffice to elicit velocity-dependent spastic electromyographic signals (EMG), but with different sensitivities. This simulation study suggests that spasticity caused by different origins may be discernable by the progression of severity, which may help individualized goalsetting and parameter-selection in rehabilitation.Clinical Relevance-Potential application of neuromorphic modeling on spasticity includes selection of parameters for therapeutic plans, such as movement range, repetition, and load.

Muscle stiffness is altered following postmastectomy breast reconstruction and breast cancer treatment. The exact mechanisms underlying these alterations are unknown; however, muscle stretch reflexes may play a role. This work examined short- (SLR) and long-latency (LLR) shoulder muscle stretch reflexes in breast cancer survivors. Forty-nine patients who had undergone postmastectomy breast reconstruction, 17 who had undergone chemoradiation, and 18 healthy, age-matched controls were enrolled. Muscle activity was recorded from the clavicular and sternocostal regions of the pectoralis major and anterior, middle, and posterior deltoids during vertical ab/adduction or horizontal flex/extension perturbations while participants maintained minimal torques. SLR and LLR were quantified for each muscle. Our major finding was that following postmastectomy breast reconstruction, SLR and LLR are impaired in the clavicular region of the pectoralis major. Individuals who had chemoradiation had impaired stretch reflexes in the clavicular and sternocostal region of the pectoralis major, anterior, middle, and posterior deltoid. These findings indicate that breast cancer treatments alter the regulation of shoulder muscle stretch reflexes and may be associated with surgical or nonsurgical damage to the pectoral fascia, muscle spindles, and/or sensory Ia afferents. NEW & NOTEWORTHY Shoulder muscle stretch reflexes may be impacted following postmastectomy breast reconstruction and chemoradiation. Here, we examined short- and long-latency shoulder muscle stretch reflexes in two experiments following common breast reconstruction procedures and chemoradiation. We show impairments in pectoralis major stretch reflexes following postmastectomy breast reconstruction and pectoralis major and deltoid muscle stretch reflexes following chemoradiation. These findings indicate that breast cancer treatments alter the regulation of shoulder muscle stretch reflexes.

Voluntary movements are prepared before they are executed. Preparatory activity has been observed across the CNS and recently documented in first-order neurons of the human PNS (i.e., in muscle spindles). Changes seen in sensory organs suggest that independent modulation of stretch reflex gains may represent an important component of movement preparation. The aim of the current study was to further investigate the preparatory modulation of short-latency stretch reflex responses (SLRs) and long-latency stretch reflex responses (LLRs) of the dominant upper limb of human subjects. Specifically, we investigated how different target parameters (target distance and direction) affect the preparatory tuning of stretch reflex gains in the context of goal-directed reaching, and whether any such tuning depends on preparation duration and the direction of background loads. We found that target distance produced only small variations in reflex gains. In contrast, both SLR and LLR gains were strongly modulated as a function of target direction, in a manner that facilitated the upcoming voluntary movement. This goal-directed tuning of SLR and LLR gains was present or enhanced when the preparatory delay was sufficiently long (>250 ms) and the homonymous muscle was unloaded [i.e., when a background load was first applied in the direction of homonymous muscle action (assistive loading)]. The results extend further support for a relatively slow-evolving process in reach preparation that functions to modulate reflexive muscle stiffness, likely via the independent control of fusimotor neurons. Such control can augment voluntary goal-directed movement and is triggered or enhanced when the homonymous muscle is unloaded., Competing Interests: The authors declare no competing financial interests., (Copyright © 2023 Torell et al.)

The spinal stretch reflex is a fundamental building block of motor function, with a sensitivity that varies continuously during movement and when changing between movement and posture. Many have investigated task-dependent reflex sensitivity, but few have provided simple, quantitative analyses of the relationship between the volitional control and stretch reflex sensitivity throughout tasks that require coordinated activity of several muscles. Here, we develop such an analysis and use it to test the hypothesis that modulation of reflex sensitivity during movement can be explained by the balance of activity within agonist and antagonist muscles better than by activity only in the muscle homonymous with the reflex. Subjects completed hundreds of flexion and extension movements as small, pseudorandom perturbations of elbow angle were applied to obtain estimates of stretch reflex amplitude throughout the movement. A subset of subjects performed a postural control task with muscle activities matched to those during movement. We found that reflex modulation during movement can be described by background activity in antagonist muscles about the elbow much better than by activity only in the muscle homonymous to the reflex ( P < 0.001). Agonist muscle activity enhanced reflex sensitivity, whereas antagonist activity suppressed it. Surprisingly, the magnitude of these effects was similar, suggesting a balance of control between agonists and antagonists very different from the dominance of sensitivity to homonymous activity during posture. This balance is due to a large decrease in sensitivity to homonymous muscle activity during movement rather than substantial changes in the influence of antagonistic muscle activity. NEW & NOTEWORTHY This study examined the sensitivity of the stretch reflexes elicited in elbow muscles to the background activity in these same muscles during movement and postural tasks. We found a heightened reciprocal control of reflex sensitivity during movement that was not present during maintenance of posture. These results help explain previous discrepancies in reflex sensitivity measured during movement and posture and provide a simple model for assessing their contributions to muscle activity in both tasks.

Stretch reflexes are crucial for performing accurate movements and providing rapid corrections for unpredictable perturbations. Stretch reflexes are modulated by supraspinal structures via corticofugal pathways. Neural activity in these structures is difficult to observe directly, but the characterization of reflex excitability during volitional movement can be used to study how these structures modulate reflexes and how neurological injuries impact this control, such as in spasticity after stroke. We have developed a novel protocol to quantify stretch reflex excitability during ballistic reaching. This novel method was implemented using a custom haptic device (NACT-3D) capable of applying high-velocity (270 °/s) joint perturbations in the plane of the arm while participants performed 3D reaching tasks in a large workspace. We assessed the protocol on four participants with chronic hemiparetic stroke and two control participants. Participants reached ballistically from a near to a far target, with elbow extension perturbations applied in random catch trials. Perturbations were applied before movement, during the early phase of movement, or near peak movement velocity. Preliminary results show that stretch reflexes were elicited in the stroke group in the biceps muscle during reaching, as measured by electromyographic (EMG) activity both before (pre-motion phase) and during (early motion phase) movement. Reflexive EMG was also seen in the anterior deltoid and pectoralis major in the pre-motion phase. In the control group, no reflexive EMG was seen, as expected. This newly developed methodology allows the study of stretch reflex modulation in new ways by combining multijoint movements with haptic environments and high-velocity perturbations.

Background: Spasticity is defined as "a motor disorder characterised by a velocity dependent increase in tonic stretch reflexes (muscle tone) with exaggerated tendon jerks". It is a highly prevalent condition following stroke and other neurological conditions. Clinical assessment of spasticity relies predominantly on manual, non-instrumented, clinical scales. Technology based solutions have been developed in the last decades to offer more specific, sensitive and accurate alternatives but no consensus exists on these different approaches., Method: A systematic review of literature of technology-based methods aiming at the assessment of spasticity was performed. The approaches taken in the studies were classified based on the method used as well as their outcome measures. The psychometric properties and usability of the methods and outcome measures reported were evaluated., Results: 124 studies were included in the analysis. 78 different outcome measures were identified, among which seven were used in more than 10 different studies each. The different methods rely on a wide range of different equipment (from robotic systems to simple goniometers) affecting their cost and usability. Studies equivalently applied to the lower and upper limbs (48% and 52%, respectively). A majority of studies applied to a stroke population (N = 79). More than half the papers did not report thoroughly the psychometric properties of the measures. Analysis identified that only 54 studies used measures specific to spasticity. Repeatability and discriminant validity were found to be of good quality in respectively 25 and 42 studies but were most often not evaluated (N = 95 and N = 78). Clinical validity was commonly assessed only against clinical scales (N = 33). Sensitivity of the measure was assessed in only three studies., Conclusion: The development of a large diversity of assessment approaches appears to be done at the expense of their careful evaluation. Still, among the well validated approaches, the ones based on manual stretching and measuring a muscle activity reaction and the ones leveraging controlled stretches while isolating the stretch-reflex torque component appear as the two promising practical alternatives to clinical scales. These methods should be further evaluated, including on their sensitivity, to fully inform on their potential., (© 2022. The Author(s).)

In targeted movements of the hand, descending activation patterns must not only generate muscle activation but also adjust spinal reflexes from stabilizing the initial to stabilizing the final postural state. We estimate descending activation patterns that change minimally while generating a targeted movement within a given movement time based on a model of the biomechanics, the muscle dynamics, and the stretch reflex. The estimated descending activation patterns predict human movement trajectories quite well. Their temporal structure varies across workspace and with movement speed, from monotonic profiles for slow movements to nonmonotonic profiles for fast movements. Descending activation patterns at different speeds thus do not result from a mere rescaling of invariant templates but reflect varying needs to compensate for interaction torques and muscle dynamics. The virtual attractor trajectories, on which active muscle torques are zero, lie within reachable workspace and are largely invariant when represented in end-effector coordinates. Their temporal structure along movement direction changes from linear ramps to "N-shaped" profiles with increasing movement speed. NEW & NOTEWORTHY The descending activation patterns driving movement must be integrated with spinal reflexes, which would resist movement if left unchanged. We estimate the descending activation patterns at different movement speeds using a model of the stretch reflex and of muscle and limb dynamics. The descending activation patterns we find are temporally structured to compensate for interaction torques as predicted by internal models but also shift the reflex threshold, solving the posture-movement problem.

The unique anatomy of the shoulder allows for expansive mobility but also sometimes precarious stability. It has long been suggested that stretch-sensitive reflexes contribute to maintaining joint stability through feedback control, but little is known about how stretch-sensitive reflexes are coordinated between the muscles of the shoulder. The purpose of this study was to investigate the coordination of stretch reflexes in shoulder muscles elicited by rotations of the glenohumeral joint. We hypothesized that stretch reflexes are sensitive to not only a given muscle's background activity but also the aggregate activity of all muscles crossing the shoulder based on the different groupings of muscles required to actuate the shoulder in three rotational degrees of freedom. We examined the relationship between a muscle's background activity and its reflex response in eight shoulder muscles by applying rotational perturbations while participants produced voluntary isometric torques. We found that this relationship, defined as gain scaling, differed at both short and long latencies based on the direction of voluntary torque generated by the participant. Therefore, gain scaling differed based on the aggregate of muscles that were active, not just the background activity in the muscle within which the reflex was measured. Across all muscles, the consideration of torque-dependent gain scaling improved model fits (Δ R 2 ) by 0.17 ± 0.12. Modulation was most evident when volitional torques and perturbation directions were aligned along the same measurement axis, suggesting a functional role in resisting perturbations among synergists while maintaining task performance. NEW & NOTEWORTHY Careful coordination of muscles crossing the shoulder is needed to maintain the delicate balance between the joint's mobility and stability. We provide experimental evidence that stretch reflexes within shoulder muscles are modulated based on the aggregate activity of muscles crossing the joint, not just the activity of the muscle in which the reflex is elicited. Our results reflect coordination through neural coupling that may help maintain shoulder stability during encounters with environmental perturbations.

Deep tendon reflexes are one of the main parameters of the neurological examination in many diseases. Reflex responses increase in upper motor neuron diseases due to a lack of suprasegmental control such as spasticity and rigidity. This information provided by the reflex response makes it an indispensable element of neurological examination. However, an important limitation is that this assessment is subjective. In this study, EMG and kinesiology measurements were recorded together during the assessment of the patellar T reflex in healthy control, spasticity, and Parkinson's disease groups. Nine kinesiologic and three electrophysiologic features were extracted. We validated the proposed method with three healthy participants by ten repeated measurements on 6 different days and we observed that angular velocity is the most stable parameter. Clustering of different groups determined with K-clustering and artificial neural network used for classification with kinesiological and EMG inputs. Our findings show that reflex grade can be determined with high accuracy (Acc = 98.6) in a large population for both pathological and healthy groups and angular velocity is sufficient for reflex grading. Therefore, we think that our study will contribute to the literature by providing an approach with high reliability and reproducibility in the quantitative assessment of reflexes., (© 2022. International Federation for Medical and Biological Engineering.)

Medical interest in the knee-jerk reflex began in about 1875 with simultaneous and independent publications by Wilhelm Heinrich Erb (1840-1921) and Carl Friedrich Otto Westphal (1833-1890) contending that the knee jerk was absent (and the ankle clonus was present) in all clear cases of locomotor ataxia (tabes dorsalis). Physicians in the medical communities of Europe, Great Britain, and North America responded with case and large group studies that tested this contention. These studies revealed the usefulness of the knee jerk and other myotatic reflexes, but also unexpected characteristics. The knee jerk, apparently so simple, proved to be a complex phenomenon depending the strength of the strike on the patella, induced muscle tension, and inhibition from the brain. Was it a reflex with afferent and efferent nerves and an intervening process in the spinal cord, or was it a local phenomenon confined to the muscle itself? Experimental studies directed at the reflex issue investigated latencies from patella strike to leg extension or muscle contraction and compared them with latencies from direct muscle strikes and theoretical calculations based on reflex components. Such studies were unable to resolve the reflex issue during the nineteenth century. The physicians were shown to be limited, like all scientific explorers of the unknown, by their knowledge, methodology, and technology.

Objectives: Stretch reflex responses were considered to be affected by the velocity of muscle fiber lengthening and angular velocity. However, the results of previous studies in vivo and in vitro are inconsistent in this regard. The purpose of the present study was to investigate the effects of the velocity of fascicle lengthening on the amplitude of the stretch reflex for each trial with a high angular velocity and wide range of motion., Methods: Thirteen healthy men volunteered for this study. While the ankle was passively moved from 100 to 80 deg at five different angular velocities (100, 200, 300, 500, and 600 deg⋅s -1 ), the velocity of fascicle lengthening in the soleus muscle was measured using ultrasonography. In addition, the amplitude of the short latency stretch reflex in the soleus muscle was also measured., Results: As angular velocity increased, the amplitude of the stretch reflex and velocity of fascicle lengthening significantly increased (both p<0.001). For each trial in all subjects, the amplitude of the stretch reflex was not correlated with the velocity of fascicle lengthening at any of the angular velocities., Conclusion: In conclusion, the stretch reflex size is not related to the fascicle behavior in each trial., Competing Interests: The authors have no conflict of interest.

In this paper, we review the legacy of Gerald (Gerry) Gottlieb in various fields related to the neural control of human movement. His studies on the myotatic (stretch) reflex and postmyotatic responses to ankle joint perturbations paved the way for current explorations of long-loop reflexes and their role in the control of movement. The dual-strategy hypothesis introduced order into a large body of literature on the triphasic muscle activation patterns seen over a variety of voluntary movements in healthy persons. The dual-strategy hypothesis continues to be important for understanding the performance of subjects with disordered motor control. The principle of linear synergy (covariance of joint torques) was an attempt to solve one of the notorious problems of motor redundancy, which remains an important topic in the field. Gerry's attitude toward the equilibrium-point hypothesis varied between rejection and using it to explore patterns of hypothetical control variables and movement variability. The discovery of reciprocal excitation in healthy neonates fostered other studies of changes in spinal cord physiology as motor skills develop. In addition, studies of people with spasticity and the effects of treatment with intrathecal baclofen were crucial in demonstrating the possibility of unmasking voluntary movements after suppression of the hyperreflexia of spasticity. Gerry Gottlieb contributed a significant body of knowledge that formed a solid foundation from which to study a variety of neurological diseases and their treatments, and a more comprehensive and parsimonious foundation to describe the neural control of human movement.

The Hoffmann reflex (H-reflex), as an electrical analog to the stretch reflex, allows electrophysiological validation of the integrity of neural circuits after injuries such as spinal cord damage or stroke. An increase of the H-reflex response, together with symptoms like non-voluntary muscle contractions, pathologically augmented stretch reflex, and hypertonia in the corresponding muscle, is an indicator of post-stroke spasticity (PSS). In contrast to rather nerve-unspecific transcutaneous measurements, here, we present a protocol to quantify the H-reflex directly at the ulnar and median nerves of the forepaw, which is applicable, with minor modifications, to the tibial and sciatic nerve of the hindpaw. Based on the direct stimulation and the adaptation to different nerves, the method represents a reliable and versatile tool to validate electrophysiological changes in spasticity-related disease models.

Purpose: The purpose of the present study was to determine the effect of the short latency stretch reflex on passive and active muscle stiffness in the soleus muscle., Methods: Fourteen males participated in this study. Passive muscle stiffness was calculated from passive muscle force and fascicle length during passive lengthening at four different angular velocities (5, 50, 100, and 200 deg s -1 ). Active muscle stiffness was also calculated according to changes in muscle force and fascicle length during fast lengthening at five different angular velocities (100, 200, 300, 500, and 600 deg s -1 ) after submaximal isometric contraction (30% of maximal voluntary contraction). During the measurements of passive and active muscle stiffness, the amplitude of the short latency stretch reflex in the soleus muscle was measured., Results: Change in passive torque and passive muscle stiffness significantly increased, whereas change in the fascicle length decreased, as angular velocity increased. At 100 and 200 deg s -1 (stretch reflex responses were not observed at 5 and 50 deg s -1 ), the amplitude of the short latency stretch reflex was highly correlated with passive muscle stiffness. Change in torque and active muscle stiffness were highest at 100 deg s -1 and decreased as angular velocity increased. At all angular velocities under active conditions, the amplitude of the short latency stretch reflex was not correlated with active muscle stiffness., Conclusion: These results suggest that the short latency stretch reflex affects passive muscle stiffness in the soleus muscle, but not active muscle stiffness., (© 2022. The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.)

Friedreich's ataxia (FA) is a hereditary system degeneration, which progressively affects sensory functions such as proprioceptive feedback, which causes progressive ataxia in FA patients. While major clinical features of movement disorders in FA patients have been identified, the underlying impaired neural control is not sufficiently understood. To elucidate the underlying control mechanism, we investigated single-joint movements of the upper limb in FA patients. Small, tolerable force perturbations were induced during voluntary single-joint arm movements to examine the compensatory reaction of the FA patient's motor system. Movement kinematics were measured, and muscle torques were quantified. We first found that as in healthy subjects, unperturbed single-joint movements in FA patients preserved similar temporal profiles of hand velocity and muscle torques, however, scaled in duration and amplitude. In addition, the small perturbations were compensated for efficiently in both groups, with the endpoint error < 0.5° (maximum displacement of 5-15°). We further quantified the differences in movement time, torque response, and displacement between patients and controls. To distinguish whether these differences were caused by a malfunction of top-down control or a malfunction of feedback control, the responses were fitted with a detailed model of the stretch reflex. The model simulations revealed that the feedback delay, but not the feedback gain was affected in FA patients. They also showed that the descending control signal was scaled in time and amplitude and co-contraction was smaller in FA patients. Thus, our study explains how the motor deficits of FA patients result from pathological alterations of both top-down and feedback control., (© 2022. The Author(s).)

Objectives: Whole-body vibration (WBV) is applied to the sole of the foot, whereas local mechanical vibration (LMV) is applied directly to the muscle or tendon. The time required for the mechanical stimulus to reach the muscle belly is longer for WBV. Therefore, the WBV-induced muscular reflex (WBV-IMR) latency may be longer than the tonic vibration reflex (TVR) latency. The aim of this study was to determine whether the difference between WBV-IMR and TVR latencies is due to the distance between the vibration application point and the target muscle., Methods: Eight volunteers participated in this study. The soleus reflex response was recorded during WBV, LMVs, and tendon tap. LMVs were applied to the Achilles tendon and sole of the foot. The latencies were calculated using the cumulative averaging technique., Results: The latency (33.4±2.8 ms) of the soleus reflex induced by the local foot vibration was similar to the soleus TVR latency (30.9±3.2 ms) and T-reflex (32.0±2.4 ms) but significantly shorter than the latency of the soleus WBV-IMR (42.3±3.4 ms) (F (3,21) =27.46, p=0.0001, partial η 2 =0.797)., Conclusions: The present study points out that the neuronal circuitries of TVR and WBV-IMR are different., Competing Interests: The authors have no conflict of interest.

Purpose: The objective of this study was to clarify the characteristics of the upper limb stretch reflex in wrestlers., Methods: Ten wrestlers and 11 control subjects participated in the study. The experiment was divided into two sessions. In the extension perturbation session, participants either relaxed or flexed the elbow when they felt a perturbation (abrupt elbow extension induced by a dynamometer). This was done 30 times by each subject for both sessions. In the flexion perturbation session, participants also relaxed or extended the elbow when they felt a perturbation (abrupt elbow flexion). During the tasks, the stretch reflex was monitored by recording the surface EMG activities of the right biceps and triceps brachii. The EMG reflex components were divided into three periods based on the time after the perturbation (M1, 20-50 ms; M2, 50-80 ms; and M3, 80-100 ms). The averaged background EMG activity just before the disturbance was subtracted from the EMG activity in each period. The resultant value was integrated to obtain reflex magnitudes of M1 to M3., Results: For the triceps brachii, in the relaxation task, the wrestler group showed a significantly smaller value for M2 than did the control group. In the extension task, the wrestler group showed a significantly larger value for M3 than did the control group. There was no difference in M1 between the two groups. For the biceps brachii, there was no significant difference between any reflex components., Conclusions: Our results suggest that high-level wrestlers have specific characteristics of the long-latency stretch reflex in the triceps brachii that are modulated in a situation-specific manner., (Copyright © 2021 The Author(s). Published by Wolters Kluwer Health, Inc. on behalf of the American College of Sports Medicine.)

Normal subjects can completely eliminate resistance upon imposed head-on-trunk rotations when they are asked to relax. It is not, however, clear how neck reflexes to stretch can be voluntarily suppressed. Reflexive responses might be modified by adjusting the gain of the reflex loop through descending control. Theoretically, necessary corrections upon interfering disturbances during coordinated motor performance requiring the interplay of relaxation/activation may be missing if muscle relaxation is taking place exclusively by this mechanism. It has been alternatively proposed that sensory information from the periphery may be allowed to "neutralize" neck reflexes if it is fed back with opposite sign to the structures driving the reflexes. Six healthy subjects were asked to relax while subjected to head-on-trunk rotations generated by a head motor. After any initial resistance had completely subsided, the head was unexpectedly exposed to "ramp-and-hold" perturbations of up to 2° amplitude and 0.7 s duration. Resistance to stretch consistently reappeared thereupon, suggesting that stretch reflex gain had not been set to zero during the previously achieved complete relaxation. Resistance to perturbations under these circumstances was compared with the forces generated when the same ramp-and-hold displacements were delivered unpredictably to the head held stationary. A quantitative model of neck proprioceptive reflexes suppression has been thus constructed. Gain scheduling or "motor set" cannot sufficiently account for the voluntary reflex suppression during slow passive head rotations. Instead, we propose as underlying mechanism, the "neutralization" of the controlling servo by means of continuous feedback tracking displacement and force signals from the periphery. NEW & NOTEWORTHY Head stabilizing neck reflexes can be voluntarily suppressed or activated depending on the task at hand. By applying brief perturbations unexpectedly, both during passive head-on-trunk movements and at rest, we investigated the mechanism of voluntary suppression of resistance to stretch. A physiologically plausible, neuromechanical model of voluntary/reflexive interactions was constructed favoring feedback over reflex gain adjustments. Accordingly, muscle relaxation during imposed head movements is based on sensory feedback similarly to muscle contractions during purposeful movements.

Introduction/aims: Motion artifact signals (MASs) created by the relative movement of intramuscular wire electrodes are an indicator of the mechanical stimulus arrival time to the muscle belly. This study proposes a method that uses wire electrodes as an intramuscular mechanosensor to determine the stretch reflex (SR) latency without lag time., Methods: Gastrocnemius SR was induced by tendon tap, heel tap, and forefoot tap. The MASs recorded by intramuscular wire electrodes were extracted from background electromyographic activity using the spike-triggered averaging technique. Simultaneous recordings were obtained from multiple sites to validate the MAS technique., Results: Using intramuscular wire electrodes, the MASs were successfully determined and extracted for all stimulus sites. In the records from the rectus femoris, MASs were also successfully extracted; thus, the reflex latency could be calculated., Discussion: Wire electrodes can be used as an intramuscular mechanosensor to determine the mechanical stimulus arrival time to the muscle belly., (© 2021 Wiley Periodicals LLC.)

Mechanical muscle tendon vibration activates multiple sensory receptors in the muscle and tendon. In particular, tendon vibration tends to activate the Ia afferents the strongest, but also will activate group II and Ib afferents. This activation can cause three main effects in the central nervous system: proprioceptive illusions, tonic vibration reflexes, and suppression of the stretch response. Noisy tendon vibration has been used to assess the frequency characteristics of proprioceptive reflexes and, interestingly there appeared to be no evidence for proprioceptive illusions or tonic vibration reflexes during standing [9]. However, it remains unknown if noisy vibration induces a suppression of the muscle stretch response. Therefore, the purpose of this study was to investigate the effects of noisy and periodic tendon vibration on the stretch response in the flexor carpi radialis muscle (FCR). We examined FCR stretch responses with and without periodic (20 and 100 Hz) and noisy (∼10-100 Hz) tendon vibration. We additionally had participants perform the task under the instruction set to either not respond to the perturbation or to respond as fast as possible. The key finding from this study was that both periodic and noisy vibration resulted in a reduced stretch response amplitude. Additionally, it was found that a participant's intent to respond did not modulate the amount of suppression observed. The findings from this study provide a more detailed understanding of the effects of tendon vibration on the muscle stretch response., (Copyright © 2021 Elsevier B.V. All rights reserved.)

Motorized assessment of the stretch reflex is instrumental to gain understanding of the stretch reflex, its physiological origin and to differentiate effects of neurological disorders, like spasticity. Both short-latency (M1) and medium-latency (M2) stretch reflexes have been reported to depend on the velocity and acceleration of an applied ramp-and-hold perturbation. In the upper limb, M2 has also been reported to depend on stretch duration. However, wrong conclusions might have been drawn in previous studies as the interdependence of perturbation parameters (amplitude, duration, velocity, and acceleration) possibly created uncontrolled, confounding effects. We disentangled the duration-, velocity-, and acceleration-dependence and their interactions of the M1 and M2 stretch reflex in the ankle plantarflexors. To disentangle the parameter interdependence, 49 unique ramp-and-hold joint perturbations elicited reflexes in 10 healthy volunteers during a torque control task. Linear mixed model analysis showed that M1 depended on acceleration, not velocity or duration, whereas M2 depended on acceleration, velocity, and duration. Simulations of the muscle spindle Ia afferents coupled to a motoneuron pool corroborated these experimental findings. In addition, this simulation model did show a nonlinear M1 velocity- and duration-dependence for perturbation parameters outside the experimental scope. In conclusion, motorized assessment of the stretch reflex or spasticity using ramp-and-hold perturbations should be systematically executed and reported. Our systematic motorized and simulation assessments showed that M1 and M2 depend on acceleration, velocity, and duration of the applied perturbation. The simulation model suggested that these dependencies emerge from: muscle-tendon unit and muscle cross-bridge dynamics, Ia sensitivity to force and yank, and motoneuron synchronization. NEW & NOTEWORTHY Previous research and definitions of the stretch reflex and spasticity have focused on velocity-dependence. We showed that perturbation acceleration, velocity, and duration all shape the M1 and M2 response, often via nonlinear or interacting dependencies. Consequently, systematic execution and reporting of stretch reflex and spasticity studies, avoiding uncontrolled parameter interdependence, is essential for proper understanding of the reflex neurophysiology.

Objective: The aim of this study was to investigate the interrater reliability and intrarater reliability of the flexor pollicis longus muscle stretch reflex (FPLR) and compare it with clinically established reflexes., Design: A total of 71 healthy volunteers participated. The FPLR, biceps reflex, brachioradialis reflex, and patellar tendon reflex of each participant were tested bilaterally and rated by eight examiners (four experienced, four inexperienced). For intrarater reliability evaluation, five examiners rated the reflexes of four volunteers at four different points in time., Results: Analysis of the interrater reliability with Gwet's AC1 demonstrated almost perfect agreement for FPLR (Gwet's AC1 = 0.90), biceps reflex (Gwet's AC1 = 0.90), and patellar tendon reflex (Gwet's AC1 = 0.95) when using binary data (reflex present vs. absent). Only fair agreement was found for the brachioradialis reflex (Gwet's AC1 = 0.56). Experienced raters had a higher agreement than inexperienced raters did when rating the biceps reflex and the patellar tendon reflex. The intrarater reliability was almost perfect for the patellar tendon reflex (Gwet's AC1 = 0.94), followed by the FPLR (Gwet's AC1 = 0.83) with substantial agreement and the biceps reflex (Gwet's AC1 = 0.57) with moderate agreement., Conclusion: The FPLR is a reliable diagnostic neuromuscular test and may therefore be useful in the clinical examination for C8/T1 nerve root lesions or pathologies of the interosseous anterior nerve., To Claim Cme Credits: Complete the self-assessment activity and evaluation online at http://www.physiatry.org/JournalCME., Cme Objectives: Upon completion of this article, the reader should be able to: (1) Elicit the flexor pollicis longus muscle stretch reflex; (2) Discuss the disadvantage of kappa statistics in assessing the interrater reliability when the prevalence of the studied trait is very high or very low; and (3) Name the spinal nerves involved in the innervation of the flexor pollicis longus muscle., Level: Advanced., Accreditation: The Association of Academic Physiatrists is accredited by the Accreditation Council for Continuing Medical Education to provide continuing medical education for physicians.The Association of Academic Physiatrists designates this Journal-based CME activity for a maximum of 1.0 AMA PRA Category 1 Credit(s)™. Physicians should only claim credit commensurate with the extent of their participation in the activity., Competing Interests: Financial disclosure statements have been obtained, and no conflicts of interest have been reported by the authors or by any individuals in control of the content of this article., (Copyright © 2021 Wolters Kluwer Health, Inc. All rights reserved.)

Objective: To determine inter-rater reliability, minimal detectable change and responsiveness of Tonic Stretch Reflex Threshold (TSRT) as a quantitative measure of elbow flexor spasticity., Methods: Elbow flexor spasticity was assessed in 55 patients with sub-acute stroke by determining TSRT, the angle of spasticity onset at rest (velocity = 0°/s). Elbow flexor muscles were stretched 20 times at different velocities. Dynamic stretch-reflex thresholds, the elbow angles corresponding to the onset of elbow flexor EMG at each velocity, were used for TSRT calculation. Spasticity was also measured with the Modified Ashworth Scale (MAS). In a sub-group of 44 subjects, TSRT and MAS were measured before and after two weeks of an upper-limb intervention., Results: The intraclass correlation coefficient was 0.65 and the 95% minimal detectable change was 32.4°. In the treated sub-group, TSRT, but not MAS significantly changed. TSRT effect size and standardized response mean were 0.40 and 0.35, respectively. Detection of clinically meaningful improvements in upper-limb motor impairment by TSRT change scores ranged from poor to excellent., Conclusions: Evaluation of stroke-related elbow flexor spasticity by TSRT has good inter-rater reliability. Test responsiveness is low, but better than that of the MAS., Significance: TSRT may be used to complement current scales of spasticity quantification., (Copyright © 2021 International Federation of Clinical Neurophysiology. Published by Elsevier B.V. All rights reserved.)

The central nervous system of humans and other animals modulates spinal cord activity to achieve several locomotion behaviors. Previous neuromechanical models investigated the modulation of human gait changing selected parameters belonging to CPGs (Central Pattern Generators) feedforward oscillatory structures or to feedback reflex circuits. CPG-based models could replicate slow and fast walking by changing only the oscillation's properties. On the other hand, reflex-based models could achieve different behaviors through optimizations of large dimensional parameter spaces. However, they could not effectively identify individual key reflex parameters responsible for gait characteristics' modulation. This study investigates which reflex parameters modulate the gait characteristics through neuromechanical simulations. A recently developed reflex-based model is used to perform optimizations with different target behaviors on speed, step length, and step duration to analyze the correlation between reflex parameters and their influence on these gait characteristics. We identified nine key parameters that may affect the target speed ranging from slow to fast walking (0.48 and 1.71 m/s) as well as a large range of step lengths (0.43 and 0.88 m) and step duration (0.51, 0.98 s). The findings show that specific reflexes during stance significantly affect step length regulation, mainly given by positive force feedback of the ankle plantarflexors' group. On the other hand, stretch reflexes active during swing of iliopsoas and gluteus maximus regulate all the gait characteristics under analysis. Additionally, the results show that the hamstrings' group's stretch reflex during the landing phase is responsible for modulating the step length and step duration. Additional validation studies in simulations demonstrated that the modulation of identified reflexes is sufficient to regulate the investigated gait characteristics. Thus, this study provides an overview of possible reflexes involved in modulating speed, step length, and step duration of human gaits., Competing Interests: The authors have declared that no competing interests exist.

Background: Hemiparesis following stroke is often accompanied by spasticity. Spasticity is one factor among the multiple components of the upper motor neuron syndrome that contributes to movement impairment. However, the specific contribution of spasticity is difficult to isolate and quantify. We propose a new method of quantification and evaluation of the impact of spasticity on the quality of movement following stroke., Methods: Spasticity was assessed using the Tonic Stretch Reflex Threshold (TSRT). TSRT was analyzed in relation to stochastic models of motion to quantify the deviation of the hemiparetic upper limb motion from the normal motion patterns during a reaching task. Specifically, we assessed the impact of spasticity in the elbow flexors on reaching motion patterns using two distinct measures of the 'distance' between pathological and normal movement, (a) the bidirectional Kullback-Liebler divergence (BKLD) and (b) Hellinger's distance (HD). These measures differ in their sensitivity to different confounding variables. Motor impairment was assessed clinically by the Fugl-Meyer assessment scale for the upper extremity (FMA-UE). Forty-two first-event stroke patients in the subacute phase and 13 healthy controls of similar age participated in the study. Elbow motion was analyzed in the context of repeated reach-to-grasp movements towards four differently located targets. Log-BKLD and HD along with movement time, final elbow extension angle, mean elbow velocity, peak elbow velocity, and the number of velocity peaks of the elbow motion were computed., Results: Upper limb kinematics in patients with lower FMA-UE scores (greater impairment) showed greater deviation from normality when the distance between impaired and normal elbow motion was analyzed either with the BKLD or HD measures. The severity of spasticity, reflected by the TSRT, was related to the distance between impaired and normal elbow motion analyzed with either distance measure. Mean elbow velocity differed between targets, however HD was not sensitive to target location. This may point at effects of spasticity on motion quality that go beyond effects on velocity., Conclusions: The two methods for analyzing pathological movement post-stroke provide new options for studying the relationship between spasticity and movement quality under different spatiotemporal constraints.

Efficiently controlling the movement of our hand requires coordinating the motion of multiple joints of the arm. Although it is widely assumed that this type of efficient control is implemented by processing that occurs in the cerebral cortex and brainstem, recent work has shown that spinal circuits can generate efficient motor output that supports keeping the hand in a static location. Here, we show that a spinal pathway can also efficiently control the hand during reaching. In our first experiment, we applied multijoint mechanical perturbations to participants' elbow and wrist as they began reaching toward a target. We found that spinal stretch reflexes evoked in elbow muscles were not proportional to how much the elbow muscles were stretched but instead were dependent on the hand's location relative to the target. In our second experiment, we applied the same elbow and wrist perturbations but had participants change how they grasped the manipulandum, diametrically altering how the same wrist perturbation moved the hand relative to the reach target. We found that changing the arm's orientation diametrically altered how spinal reflexes in the elbow muscles were evoked, and in such a way that were again dependent on the hand's location relative to the target. These findings demonstrate that spinal circuits can help efficiently control the hand during dynamic reaching actions and show that efficient and flexible motor control is not exclusively dependent on processing that occurs within supraspinal regions of the nervous system. NEW & NOTEWORTHY We have previously shown that spinal circuits can rapidly generate reflex responses that efficiently engage multiple joints to support postural hand control of the upper limb. Here, we show that spinal circuits can also rapidly generate such efficient responses during reaching actions.

Both linear and nonlinear electromyographic (EMG) connectivity has been reported during the expression of stretch reflexes, though it is not clear whether they are generated by the same neural pathways. To answer this question, we aim to distinguish linear and nonlinear connectivity, as well as their delays in muscle responses, resulting from continuous elbow joint perturbations. We recorded EMG from Biceps Brachii muscle when eight able-bodied participants were performing a steady elbow flexion torque while simultaneously receiving a continuous position perturbation. Using a recently developed phase coupling metric, we estimated linear and nonlinear connectivity as well as their associated delays between Biceps EMG responses and perturbations. We found that the time delay for linear connectivity (24.5 ± 5.4 ms) is in the range of short-latency stretch reflex period (< 35 ms), while that for nonlinear connectivity (53.8 ± 3.2 ms) is in the range of long-latency stretch reflex period (40-70 ms). These results suggest that the estimated linear connectivity between EMG and perturbations is very likely generated by the mono-synaptic spinal stretch reflex loop, while the nonlinear connectivity may be associated with multi-synaptic supraspinal stretch reflex loops. As such, this study provides new evidence of the nature of neural connectivity related to the stretch reflex.

Sudden foot dorsiflexion lengthens soleus muscle and activates stretch-based spinal reflexes. Dorsiflexion can be triggered by activating tibialis anterior (TA) muscle through peroneal nerve stimulation or transcranial magnetic stimulation (TMS) which evokes a response in the soleus muscle referred to as Medium Latency Reflex (MLR) or motor-evoked potential-80 (Soleus MEP80), respectively. This study aimed to examine the relationship between these responses in humans. Therefore, latency characteristics and correlation of responses between soleus MEP80 and MLR were investigated. We have also calculated the latencies from the onset of tibialis activity, i.e., subtracting of TA-MEP from MEP80 and TA direct motor response from MLR. We referred to these calculations as Stretch Loop Latency Central (SLLc) for MEP80 and Stretch Loop Latency Peripheral (SLLp) for MLR. The latency of SLLc was found to be 61.4 ± 5.6 ms which was significantly shorter (P = 0.0259) than SLLp (64.0 ± 4.2 ms) and these latencies were correlated (P = 0.0045, r = 0.689). The latency of both responses was also found to be inversely related to the response amplitude (P = 0.0121, r = 0.451) probably due to the activation of large motor units. When amplitude differences were corrected, i.e. investigating the responses with similar amplitudes, SLLp, and SLLc latencies found to be similar (P = 0.1317). Due to the identical features of the soleus MEP80 and MLR, we propose that they may both have spinal origins., (Copyright © 2020 Elsevier Ltd. All rights reserved.)

No studies in ACL-D individuals have examined neuromuscular adaptations during landing from a jump where an unexpected mechanical event changes the pre-programmed course of movement. The purpose of this study was to compare pre- and post-landing muscle activation in ACL-D individuals and uninjured controls during normal and surprise landings. Nineteen ACL-D and 17 uninjured volunteered. Participants performed repeated single leg landings from 30 and 15 cm heights. During 15 cm landings, a single surprise landing was performed where participants unexpectedly fell through a false surface at 15 cm to the solid floor a further 15 cm below. Electromyography (EMG) amplitude from vastus lateralis (VL), lateral hamstrings (LH), and soleus (Sol) was recorded. Pre-landing (-60 to 0 ms), post-landing short latency (31-60 ms), and post-landing medium latency (61-90 ms) periods were examined. Comparisons in EMG amplitudes were made across limbs (ACL-D, ACL intact, and control) in 30 cm landings. Additionally, the ratio of EMG amplitude in surprise:30 cm normal landings was analyzed. Post-landing LH EMG was reduced in the ACL-D compared to control limbs at short latencies (P < 0.05). Post-landing VL EMG was reduced in the ACL-D and ACL intact compared to the control limb at both latencies (P < 0.05). Surprise landings notably increased post-landing EMG in all muscles, across all limbs (P < 0.001). However, the gain in VL EMG was significantly greater in ACL-D and ACL intact limbs (P < 0.05). These changes in neuromuscular control of ACL-D individuals during expected and surprise landings may have important implications for rehabilitation, instability, and the risk of secondary injury., (© 2020 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd.)

Spasticity is one of the major problems that arise in different neurological diseases and seriously affect the quality of human life. Research on the understanding of mechanism of spasticity remains as important as the studies on the spasticity therapy and rehabilitation. In this study, the spasticity mechanism which develops concerning the upper motor neuron lesions is investigated by modelling "Patella tendon reflex triggered patella pendulum". The mathematical model based on the pendulum phenomenon is developed by solving the curve-fitting problem as finding the curve that best fits a set of data points. Electrophysiological and dynamic measurement data were taken from 76 spastic subjects and 20 healthy participants. The mathematical model is determined by the morphological properties of the goniometric variations. The results denote that the mathematical model containing two clinically relevant parameters -frequency component of the damped oscillatory motion defined as "f 0 " with the maximum angle of the reflex defined as "a 0 " ensures to distinguish spasticity from healthy subjects.

Many of us know about stretch reflexes from the doctor's office, when a physician taps the tendon near our kneecap to elicit a quick knee extension. This procedure is used as a diagnostic tool to determine the integrity of the spinal cord and the extension response it elicits may seem otherwise useless. In fact, the tendon tap taps into one aspect of a critical building block of mammalian motor control, the stretch reflexes. Stretch reflexes are often thought to quickly resist unexpected changes in muscle length via a very simple circuit in the spinal cord, and this is one circuit that the tendon tap engages. It turns out, however, that stretch reflexes support a myriad of functions and are highly flexible. Under naturalistic conditions, stretch reflexes are shaped by peripheral physiology and engage neural circuits spanning the spinal cord, brainstem and cerebral cortex. In this Primer, we outline what is currently known about stretch reflex function and its underlying mechanisms, with a specific focus on how the cascade of nested responses collectively known as stretch reflexes interact with and build off of one another to support real-world motor behavior., (Copyright © 2020 Elsevier Inc. All rights reserved.)

Aim: To assess the neural and non-neural contributions to spasticity in the impaired ankle of children with cerebral palsy (CP)., Method: Instrumented tapping of the Achilles tendon was done isometrically to minimize non-neural contributions and elicit neural contributions. Robot-controlled ankle stretching was done at various velocities, including slow stretching, with minimized neural contributions. Spasticity was assessed as having neural (phasic and tonic stretch reflex torque, tendon reflex gain, contraction rate, and half relaxation rate) and non-neural origin (elastic stiffness and viscous damping) in 17 children with CP (six females and 11 males; mean age [SD] 10y 8mo [3y 11mo], range 4y-18y) and 17 typically developing children (six females and 11 males; mean age [SD] 12y 7mo [2y 9mo], range 7y-18y). All torques were normalized to weight×height., Results: Children with CP showed increased phasic and tonic stretch reflex torque (p=0.004 and p=0.001 respectively), tendon reflex gain (p=0.02), contraction rate (p=0.038), half relaxation rate (p=0.02), elastic stiffness (p=0.01), and viscous damping (p=0.01) compared to typically developing children., Interpretation: Controlled stretching and instrumented tendon tapping allow the systematic quantification of various neural and non-neural changes in CP, which can be used to guide impairment-specific treatment., What This Paper Adds: Ankle spasticity is associated with increased phasic and tonic stretch reflexes, tendon reflex gain, and contraction and half relaxation rates. Ankle spasticity is also associated with increased elastic stiffness and viscous damping., (© 2020 Mac Keith Press.)

Spasticity is one of the most common symptoms present in humans with spinal cord injury (SCI); however, its clinical assessment remains underdeveloped. The purpose of the study was to examine the contribution of passive muscle stiffness and active spinal reflex mechanisms to clinical outcomes of spasticity after SCI. It is important that passive and active contributions to increased muscle stiffness are distinguished to make appropriate decisions about antispastic treatments and to monitor its effectiveness. To address this question, we combined biomechanical and electrophysiological assessments of ankle plantarflexor muscles bilaterally in individuals with and without chronic SCI. Spasticity was assessed using the Modified Ashworth Scale (MAS) and a self-reported questionnaire. We performed slow and fast dorsiflexion stretches of the ankle joint to measure passive muscle stiffness and reflex-induced torque using a dynamometer and the soleus H reflex using electrical stimulation over the posterior tibial nerve. All SCI participants reported the presence of spasticity. While 96% of them reported higher spasticity on one side compared with the other, the MAS detected differences across sides in only 25% of the them. Passive muscle stiffness and the reflex-induced torque were larger in SCI compared with controls more on one side compared with the other. The soleus stretch reflex, but not the H reflex, was larger in SCI compared with controls and showed differences across sides, with a larger reflex in the side showing a higher reflex-induced torque. MAS scores were not correlated with biomechanical and electrophysiological outcomes. These findings provide evidence for bilateral and asymmetric contributions of passive and active ankle plantar flexors stiffness to spasticity in humans with chronic SCI and highlight a poor agreement between a self-reported questionnaire and the MAS for detecting asymmetries in spasticity across sides. NEW & NOTEWORTHY Spasticity affects a number of people with spinal cord injury (SCI). Using biomechanical, electrophysiological, and clinical assessments, we found that passive muscle properties and active spinal reflex mechanisms contribute bilaterally and asymmetrically to spasticity in ankle plantarflexor muscles in humans with chronic SCI. A self-reported questionnaire had poor agreement with the Modified Ashworth Scale in detecting asymmetries in spasticity. The nature of these changes might contribute to the poor sensitivity of clinical exams.

Plastic adaptations are known to take place in muscles, tendons, joints, and the nervous system in response to changes in muscle activity. However, few studies have addressed how these plastic adaptations are related. Thus this study focuses on changes in the mechanical properties of the ankle plantarflexor muscle-tendon unit, stretch reflex activity, and spinal neuronal pathways in relation to cast immobilization. The left rat hindlimb from toes to hip was immobilized with a plaster cast for 1, 2, 4, or 8 wk followed by acute electrophysiological recordings to investigate muscle stiffness and stretch reflex torque. Moreover, additional acute experiments were performed after 4 wk of immobilization to investigate changes in the central gain of the stretch reflex. Monosynaptic reflexes (MSR) were recorded from the L4 and L5 ventral roots following stimulation of the corresponding dorsal roots. Rats developed reduced range of movement in the ankle joint 2 wk after immobilization. This was accompanied by significant increases in the stiffness of the muscle-tendon complex as well as an arthrosis at the ankle joint at 4 and 8 wk following immobilization. Stretch reflexes were significantly reduced at 4-8 wk following immobilization. This was associated with increased central gain of the stretch reflex. These data show that numerous interrelated plastic changes occur in muscles, connective tissue, and the central nervous system in response to changes in muscle use. The findings provide an understanding of coordinated adaptations in multiple tissues and have important implications for prevention and treatment of the negative consequences of immobilization following injuries of the nervous and musculoskeletal systems. NEW & NOTEWORTHY Immobilization leads to multiple simultaneous adaptive changes in muscle, connective tissue, and central nervous system.

Objectives: The corticospinal system (CS) regulates muscle activation through shifts in muscle-level tonic stretch-reflex thresholds (TSRT). This ability is impaired in stroke and contributes to sensorimotor impairments such as spasticity. We determined the role of CS in elbow flexor activity regulation in healthy and post-stroke subjects. We also determined whether CS modulation deficits were related to sensorimotor impairment intensity in post-stroke individuals., Methods: Seventeen healthy (59.8 ± 12.2 yr) and 27 stroke subjects (58.7 ± 10.1 yr) had transcranial magnetic stimulation (TMS) applied over the primary motor cortex (M1) flexor representation to elicit motor-evoked potentials (MEPs) in elbow flexors in different angular positions. In a subset of post-stroke subjects (n = 12), flexor TSRTs were measured in passive and active conditions, and TSRT modulation was determined., Results: Position-related MEP amplitude modulation was similar in healthy and mild stroke subjects, while subjects with more severe stroke exhibited less consistent modulation. MEP modulation in stroke was related to clinical upper limb motor impairment, spasticity, and the ability to modulate TSRTs between passive and active elbow movements., Conclusions: CS output was closely related to TSRT modulation. Impairments in TSRT regulation may underlie motor deficits in moderate-to-severe post-stroke individuals., Significance: Translation of these neurophysiological findings to clinical applications may enhance post-stroke motor recovery., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2020 International Federation of Clinical Neurophysiology. Published by Elsevier B.V. All rights reserved.)

Background: Increment of compound muscle action potential amplitude is a diagnostic hallmark of Lambert-Eaton myasthenic syndrome (LEMS). Making a diagnosis can be challenging, therefore, a proper cutoff for abnormal increment is highly relevant for improved recognition of this rare disease., Methods: We determined the sensitivity and specificity of 60% and 100% cutoff values in all consecutive patients who underwent increment testing in our hospital from 1999 to 2016., Results: We included 156 patients, 63 with LEMS and 93 without LEMS. Sensitivity of a 60% cutoff for increment testing was 77.8% (95% confidence interval 65.5%-87.3%) and 58.7% (45.6%-71.0%) for 100%. Specificity was 98.9% (94.2%-100%) and 100% (96.1%-100%) using a threshold of 60% and 100%, respectively., Conclusions: Lowering the cutoff value for abnormal increment to 60% greatly increases sensitivity to diagnose LEMS without an overt loss in specificity., (© 2020 The Authors. Muscle & Nerve published by Wiley Periodicals, Inc.)

Study Design: An experimental design., Objectives: The aim of this study was to determine the latencies of vibration-induced reflexes in individuals with and without spinal cord injury (SCI), and to compare these latencies to identify differences in reflex circuitries., Setting: A tertiary rehabilitation center in Istanbul., Methods: Seventeen individuals with chronic SCI (SCI group) and 23 participants without SCI (Control group) were included in this study. Latency of tonic vibration reflex (TVR) and whole-body vibration-induced muscular reflex (WBV-IMR) of the left soleus muscle was tested for estimating the reflex origins. The local tendon vibration was applied at six different vibration frequencies (50, 85, 140, 185, 235, and 265 Hz), each lasting for 15 s with 3-s rest intervals. The WBV was applied at six different vibration frequencies (35, 37, 39, 41, 43, and 45 Hz), each lasting for 15 s with 3-s rest intervals., Results: Mean (SD) TVR latency was 39.7 (5.3) ms in the SCI group and 35.9 (2.7) ms in the Control group with a mean (95% CI) difference of -3.8 (-6.7 to -0.9) ms. Mean (SD) WBV-IMR latency was 45.8 (7.4) ms in the SCI group and 43.3 (3.0) ms in the Control group with a mean (95% CI) difference of -2.5 (-6.5 to 1.4) ms. There were significant differences between TVR latency and WBV-IMR latency in both the groups (mean (95% CI) difference; -6.2 (-9.3 to -3.0) ms, p = 0.0001 for the SCI group and -7.4 (-9.3 to -5.6) ms, p = 0.011 for Control group)., Conclusions: The results suggest that the receptor of origin of TVR and WBV-IMR may be different.

In an uncertain external environment, the motor system may need to respond rapidly to an unexpected stimulus. Limb displacement causes muscle stretch; the corrective response has multiple activity bursts, which are suggested to originate from different parts of the neuraxis. The earliest response is so fast, it can only be produced by spinal circuits; this is followed by slower components thought to arise from primary motor cortex (M1) and other supraspinal areas. Spinal cord (SC) contributions to the slower components are rarely considered. To address this, we recorded neural activity in M1 and the cervical SC during a visuomotor tracking task, in which 2 female macaque monkeys moved their index finger against a resisting motor to track an on-screen target. Following the behavioral trial, an increase in motor torque rapidly returned the finger to its starting position (lever velocity >200°/s). Many cells responded to this passive mechanical perturbation (M1: 148 of 211 cells, 70%; SC: 67 of 119 cells, 56%). The neural onset latency was faster for SC compared with M1 cells (21.7 ± 11.2 ms vs 25.5 ± 10.7 ms, respectively, mean ± SD). Using spike-triggered averaging, some cells in both regions were identified as likely premotor cells, with monosynaptic connections to motoneurons. Response latencies for these cells were compatible with a contribution to the muscle responses following the perturbation. Comparable fractions of responding neurons in both areas were active up to 100 ms after the perturbation, suggesting that both SC circuits and supraspinal centers could contribute to later response components. SIGNIFICANCE STATEMENT Following a limb perturbation, multiple reflexes help to restore limb position. Given conduction delays, the earliest part of these reflexes can only arise from spinal circuits. By contrast, long-latency reflex components are typically assumed to originate from supraspinal centers. We recorded from both spinal and motor cortical cells in monkeys responding to index finger perturbations. Many spinal interneurons, including those identified as projecting to motoneurons, responded to the perturbation; the timing of responses was compatible with a contribution to both short- and long-latency reflexes. We conclude that spinal circuits also contribute to long-latency reflexes in distal and forearm muscles, alongside supraspinal regions, such as the motor cortex and brainstem., (Copyright © 2020 Soteropoulos and Baker.)

In normal humans, tapping the forehead produces a neck muscle reflex that is used clinically to test vestibular function, the cervical vestibular evoked myogenic potential (cVEMP). As stretch receptors can also be activated by skull taps, we investigated the origin of the early and late peaks of the bone-conducted cVEMP. In twelve normal participants, we differentially stimulated the vestibular and neck stretch receptors by applying vibration to the forehead (activating both vestibular and stretch receptors) and to the sternum (activating mainly stretch receptors). Patients with bilateral vestibulopathy (BVP; n = 26) and unilateral vestibular loss (uVL; n = 17) were also investigated for comparison. Comparison of peaks in normal subjects suggested that the early peaks were vestibular-dependent, while the later peaks had mixed vestibular and stretch input. The late peaks were present but small (1.1 amplitude ratio) in patients with BVP and absent VEMPs, confirming that they do not strictly depend on vestibular function, and largest in age-matched controls (1.5 amplitude ratio, p = 0.049), suggesting that there is an additional vestibular reflex at this latency (approx. 30 ms). Patients with uVL had larger late peaks on the affected than the normal side (1.4 vs 1.0 amplitude ratio, p = 0.034). The results suggest that the early responses in SCM to skull vibration in humans are vestibular-dependent, while there is a late stretch reflex bilaterally and a late vestibular reflex in the contralateral muscle.