Deficits in corticospinal control of stretch reflex thresholds in stroke: Implications for motor impairment

Predicting response to non-invasive brain stimulation in post-stroke upper extremity motor impairment: the importance of neurophysiological and clinical biomarkers

BACKGROUND

Non-invasive brain stimulation (NIBS) is a promising approach to enhance upper extremity motor impairment (UEMI) recovery in post-stroke individuals. However, variability in treatment response poses a significant challenge. Identifying neurophysiological and clinical biomarkers that predict NIBS response could improve personalization and treatment efficacy.

OBJECTIVES

This study aims to determine the predictive relevance of neurophysiological and clinical biomarkers for responses to NIBS in post-stroke UEMI using a machine learning model.

METHODS

This secondary analysis involved 63 post-stroke individuals with UEMI (age 56.9 ± 11.1 years). A support vector machine model was used to assess the importance of two neurophysiological biomarkers-brain activity in the lesioned hemisphere quantified using quantitative electroencephalography (power ratio index, PRI) and corticospinal tract (CST) integrity assessed via transcranial magnetic stimulation-and one clinical biomarker-the level of UEMI assessed with Fugl-Meyer upper extremity (FMA-UE)-in predicting responders (ΔFMA-UE ≥ 5 points) and those with excellent response (ΔFMA-UE ≥ 10 points) to NIBS based on the change of FMA-UE before and after treatment.

RESULTS

Of the 63 participants, 42 (65%) were classified as responders, and 14 (22%) demonstrated excellent responses. Predictive importance for responders was 0.78 for PRI-LH, 0.21 for UEMI level, and 0.01 for CST integrity. For predicting excellent responses, PRI-LH had an importance of 0.39, UEMI level 0.37, and CST integrity 0.24.

CONCLUSIONS

The study highlights the importance of electrical brain activity in the LH and UEMI level in predicting NIBS responders and excellent responses, with CST integrity being particularly valuable for excellent outcomes.

An objective method to quantify elbow flexor spasticity using surface EMG and 3D motion analysis

Spasticity in the upper extremities, particularly elbow flexor spasticity, significantly impairs motor control. Evaluating the extent of spasticity is crucial for effective therapy planning and assessing treatment outcomes. However, there are currently no accurate and reliable measures to quantify upper extremity spasticity. This study aims to introduce an instrumented assessment method for evaluating elbow flexor spasticity using an integrated approach tailored for spasticity assessment. This clinical study included 17 patients with elbow flexor spasticity (mean age 40 ± 20 years) and 20 arms of 10 healthy adults (mean age 33 ± 8 years). The elbow flexors were passively stretched at low and high velocities, and kinematic data were recorded using 3D motion analysis (U.L.E.M.A. model). Muscle excitations of the biceps brachii were assessed via surface EMG. Outcome parameters included the maximum elbow extension deficit during slow and fast passive stretch, EMG data normalized to maximum voluntary isometric contraction (MVIC) at low and high velocities, and the difference between the two (EMGchange). All outcome parameters showed significant differences (p < 0.05) between patients with elbow flexor spasticity and healthy adults. The proposed instrumented assessment tool is a suitable measurement method for evaluating elbow flexor spasticity.

Clinical Relevance of the Tonic Stretch Reflex Threshold and μ as Measures of Upper Limb Spasticity and Motor Impairment After Stroke

BACKGROUND

After a central nervous system lesion, the ability to control muscle activation and relaxation in specific joint ranges may be impaired. The underlying mechanism of this sensorimotor impairment is related to a decreased ability to regulate the tonic stretch reflex threshold (TSRT) through descending and peripheral control processes. In dynamics, the reflex threshold and its velocity-sensitivity (μ) describe how movement in specific upper limb (UL) joint ranges is impaired after stroke.

OBJECTIVE

To examine the relationships between measures of elbow flexor impairment using TSRT and μ, and clinical scores of spasticity and motor function. We hypothesized that TSRT and μ would be related to clinical spasticity and motor impairment scores in patients with acute and chronic stroke.

METHODS

TSRT, μ, and clinical data of the resistance to passive movement (Modified Ashworth Scale) and UL motor function (Fugl-Meyer Assessment [FMA]) were collected from 120 patients. Relationships between variables were determined using simple correlations and multiple regression analysis.

RESULTS

TSRT and μ explained 72.0% of the variance in the FMA of the Upper Extremity [FMA-UE] describing only in-synergy and out-of-synergy movements and reflex function. TSRT explained 68.7% of the variance in the total score of the FMA-UE.

CONCLUSIONS

This study shows for the first time, a significant relationship between deficits in TSRT regulation and μ with UL motor impairment after stroke. TSRT and μ may be valuable clinical biomarkers of sensorimotor impairment for monitoring spontaneous or treatment-induced motor recovery.

Useful and Useless Misnomers in Motor Control

This article addresses the issue of using terms and concepts in motor control that are ill-defined, undefined, and/or imported from nonbiological fields. In many of such cases, the discourse turns nonscientific and unproductive. Some of such terms are potentially useful but need to be properly and exactly defined. Other terms seem to be misleading and nonfixable. There is also an intermediate group with terms that may or may not be useful if defined properly. The paper presents three examples per group: "reflex," "synergy," and "posture" versus "motor program," "efference copy," and "internal model" versus "muscle tone," "stiffness and impedance," and "redundancy." These terms are analyzed assuming that motor control is a branch of natural science, which must be analyzed using laws of nature, not a subfield of the control theory. In the discussion, we also accept the framework of the theory of movement control with spatial referent coordinates as the only example built on laws of nature with clearly formulated physical and physiological nature of the control parameters.

Tonic stretch reflex threshold as a measure of disordered motor control and spasticity - A critical review

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.

ENHANCE proof-of-concept three-arm randomized trial: effects of reaching training of the hemiparetic upper limb restricted to the spasticity-free elbow range

Post-stroke motor recovery processes remain unknown. Timescales and patterns of upper-limb (UL) recovery suggest a major impact of biological factors, with modest contributions from rehabilitation. We assessed a novel impairment-based training motivated by motor control theory where reaching occurs within the spasticity-free elbow range. Patients with subacute stroke (≤ 6 month; n = 46) and elbow flexor spasticity were randomly allocated to a 10-day UL training protocol, either personalized by restricting reaching to the spasticity-free elbow range defined by the tonic stretch reflex threshold (TSRT) or non-personalized (non-restricted) and with/without anodal transcranial direct current stimulation. Outcomes assessed before, after, and 1 month post-intervention were elbow flexor TSRT angle and reach-to-grasp arm kinematics (primary) and stretch reflex velocity sensitivity, clinical impairment, and activity (secondary). Results were analyzed for 3 groups as well as those of the effects of impairment-based training. Clinical measures improved in both groups. Spasticity-free range training resulted in faster and smoother reaches, smaller (i.e., better) arm-plane path length, and closer-to-normal shoulder/elbow movement patterns. Non-personalized training improved clinical scores without improving arm kinematics, suggesting that clinical measures do not account for movement quality. Impairment-based training within a spasticity-free elbow range is promising since it may improve clinical scores together with arm movement quality.Clinical Trial Registration: URL: http://www.clinicaltrials.gov . Unique Identifier: NCT02725853; Initial registration date: 01/04/2016.

Sensory and motor cortical excitability changes induced by rTMS and sensory stimulation in stroke: A randomized clinical trial

Background

The ability to produce coordinated movement is dependent on dynamic interactions through transcallosal fibers between the two cerebral hemispheres of the brain. Although typically unilateral, stroke induces changes in functional and effective connectivity across hemispheres, which are related to sensorimotor impairment and stroke recovery. Previous studies have focused almost exclusively on interhemispheric interactions in the primary motor cortex (M1).

Objective

To identify the presence of interhemispheric asymmetry (ASY) of somatosensory cortex (S1) excitability and to investigate whether S1 repetitive transcranial magnetic stimulation (rTMS) combined with sensory stimulation (SS) changes excitability in S1 and M1, as well as S1 ASY, in individuals with subacute stroke.

Methods

A randomized clinical trial. Participants with a single episode of stroke, in the subacute phase, between 35 and 75 years old, were allocated, randomly and equally balanced, to four groups: rTMS/sham SS, sham rTMS/SS, rTMS/SS, and sham rTMS/Sham SS. Participants underwent 10 sessions of S1 rTMS of the lesioned hemisphere (10 Hz, 1,500 pulses) followed by SS. SS was applied to the paretic upper limb (UL) (active SS) or non-paretic UL (sham SS). TMS-induced motor evoked potentials (MEPs) of the paretic UL and somatosensory evoked potential (SSEP) of both ULs assessed M1 and S1 cortical excitability, respectively. The S1 ASY index was measured before and after intervention. Evaluator, participants and the statistician were blinded.

Results

Thirty-six participants divided equally into groups (nine participants per group). Seven patients were excluded from MEP analysis because of failure to produce consistent MEP. One participant was excluded in the SSEP analysis because no SSEP was detected. All somatosensory stimulation groups had decreased S1 ASY except for the sham rTMS/Sham SS group. When compared with baseline, M1 excitability increased only in the rTMS/SS group.

Conclusion

S1 rTMS and SS alone or in combination changed S1 excitability and decreased ASY, but it was only their combination that increased M1 excitability.

Clinical trial registration

clinicaltrials.gov, identifier (NCT03329807).

Brain-computer interface combined with mental practice and occupational therapy enhances upper limb motor recovery, activities of daily living, and participation in subacute stroke

Background

We investigated the effects of brain-computer interface (BCI) combined with mental practice (MP) and occupational therapy (OT) on performance in activities of daily living (ADL) in stroke survivors.

Methods

Participants were randomized into two groups: experimental (n = 23, BCI controlling a hand exoskeleton combined with MP and OT) and control (n = 21, OT). Subjects were assessed with the functional independence measure (FIM), motor activity log (MAL), amount of use (MAL-AOM), and quality of movement (MAL-QOM). The box and blocks test (BBT) and the Jebsen hand functional test (JHFT) were used for the primary outcome of performance in ADL, while the Fugl-Meyer Assessment was used for the secondary outcome. Exoskeleton activation and the degree of motor imagery (measured as event-related desynchronization) were assessed in the experimental group. For the BCI, the EEG electrodes were placed on the regions of FC3, C3, CP3, FC4, C4, and CP4, according to the international 10-20 EEG system. The exoskeleton was placed on the affected hand. MP was based on functional tasks. OT consisted of ADL training, muscle mobilization, reaching tasks, manipulation and prehension, mirror therapy, and high-frequency therapeutic vibration. The protocol lasted 1 h, five times a week, for 2 weeks.

Results

There was a difference between baseline and post-intervention analysis for the experimental group in all evaluations: FIM (p = 0.001, d = 0.56), MAL-AOM (p = 0.001, d = 0.83), MAL-QOM (p = 0.006, d = 0.84), BBT (p = 0.004, d = 0.40), and JHFT (p = 0.001, d = 0.45). Within the experimental group, post-intervention improvements were detected in the degree of motor imagery (p < 0.001) and the amount of exoskeleton activations (p < 0.001). For the control group, differences were detected for MAL-AOM (p = 0.001, d = 0.72), MAL-QOM (p = 0.013, d = 0.50), and BBT (p = 0.005, d = 0.23). Notably, the effect sizes were larger for the experimental group. No differences were detected between groups at post-intervention.

Conclusion

BCI combined with MP and OT is a promising tool for promoting sensorimotor recovery of the upper limb and functional independence in subacute post-stroke survivors.

Spin of information and inconsistency between abstract and full text in RCTs investigating upper limb rehabilitation after stroke: An overview study

BACKGROUND

Researchers may be tempted to favorably distort the interpretation of their findings when reporting the abstract (i.e., spin). Spin bias overemphasizes the beneficial effects of the intervention compared with the results shown in the full text.

OBJECTIVE

To assess the occurrence of spin bias and incompleteness in reporting abstracts in post-stroke upper limb (UL) rehabilitation randomized clinical trials (RCTs).

METHODS

A sample of 120 post-stroke UL rehabilitation RCTs (indexed in PEDro database), published in English between 2012 and 2020, was included. The completeness of reporting and spin were assessed using the Consolidated Standards of Reporting Trials for Abstracts (CONSORT-A) and the spin checklist. The relationship between CONSORT-A and spin checklist scores with RCT and journal characteristics was assessed.

RESULTS

CONSORT-A and spin checklist scored 5.3±2.4 (max 15-points, higher scores indicating better reporting) and 5.5±2.0 (max 7-points, higher scores indicating presence of spin), respectively; Significant differences were detected between abstract and full-text scores in the CONSORT-A checklist (p < 0.01) and the spin checklist (p < 0.01). Items of the CONSORT-A checklist in the abstracts and full text showed a fair agreement (k = 0.31), while a moderate agreement (k = 0.59) for the spin checklist was detected. Completeness of abstract was associated (R2 = 0.46) with journal Impact Factor (p < 0.01), CONSORT Guideline endorsement (p = 0.04), and abstract word number (p = 0.02). A lower spin was associated with a higher journal Impact Factor (p = 0.01) and CONSORT Guideline endorsement (p = 0.01).

CONCLUSIONS

Post-stroke UL rehabilitation RCTs abstracts were largely incomplete showing spin. Authors, reviewers, publishers, and stakeholders should be aware of this phenomenon. Publishers should consider allowing more words in abstracts to improve the completeness of reporting abstracts. Although we have investigated only stroke rehabilitation, our results suggest that health care professionals of all disciplines should avoid clinical decision-making based solely upon abstracts.

Post-Stroke Treatment with Neuromuscular Functional Electrostimulation of Antagonistic Muscles and Kinesiotherapy Evaluated with Electromyography and Clinical Studies in a Two-Month Follow-Up

The aim of this study was to determine the sustained influence of personalized neuromuscular functional electrical stimulation (NMFES) combined with kinesiotherapy (mainly, proprioceptive neuromuscular facilitation (PNF)) on the activity of muscle motor units acting antagonistically at the wrist and the ankle in a large population of post-stroke patients. Clinical evaluations of spasticity (Ashworth scale), manual muscle testing (Lovett scale), and surface electromyography recordings at rest (rEMG) and during attempts of maximal muscle contraction (mcEMG) were performed three times in 120 post-stroke patients (T0: up to 7 days after the incidence; T1: after 21 days of treatment; T2: after 60 days of treatment). Patients (N = 120) were divided into two subgroups-60 patients received personalized NMFES and PNF treatment (NMFES+K), and the other 60 received only PNF (K). The NMFES+K therapy resulted in a decrease in spasticity and an increase in muscle strength of mainly flexor muscles, in comparison with the K group. A positive correlation between the increase of rEMG amplitudes and high Ashworth scale scores and a positive correlation between low amplitudes of mcEMG and low Lovett scale scores were found in the wrist flexors and calf muscles on the paretic side. Negative correlations were found between the rEMG and mcEMG amplitudes in the recordings. The five-grade alternate activity score of the antagonists' actions improved in the NMFES+K group. These improvements in the results of controlled NMFES treatment combined with PNF in patients having experienced an ischemic stroke, in comparison to the use of kinesiotherapy alone, might justify the application of conjoined rehabilitation procedures based on neurophysiological approaches. Considering the results of clinical and neurophysiological studies, we suppose that NMFES of the antagonistic muscle groups acting at the wrist and the ankle may evoke its positive effects in post-stroke patients by the modulation of the activity more in the spinal motor centers, including the level of Ia inhibitory neurons, than only at the muscular level.

Effect of Object Texture and Weight on Ipsilateral Corticospinal Influences During Bimanual Holding in Humans

We tested the hypothesis that the ipsilateral corticospinal system, like the contralateral corticospinal system, controls the threshold muscle length at which wrist muscles and the stretch reflex begin to act during holding tasks. Transcranial magnetic stimulation was applied over the right primary motor cortex in 21 healthy subjects holding a smooth or coarse block between the hands. Regardless of the lifting force, motor evoked potentials in right wrist flexors were larger for the smooth block. This result was explained based on experimental evidence that motor actions are controlled by shifting spatial stretch reflex thresholds. Thus, the ipsilateral corticospinal system is involved in threshold position control by modulating facilitatory influences of hand skin afferents on motoneurons of wrist muscles during bimanual object manipulation.

Changes in Cortical Activity in Stroke Survivors Undergoing Botulinum Neurotoxin Therapy for Treatment of Focal Spasticity

Background: Botulinum NeuroToxin-A (BoNT-A) relieves muscle spasticity and increases range of motion necessary for stroke rehabilitation. Determining the effects of BoNT-A therapy on brain neuroplasticity could help physicians customize its use and predict its outcome.

Objective: The purpose of this study was to investigate the effects of Botulinum Toxin-A therapy for treatment of focal spasticity on brain activation and functional connectivity.

Design: We used functional Magnetic Resonance Imaging (fMRI) to track changes in blood oxygen-level dependent (BOLD) activation and functional connectivity associated with BoNT-A therapy in nine chronic stroke participants, and eight age-matched controls. Scans were acquired before BoNT-A injections (W0) and 6 weeks after the injections (W6). The task fMRI scan consisted of a block design of alternating mass finger flexion and extension. The voxel-level changes in BOLD activation, and pairwise changes in functional connectivity were analyzed for BoNT-A treatment (stroke W0 vs. W6).

Results: BoNT-A injection therapy resulted in significant increases in brain activation in the contralesional premotor cortex, cingulate gyrus, thalamus, superior cerebellum, and in the ipsilesional sensory integration area. Lastly, cerebellar connectivity correlated with the Fugl-Meyer assessment of motor impairment before injection, while premotor connectivity correlated with the Fugl-Meyer score after injection.

Conclusion: BoNT-A therapy for treatment of focal spasticity resulted in increased brain activation in areas associated with motor control, and cerebellar connectivity correlated with motor impairment before injection. These results suggest that neuroplastic effects might take place in response to improvements in focal spasticity.

A Mirror Bilateral Neuro-Rehabilitation Robot System with the sEMG-Based Real-Time Patient Active Participant Assessment

In this paper, a novel mirror visual feedback-based (MVF) bilateral neurorehabilitation system with surface electromyography (sEMG)-based patient active force assessment was proposed for upper limb motor recovery and improvement of limb inter-coordination. A mirror visual feedback-based human–robot interface was designed to facilitate the bilateral isometric force output training task. To achieve patient active participant assessment, an sEMG signals-based elbow joint isometric force estimation method was implemented into the proposed system for real-time affected side force assessment and participation evaluation. To assist the affected side limb efficiently and precisely, a mirror bilateral control framework was presented for bilateral limb coordination. Preliminary experiments were conducted to evaluate the estimation accuracy of force estimation method and force tracking accuracy of system performance. The experimental results show the proposed force estimation method can efficiently calculate the elbow joint force in real-time, and the affected side limb of patients can be assisted to track output force of the non-paretic side limb for better limb coordination by the proposed bilateral rehabilitation system.

Central pattern generator and human locomotion in the context of referent control of motor actions

Unperturbed human locomotion presumably results from feedforward shifts in stable body equilibrium in the environment, thus avoiding falling and subsequent catching considered in alternative theories of locomotion. Such shifts are achieved by relocation of the referent body configuration at which multiple muscle recruitment begins. Rather than being directly specified by a central pattern generator, multiple muscles are activated depending on the extent to which the body is deflected from the referent, threshold body configuration, as confirmed in previous studies. Based on the referent control theory of action and perception, solutions to classical problems in motor control are offered, including the previously unresolved problem of the integration of central and reflex influences on motoneurons and the problem of how posture and movement are related. The speed of locomotion depends on the rate of shifts in the referent body configuration. The transition from walking to running results from increasing the rate of referent shifts. It is emphasised that there is a certain hierarchy between reciprocal and co-activation of agonist and antagonist muscles during locomotion and other motor actions, which is also essential for the understanding of how locomotor speed is regulated. The analysis opens a new avenue in neurophysiological approaches to human locomotion with clinical implications.

Recent Advances in the Neural Control of Movements: Lessons for Functional Recovery

We review the current views on the control and coordination of movements following the traditions set by Nikolai Bernstein. In particular, we focus on the theory of neural control of effectors - from motor units to individual muscles, to joints, limbs, and to the whole body - with spatial referent coordinates organized into a hierarchy with multiple few-to-many mappings. Further, we discuss synergies ensuring stability of natural human movements within the uncontrolled manifold hypothesis. Synergies are organized within the neural control hierarchy based on the principle of motor abundance. Movement disorders are discussed as consequences of an inability to use the whole range of changes in referent coordinates (as in spasticity) and an inability to ensure controlled stability of salient variables as reflected in indices of multi-element synergies and their adjustments in preparation to actions (as in brain disorders, including Parkinson's disease, multiple-system atrophy, and stroke). At the end of the review, we discuss possible implications of this theoretical approach to peripheral disorders and their rehabilitations using, as an example, osteoarthritis. In particular, "joint stiffening" is viewed as a maladaptive strategy, which can compromise stability of salient variables during walking.

A New Definition of Poststroke Spasticity and the Interference of Spasticity With Motor Recovery From Acute to Chronic Stages

The relationship of poststroke spasticity and motor recovery can be confusing. "True" motor recovery refers to return of motor behaviors to prestroke state with the same end-effectors and temporo-spatial pattern. This requires neural recovery and repair, and presumably occurs mainly in the acute and subacute stages. However, according to the International Classification of Functioning, Disability and Health, motor recovery after stroke is also defined as "improvement in performance of functional tasks," i.e., functional recovery, which is mainly mediated by compensatory mechanisms. Therefore, stroke survivors can execute motor tasks in spite of disordered motor control and the presence of spasticity. Spasticity interferes with execution of normal motor behaviors ("true" motor recovery), throughout the evolution of stroke from acute to chronic stages. Spasticity reduction does not affect functional recovery in the acute and subacute stages; however, appropriate management of spasticity could lead to improvement of motor function, that is, functional recovery, during the chronic stage of stroke. We assert that spasticity results from upregulation of medial cortico-reticulo-spinal pathways that are disinhibited due to damage of the motor cortex or corticobulbar pathways. Spasticity emerges as a manifestation of maladaptive plasticity in the early stages of recovery and can persist into the chronic stage. It coexists and shares similar pathophysiological processes with related motor impairments, such as abnormal force control, muscle coactivation and motor synergies, and diffuse interlimb muscle activation. Accordingly, we propose a new definition of spasticity to better account for its pathophysiology and the complex nuances of different definitions of motor recovery.

Baseline Motor Impairment Predicts Transcranial Direct Current Stimulation Combined with Physical Therapy-Induced Improvement in Individuals with Chronic Stroke

Transcranial direct current stimulation (tDCS) can enhance the effect of conventional therapies in post-stroke neurorehabilitation. The ability to predict an individual's potential for tDCS-induced recovery may permit rehabilitation providers to make rational decisions about who will be a good candidate for tDCS therapy. We investigated the clinical and biological characteristics which might predict tDCS plus physical therapy effects on upper limb motor recovery in chronic stroke patients. A cohort of 80 chronic stroke individuals underwent ten to fifteen sessions of tDCS plus physical therapy. The sensorimotor function of the upper limb was assessed by means of the upper extremity section of the Fugl-Meyer scale (UE-FM), before and after treatment. A backward stepwise regression was used to assess the effect of age, sex, time since stroke, brain lesion side, and basal level of motor function on UE-FM improvement after treatment. Following the intervention, UE-FM significantly improved (p < 0.05), and the magnitude of the change was clinically important (mean 6.2 points, 95% CI: 5.2-7.4). The baseline level of UE-FM was the only significant predictor (R 2 = 0.90, F (1, 76) = 682.80, p < 0.001) of tDCS response. These findings may help to guide clinical decisions according to the profile of each patient. Future studies should investigate whether stroke severity affects the effectiveness of tDCS combined with physical therapy.