Impaired Regulation Post-stroke of Motor Unit Firing Behavior during Volitional Relaxation of Knee Extensor Torque Assessed Using High Density Surface EMG Decomposition

The purpose of this study was to use high density surface EMG recordings to quantify stroke-related abnormalities in motor unit firing behavior during repeated sub-maximal knee extensor contractions. A high density surface EMG system (sEMG) was used to record and extract single motor unit firing behavior in the vastus lateralis muscle of 6 individuals with chronic stroke and 8 controls during repeated sub-maximal isometric knee extension contractions. Paretic motor unit firing rates were increased with subsequent contractions (6.19±0.35 pps vs 7.89±0.66 pps,

Altered motor unit discharge patterns in paretic muscles of stroke survivors assessed using surface electromyography

Hemispheric stroke survivors often show impairments in voluntary muscle activation. One potential source of these impairments could come from altered control of muscle, via disrupted motor unit (MU) firing patterns. In this study, we sought to determine whether MU firing patterns are modified on the affected side of stroke survivors, as compared with the analogous contralateral muscle

Adaptive real-time identification of motor unit discharges from non-stationary high-density surface electromyographic signals

Objective. Estimation of the discharge pattern of motor units by electromyography (EMG) decomposition has been applied for neurophysiologic investigations, clinical diagnosis, and human-machine interfacing. However, most of the methods for EMG decomposition are currently applied offline. Here, we propose an approach for high-density surface EMG decomposition in real-time. Methods. A real-time decomposition scheme including two sessions, offline training and online decomposition, is proposed based on the convolutional kernel compensation algorithm. The estimation parameters, separation vectors and the thresholds for spike extraction, are first computed during offline training, and then they are directly applied to estimate motor unit spike trains (MUSTs) during the online decomposition. The estimation parameters are updated with the identification of new discharges to adapt to non-stationary conditions. The decomposition accuracy was validated on simulated EMG signals by convolving synthetic MUSTs with motor unit action potentials (MUAPs). Moreover, the accuracy of the online decomposition was assessed from experimental signals recorded from forearm muscles using a signal-based performance metrics (pulse-to-noise ratio, PNR). Main results. The proposed algorithm yielded a high decomposition accuracy and robustness to non-stationary conditions. The accuracy of MUSTs identified from simulated EMG signals was > 80% for most conditions. From experimental EMG signals, on average, 12±2 MUSTs were identified from each electrode grid with PNR of 25.0±1.8 dB, corresponding to an estimated decomposition accuracy > 75%. Conclusion and Significance. These results indicate the feasibility of real-time identification of motor unit activities non-invasively during variable force contractions, extending the potential applications of high-density EMG as a neural interface

Stroke increases ischemia-related decreases in motor unit discharge rates

Following stroke, hyperexcitable sensory pathways, such as the group III/IV afferents that are sensitive to ischemia, may inhibit paretic motor neurons during exercise. We quantified the effects of whole leg ischemia on paretic vastus lateralis motor unit firing rates during submaximal isometric contractions. Ten chronic stroke survivors (>1 yr poststroke) and 10 controls participated. During conditions of whole leg occlusion, the discharge timings of motor units were identified from decomposition of high-density surface electromyography signals during repeated submaximal knee extensor contractions. Quadriceps resting twitch responses and near-infrared spectroscopy measurements of oxygen saturation as an indirect measure of blood flow were made. There was a greater decrease in paretic motor unit discharge rates during the occlusion compared with the controls (average decrease for stroke and controls, 12.3 ± 10.0% and 0.1 ± 12.4%, respectively; P < 0.001). The motor unit recruitment thresholds did not change with the occlusion (stroke: without occlusion, 11.68 ± 5.83%MVC vs. with occlusion, 11.11 ± 5.26%MVC; control: 11.87 ± 5.63 vs. 11.28 ± 5.29%MVC). Resting twitch amplitudes declined similarly for both groups in response to whole leg occlusion (stroke: 29.16 ± 6.88 vs. 25.75 ± 6.78 Nm; control: 38.80 ± 13.23 vs 30.14 ± 9.64 Nm). Controls had a greater exponential decline (lower time constant) in oxygen saturation compared with the stroke group (stroke time constant, 22.90 ± 10.26 min vs. control time constant, 5.46 ± 4.09 min; P < 0.001). Ischemia of the muscle resulted in greater neural inhibition of paretic motor units compared with controls and may contribute to deficient muscle activation poststroke. NEW & NOTEWORTHY Hyperexcitable inhibitory sensory pathways sensitive to ischemia may play a role in deficient motor unit activation post stroke. Using high-density surface electromyography recordings to detect motor unit firing instances, we show that ischemia of the exercising muscle results in greater inhibition of paretic motor unit firing rates compared with controls. These findings are impactful to neurophysiologists and clinicians because they implicate a novel mechanism of force-generating impairment poststroke that likely exacerbates baseline weakness

Mechanisms of Impaired Motor Unit Firing Behavior in the Vastus Lateralis Muscle after Stroke

The purpose of this dissertation research project was to examine the role of impaired motor unit firing behavior on force generation after a stroke. We studied the relationship between intrinsic motoneuron properties and inhibitory sensory pathways to deficient motoneuron activity in the vastus lateralis muscle after a stroke. Individuals with stroke often have deficits with force generation and volitional relaxation. Current models of impaired force output after a stroke focus primarily on the pathology within the corticospinal pathway because of decreased descending drive. Though this is an important aspect of deficient motoneuron output, it is incomplete because motoneurons receive other inputs that can shape motor output. Because the motoneuron is the last site of signal integration for muscle contractions, using methods that study motor unit activity can provide a window to the activity in the spinal circuitry. This research study utilized a novel algorithm that decomposed electromyography (EMG) signals into the contributions of the individual motor units. This provided the individual firing instances for a large number of concurrently active motor units during isometric contractions of the knee extensors. In the first aim, the association between the hyperemic response and motor unit firing rate modulation to intermittent, fatiguing contractions was investigated. It was found that the magnitude of blood flow was lower for individuals with stroke compared to healthy controls, but both groups increased blood flow similarly in response to fatiguing contractions. This did not relate to changes in muscle fiber contractibility for the participants with stroke; rather, participants better able to increase blood flow showed greater modulation in motor unit firing rates. To further investigate how ischemic conditions impact motor unit output, the second aim used a blood pressure cuff to completely occlude blood flow through the femoral artery with the intent of activating inhibitory afferent pathways. We found that ischemic conditions had a greater inhibitory impact on motor unit output for individuals with stroke compared to healthy controls, possibly because of hyper-excitable group III/IV afferent pathways. The final aim investigated how stroke related changes in the intrinsic excitability of the motoneurons impacted prolonged motor unit firing during voluntary relaxation. A serotonin reuptake inhibitor was administered to quantify motoneuron sensitivity to neuromodulatory inputs. This study found that the serotonin reuptake inhibitor increased muscle relaxation and may have reduced persistent inward current contributions to prolonged motor unit firing. In conclusion, while damage to the corticospinal tract is a major component to poor functionality, the intrinsic properties of the motoneuron and sensory pathways to the motoneuron pool are essential for understanding deficient motor control after a stroke

Tracking motor units longitudinally across experimental sessions with high-density surface electromyography.

KEY POINTS: Classic motor unit (MU) recording and analysis methods do not allow the same MUs to be tracked across different experimental sessions, and therefore, there is limited experimental evidence on the adjustments in MU properties following training or during the progression of neuromuscular disorders. We propose a new processing method to track the same MUs across experimental sessions (separated by weeks) by using high-density surface electromyography. The application of the proposed method in two experiments showed that individual MUs can be identified reliably in measurements separated by weeks and that changes in properties of the tracked MUs across experimental sessions can be identified with high sensitivity. These results indicate that the behaviour and properties of the same MUs can be monitored across multiple testing sessions. The proposed method opens new possibilities in the understanding of adjustments in motor unit properties due to training interventions or the progression of pathologies. ABSTRACT: A new method is proposed for tracking individual motor units (MUs) across multiple experimental sessions on different days. The technique is based on a novel decomposition approach for high-density surface electromyography and was tested with two experimental studies for reliability and sensitivity. Experiment I (reliability): ten participants performed isometric knee extensions at 10, 30, 50 and 70% of their maximum voluntary contraction (MVC) force in three sessions, each separated by 1 week. Experiment II (sensitivity): seven participants performed 2 weeks of endurance training (cycling) and were tested pre-post intervention during isometric knee extensions at 10 and 30% MVC. The reliability (Experiment I) and sensitivity (Experiment II) of the measured MU properties were compared for the MUs tracked across sessions, with respect to all MUs identified in each session. In Experiment I, on average 38.3% and 40.1% of the identified MUs could be tracked across two sessions (1 and 2 weeks apart), for the vastus medialis and vastus lateralis, respectively. Moreover, the properties of the tracked MUs were more reliable across sessions than those of the full set of identified MUs (intra-class correlation coefficients ranged between 0.63-0.99 and 0.39-0.95, respectively). In Experiment II, ∼40% of the MUs could be tracked before and after the training intervention and training-induced changes in MU conduction velocity had an effect size of 2.1 (tracked MUs) and 1.5 (group of all identified motor units). These results show the possibility of monitoring MU properties longitudinally to document the effect of interventions or the progression of neuromuscular disorders

Evaluation of the Long-term Reliability of Motor Uni Discharge Rates obtained by Decomposition of the Surface Electromyographic Signal

This study evaluated the long-term reliability of motor unit discharge rates (MUDRs) during isometric contractions at 60% maximum voluntary contraction (MVC), obtained by decomposition of the surface electromyographic (sEMG) signal from the flexor carpi radialis (FCR) and tibialis anterior (TA). There were four test sessions: one week between sessions 1 and 2; six weeks between sessions 2, 3, and 4. Participants performed 3 maximal isometric contractions of the wrist flexors and 3-isometric ramp contractions to 60% MVC. A load cell and 5-pin electrode (dEMG System, Delsys, Inc., Boston, MA) were used to monitor force and sEMG, respectively. The MUDRs were obtained using the Precision Decomposition Algorithm III in the dEMG Analysis software, and calculated as the inverse of the smoothed firing intervals. The mean discharge rate was calculated during a one-second window centered at the plateau portion of the 60% MVC ramp contraction. Maximal isometric strength during wrist flexion and dorsiflexion was also monitored. Across the four test sessions, maximal isometric strength of the wrist flexors and dorsiflexors increased 10 and 11.85%, respectively (p’s0.94). The MUDRs for the FCR (5.2%) and TA (7.8%) also exhibited slight fluctuations across the four test sessions (p’s<0.01). The consistency of MUDR values within each subject was still considered good, as the intraclass correlation coefficient for both measures was R=0.79. It was concluded that the overall long-term reliability of MUDRs in both the FCR and TA was good

Motor Unit Properties of the First Dorsal Interosseous in Chronic Stroke Subjects: Concentric Needle and Single Fiber EMG Analysis

The purpose of this study was to better understand changes in motor unit electrophysiological properties in people with chronic stroke based on concentric needle electromyography (EMG) and single fiber EMG recordings. The first dorsal interosseous (FDI) muscle was studied bilaterally in eleven hemiparetic stroke subjects. A significant increase in mean fiber density (FD) was found in the paretic muscle compared with the contralateral side based on single fiber EMG (1.6 ± 0.2 vs. 1.3 ± 0.1, respectively, P = 0.003). There was no statistically significant difference between the paretic and contralateral sides in most concentric needle motor unit action potential (MUAP) parameters, such as amplitude (768.7 ± 441.7 vs. 855.0 ± 289.9 μV), duration (8.9 ± 1.8 vs. 8.68 ± 0.9 ms) and size index (1.2 ± 0.5 vs. 1.1 ± 0.3) (P &gt; 0.18), nor was there a significant difference in single fiber EMG recorded jitter (37.0 ± 9.6 vs. 39.9 ± 10.6 μs, P = 0.45). The increase in FD suggests motor units of the paretic FDI have enlarged due to collateral reinnervation. However, sprouting might be insufficient to result in a statistically significant change in the concentric needle MUAP parameters. Single fiber EMG appears more sensitive than concentric needle EMG to reflect electrophysiological changes in motor units after stroke. Both single fiber and concentric needle EMG recordings may be necessary to better understand muscle changes after stroke, which is important for development of appropriate rehabilitation strategies. The results provide further evidence that motor units are remodeled after stroke, possibly in response to a loss of motoneurons

Markers of physical functioning and neuromuscular fatigue for the post-discharge follow-up of subjects already assisted in intensive care for COVID-19 and non-communicable diseases.

L'insorgenza di malattie non trasmissibili con incidenza neuromuscolare e il prolungato ricovero in terapia intensiva comportano implicazioni negative sulla capacità funzionale e l'autonomia dei pazienti. Queste alterazioni sono causate da fattori come l'immobilità prolungata, l'infiammazione sistemica, le disfunzioni neuromuscolari e gli effetti diretti della patologia stessa. Ciò porta a fatica e debolezza muscolare, contribuendo a un significativo declino nelle abilità motorie, ostacolando il recupero e peggiorando la qualità della vita dei pazienti. Le principali disfunzioni motorie si manifestano con l’alterata deambulazione, limitando l'autonomia nelle attività quotidiane. Tali disfunzioni sono causate dalla compromissione delle vie motorie del sistema nervoso e si manifestano già durante la fase acuta della malattia, peggiorando successivamente con il suo evolversi. Tale quadro clinico è aggravato ulteriormente dalla conseguente inattività fisica, favorita dalle condizioni psicofisiche dei singoli. Di conseguenza, tutto ciò incide notevolmente sulle proprietà contrattili dei muscoli, portando ad alterazioni critiche che influenzano la capacità del sistema nervoso centrale e periferico di reclutare e modulare l'attività delle unità motorie. Date le complessità associate a queste considerazioni, diventa fondamentale identificare marcatori in grado di quantificare e caratterizzare l’alterazione della capacità funzionale e l’insorgenza della debolezza muscolare e della fatica. Ciò faciliterebbe una diagnosi precoce e, in modo cruciale, il monitoraggio continuo di tali problematiche. Pertanto, l'obiettivo primario di questa ricerca di Dottorato è identificare e analizzare marcatori in grado di supportare efficacemente gli operatori sanitari nella progettazione e nell'implementazione di approcci terapeutici personalizzati per accelerare il recupero di questi individui. Nonostante, infatti, la pratica clinica attualmente in uso negli ospedali offra continui miglioramenti, questa presenta ancora delle limitazioni. Sebbene le valutazioni attualmente impiegate riescano ad identificare la presenza di fatica e debolezza muscolare nei pazienti post-ricovero in terapia intensiva o in soggetti affetti da malattie non trasmissibili, non riescono tuttavia ad indagare a fondo su quali siano le effettive cause che innescano la perdita della forza muscolare o ad esaminare in modo esaustivo i fattori centrali e/o periferici che contribuiscono all'insorgenza della fatica. Per colmare in modo esaustivo questa lacuna, lo studio ha condotto un'ampia ricerca combinando l'elettromiografia di superficie con la capacità di generare forza muscolare in diverse condizioni patologiche, includendo attivazioni muscolari sia volontarie che indotte elettricamente. Il muscolo oggetto di studio è stato il tibiale anteriore, scelto per il suo ruolo cruciale nella biomeccanica della deambulazione e quindi fondamentale per il mantenimento dell’autonomia motoria. I risultati hanno mostrato che le variazioni nella forza e nei parametri delle unità motorie possono servire da indicatori per le alterazioni neuromuscolari e il recupero progressivo, facilitando il monitoraggio a breve e lungo termine. Questo studio ha quindi un'importanza fondamentale per le popolazioni coinvolte e può suggerire approcci più ampi per la gestione delle alterazioni neuromuscolari in diversi contesti clinici. In particolare, sottolinea l'importanza di programmi di riabilitazione personalizzati e soggettivati alle esigenze specifiche di ciascun individuo.The onset of non-communicable neuromuscular diseases and prolonged stays in the intensive care unit have deep implications for physical functioning and neuromuscular health. These repercussions arise from muscle deconditioning, systemic inflammation, and the direct impact of the pathology. Moreover, resulting fatigue and acquired muscle weakness contribute to reduced muscular performance, significantly hampering recovery and diminishing overall quality of life. The predominant motor impairments observed in these patients primarily manifest in their ability to perform correct walking, substantially limiting their independent execution of daily activities. This compromised excitability in descending motor pathways becomes evident during the acute phase of the disease and intensifies as the condition progresses chronically, exacerbated by prolonged physical inactivity. Consequently, this significantly affects the muscle's contractile properties, leading to critical alterations that influence the nervous systems' capacity to recruit and modulate the activity of motor units, the fundamental functional units responsible for planning, executing, and maintaining motor gestures. Given these considerations, it becomes crucial to identify markers that enable the quantification and characterization of physical functioning impairment, muscle weakness and fatigue. This would facilitate early diagnosis and, crucially, the ongoing monitoring of these issues. Thus, the primary goal of this PhD research is to identify and analyze markers that can effectively support healthcare practitioners in devising and delivering personalized therapeutic approaches to expedite the recovery of these individuals. The overarching objective is to optimize the current clinical practice commonly employed in hospitals. Despite ongoing refinements, these practices still exhibit limitations. While standard assessments succeed in identifying the presence of fatigue and muscle weakness in ICU patients or those afflicted by non-communicable diseases, they fall short of investigating the root causes of muscle strength deterioration or thoroughly probing the central and/or peripheral factors contributing to the emergence of pathological fatigue. To comprehensively bridge this existing gap, the study undertook an extensive exploration by measuring concurrent joint torques and surface electromyography across various pathological conditions, encompassing both voluntary and electrically induced muscle activations. The focal point was the tibialis anterior muscle, chosen for its pivotal role in gait patterns and consequential influence on individual autonomy. The presented results were mainly achieved through the decomposition of signals recorded using the High-Density Surface EMG technique. This technique enabled the analysis of individual motor units recruited during motor tasks administered to patients within the studied populations. The process of data collection and analysis revealed that variations in muscle strength values and motor unit parameters can serve as indicators of neuromuscular system alterations and progressive recovery. These factors are pivotal for subsequent follow-up procedures. Indeed, by establishing a robust framework of markers, is possible to contribute to the development of evidence-based protocols that enhance the post-discharge care of these individuals. This study is not only pivotal for these specific cohorts but also holds the potential to inform broader strategies for managing physical impairment and neuromuscular challenges in diverse clinical settings. Notably, the study highlights that hospitalization in intensive care, as well as the onset of non-communicable pathologies with high motor impact, leads to specific alterations in parameters of both central and peripheral neuromuscular pathways. This underscores the imperative for devising personalized rehabilitation regimens tailored to each patient's needs

A Longitudinal Electromyography Study of Complex Movements in Poststroke Therapy. 1: Heterogeneous Changes Despite Consistent Improvements in Clinical Assessments

Poststroke weakness on the more-affected side may arise from reduced corticospinal drive, disuse muscle atrophy, spasticity, and abnormal coordination. This study investigated changes in muscle activation patterns to understand therapy-induced improvements in motor-function in chronic stroke compared to clinical assessments and to identify the effect of motor-function level on muscle activation changes. Electromyography (EMG) was recorded from five upper limb muscles on the more-affected side of 24 patients during early and late therapy sessions of an intensive 14-day program of Wii-based Movement Therapy (WMT) and for a subset of 13 patients at 6-month follow-up. Patients were classified according to residual voluntary motor capacity with low, moderate, or high motor-function levels. The area under the curve was calculated from EMG amplitude and movement duration. Clinical assessments of upper limb motor-function pre- and post-therapy included the Wolf Motor Function Test, Fugl-Meyer Assessment and Motor Activity Log Quality of Movement scale. Clinical assessments improved over time (p &lt; 0.01) with an effect of motor-function level (p &lt; 0.001). The pattern of EMG change by late therapy was complex and variable, with differences between patients with low compared to moderate or high motor-function levels. The area under the curve (p = 0.028) and peak amplitude (p = 0.043) during Wii-tennis backhand increased for patients with low motor-function, whereas EMG decreased for patients with moderate and high motor-function levels. The reductions included movement duration during Wii-golf (p = 0.048, moderate; p = 0.026, high) and Wii-tennis backhand (p = 0.046, moderate; p = 0.023, high) and forehand (p = 0.009, high) and the area under the curve during Wii-golf (p = 0.018, moderate) and Wii-baseball (p = 0.036, moderate). For the pooled data over time, there was an effect of motor-function (p = 0.016) and an interaction between time and motor-function (p = 0.009) for Wii-golf movement duration. Wii-baseball movement duration decreased as a function of time (p = 0.022). There was an effect on Wii-tennis forehand duration for time (p = 0.002), an interaction of time and motor-function (p = 0.005) and an effect of motor-function level on the area under the curve (p = 0.034) for Wii-golf. This study demonstrated different patterns of EMG changes according to residual voluntary motor-function levels, despite heterogeneity within each level that was not evident following clinical assessments alone. Thus, rehabilitation efficacy might be underestimated by analyses of pooled data