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,

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

Ischemic Conditioning Increases Strength and Volitional Activation of Paretic Muscle in Chronic Stroke: A Pilot Study

7siIschemic conditioning (IC) on the arm or leg has emerged as an intervention to improve strength and performance in healthy populations, but the effects on neurologic populations are unknown. The purpose of this study was to quantify the effects of a single session of IC on knee extensor strength and muscle activation in chronic stroke survivors. Maximal knee extensor torque measurements and surface EMG were quantified in 10 chronic stroke survivors (>1 year post-stroke) with hemiparesis before and after a single session of IC or Sham on the paretic leg. IC consisted of five minutes of compression with a proximal thigh cuff (inflation pressure = 225 mmHg for IC or 25 mmHg for Sham) followed by five minutes of rest. This was repeated five times. Maximal knee extensor strength, EMG magnitude, and motor unit firing behavior were measured before and immediately after IC or Sham. IC increased paretic leg strength by 10.6plus minus8.5 Nm while no difference was observed in the Sham group (change in Sham = 1.3plus minus2.9 Nm; p = 0.001 IC vs. Sham). IC-induced increases in strength were accompanied by a 31plus minus15% increase in the magnitude of muscle EMG during maximal contractions and a 5% decrease in motor unit recruitment thresholds during sub-maximal contractions. Individuals who had the most asymmetry in strength between their paretic and non-paretic legs had the largest increases in strength (r2= 0.54). This study provides evidence that a single session of IC can increase strength through improved muscle activation in chronic stroke survivors.openembargoed_20190204Hyngstrom, Allison S; Murphy, Spencer A; Nguyen, Jennifer; Schmit, Brian D; Negro, Francesco; Gutterman, David D; Durand, Matthew JHyngstrom, Allison S; Murphy, Spencer A; Nguyen, Jennifer; Schmit, Brian D; Negro, Francesco; Gutterman, David D; Durand, Matthew

The Relationship Between Blood Flow and Motor Unit Firing Rates in Response to Fatiguing Exercise Post-stroke

We quantified the relationship between the change in post-contraction blood flow with motor unit firing rates and metrics of fatigue during intermittent, sub-maximal fatiguing contractions of the knee extensor muscles after stroke. Ten chronic stroke survivors (>1-year post-stroke) and nine controls participated. Throughout fatiguing contractions, the discharge timings of individual motor units were identified by decomposition of high-density surface EMG signals. After five consecutive contractions, a blood flow measurement through the femoral artery was obtained using an ultrasound machine and probe designed for vascular measurements. There was a greater increase of motor unit firing rates from the beginning of the fatigue protocol to the end of the fatigue protocol for the control group compared to the stroke group (14.97 ± 3.78% vs. 1.99 ± 11.90%, p = 0.023). While blood flow increased with fatigue for both groups (p = 0.003), the magnitude of post-contraction blood flow was significantly greater for the control group compared to the stroke group (p = 0.004). We found that despite the lower magnitude of muscle perfusion through the femoral artery in the stroke group, blood flow has a greater impact on peripheral fatigue for the control group; however, we observed a significant correlation between change in blood flow and motor unit firing rate modulation (r2 = 0.654, p = 0.004) during fatigue in the stroke group and not the control group (r2 = 0.024, p < 0.768). Taken together, this data showed a disruption between motor unit firing rates and post-contraction blood flow in the stroke group, suggesting that there may be a disruption to common inputs to both the reticular system and the corticospinal tract. This study provides novel insights in the relationship between the hyperemic response to exercise and motor unit firing behavior for post-stroke force production and may provide new approaches for recovery by improving both blood flow and muscle activation simultaneously

Neuromuscular Markers of High Performance Sport Preparation: Muscle Contractile Mechanics

Assessments of skeletal muscle functional capacity or bilateral muscular asymmetry often necessitate maximal contractile effort, which exacerbates muscle fatigue or injury. Tensiomyography (TMG) has been investigated in laboratory settings, as a means to assess muscle contractile function following fatigue; however observations have not been contextualised by concurrent physiological measures. TMG has more sparingly been applied in the field, with elite athletes. The aim of this thesis was to examine acute alterations and underlying variations in muscle contractile mechanics, through the application of TMG, contextualised with established physiological measures; and to apply TMG within high performance sports programmes. TMG successfully detected fatigue, evident from reduced strength, by displaying impaired muscle displacement, accompanied by elevated resting muscle tension. Greater asymmetry was detected in individuals with asymmetric strength; however, symmetry was masked during more complex tasks. Increased day-to-day variability was detected among highly trained athletes compared to recreationally active individuals. Acute training adaptations were detected, in contractile mechanics, in individual muscles. TMG could be useful in establishing fatigue status of skeletal muscle without exacerbating the functional decrements of the muscle, whilst also providing useful screening information for detecting asymmetry which may not be apparent during functional actions

Mechanisms of Fatigability in People with Type 2 Diabetes and Prediabetes

Dynamic fatiguing exercise of limb muscles is the basis of exercise training and a cornerstone of management of type 2 diabetes mellitus (T2D) and prediabetes. Little is known however, about the fatigability of limb muscles (the acute exercise induced reduction in force or power) and the involved mechanisms in people with T2D and prediabetes. Current evidence suggests that people with T2D have reduced muscle strength and power, are more fatigable after static contractions, and have physical impairments affecting activities of daily living. However, impaired function in people with T2D compared with controls is larger for dynamic than static tasks. The purpose of this dissertation was to determine the magnitude and mechanisms of fatigability in people with T2D and prediabetes after a dynamic exercise task with the knee extensor muscles. Importantly, these studies matched people with T2D and prediabetes to controls based on age, sex, physical activity and body size. The first studies determined the magnitude of fatigability and the neural and muscular mechanisms in people with T2D and controls (Study 1) and in prediabetes (Study 2). People with T2D had approximately twice the decline in both power (fatigability) and electrically-evoked muscle contractile properties than controls after the six-minute dynamic task with the knee extensor muscles. People with prediabetes also had greater fatigability (~50%) and reductions in contractile properties than controls, but less than people with T2D. The reduction in voluntary activation (neural drive to the muscle) after fatiguing exercise was not different between people with T2D, prediabetes and controls. Thus, the greater fatigability in people with T2D was due to mechanisms within the skeletal muscle rather than neural drive. Study 3 determined whether skeletal muscle blood flow could explain the greater fatigability in people with T2D. People with T2D had greater fatigability and lower blood flow after exercise than controls, and there was an association between fatigability and the exercise-induced increase in muscle blood flow after exercise. Collectively, these data suggest that people with T2D and prediabetes have greater fatigability during dynamic exercise with knee extensor muscles due to mechanisms effecting muscle contractile properties, including impaired skeletal muscle blood flo

Investigation of the feasibility of using focal vibratory stimulation with robotic aided therapy for spasticity rehabilitation in spinal cord injury

The occurrence of a traumatic spinal cord injury is in hundreds of thousands of people every year. Survivors are left with loss of many bodily functions, loss of sensation below the point of injury and many more painful and uncomfortable repercussions which interfere with activities of daily living. Over 70% of people with SCI develop spasticity: abnormally increased muscle tone and connected joint stiffness that interfere with residual volitional control of the limbs. Treatments for spasticity include many pharmacological and non-pharmacological techniques, however many of them have severe sideeffects. Evidence suggest the use of vibratory stimulation to relieve repercussions of spasticity, despite not agreeing on the most advantageous protocol. This thesis evaluated effects that focal vibratory stimulation have on the muscle performance. Within two studies, focal muscle vibration is compared against different application conditions such as timing and location. The results suggests that if focal vibrations are applied to the relaxed muscle, the increase in muscle's force is observed. Analysis of the cortical waves indicates minimal cortical involvement in vibratory stimulation modulation. On the other hand, FV applied of the connected tendon/bone imposed to a contraction seems to have a potential to increase muscle's activation. There is evidence that motor cortex is responding to this stimulation to stabilise the muscle in order to perform the contraction. Within clinical trial, focal muscle vibratory stimulation is employed in total of 6 interventional sessions while a joint's spastic exor and extensor muscles were relaxed. Spasticity appears to be reduced as a consequence of the stimulation. Moreover, engaging the joint into robotic-aided therapy increase volitional control of the wrist, according to the analysis of the active range of motion, joint stiffness and kinematic parameters associated to the movement. The measurement and movement facilitation device used in the clinical trial was designed and developed in accordance to the spasticity and spinal cord injury repercussions consideration. The studies conducted for this thesis demonstrated feasibility and potential for the use of focal muscle vibratory stimulation to enhance muscle power in healthy muscles but also relieve consequences of spasticity. Vibrations combined with movement robotic-aided therapy have a prospects to enhance motor control

Study of the mechanisms involved in the regulation of O2 consumption kinetics during exercise.

Lors d’un exercice à une intensité au-dessous du seuil ventilatoire 1 (V1), la réponse fondamentale de la cinétique de la consommation de dioxygène (V̇ O2) s'élève de manière mono- exponentielle, atteignant un état stable après quelques minutes. Cependant, lors de l'exercice à une charge de travail constante au-dessus de V1, la cinétique de V̇ O2 est caractérisée par un début de l’état stable retardé et une deuxième augmentation de V̇ O2 superposée à la réponse initiale de V̇ O2.Cette augmentation lente de V̇ O2 est appelée la composante lente (V̇ O2sc). Il a été proposé que cet excès de V̇ O2, reflet de l'inefficacité musculaire, provienne principalement des muscles exercitant; cependant, à ce jour, les mécanismes putatifs à cette augmentation sont toujours mal compris. Plusieurs théories ont été proposées, parmi lesquels : a) la combinaison de processus liés à la fatigue nécessitant un recrutement supplémentaire de fibres pour compenser les fibres déjà fatiguées, et b) l'influence potentielle des différents profils métaboliques de différentes populations de types de fibres. Le but de cette thèse est de clarifier et de nourrir le débat sur les causes de V̇ O2sc, en particulier pour ces deux derniers paradigmes. Trois expérimentations ont été réalisées pour mesurer la concordance et les interférences de différentes cinétiques de fibres musculaires et la fatigue musculaire avec la V̇ O2sc. Les résultats de cette thèse sont les suivants : 1) Lors d’un exercice difficile, l'altération des propriétés neuromusculaires des extenseurs du genou (reflétant les processus de fatigue) n’a été significativement réduite qu’après 20-30 min d'exercice, alors que la V̇ O2sc avait fini de croitre. Ce résultat suggère qu'une relation temporelle entre la fatigue et la V̇ O2sc ne semble pas exister et, par conséquent, le développement de la fatigue n'est pas une condition essentielle pour le développement de la V̇ O2sc. 2) La fonction neuromusculaire évaluée à l’aide d’une stimulation double (Ddb, 100 Hz) pendant l'exercice d'extension du genou n'a pas été altérée dans le domaine difficile. En revanche, dans le domaine intense, la diminution significative de la force maximale et du taux maximal de développement de la force lors de la Ddb, reflétaient des processus de fatigue et étaient partiellement corrélées au développement de V̇ O2sc relatif. Par conséquent, les résultats suggéraient que la V̇ O2sc dans les domaines difficiles et intenses n'est pas le produit d'un mécanisme identique. 3) Afin de construire une nouvelle courbe combinant les principes de Henneman et de superposition, les trois courbes de transitions (repos-modérée, modérée-difficile et difficile-intense) ont été alignées dans le temps et sommées. Les résultats ont montré que globalement les paramètres de la cinétique de la courbe reconstruite n'étaient pas significativement différents d'une transition depuis le repos à un exercice d'intensité intense. Cela suggère que le recrutement supplémentaire de fibres n'était pas présent et que l'apparition de V̇ O2sc est au moins liée, sinon le résultat, des différentes propriétés métaboliques des fibres musculaires. Ces résultats évidence, lors de l'exercice chez l'homme, que les processus de fatigue représentés par des altérations des propriétés neuromusculaires ne sont pas une condition sine qua non pour le développement de la V̇ O2sc dans le domaine difficile, et que l'apparition du V̇ O2sc pourrait être le iétés métaboliques des fibres musculaires. -- Below the gas exchange threshold (GET), the fundamental response of O2 consumption (V̇ O2) kinetics rises monoexponentially, reaching a steady state after a few minutes. However, at a constant work rate exceeding the GET, the response is characterized by a delayed onset and a second rise in V̇ O2 superimposed on the initial V̇ O2 response. This slowly developing rise in V̇ O2 is termed the slow component (V̇ O2SC). This excess of V̇ O2, a reflection of muscle inefficiency, has been proposed to arise primarily from the exercising muscles; however, to date, the putative mechanisms are poorly understood. Several theories have been proposed, including the combination of fatigue-related processes requiring additional fiber recruitment to compensate for the already fatigued fibers and the potential influence of the different metabolic profiles of different fiber-type populations. The aim of this thesis is to clarify and nourish the debate on the causes of the V̇ O2SC, especially for these last two paradigms. Three different experiments were performed to measure the concordance and interferences of different kinetics of muscle fibers and muscle fatigue with the V̇ O2SC. The findings of this thesis are as follows: 1) During exercise at heavy intensity, the alteration in the neuromuscular properties of the knee extensors (reflecting fatigue processes) was significantly reduced after only 20-30 min of exercise, while the V̇ O2SC was stable. The results suggest that a temporal relationship between fatigue and the V̇ O2SC does not appear to exist; therefore, the development of fatigue is not an essential requirement to elicit the V̇ O2SC. 2) Neuromuscular function assessed through doublet stimulation (Ddb, 100Hz) during knee extension exercise was not altered in the heavy domain. In contrast, in the severe domain, the significant diminution in maximal force and maximal rate of force development during the Ddb, reflected fatigue processes and were partially correlated with the development of the relative V̇ O2sc. Therefore, the results suggest that the V̇ O2sc in the heavy and severe domains is not the product of an identical mechanism. 3) After constructing a new kinetics curve combining the Henneman and superposition principles, the three different intensity curves (moderate, heavy and severe) were time aligned and summed. The results showed that overall kinetics parameters from the reconstructed curve were not significantly different from one transition to severe-intensity exercise. This suggests that additional fiber recruitment was not present and that the appearance of the V̇ O2sc is at least related to, if not the result of, the different metabolic properties of muscle fibers. These results provide evidence in exercising humans that fatigue processes portrayed by alterations in neuromuscular properties are not a sine qua non for the development of the slow component in the heavy domain, and that, the appearance of the V̇ O2sc could be the result of the different metabolic properties of muscle fibers

Applications of EMG in Clinical and Sports Medicine

This second of two volumes on EMG (Electromyography) covers a wide range of clinical applications, as a complement to the methods discussed in volume 1. Topics range from gait and vibration analysis, through posture and falls prevention, to biofeedback in the treatment of neurologic swallowing impairment. The volume includes sections on back care, sports and performance medicine, gynecology/urology and orofacial function. Authors describe the procedures for their experimental studies with detailed and clear illustrations and references to the literature. The limitations of SEMG measures and methods for careful analysis are discussed. This broad compilation of articles discussing the use of EMG in both clinical and research applications demonstrates the utility of the method as a tool in a wide variety of disciplines and clinical fields