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Single Motor Unit Laboratory: Current Projects


Mechanisms of muscle weakness in hemispheric stroke
Xiaogang Hu, Nina Suresh

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This project aims to determine whether the motor unit pool organization in the paretic muscle is modified in hemispheric stroke survivors. We use surface EMG recording and decomposition techniques (Delsys) to extract information regarding altered motor unit recruitment, discharge, and action potential morphology in paretic muscles.


Neural mechanisms of spasticity in stroke
Xiaogang Hu, Nina Suresh, Matthieu Chardon

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This project aims to identify the evidence of motoneuron hyperexcitability as potential contributions of muscle hypertonia in hemispheric stroke survivors. A motoneuron hyperexcitability can arise from a sustained depolarization of the passive membrane potential, an excitatory postsynaptic potential (EPSP) with larger amplitude and/or longer duration than normal EPSPs. We estimate possible altered properties of EPSP and resting potentials using precisely controlled mechanical stimulation (Linmot) and subthreshold sequential electrical stimulation protocols.


Impact of intermittent hypoxia and prednisolone on motor performance in persons with Spinal Cord Injury
Milap Sandhu, William Z Rymer

Mechanisms of muscle weakness in hemispheric stroke

The aim of this study is to investigate intermittent hypoxia therapy (i.e. breathing low oxygen) for induction of functional neuroplasticity and enhancement of spinal motor output. Previous work has shown that intermittent hypoxia therapy can improve lower extremity function in humans with spinal cord injury (SCI). As SCI is frequently associated with a chronic systemic inflammation - a condition that impairs the ability of hypoxia to improve motor function - we are investigating if pre-treatment with an anti-inflammatory agent would enhance the effects of intermittent hypoxia therapy.


Effect of Acute Intermittent Hypoxia on Upper Limb Function in Individuals with Spinal Cord Injury
Milap Sandhu, Sofia Anastasopoulos, William Z Rymer

Neural mechanisms of spasticity in stroke

In recent years, it has become clear that repeated presentations of transient and mild hypoxia can elicit neuroplasticity, and boost the efficacy of standard therapeutic strategies in certain cases of neurologic dysfunction. An array of animal preparations have taught us a great deal about molecular cascades responsible for hypoxia-induced plasticity, and recent work in humans demonstrates the feasibility of translating this approach in individuals with chronic spinal cord injury (SCI). In this study, we are investigating the efficacy of acute intermittent hypoxia (AIH) to improve upper extremity function in persons with tetraplegia due to cervical SCI. Our hypothesis is that administration of AIH will improve arm and hand strength, as well as electromyogram activity in the upper limb. This study will help create a framework for the development of practical techniques to harness beneficial neuroplasticity in individuals with SCI.


Muscle pathologies: How high density EMG grids can help us?
Ghulam Rasool, William Z Rymer

Impaired control of individualized finger movement in stroke survivors

Weakness, muscle atrophy, and motor impairments generally follow cerebrovascular injury such as stroke. We are employing multichannel high-density surface EMG grids to analyze muscular activation patterns during non-fatiguing sustained isometric voluntary contractions in stroke survivors. In addition to conventional temporal and spectral EMG signal analysis, we are interested in tools for spatial and spatiotemporal analysis. In the figure, we present a muscle activation map of the biceps brachii where the impaired muscle side shows a non-homogenous activation pattern as compared to the healthy one.

Our investigation shows that, for a given subject, muscle activity maps are consistent across all measured contraction levels differing only in the RMS activity levels. However, the maps from opposite arms (impaired vs. contralateral) of stroke survivors are significantly different from each other, especially when compared to intact participants. Our analyses revealed that chronic stroke altered the size and the location of the active region in these maps.

The former is potentially related to disruption of fiber and tissue structure, possibly linked to factors such as extracellular fat accumulation, connective tissue infiltration, muscle fiber atrophy, shortening of fibers and muscle fiber loss. Changes in spatial patterns in muscle activity maps may also be related to a shift in the location of the innervation zone or the endplate region. Alterations in muscle activity maps are also linked to the functional impairment levels (Fugl-Meyer) and spasticity (modified Ashworth scale).

Overall, our investigation demonstrates that the muscle architecture and morphology are significantly altered after the brain injury.


Cervical motor level of injury identification and characterization based on surface EMG
Babak Afsharipour, Ghulam Rasool, William Z Rymer

Computational strategies for mixed reality rehabilitation

The primary objective of this study is to develop and evaluate a quantitative tool for characterizing and tracking the level of cervical motor injury, based on surface EMG (sEMG) distribution over the upper extremity. We propose to use a dense grid of sEMG electrodes, placed over the upper extremities, to accurately mark the level and extent of cervical motor injury. We are characterizing the muscle(s) region(s) innervated by damaged ventral roots or damaged ventral cord grey matter. The vision of this study is mapping anatomical level of injury to physiological-functional level of injury.


Length dependence of the shear elastic properties of the biceps brachii after stroke
Andrew Lai, Xiaogang Hu, Nina Suresh

Hypertonia post-stroke is a motor control disorder associated with poor patient outcome. The etiology of hypertonia is unclear; usually the increase in tone is attributed to enhanced stretch reflex responses, however, the potential role of non-neural mechanisms is relatively uninvestigated. Currently we are examining the hypothesis that altered intrinsic muscle elasticity plays a role in mediating clinical hypertonia. The goal of this project was to determine if the stroke affected muscle has altered elastic properties within the muscle’s range of motion. To this end, muscle’s elastic properties were non-invasively estimated in the biceps brachii using supersonic shear wave elastography (SWE).


Alterations in biceps brachii motor unit behavior after stroke
Andrew Lai, Xiaogang Hu, Nina Suresh

   
Impact of intermittent hypoxia and prednisolone on motor performance in persons with Spinal Cord Injury

After stroke, motor control is impaired in many hemispheric stroke survivors. The goal of this project is quantify impaired motor unit activity and to interpret the motor unit behavior in context of the underlying motor pathology. Using an electrode grid and Delsys decomposition techniques, we record and analyze surface EMG during isometric contractions of different intensities.


Altered Rheological Properties of Passive Skeletal Muscles in Chronic Stroke
Ghulam Rasool, Sabrina S. M. Lee

We are investigating changes in rheological properties of skeletal muscles in the hemiplegic chronic stroke population. Our objective was to quantify viscoelastic properties of stroke-affected muscles using ultrasound shear wave velocity as a surrogate for tissue mechanical properties. We hypothesize that after the cerebrovascular accident, in addition to well-known changes in neural and contractile properties, the local muscle rheological properties are also changed. We also hypothesize that these changes in rheological characteristics of muscle are linked to clinically observed hypertonia, spasticity, muscle weakness, poor biomechanical output and impaired motor control.

We are quantifying rheological properties of the biceps muscle by measuring the shear wave propagation, i.e., group and phase (dispersion) velocities. The group velocity represents an average propagation speed of the shear wave over all frequencies and quantifies tissue elasticity. Using SuperSonic Imaging technology, we generate shear waves and measure their propagation in biceps muscles in a passive state as shown in Figure 1.

In our preliminary data, we observed significantly higher group velocity values in stroke-affected muscles (except one stroke survivor) (Figure. 2(A)). We present phase velocity data from two representative stroke survivors in (Figure. 1(B)). In both cases, we observed frequency-dependent changes in the shear wave propagation, which provided evidence of a significant contribution from viscous components. Therefore, an analysis of muscle passive mechanical properties would be incomplete without characterization of viscous components. We further noted that phase velocity (dispersion) values were significantly greater in stroke-affected muscles, especially at higher frequencies, which highlighted the importance of measuring rheological properties over elastic only.

We conclude that alterations in skeletal muscle post-stroke were mediated by changes in tissue rheological parameters, i.e., in both elastic and viscous components, that may have originated from changes in the extracellular matrix, or from connective tissue infiltration giving rise to tissue fibrosis. These alterations, in part, played a significant role in inducing muscle weakness and caused impaired motor control after brain injury.



 


 
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