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

Effects of Random Oscillations on Balance Control in Healthy Young Adults

In human walking, balance control is managed through proactive changes in spatio-temporal parameters of stepping [1]. It has been suggested that continuous disruptions to healthy young adult balance cause greater changes to overall variability of these parameters than a shift in the mean stepping parameters [2]. This suggests that walking may be occurring in a more reactive manner, modulating to maintain balance without increasing the mean significantly. Work using continuous oscillations to treadmill walking suggest there is an interplay between the predictability of a signal used to disrupt subject balance and the degree to which compensation occurs [3]. To determine how balance compensation occurs during continuous, unpredictable oscillations this work investigated the effects of unpredictable oscillations on human walking. A 6 Degree of Freedom Motion base was used to oscillate 12 subjects walking on a treadmill for seven different balance conditions: (1) Normal Walking (2) Pitch Amplitude Oscillations, (3) Pitch Frequency Oscillations, (4) Roll Amplitude Oscillations, (5) Roll Frequency Oscillations, (6) Medial-Lateral Amplitude Oscillations, and (7) Medial-Lateral Frequency Oscillations. Amplitude perturbations used a probabilistic multiplier to change the amplitude of an applied sine wave each period, maintaining timing, while frequency perturbations used the same multiplier to vary the timing of sine waves for each period. Amplitude oscillations caused a greater degree of proactive control characterized by changes in temporal stepping parameters. Frequency oscillations resulted in a greater change in reactive control, demonstrating variability in stepping parameters immediately preceding and following peaks in accelerations peaks which exceed 0.5 m/s2. These observations suggest that healthy young adults shift to a reactive strategy of balance compensation when subject to more difficult, higher acceleration oscillations of support surface while maintaining a proactive rate of level walking at low accelerations

Voluntary control of wearable robotic exoskeletons by patients with paresis via neuromechanical modeling.

BACKGROUND: Research efforts in neurorehabilitation technologies have been directed towards creating robotic exoskeletons to restore motor function in impaired individuals. However, despite advances in mechatronics and bioelectrical signal processing, current robotic exoskeletons have had only modest clinical impact. A major limitation is the inability to enable exoskeleton voluntary control in neurologically impaired individuals. This hinders the possibility of optimally inducing the activity-driven neuroplastic changes that are required for recovery. METHODS: We have developed a patient-specific computational model of the human musculoskeletal system controlled via neural surrogates, i.e., electromyography-derived neural activations to muscles. The electromyography-driven musculoskeletal model was synthesized into a human-machine interface (HMI) that enabled poststroke and incomplete spinal cord injury patients to voluntarily control multiple joints in a multifunctional robotic exoskeleton in real time. RESULTS: We demonstrated patients' control accuracy across a wide range of lower-extremity motor tasks. Remarkably, an increased level of exoskeleton assistance always resulted in a reduction in both amplitude and variability in muscle activations as well as in the mechanical moments required to perform a motor task. Since small discrepancies in onset time between human limb movement and that of the parallel exoskeleton would potentially increase human neuromuscular effort, these results demonstrate that the developed HMI precisely synchronizes the device actuation with residual voluntary muscle contraction capacity in neurologically impaired patients. CONCLUSIONS: Continuous voluntary control of robotic exoskeletons (i.e. event-free and task-independent) has never been demonstrated before in populations with paretic and spastic-like muscle activity, such as those investigated in this study. Our proposed methodology may open new avenues for harnessing residual neuromuscular function in neurologically impaired individuals via symbiotic wearable robots

Sensorimotor Processing in Post-Stroke Fatigue

Chronic pathological fatigue is a highly debilitating symptom with a significant impact on quality of life of stroke survivors. Despite its high prevalence, research into the mechanisms that underlie post-stroke fatigue is lacking. This thesis outlines how changes in cortical neurophysiology results in alterations in sensorimotor processing associated with the perception of effort and how prolonged experience of high effort can subsequently result in chronic pathological fatigue. I show that the perception of effort for what are usually low effort activities is altered in non-depressed, chronic stroke survivors with minimal physical impairment. Low effort voluntary contractions are perceived as more effortful in stroke survivors with high fatigue compared to those with low fatigue. Sensory attenuation, the ability to attend away from predictable sensory input, is thought to underlie altered effort perception. If one is unable to attend away from predictable sensory input associated with a voluntary movement, this will give rise to the perception of higher effort afforded to the movement. I show that stroke survivors with high fatigue do not show reduced sensory attenuation of sensory input arising from mechanoreceptors as quantified using a force matching task and suggest that high effort afforded to simple voluntary movements may be a result of reduced sensory attenuation of information arising from within the body, namely proprioceptive afferent information from the contracting muscle. Using TMS, I show that cortical excitability both at rest and during movement preparation is altered in stroke survivors with high fatigue and propose that cortical excitability reflects the degree of sensory attenuation at the level of the sensorimotor cortex. Finally, I show that neuromodulatory techniques such as transcranial direct current stimulation, are potential tools that can be used to reduce fatigue severity by potentially resetting cortical neurophysiology and reducing perceived effort. Overall, the data provides some evidence in support of the sensory attenuation model of fatigue and provides a novel insight into the mechanisms implicated in post- stroke fatigue

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 > 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

Effects of Movement Context on Reach-Grasp-Lift Motion and Grip Force after Stroke

Loss of upper extremity function after stroke is a significant problem resulting in enormous personal, societal, and economic costs. Neurophysiological discoveries over several decades have revealed great potential for use-dependent neural adaptation, and have revitalized the search for training strategies that optimize recovery. Although task-specific repetitive practice is recognized as a key stimulus to promote upper extremity function after stroke, choices of what to practice and how to practice remain challenging and poorly guided by evidence. This research was inspired by evidence in healthy individuals, that movement can be altered by characteristics of the task and the environment, together referred to as the movement context. The purpose of this research was to determine whether motor performance of the paretic upper extremity is affected by two specific movement context variations: 1) preferred speed versus fast, and 2) unilateral versus bilateral. Using electromagnetic motion tracking and pressure sensor quantification of grip force, we assessed upper extremity task performance in people with post-stroke hemiparesis. To evaluate effects of movement speed, we compared paretic-limb performance of a reach-grasp-lift task at a self-selected preferred speed to the same task performed as fast as possible. People with hemiparesis were able to move faster than their preferred speed, and when they did, movement quality was better. Reach paths were straighter, finger movements were more efficient, and the fingers opened wider. To evaluate effects of the bilateral movement context, we compared paretic-limb performance of a reach-grasp-lift-release task unilaterally versus bilaterally. We found no immediate improvement in the bilateral context. We further explored effects of the bilateral movement context by measuring maximal and submaximal grip force capacity using grip dynamometers. Unlike healthy controls and unlike the non-paretic side, the paretic side of people with hemiparesis produced more maximal force in the bilateral condition. In a submaximal task, however, the bilateral condition did not enhance the paretic side\u27s contribution. These results suggest that emphasizing speed during post-stroke rehabilitation may be worthwhile, that the bilateral movement context has little immediate impact on task performance, and that the paretic limb may benefit from the bilateral condition only at high force levels

Multichannel surface EMG decomposition based on measurement correlation and LMMSE

A method based on measurement correlation (MC) and linear minimum mean square error (LMMSE) for multichannel surface electromyography (sEMG) signal decomposition was developed in this study. This MC-LMMSE method gradually and iteratively increases the correlation between an optimized vector and a reconstructed matrix that is correlated with the measurement matrix. The performance of the proposed MC-LMMSE method was evaluated with both simulated and experimental sEMG signals. Simulation results show that the MC-LMMSE method can successfully reconstruct up to 53 innervation pulse trains with a true positive rate greater than 95%. The performance of the MC-LMMSE method was also evaluated using experimental sEMG signals collected with a 64-channel electrode array from the first dorsal interosseous muscles of three subjects at different contraction levels. A maximum of 16 motor units were successfully extracted from these multichannel experimental sEMG signals. The performance of the MC-LMMSE method was further evaluated with multichannel experimental sEMG data by using the “two sources” method. The large population of common MUs extracted from the two independent subgroups of sEMG signals demonstrates the reliability of the MC-LMMSE method in multichannel sEMG decomposition

Changes in muscle function and motorneuron excitability of the triceps surae following a bout of fatiguing eccentric exercise

A reduction in capacity of the neuromuscular system associated with exercise can occur from a wide range of physiological and psychological factors. Many researchers have investigated neural activation during exercise, or the effects of muscle damage associated with eccentric exercise, but few have studied the prolonged effects of a bout of eccentric exercise on strength and motorneuron excitability. Eleven male and female subjects (aged 20-43 years) were tested to determine the effects of a fatiguing bout of eccentric exercise upon maximal isometric plantarflexion strength, motorneuron excitability, and neural activation of the soleus (SOL) and medial gastrocnemius (MG). The exercise consisted of two hours on a calf raise machine, the only the right leg performing eccentric repetitions, with three sets of 60 repetitions at 60% of the concentric one repetition maximum (1RM). Hoffman reflex (H-reflex), evoked responses, maximum voluntary contraction (MVC) torque, voluntary root mean squared electromyography (nnsEMG), Creatine Kinase (CK), and the Achilles tendon reflex (T -reflex) were tested immediately prior to, immediately post, and l, 24, 48 and 72 hours post exercise. Results indicated that there were significant (R \u3c 0.05) decreases of 18% and 23% in MVC torque and SOL rmsEMG respectively following the fatiguing protocol. There were also significant declines of 31% in the SOL H-reflex, 25% in the SOL HMAX:MMAX (the ratio of the maximum H-reflex to the maximum M-response ), as well as a 21% decline in the amplitude of the evoked twitch. There were no significant decreases in the M-response or T-reflex, or in any of the variables of the control leg, following the exercise bout. The reduced voluntary torque and EMG suggests that the force loss was due to a decreased neural drive. The decline in the H-reflex following exercise indicates a reduction in the excitability of the α-motorneuron pool (since altered M-waves suggest no impairment in neuromuscular propagation). The change in strength may in part be due to alterations in spinal excitability, but other factors must also contribute since the correlation between the two (although significant) is relatively weak (r2 = 0.2). The lack of change in the T-reflex may suggest that, with the combined effect of a decrease in spinal excitability and increase in spindle responsiveness and/or muscle compliance, which in part compensate for the decline in α-motorneuron excitability, the resultant net change was zero. Result suggests that alterations in motor drive associated with fatiguing eccentric exercise probably represent a combination of the modulatory effects of a number of inputs (both excitatory and inhibitory) to the α-motorneuron

Fatigue and Mobility Post-Stroke

Fatigue post-stroke is a disabling and persistent symptom affecting many stroke survivors. Despite its high prevalence, the pathophysiology underlying this phenomenon remains obscure. Thus, the aim of this thesis was to study the neuromuscular basis underlying fatigue post-stroke and its association with self-reported fatigue and with the performance of tasks incorporating balance and mobility components. Community-dwelling stroke survivors who had mild to moderate deficits in functional balance and mobility participated in a series of investigations. Chapter 2 describes the initial validation of the Community Balance and Mobility (CB&M) scale for use in persons with chronic stroke. Chapter 3 reported the presence of self-reported fatigue, assessed with the Fatigue Assessment Scale and restricted functional balance and mobility, measured with the 6-minute walk test and with the CB&M. Based on the findings obtained from the twitch interpolation and transcranial magnetic stimulation techniques, stroke resulted in a shift of the origin of neuromuscular fatigue such that the participants with stroke were more susceptible to the development of central fatigue following a standardized fatigue task, whereas healthy subjects had more evidence of peripheral fatigue. Also, the results from Chapter 3 demonstrated that the susceptibility to central failure was positively associated with the increased self-reported fatigability and negatively with the 6-MWT and CB&M scores. In Chapter 4 changes in the intrinsic properties of the spinal motoneurons, manifested as prolongation of the afterhyperpolarization time-course estimated with the interval death rate transform method were demonstrated. Prolonged afterhyperpolarization may have contributed to the increased central fatigue observed on the paretic side of the participants with stroke. In summary, the stroke-induced disturbances along the neuromuscular system together with the post-stroke deficits in functional balance and mobility may compromise the ability of the central nervous system to cope with the increased physiological demands during physical activities. This may lead to the increased perception of effort, which could influence the performance of activities of daily living and may partially underlie the general complaint of fatigue experienced by people with stroke. The findings reported in this thesis have advanced the understanding of a pathophysiological basis of fatigue post-stroke, which is essential for developing and guiding effective rehabilitation treatments

Improving Precision Force Control With Low-Frequency Error Amplification Feedback: Behavioral and Neurophysiological Mechanisms

Although error amplification (EA) feedback has been shown to improve performance on visuomotor tasks, the challenge of EA is that it concurrently magnifies task-irrelevant information that may impair visuomotor control. The purpose of this study was to improve the force control in a static task by preclusion of high-oscillatory components in EA feedback that cannot be timely used for error correction by the visuomotor system. Along with motor unit behaviors and corticomuscular coherence, force fluctuations (Fc) were modeled with non-linear SDA to contrast the reliance of the feedback process and underlying neurophysiological mechanisms by using real feedback, EA, and low-frequency error amplification (LF-EA). During the static force task in the experiment, the EA feedback virtually potentiated the size of visual error, whereas the LF-EA did not channel high-frequency errors above 0.8 Hz into the amplification process. The results showed that task accuracy was greater with the LF-EA than with the real and EA feedback modes, and that LF-EA led to smaller and more complex Fc. LF-EA generally led to smaller SDA variables of Fc (critical time points, critical point of Fc, the short-term effective diffusion coefficient, and short-term exponent scaling) than did real feedback and EA. The use of LF-EA feedback increased the irregularity of the ISIs of MUs but decreased the RMS of the mean discharge rate, estimated with pooled MU spike trains. Beta-range EEG–EMG coherence spectra (13–35 Hz) in the LF-EA condition were the greatest among the three feedback conditions. In summary, amplification of low-frequency errors improves force control by shifting the relative significances of the feedforward and feedback processes. The functional benefit arises from the increase in the common descending drive to promote a stable state of MU discharges