Quantifying Muscle Co-activation for Impaired Finger Independence in Stroke Survivors.

OBJECTIVE: Hand impairment frequently occurs in individuals following a stroke. There is evidence of abnormal muscle co-activation that contributes to impaired control of finger independence. This study quantitatively analyzed hand muscle co-activation patterns of chronic stroke survivors. Systematically quantifying the degree of muscle co-activation patterns in stroke survivors can help us to better understand the mechanisms behind compromised finger independence and enables a more accurate assessment of hand impairment. METHODS: We analyzed muscle co-activation patterns both macroscopically and microscopically using high-density surface electromyographic (HD-sEMG) signals and decomposed motor unit signals from extrinsic and intrinsic flexor/extensor muscles. The muscle co-activation patterns between both sides of stroke survivors and neurologically intact controls were compared. RESULTS: We observed increased levels of co-activation in the affected sides of stroke survivors compared with their contralateral sides and the control groups, with a higher degree in the extrinsic muscles than the intrinsic muscles. The asymmetry in muscle co-activation between hands correlated with impaired finger force independence and clinical assessment scales. In the micro-level analysis of motor unit action potentials (MUAPs) distributions, we observed a notable increase in action potential spread of MUAPs in the individual affected extrinsic muscles, but the altered MUAP distribution did not correlate with clinical assessment scales. CONCLUSION: We systematically quantified abnormal muscle co-activation patterns in impaired finger independence after stroke. SIGNIFICANCE: With further development, the outcomes provide a comprehensive understanding of hand dexterity deficits in stroke survivors, which may provide guidance for targeted rehabilitation strategies and offer a potential for automated impairment evaluations.

The interplay of environmental luminance and genetics in the retinal dystrophy induced by the dominant RPE65 mutation.

SignificanceIn humans, genetic mutations in the retinal pigment epithelium (RPE) 65 are associated with blinding diseases, for which there is no effective therapy alleviating progressive retinal degeneration in affected patients. Our findings uncovered that the increased free opsin caused by enhancing the ambient light intensity increased retinal activation, and when compounded with the RPE visual cycle dysfunction caused by the heterozygous D477G mutation and aggregation, led to the onset of retinal degeneration.

Activation of Superficial and Deep Finger Flexors Through Transcutaneous Nerve Stimulation.

OBJECTIVE: Functional electrical stimulation (FES) is a common technique to elicit muscle contraction and help improve muscle strength. Traditional FES over the muscle belly typically only activates superficial muscle regions. In the case of hand FES, this prevents the activation of the deeper flexor muscles which control the distal finger joints. Here, we evaluated whether an alternative transcutaneous nerve-bundle stimulation approach can activate both superficial and deep extrinsic finger flexors using a high-density stimulation grid. METHODS: Transverse ultrasound of the forearm muscles was used to obtain cross-sectional images of the underlying finger flexors during stimulated finger flexions and kinematically-matched voluntary motions. Finger kinematics were recorded, and an image registration method was used to capture the large deformation of the muscle regions during each flexion. This deformation was used as a surrogate measure of the contraction of muscle tissue, and the regions of expanding tissue can identify activated muscles. RESULTS: The nerve-bundle stimulation elicited contractions in the superficial and deep finger flexors. Both separate and concurrent activation of these two muscles were observed. Joint kinematics of the fingers also matched the expected regions of muscle contractions. CONCLUSIONS: Our results showed that the nerve-bundle stimulation technique can activate the deep extrinsic finger flexors, which are typically not accessible via traditional surface FES. SIGNIFICANCE: Our nerve-bundle stimulation method enables us to produce the full range of motion of different joints necessary for various functional grasps, which could benefit future neuroprosthetic applications.

Automatic Detection of Contracting Muscle Regions via the Deformation Field of Transverse Ultrasound Images: A Feasibility Study.

Accurate identification of contracting muscles can help us to understand the muscle function in both physiological and pathological conditions. Conventional electromyography (EMG) have limited access to deep muscles, crosstalk, or instability in the recordings. Accordingly, a novel framework was developed to detect contracting muscle regions based on the deformation field of transverse ultrasound images. We first estimated the muscle movements in a stepwise calculation, to derive the deformation field. We then calculated the divergence of the deformation field to locate the expanding or shrinking regions during muscle contractions. Two preliminary experiments were performed to evaluate the feasibility of the developed algorithm. Using concurrent intramuscular EMG recordings, Experiment I verified that the divergence map can capture the activity of superficial and deep muscles, when muscles were activated voluntarily or through electrical stimulation. Experiment II verified that the divergence map can only capture contracting muscles but not muscle shortening during passive movements. The results demonstrated that the divergence can individually capture the activity of muscles at different depths, and was not sensitive to muscle shortening during passive movements. The proposed framework can automatically detect the regions of contracting muscle, and could potentially serve as a tool to assess the functions of a group of muscles concurrently.

Evolution of a 4-Benzyloxy-benzylamino Chemotype to Provide Efficacious, Potent, and Isoform Selective PPARα Agonists as Leads for Retinal Disorders.

Peroxisome proliferator-activated receptor alpha (PPARα) is expressed in retinal Müller cells, endothelial cells, and in retinal pigment epithelium; agonism of PPARα with genetic or pharmacological tools ameliorates inflammation, vascular leakage, neurodegeneration, and neovascularization associated with retinal diseases in animal models. As such, PPARα is a promising drug target for diabetic retinopathy and age-related macular degeneration. Herein, we report proof-of-concept in vivo efficacy in an streptozotocin-induced vascular leakage model (rat) and preliminary pharmacokinetic assessment of a first-generation lead 4a (A91). Additionally, we present the design, synthesis, and evaluation of second-generation analogues, which led to the discovery of 4u and related compounds that reach cellular potencies <50 nM and exhibit >2,700-fold selectivity for PPARα over other PPAR isoforms. These studies identify a pipeline of candidates positioned for detailed PK/PD and pre-clinical evaluation.

Object stiffness recognition using haptic feedback delivered through transcutaneous proximal nerve stimulation.

OBJECTIVE: Haptic feedback is crucial when we manipulate objects. Information pertaining to an object's stiffness in particular can help facilitate fine motor control. In this study, we seek to determine whether objects of different stiffness levels can be recognized using haptic feedback provided by transcutaneous electrical stimulation of peripheral nerves. APPROACH: Using a stimulation electrode grid placed along the medial side of the upper arm, the median and ulnar nerve bundles were targeted to evoke haptic sensation on the palmar side of the hand. Stimulation current amplitude was modulated in real-time with the fingertip force recorded from a sensorized prosthetic hand. In order to evaluate which stimulation pattern was more critical, object stiffness was encoded either by the rate of change of the stimulus amplitude or the level of peak stimulus amplitude, as the prosthesis grasped the objects. MAIN RESULTS: Both encoding methods allowed the subjects to differentiate objects of different stiffness levels with >90% accuracy. No significant difference was observed between the two encoding methods, which indicated that both the rate of change of the stimulation amplitude and the peak stimulation amplitude could effectively provide stiffness information of the objects. SIGNIFICANCE: The outcomes suggest that it is possible to elicit haptic sensations describing various object stiffness levels using transcutaneous nerve stimulation. The haptic feedback associated with object stiffness can facilitate object manipulation/interactions. It may also improve user experience during human-machine interactions, when object stiffness information is incorporated.

Multichannel Nerve Stimulation for Diverse Activation of Finger Flexors.

OBJECTIVE: Neuromuscular electrical stimulation (NMES) is a common approach to restore muscle strength of individuals with a neurological injury but restoring hand dexterity is still a challenge. This study sought to quantify the diversity of finger movements elicited by a multichannel nerve stimulation technique. METHODS: A 2 × 8 stimulation grid, placed on the upper arm along the ulnar and median nerves, was used to activate different finger flexors by automatically switching between randomized bipolar electrodes. The forces from each individual finger as well as the high-density electromyogram (HDEMG) of the intrinsic and extrinsic flexors were recorded. The elicited finger forces were categorized using hierarchical clustering, and the 2D correlation of the spatial patterns of muscle activation was also calculated. RESULTS: A wide range of movement patterns were identified, including multi-finger and single-digit movements. Additionally, a number of electrode pairs elicited similar finger movements. The muscle activation patterns showed similar and distinct spatial patterns, signifying activation redundancy. CONCLUSION: These results revealed the diversity of elicitable finger movements and muscle activations. The system redundancy can be explored to compensate for system instability due to fatigue or electrode shift. The outcomes can also enable the development of an automatic calibration of the stimulation.

Muscle Fatigue Post-stroke Elicited From Kilohertz-Frequency Subthreshold Nerve Stimulation.

Purpose: Rapid muscle fatigue limits clinical applications of functional electrical stimulation (FES) for individuals with motor impairments. This study aimed to characterize the sustainability of muscle force elicited with a novel transcutaneous nerve stimulation technique. Method: A hemiplegic chronic stroke survivor was recruited in this case study. Clustered subthreshold pulses of 60-µs with kilohertz (12.5 kHz) carrier frequency (high-frequency mode, HF) were delivered transcutaneously to the proximal segment of the median/ulnar nerve bundles to evaluate the finger flexor muscle fatigue on both sides of the stroke survivor. Conventional nerve stimulation technique with 600-µs pulses at 30 Hz (low-frequency mode, LF) served as the control condition. Fatigue was evoked by intermittently delivering 3-s stimulation trains with 1-s resting. For fair comparison, initial contraction forces (approximately 30% of the maximal voluntary contraction) were matched between the HF and LF modes. Muscle fatigue was evaluated through elicited finger flexion forces (amplitude and fluctuation) and muscle activation patterns quantified by high-density electromyography (EMG). Result: Compared with those from the LF stimuli, the elicited forces declined more slowly, and the force plateau was higher under the HF stimulation for both the affected and contralateral sides, resulting in a more sustainable force output at higher levels. Meanwhile, the force fluctuation under the HF stimulation increased more slowly, and, thus, was smaller after successive stimulation trains compared with the LF stimuli, indicating a less synchronized activation of muscle fibers. The efficiency of the muscle activation, measured as the force-EMG ratio, was also higher in the HF stimulation mode. Conclusion: Our results indicated that the HF nerve stimulation technique can reduce muscle fatigue in stroke survivors by maintaining a higher efficiency of muscle activations compared with the LF stimulation. The technique can help improve the performance of neurorehabilitation methods based on electrical stimulation, and facilitate the utility of FES in clinical populations.

Flexibility of Finger Activation Patterns Elicited through Non-invasive Multi-Electrode Nerve Stimulation.

The inability to effectively activate and control skeletal muscles is a common impairment following a variety of neurological conditions or injuries. One common approach to restoring or augmenting this impairment is the use of external electrical stimulation of the muscles, called functional electrical stimulation (FES). Typically targeted directly at the anatomical muscle belly, existing methodologies often involve high current amplitudes, limited superficial muscle activation, and early onset of muscle fatigue. We have recently explored the capabilities of a non-invasive peripheral nerve stimulation method for the dexterous control of finger and hand muscles. Further development of our stimulation system has enabled us to manually search across a variety of stimulation locations with increased consistency and efficiency. This study examined the preliminary results in two subjects of an automated stimulation system which can rapidly characterize a large combination of stimulation electrodes. Our preliminary findings suggested that the stimulation grid was able to produce a number of clustered EMG activities and finger forces. This robust ability to flexibly generate different grasp patterns demonstrates the promise of the methodology in future applications for FES and rehabilitation.

Delayed fatigue in finger flexion forces through transcutaneous nerve stimulation.

OBJECTIVE: Weakness of the hand is a major impairment which limits independent living. Neuromuscular electrical stimulation (NMES) is a common approach to help restore muscle strength. Traditional NMES directly over the muscle often leads to a rapid onset of muscle fatigue. In this study, we investigated the force sustainability of finger flexor muscles using a transcutaneous nerve stimulation approach. APPROACH: Finger flexion forces and high-density electromyogram (HD EMG) signals were obtained while electrical stimulation was applied to the ulnar and median nerve bundles through a stimulation grid on the proximal arm segment. Stimulation was also applied to the finger flexor muscle belly targeting the motor point, serving as a control condition. The force produced from the two stimulation approaches were initially matched, and muscle fatigue was subsequently induced with 5 min of continuous stimulation. The rate of decay of the force and EMG amplitude were quantified, and the spatial distribution of the muscle activation during the sustained contraction was also evaluated. MAIN RESULTS: The proximal nerve stimulation approach induced a slower decay in both force and EMG, compared with the stimulation at the motor point. The spatial distribution of the elicited muscle activation showed that the proximal nerve stimulation led to a distributed activation across the intrinsic and extrinsic finger flexor muscles and also activated a wider area within the extrinsic muscle. SIGNIFICANCE: Our findings demonstrated that the stimulation of the proximal nerve bundles can elicit sustained force output and delayed decrease in the rate of force decline. This is potentially due to a spatially distributed activation of the muscle fibers, compared with the traditional motor point stimulation. Future development of our nerve stimulation approach may enable prolonged usage during rehabilitation or assistance for better functional outcomes.

Assessing Hand Muscle Structural Modifications in Chronic Stroke.

The purpose of the study is to assess poststroke muscle structural alterations by examining muscular electrical conductivity and inherent electrophysiological properties. In particular, muscle impedance and compound muscle action potentials (CMAP) were measured from the hypothenar muscle bilaterally using the electrical impedance myography and the electrophysiological techniques, respectively. Significant changes of muscle impedance were observed in the paretic muscle compared with the contralateral side (resistance: paretic: 27.54 ± 0.97 Ω, contralateral: 25.46 ± 0.91 Ω, p < 0.05; phase angle: paretic: 8.81 ± 0.61°, contralateral: 10.79 ± 0.69°, p < 0.05). In addition, impedance changes correlated moderately with the CMAP amplitude in the paretic hand (phase angle: r = 0.66, p < 0.05; reactance: r = 0.58, p < 0.05). The study discloses significant muscle rearrangements as a result of fiber loss or atrophy, fat infiltration or impaired membrane integrity in chronic stroke.

Evoked haptic sensations in the hand via non-invasive proximal nerve stimulation.

OBJECTIVE: Haptic perception of a prosthetic limb or hand is a crucial, but often unmet, need which impacts the utility of the prostheses. In this study, we seek to evaluate the feasibility of a non-invasive transcutaneous nerve stimulation method in generating haptic feedback in a transradial amputee subject as well as intact able-bodied subjects. APPROACH: An electrode grid was placed on the skin along the medial side of the upper arm beneath the short head of the biceps brachii, in proximity to the median and ulnar nerves. Varying stimulation patterns were delivered to different electrode pairs, in order to emulate different types of sensations (Single Tap, Press-and-Hold, Double Tap) at different regions of the hand. Subjects then reported the magnitude of sensation by pressing on a force transducer to transform the qualitative haptic perception into a quantitative measurement. MAIN RESULTS: Altering current stimulations through electrode pairs on the grid resulted in repeatable alterations in the percept regions of the hand. Most subjects reported spatial coverage of individual fingers or phalanges, which can resemble the whole hand through different pairs of stimulation electrodes. The different stimulation patterns were also differentiable by all subjects. The amputee subject also reported haptic sensations similar to the able-bodied subjects. SIGNIFICANCE: Our findings demonstrated the capabilities of our transcutaneous stimulation method. Subjects were able to perceive spatially distinct sensations with graded magnitudes that emulated tapping and holding sensation in their hands. The elicitation of haptic sensations in the phantom hand of an amputee is a significant step in the development of our stimulation method, and provides insight into the future adaptation and implementation of prostheses with non-invasive sensory feedback to the users.

Variation of Finger Activation Patterns Post-stroke Through Non-invasive Nerve Stimulation.

Purpose: A transcutaneous proximal nerve stimulation technique utilizing an electrode grid along the nerve bundles has previously shown flexible activation of multiple fingers. This case study aimed to further demonstrate the ability of this novel stimulation technique to induce various finger grasp patterns in a stroke survivor. Methods: An individual with chronic hemiplegia and severe hand impairment was recruited. Electrical stimulation was delivered to different pairs of an electrode grid along the ulnar and median nerves to selectively activate different finger flexor muscles, with an automated electrode switching method. The resultant individual isometric flexion forces and forearm flexor high-density electromyography (HDEMG) were acquired to evaluate the finger activation patterns. A medium and low level of overall activation were chosen to gauge the available finger patterns for both the contralateral and paretic hands. All the flexion forces were then clustered to categorize the different types of grasp patterns. Results: Both the contralateral and paretic sides demonstrated various force clusters including single and multi-finger activation patterns. The contralateral hand showed finger activation patterns mainly centered on median nerve activation of the index, middle, and ring fingers. The paretic hand exhibited fewer total activation patterns, but still showed activation of all four fingers in some combination. Conclusion: Our results show that electrical stimulation at multiple positions along the proximal nerve bundles can elicit a select variety of finger activation patterns even in a stroke survivor with minimal hand function. This system could be further implemented for better rehabilitative training to help induce functional grasp patterns or to help regain muscle mass.

Relative contribution of different altered motor unit control to muscle weakness in stroke: a simulation study.

OBJECTIVE: Chronic muscle weakness impacts the majority of individuals after a stroke. The origins of this hemiparesis is multifaceted, and an altered spinal control of the motor unit (MU) pool can lead to muscle weakness. However, the relative contribution of different MU recruitment and discharge organization is not well understood. In this study, we sought to examine these different effects by utilizing a MU simulation with variations set to mimic the changes of MU control in stroke. APPROACH: Using a well-established model of the MU pool, this study quantified the changes in force output caused by changes in MU recruitment range and recruitment order, as well as MU firing rate organization at the population level. We additionally expanded the original model to include a fatigue component, which variably decreased the output force with increasing length of contraction. Differences in the force output at both the peak and fatigued time points across different excitation levels were quantified and compared across different sets of MU parameters. MAIN RESULTS: Across the different simulation parameters, we found that the main driving factor of the reduced force output was due to the compressed range of MU recruitment. Recruitment compression caused a decrease in total force across all excitation levels. Additionally, a compression of the range of MU firing rates also demonstrated a decrease in the force output mainly at the higher excitation levels. Lastly, changes to the recruitment order of MUs appeared to minimally impact the force output. SIGNIFICANCE: We found that altered control of MUs alone, as simulated in this study, can lead to a substantial reduction in muscle force generation in stroke survivors. These findings may provide valuable insight for both clinicians and researchers in prescribing and developing different types of therapies for the rehabilitation and restoration of lost strength after stroke.

Exploration of Hand Grasp Patterns Elicitable Through Non-Invasive Proximal Nerve Stimulation.

Various neurological conditions, such as stroke or spinal cord injury, result in an impaired control of the hand. One method of restoring this impairment is through functional electrical stimulation (FES). However, traditional FES techniques often lead to quick fatigue and unnatural ballistic movements. In this study, we sought to explore the capabilities of a non-invasive proximal nerve stimulation technique in eliciting various hand grasp patterns. The ulnar and median nerves proximal to the elbow joint were activated transcutanously using a programmable stimulator, and the resultant finger flexion joint angles were recorded using a motion capture system. The individual finger motions averaged across the three joints were analyzed using a cluster analysis, in order to classify the different hand grasp patterns. With low current intensity (<5 mA and 100 µs pulse width) stimulation, our results show that all of our subjects demonstrated a variety of consistent hand grasp patterns including single finger movement and coordinated multi-finger movements. This study provides initial evidence on the feasibility of a proximal nerve stimulation technique in controlling a variety of finger movements and grasp patterns. Our approach could also be developed into a rehabilitative/assistive tool that can result in flexible movements of the fingers.

Origins of Common Neural Inputs to Different Compartments of the Extensor Digitorum Communis Muscle.

The extensor digitorum communis (EDC) is a multi-compartment muscle that allows dexterous extension of the four digits. However, the level of common input shared across different compartments of this muscle is not well understood. We seek to systematically characterize the common and independent neural input, originated from different levels of the central nervous system, to the different compartments. A motor unit (MU) coherence analysis was used to capture the different sources of common and independent input, by quantifying the coherence of MU discharge between different compartments. The MU activities were obtained from decomposition of surface electromyogram recordings. Our results showed that the MU coherence across different muscle compartments accounted for only a small proportion (<20%) of the total input in the alpha (5-12 Hz) and beta (15-30 Hz) bands, but was a major driver (>60%) in the delta (1-4 Hz) band. Additionally, cross-compartment coherence between the middle and ring-little fingers tended to be higher as compared with other finger combinations. Overall, the common input shared across different fingers are found to be at low to moderate levels, in comparison with the total input, which allows dexterous control of individual digits with some degree of coordinated control of multiple digits.

Assessing muscle compliance in stroke with the Myotonometer.

BACKGROUND: This study explores changes of the intrinsic biomechanical property in the biceps brachii muscle after a hemispheric stroke using the Myotonometry technique. METHODS: Nineteen subjects with chronic hemiplegia participated in the study. Myotonometer was used to measure tissue displacement when compression force was applied at 8 levels from 2.45N to 19.6N. Muscle displacement and compliance were determined and averaged over multiple trials. FINDINGS: Statistical analysis indicated a significant decrease in muscle displacement and compliance in the spastic muscles compared with the contralateral side (muscle displacements: spastic: 4.51 (0.31) mm, contralateral: 5.74 (0.37) mm, p<0.005; compliance: spastic: 0.17 (0.011) mm/N, contralateral: 0.22 (0.014) mm/N, p<0.005). Correlation analysis, however, did not show any association between clinical assessments and myotonometric measurement (p>0.1). INTERPRETATION: Alterations of muscle compliance in the spastic side reflect changes in the contractile or intrinsic mechanical properties after a stroke. Findings of the study have demonstrated high sensitivity and effectiveness of the Myotonometer in assessing muscle compliance changes.

Electrical impedance myography changes after incomplete cervical spinal cord injury: An examination of hand muscles.

OBJECTIVE: This study was to apply the newly developed electrical impedance myography (EIM) technique to examine hand muscles in patients with an incomplete cervical spinal cord injury (SCI). METHODS: EIM was performed on the thenar, hypothenar, and first dorsal interosseous (FDI) muscles of SCI (n=16) and age-matched healthy control (n=18) subjects. By sending low intensity and high frequency current through the skin and measuring the consequent voltage, EIM estimates the major impedance parameters, which include resistance (R), reactance (X) and phase angle (θ). RESULTS: The SCI group had lower reactance and phase angle (p<0.0001) as compared to the control group in all three muscles, and lower resistance in the thenar muscle (p<0.05). The SCI group also demonstrated a smaller anisotropy in resistance (p<0.0001) and larger anisotropy in phase angle (p<0.05) compared to those from healthy controls. CONCLUSION: The reduced reactance and phase angle of paralyzed muscles could be due tochanges of membrane integrity and fat infiltration, whereas the change in the anisotropy may reflect the rearrangement of muscle fiber geometry. SIGNIFICANCE: The EIM provides a quick and convenient tool for examination of muscle alterations after SCI.

Alterations in Localized Electrical Impedance Myography of Biceps Brachii Muscles Paralyzed by Spinal Cord Injury.

This study assessed electrical impedance myography (EIM) changes after spinal cord injury (SCI) with a localized multifrequency technology. The EIM measurement was performed on the biceps brachii muscle at rest condition of 17 cervical SCI subjects, and 23 neurologically intact subjects as control group. The results showed that there was a significant decrease in muscle reactance (X) and phase angle (θ) at selected frequencies (i.e., 50 and 100 kHz) in SCI compared to control. There was no significant difference in muscle resistance (R) between the two groups. The anisotropy examination revealed that SCI group had a decreased anisotropy ratio in resistance. In addition, the multifrequency spectrum analysis showed a decreased slope of the log(freq)-resistance regression in SCI group when compared to healthy control. Findings of the EIM changes are related to inherit muscle changes after the injury. Since EIM requires no patient effort and is quick and convenient to conduct, it may provide a useful tool for examination of paralyzed muscle changes after SCI.

Robotic Hand-Assisted Training for Spinal Cord Injury Driven by Myoelectric Pattern Recognition: A Case Report.

A 51-year-old man with an incomplete C6 spinal cord injury sustained 26 yrs ago attended twenty 2-hr visits over 10 wks for robot-assisted hand training driven by myoelectric pattern recognition. In each visit, his right hand was assisted to perform motions by an exoskeleton robot, while the robot was triggered by his own motion intentions. The hand robot was designed for this study, which can perform six kinds of motions, including hand closing/opening; thumb, index finger, and middle finger closing/opening; and middle, ring, and little fingers closing/opening. After the training, his grip force increased from 13.5 to 19.6 kg, his pinch force remained the same (5.0 kg), his score of Box and Block test increased from 32 to 39, and his score from the Graded Redefined Assessment of Strength, Sensibility, and Prehension test Part 4.B increased from 22 to 24. He accomplished the tasks in the Graded Redefined Assessment of Strength, Sensibility, and Prehension test Part 4.B 28.8% faster on average. The results demonstrate the feasibility and effectiveness of robot-assisted training driven by myoelectric pattern recognition after spinal cord injury.