BiLSTM-Based Joint Torque Prediction From Mechanomyogram During Isometric Contractions: A Proof of Concept Study.

Quantifying muscle strength is an important measure in clinical settings; however, there is a lack of practical tools that can be deployed for routine assessment. The purpose of this study is to propose a deep learning model for ankle plantar flexion torque prediction from time-series mechanomyogram (MMG) signals recorded during isometric contractions (i.e., a similar form to manual muscle testing procedure in clinical practice) and to evaluate its performance. Four different deep learning models in terms of model architecture (based on a stacked bidirectional long short-term memory and dense layers) were designed with different combinations of the number of units (from 32 to 512) and dropout ratio (from 0.0 to 0.8), and then evaluated for prediction performance by conducting the leave-one-subject-out cross-validation method from the 10-subject dataset. As a result, the models explained more variance in the untrained test dataset as the error metrics (e.g., root-mean-square error) decreased and as the slope of the relationship between the measured and predicted joint torques became closer to 1.0. Although the slope estimates appear to be sensitive to an individual dataset, >70% of the variance in nine out of 10 datasets was explained by the optimal model. These results demonstrated the feasibility of the proposed model as a potential tool to quantify average joint torque during a sustained isometric contraction.

Scaling relationships between human leg muscle architectural properties and body size.

A skeletal muscle's peak force production and excursion are based on its architectural properties that are, in turn, determined by its mass, muscle fiber length and physiological cross-sectional area (PCSA). In the classic interspecific study of mammalian muscle scaling, it was demonstrated that muscle mass scales positively allometrically with body mass whereas fiber length scales isometrically with body mass, indicating that larger mammals have stronger leg muscles than they would if they were geometrically similar to smaller ones. Although this relationship is highly significant across species, there has never been a detailed intraspecific architectural scaling study. We have thus created a large dataset of 896 muscles across 34 human lower extremities (18 females and 16 males) with a size range including approximately 90% and 70% of the United States population height and mass, respectively, across the range 36-103 years. Our purpose was to quantify the scaling relationships between human muscle architectural properties and body size. We found that human muscles depart greatly from isometric scaling because muscle mass scales with body mass1.3 (larger exponent than isometric scaling of 1.0) and muscle fiber length scales with negative allometry with body mass0.1 (smaller exponent than isometric scaling of 0.33). Based on the known relationship between architecture and function, these results suggest that human muscles place a premium on muscle force production (mass and PCSA) at the expense of muscle excursion (fiber length) with increasing body size, which has implications for understanding human muscle design as well as biomechanical modeling.

Ankle Joint Angle Influences Relative Torque Fluctuation during Isometric Plantar Flexion.

The purpose of this study was to investigate the influence of changes in muscle length on the torque fluctuations and on related oscillations in muscle activity during voluntary isometric contractions of ankle plantar flexor muscles. Eleven healthy individuals were asked to perform voluntary isometric contractions of ankle muscles at five different contraction intensities from 10% to 70% of maximum voluntary isometric contraction (MVIC) and at three different muscle lengths, implemented by changing the ankle joint angle (plantar flexion of 26°-shorter muscle length; plantar flexion of 10°-neutral muscle length; dorsiflexion of 3°-longer muscle length). Surface electromyogram (EMG) signals were recorded from the skin surface over the triceps surae muscles, and rectified-and-smoothed EMG (rsEMG) were estimated to assess the oscillations in muscle activity. The absolute torque fluctuations (quantified by the standard deviation) were significantly higher during moderate-to-high contractions at the longer muscle length. Absolute torque fluctuations were found to be a linear function of torque output regardless of muscle length. In contrast, the relative torque fluctuations (quantified by the coefficient of variation) were higher at the shorter muscle length. However, both absolute and relative oscillations in muscle activities remained relatively consistent at different ankle joint angles for all plantar flexors. These findings suggest that the torque steadiness may be affected by not only muscle activities, but also by muscle length-dependent mechanical properties. This study provides more insights that muscle mechanics should be considered when explaining the steadiness in force output.

Contribution of stroke-related changes in neuromuscular factors to gear ratio during isometric contraction of medial gastrocnemius muscle: A simulation study.

BACKGROUND: It is not clear which neuromuscular factors are most closely associated with the loss of variable fascicle gearing after chronic stroke. The purpose of this simulation study is to determine the effects of stroke-related changes in key neuromuscular factors on the gear ratio. METHODS: A modified Hill-type model of the medial gastrocnemius was developed to determine the gear ratio for a given muscle activation level and musculotendon length. Model parameters were then systematically adjusted to simulate known stroke-related changes in neuromuscular factors, and the gear ratio was computed for each change in the parameters. A Monte Carlo simulation was performed to understand which neuromuscular factors and fiber behavior-related parameters are most relevant to the loss of variable gearing. Dominance analyses were also conducted to quantify the relative importance of fiber behavior-related parameters on the gear ratio. FINDINGS: The gear ratio decreases significantly with smaller pennation angle and with shorter optimal fiber length. In addition, muscle thickness and pennation angle at optimal fiber length appear to be the most important muscle architectural parameters. Dominance analyses further suggest that primary determinants of gear ratio include initial pennation angle, fiber rotation-shortening ratio, initial muscle thickness, and fiber rotation. INTERPRETATION: Our findings provide insight that the pennation angle may play an important role for efficient muscular contraction, implying that maintaining muscle architecture and/or improving fiber/fascicle rotation could a key goal in rehabilitation interventions. Our findings will help us to better interpret altered gearing behavior in aging and pathological muscles.

Biomechanical Modeling of Brachialis-to-Wrist Extensor Muscle Transfer Function for Daily Activities in Tetraplegia.

We recently reported a novel case demonstrating the feasibility of a brachialis (BRA)-to-extensor carpi radialis brevis (ECRB) tendon transfer, but it is not yet known whether this transfer provides robust functional results across activities. The purpose of this study was to use biomechanical modeling to define the functional capacity of the BRA-to-ECRB tendon transfer in terms of enabling the performance of several activities of daily living. Methods: A model of the transferred BRA-ECRB muscle-tendon unit was developed to calculate isometric elbow and wrist joint torque as a function of elbow and wrist angles resulting from different BRA reattachment locations from 50 to 80 mm proximal to the wrist joint crease. Using this model, mathematical optimization predicted the optimal location for BRA reattachment in order to perform each of a number of important upper extremity tasks as well as to calculate a global optimum for performing all of the tasks. Results: Analysis of active joint torque showed that the entire elbow torque-angle curve surface shifted "diagonally" toward elbow flexion and wrist extension as the attachment location approached the wrist joint; peak wrist torque was produced at extended wrist angles. Our model predicted that the optimal attachment location for each different task ranged from 54.3 to 74.6 mm proximal to the wrist joint, which is feasible given the anatomy of the muscle-tendon unit. The attachment location to optimize performing all tasks was calculated as 63.5 mm proximal to the wrist joint. Conclusions: This study clearly demonstrates that the BRA, which is underused as a donor in tetraplegia surgery, is an excellent donor muscle to provide wrist extension. Biomechanical simulation further highlighted the need to consider not only donor-muscle appropriateness but the patient's desired function when planning surgical tendon transfers. Clinical Relevance: Quantitative evaluation of the way that surgery affects daily tasks rather than simply matching muscle properties may be a more appropriate approach for surgeons to use when choosing and tensioning donor muscles.

Relative contribution of altered neuromuscular factors to muscle activation-force relationships following chronic stroke: A simulation study.

The purpose of this study was to investigate the potential effects of key neuromuscular factors on muscle activation-force relationships, thereby helping us understand abnormal EMG-force relationships often reported in chronic stroke-impaired muscles. A modified Hill-type muscle model was developed to calculate muscle force production for a given muscle activation level and musculotendon length. Model parameters used to characterize musculotendon unit properties of medial gastrocnemius were adjusted to simulate known stroke-related changes in neuromuscular factors (e.g., voluntary activation and muscle mechanical properties). The muscle activation-force slope (i.e., muscle activation over force) was computed as a function of ankle joint angle. A Monte Carlo simulation approach was implemented to understand which neuromuscular factors are closely associated with the activation-force slope. Our simulations showed that a reduction in factors linked to voluntary activation capacity and to maximum force-generating capacity may be the primary contributors that increase the activation-force slope in dorsiflexed positions, and that a narrower active force-length curve appears to be the most significant factor that increases the slope in plantar flexed positions. In addition, our Monte Carlo simulation results demonstrated that an increase in the activation-force slope is strongly correlated with a reduction in voluntary activation capacity, in the maximum force-generating capacity, and in the active force-length curve width. These findings will help us to better interpret altered EMG-force relationships following chronic stroke.

Mechanomyogram amplitude vs. isometric ankle plantarflexion torque of human medial gastrocnemius muscle at different ankle joint angles.

The purpose of this study was to investigate the influence of changes in ankle joint angle on the mechanomyogram (MMG) amplitude of the human medial gastrocnemius (MG) muscle during voluntary isometric plantarflexion contractions. Ten healthy individuals were asked to perform voluntary isometric contractions at six different contraction intensities (from 10% to 100%) and at three different ankle joint angles (plantarflexion of 26°; plantarflexion of 10°; dorsiflexion of 3°). MMG signals were recorded from the surface over the MG muscle, using a 3-axis accelerometer. The relations between root mean square (RMS) MMG and isometric plantarflexion torque at different ankle joint angles were characterized to evaluate the effects of altered muscle mechanical properties on RMS MMG. We found that the relation between RMS MMG and plantarflexion torque is changed at different ankle joint angles: RMS MMG increases monotonically with increasing the plantarflexion torque but decreases as the ankle joint became dorsiflexed. Moreover, RMS MMG shows a negative correlation with muscle length, with passive torque, and with maximum voluntary torque, which were all changed significantly at different ankle joint angles. Our findings demonstrate the potential effects of changing muscle mechanical properties on muscle vibration amplitude. Future studies are required to explore the major sources of this muscle vibration from the perspective of muscle mechanics and muscle activation level, attributable to changes in the neural command.

Outcome from a brachialis donor for wrist extension in tetraplegia-time to reconsider the International Classification for Surgery of the Hand in Tetraplegia (ICSHT).

INTRODUCTION: Surgical reconstruction after quadriplegia represents a powerful solution to restore lost function by injury. A case is presented in which surgical reconstruction of a patient with a C4 level spinal cord injury is performed using the brachialis (BRA) muscle as the donor. CASE PRESENTATION: The patient previously had no hand function. This transfer, in combination with fusion of the thumb CMC joint and transfer of the flexor pollicis longus (FPL) tendon to the radius, gives the patient full thumb key pinch powered by BRA transferred to the wrist extensors. Theoretical analysis of muscle architectural properties demonstrates that the BRA has sufficient force and excursion to substitute for both the long and short radial wrist extensors. Furthermore, based on the fact that the BRA has almost twice the excursion compared to the extensor carpi radialis longus (ECRL), wrist extension can occur throughout the entire wrist and elbow ranges of motion. Finally, peak tension is lower than the rupture tension previously measured by us using this type of tendon-to-tendon attachment technique, suggesting that the transfer itself is safe and, importantly, can be immediately mobilized for neuromuscular rehabilitation. DISCUSSION: This procedure can thus restore tremendous functional capacity in patients who were previously categorized as group 0 by the International Classification of Hand Surgery in Tetraplegia (ICSHT). We suggest that, based on the BRA being an excellent donor for surgical reconstruction, that the ICHST system be reconsidered.

Longer electromechanical delay in paretic triceps surae muscles during voluntary isometric plantarflexion torque generation in chronic hemispheric stroke survivors.

Electromechanical delay (EMD) is the time delay between the onset of muscle activity and the onset of force/joint torque. This delay appears to be linked to muscular contraction efficiency. However, to our knowledge, limited evidence is available regarding the magnitude of the EMD in stroke-impaired muscles. Accordingly, this study aims to quantify the EMD in both paretic and non-paretic triceps surae muscles of chronic hemispheric stroke survivors, and to investigate whether the EMD is related to voluntary force-generating capacity in this muscle group. Nine male chronic stroke survivors were asked to perform isometric plantarflexion contractions at different force levels and at different ankle joint angles ranging from maximum plantarflexion to maximum dorsiflexion. The surface electromyograms were recorded from triceps surae muscles. The longest EMD among triceps surae muscles was chosen as the EMD for each side. Our results revealed that the EMD in paretic muscles was significantly longer than in non-paretic muscles. Moreover, both paretic and non-paretic muscles showed a negative correlation between the EMD and maximum torque-generating capacity. In addition, there was a strong positive relationship between the EMD and shear wave speed in paretic muscles as well as a negative relationship between the EMD and passive ankle joint range of motion. These findings imply that the EMD may be a useful biomarker, in part, associated with contractile and material properties in stroke-impaired muscles.

Loss of variable fascicle gearing during voluntary isometric contractions of paretic medial gastrocnemius muscles in male chronic stroke survivors.

KEY POINTS: Maximum fascicle shortening/rotation was significantly decreased in paretic medial gastrocnemius (MG) muscles compared to non-paretic MG muscles. The fascicle gear ratio on both sides decreased as the ankle became dorsiflexed, but the slope of the fascicle gear ratio over ankle joint angle was significantly lower on the paretic side. The side-to-side slope difference was strongly correlated with the relative maximum joint torque and with the relative shear wave speed, suggesting that variable gearing may explain muscle weakness after stroke. ABSTRACT: The present study aimed to understand variable fascicle gearing during voluntary isometric contractions of the medial gastrocnemius (MG) muscle in chronic stroke survivors. Using ultrasonography, we characterized fascicle behaviour on both paretic and non-paretic sides during plantarflexion contractions at different intensities and at different ankle joint angles. Shear wave speed was also recorded from the MG muscle belly under passive conditions. Fascicle gear ratios were then calculated as the ratio of muscle belly shortening velocity to fascicle shortening velocity, and variable fascicle gearing was quantified from the slope of gear ratio vs. joint angle relations. This slope was used to establish associations with maximum joint torques and with shear wave speeds. At all measured angles, we found a significant reduction in both maximum fascicle shortening and maximum fascicle rotation on the paretic side compared to the non-paretic side on our stroke survivor cohort. The fascicle rotation per fascicle shortening on the paretic side was also significantly smaller than on the non-paretic side, especially at plantarflexed positions. Furthermore, the fascicle gear ratio on both sides decreased as the ankle became dorsiflexed, but the change in the fascicle gear ratio was significantly lower on the paretic side. The side-to-side difference in the gear ratio slope was also strongly correlated with the relative maximum joint torque and with the relative shear wave speed, suggesting that variable gearing may explain muscle weakness after stroke. Further studies are needed to investigate how muscular changes after stroke may impede variable gearing and adversely impact muscle performance.

Bioinspired Micro Glue Threads Fabricated by Liquid Bridge-to-Solidification as an Effective Sensing Platform.

Spiders synthesize their web using a liquid bridge-to-solidification mechanism at the end of their glands. Inspired by this process, in this work, we fabricated micro-glue threads (µGTs, polymer microwires) by a simple "pinch and spread" process using just two fingertips. The µGTs exhibited excellent tensile strength (∼50 GPa), comparable to those of spider silk and biological fibers. The chemical, physical, and mechanical properties of the µGTs were investigated, and it was confirmed that the thickness of the µGTs could be controlled by ethanol treatment in varying concentrations. Moreover, electrically conductive µGTs were easily fabricated by simply mixing them with various nanomaterials such as gold nanoparticles, zinc oxide nanowires, and reduced graphene oxide (rGO). Interestingly, the conductive µGTs, fabricated using rGO, exhibited remarkable electrical conductivity (0.45 µS) compared to those fabricated using other materials. The conductive µGTs are applicable not only to NO2 gas sensing but also as electrical fuselike materials that melt when the humidity increases. Collectively, the results present µGTs as cost-effective, simple, and versatile materials, which enables their application in a variety of sensors.

Effects of Changes in Ankle Joint Angle on the Relation Between Plantarflexion Torque and EMG Magnitude in Major Plantar Flexors of Male Chronic Stroke Survivors.

The slope of the EMG-torque relation is potentially useful as a parameter related to muscular contraction efficiency, as a greater EMG-torque slope has often been reported in stroke-impaired muscles, compared to intact muscles. One major barrier limiting the use of this parameter on a routine basis is that we do not know how the EMG-torque slope is affected by changing joint angles. Thus, the primary purpose of this study is to characterize the EMG-torque relations of triceps surae muscles at different ankle joint angles in both paretic and non-paretic limbs of chronic hemispheric stroke survivors. Nine male chronic stroke survivors were asked to perform isometric plantarflexion contractions at different contraction intensities and at five different ankle joint angles, ranging from maximum plantarflexion to maximum dorsiflexion. Our results showed that the greater slope of the EMG-torque relations was found on the paretic side compared to the non-paretic side at comparable ankle joint angles. The EMG-torque slope increased as the ankle became plantarflexed on both sides, but an increment of the EMG-torque slope (i.e., the coefficient a) was significantly greater on the paretic side. Moreover, the relative (non-paretic/paretic) coefficient a was also strongly correlated with the relative (paretic/non-paretic) maximum ankle plantarflexion torque and with shear wave speed in the medial gastrocnemius muscle. Conversely, the relative coefficient a was not well-correlated with the relative muscle thickness. Our findings suggest that muscular contraction efficiency is affected by hemispheric stroke, but in an angle-dependent and non-uniform manner. These findings may allow us to explore the relative contributions of neural factors and muscular changes to voluntary force generating-capacity after stroke.

Limited fascicle shortening and fascicle rotation may be associated with impaired voluntary force-generating capacity in pennate muscles of chronic stroke survivors.

BACKGROUND: Muscle weakness is one of the most common motor impairments after stroke. A variety of progressive muscular changes are reported in chronic stroke survivors, and it is now feasible to consider these changes as an added source of weakness. However, the net contributions of such muscular changes towards muscle weakness have not been fully quantified. METHODS: Accordingly, this study aims: (1) to compare muscle architecture of the human medial gastrocnemius between paretic and non-paretic sides in seven chronic hemispheric stroke survivors under passive conditions; (2) to characterize fascicle behavior (i.e., fascicle shortening and fascicle rotation) of the muscle during voluntary isometric contractions; and (3) to assess potential associations between muscle architectural parameters and muscle weakness. Muscle architecture of the medial gastrocnemius (including fascicle length, fascicle pennation angle, and muscle thickness) was characterized using B-mode ultrasonography, and fascicle behavior was then quantified as a function of isometric plantarflexion torque normalized to body mass. FINDINGS: Our experimental results showed that under passive conditions, there was a significant difference in fascicle length and muscle thickness between paretic and non-paretic muscles, but no difference in resting fascicle pennation angle. However, during isometric contraction, both fascicle shortening and fascicle rotation on the paretic side were significantly decreased, compared to the non-paretic side. Moreover, the relative (i.e., paretic/non-paretic) fascicle rotation-shortening ratio (i.e., fascicle rotation per fascicle shortening) was strongly correlated with the relative maximum voluntary isometric plantarflexion torque. INTERPRETATION: This association implies that such fascicle changes could impair the force-generating capacity of the muscle in chronic stroke survivors.

Prolonged time course of population excitatory postsynaptic potentials in motoneurons of chronic stroke survivors.

Hyperexcitability of spinal motoneurons may contribute to muscular hypertonia after hemispheric stroke. The origins of this hyperexcitability are not clear, but we hypothesized that prolongation of the Ia excitatory postsynaptic potential (EPSP) in spastic motoneurons may be one potential mechanism, by enabling more effective temporal summation of Ia EPSPs, making action potential initiation easier. Thus, the purpose of this study is to quantify the time course of putative EPSPs in spinal motoneurons of chronic stroke survivors. To estimate the EPSP time course, a pair of low-intensity electrical stimuli was delivered sequentially to the median nerve in seven hemispheric stroke survivors and in six intact individuals, to induce an H-reflex response from the flexor carpi radialis muscle. H-reflex response probability was then used to quantify the time course of the underlying EPSPs in the motoneuron pool. A population EPSP estimate was then derived, based on the probability of evoking an H-reflex from the second test stimulus in the absence of a reflex response to the first conditioning stimulus. Our experimental results showed that in six of seven hemispheric stroke survivors, the apparent rate of decay of the population EPSP was markedly slower in spastic compared with contralateral (stroke) and intact motoneuron pools. There was no significant difference in EPSP time course between the contralateral side of stroke survivors and control subject muscles. We propose that one potential mechanism for hyperexcitability of spastic motoneurons in chronic stroke survivors may be associated with this prolongation of the Ia EPSP time course. Our subthreshold double-stimulation approach could provide a noninvasive tool for quantifying the time course of EPSPs in both healthy and pathological conditions. NEW & NOTEWORTHY Spastic motoneurons in stroke survivors showed a prolonged Ia excitatory postsynaptic potential (EPSP) time course compared with contralateral and intact motoneurons, suggesting that one potential mechanism for hyperexcitability of spastic motoneurons in chronic stroke survivors may be associated with this prolongation of the Ia EPSP time course.

Evaluation of three-dimensional in vivo scapular kinematics and scapulohumeral rhythm between shoulders with a clavicle hook plate and contralateral healthy shoulders.

PURPOSE: Acromioclavicular-coracoclavicular ligament injury occurs frequently, and the clavicle hook plate technique is an easy-to-use treatment method. However, complications such as subacromial impingement syndrome, synovitis, erosion, osteolysis, post-operative pain, and post-operative limitations in range of motion have been reported. We aimed to evaluate the use of the clavicle hook plate in the shoulder joints and to compare in vivo three-dimensional (3D) scapular kinematics and scapulohumeral rhythm between the shoulders with a clavicle hook plate and contralateral normal shoulder joints. METHODS: Ten male patients (aged 40.5 ± 14.4 years) who underwent clavicle hook plate fixation for an acromioclavicular-coracoclavicular ligament injury were selected. Computed tomography and fluoroscopy were conducted on both the shoulder joints, and 3D models were created. Using a 3D-2D model-image registration technique, we determined the 3D coordinates of the scapula, and we measured the scapular kinematics and scapulohumeral rhythm. RESULTS: The values for upward rotation, posterior tilt, and external rotation in the two groups increased in proportion with humeral elevation, showing significant differences between the two groups (p < 0.05). Overall, the value in the clavicle hook plate group (group H) was smaller than that in the control group (group C) by 23.5% (6.7°) of upward rotation and 64.8% (18.9°) of posterior tilt. However, the external rotation in group H was greater than that in group C by 32.3% (2.3°). In overall value, there was a significant difference not in upward rotation and external rotation, but in posterior tilt. During humeral elevation, the overall changes in scapulohumeral rhythm were 4.65 ± 2.45 in group H and 3.8 ± 0.8 in group C, and statistical differences were not detected between the two groups. CONCLUSIONS: Clavicle hook plate fixation changes the scapular kinematics and scapulohumeral rhythm; thus, when clavicle hook plate fixation is complete, the implant should be promptly removed.

Effect of Botulinum Toxin on the Spatial Distribution of Biceps Brachii EMG Activity Using a Grid of Surface Electrodes: A Case Study.

Botulinum toxin (BT) is widely prescribed by physicians for managing spasticity post stroke. In an ongoing study, we examine the spatial pattern of muscle activity in biceps brachii of stroke survivors before and after receiving BT, examined over the course of 11 weeks (2 weeks before - 9 weeks after). We hypothesize that BT alters muscle electrophysiology by disrupting fiber neuromuscular transmission in an inhomogeneous manner and we seek to detect these changes using grid surface electromyography (sEMG). Also, we obtained B-mode ultrasound images to have an accurate interpretation of sEMG data by looking at the fiber angle and subcutaneous fat thickness distribution across muscle. Here, we are reporting a single case where a chronic stroke survivor received BT injection in the biceps brachii (BB). A 16x8 sEMG electrode grid was used to capture the muscle activity distribution of BB during sustained non-fatiguing isometric contraction at 40% of maximal voluntary (MVC) elbow flexion. We obtained the root mean squared (RMS) maps of the signal recorded at each of the $16 \times 8$ electrodes. We observed substantial changes in the RMS pattern of BB muscle after receiving BT. More than 80% decrease in sEMG amplitude (RMS) was observed for the channels around the BT injection site as well as about 74% elbow flexion force reduction at the time point of 3-4 weeks post-injection. We also found significant differences between the spatial voluntary activation pattern of pre and post BT RMS maps. We further observed a non-uniform effect and recovery caused by the BT on the distribution of muscle activity. In conclusion, we observed evidence of alteration of the amplitude and pattern of muscle activity after botulinum toxin injection and can document the capability of grid recordings to detect these pattern changes. Our major goals target further investigation to provide an indepth understanding of the effect of botulinum toxin injection at motor unit level.

Frequency Dependence of Shear Wave Velocity in Stroke-Affected Muscles During Isometric Contraction- Preliminary Data.

In addition to changes in the central nervous system, many changes can occur in the composition and structure of skeletal muscles after a hemispheric stroke. The mechanical behavior of skeletal muscles is linked to the density and structural arrangement of key constituents. Yet, little is known about changes in post-stroke muscle mechanical properties such as viscoelasticity. The aim of this study was to explore the frequency-dependent changes in shear wave (SW) velocity as a potentially informative feature accompanying changes in muscle viscoelastic properties under passive and active conditions in hemiplegic stroke. We used the ultrasound SuperSonic Imaging technique to induce and measure SW propagation in the biceps brachii muscle for both the paretic and contralateral limbs in three hemiplegic stroke survivors during passive and submaximal voluntary muscle contractions. We found that for all subjects, the muscles on both the paretic and non-paretic sides demonstrated large dispersion (i.e., a change in SW phase velocities as a function of frequency within each contraction level) under both passive and active conditions, although muscles on the paretic side displayed larger dispersion. In addition, for a range of frequencies from 108-756 Hz, the SW phase velocity was higher in active nonparetic muscles compared to those of paretic side with an increase of 42% at 756 Hz. This is in contrast with the muscle response under passive condition where the SW phase velocity exhibited a 97 % increase at 765Hz on the paretic side compared to the non-paretic side. These results suggest the mechanical properties are altered for stroke-affected muscles, which may be a result of changes in the muscle extracellular matrix composition. Further, this study provides evidence that there are changes in tissue mechanical properties and that may consequently influence muscle function.

Spatial correlation and segregation of multimodal MRI abnormalities in multiple system atrophy.

OBJECTIVE: The variability of the severity and regional distribution of pathological process in basal ganglia (BG) and brainstem-cerebellar systems results in clinical heterogeneity and represents the motor subtype of multiple system atrophy (MSA). This study aimed to quantify spatial patterns of multimodal MRI abnormalities in BG and stem-CB regions and define structural MRI findings that correlate with clinical characteristics. METHODS: We simultaneously measured R2*, mean diffusivity (MD), and volume in the subcortical structures (BG, thalamus, brainstem-cerebellar regions) of 39 probable MSA and 22 control subjects. Principal component analysis (PCA) and structural equation modeling (SEM) were performed to show a model consisting of multiple inter-dependencies. RESULTS: Structural MRI alterations were found to be significantly interrelated within BG as well as brainstem-cerebellar regions in MSA patients. PCA extracted four factors: three factors reflected alterations in R2*, MD and volume of the BG region including the caudate nucleus, putamen, and pallidum, and the remaining one factor represented degenerative changes in MD and volume of stem-CB region. In SEM, a latent variable reflecting brainstem-cerebellar degeneration did not show a significant correlation with the other latent variables associated with BG degeneration. Putaminal MD values and a PCA-driven factor reflecting MD values in the BG showed a significant correlation with UPDRS and UMSARS scores. CONCLUSION: Multimodal structural MRI abnormalities in MSA appear to be segregated into BG and stem-CB-related factors that can be associated with the clinical phenotype and motor severity.