EMG-force relation in the first dorsal interosseous muscle of patients with amyotrophic lateral sclerosis

BACKGROUND AND PURPOSE

The relationship between surface electromyography (EMG) and muscle force is essential to assess muscle function and its deficits. However, few studies have explored the EMG-force relation in patients with amyotrophic lateral sclerosis (ALS). The purpose of this study was to examine the EMG-force relation in ALS subjects and its alteration in comparison with healthy control subjects.

METHODS

Surface EMG and force signals were recorded while 10 ALS and 10 age-matched healthy control subjects produced isometric voluntary contractions in the first dorsal interosseous (FDI) muscle over the full range of activation. A linear fit of the EMG-force relation was evaluated through the normalized root mean square error (RMSE) between the experimental and predicted EMG amplitudes. The EMG-force relation was compared between the ALS and the healthy control subjects.

RESULTS

With a linear fit, the normalized RMSE between the experimental and predicted EMG amplitudes was 9.6 ± 3.6% for the healthy control subjects and 12.3 ± 8.0% for the ALS subjects. The slope of the linear fit was 2.9 ± 2.2 μVN-1 for the ALS subjects and was significantly shallower (p < 0.05) than the control subjects (5.1 ± 1.8 μVN-1). However, after excluding the four ALS subjects who had very weak maximum force, the slope for the remaining ALS subjects was 3.5 ± 2.2 μVN-1 and was not significantly different from the control subjects (p > 0.05).

CONCLUSIONS

A linear fit can be used to well describe the EMG-force relation for the FDI muscle of both ALS and healthy control subjects. A variety of processes may work together in ALS that can adversely affect the EMG-force relation.

Estimating the time course of population excitatory postsynaptic potentials in motoneurons of spastic stroke survivors

Hyperexcitable motoneurons are likely to contribute to muscle hypertonia after a stroke injury; however, the origins of this hyperexcitability are not clear. One possibility is that the effective duration of the Ia excitatory postsynaptic potential (EPSP) is prolonged, increasing the potential for temporal summation of EPSPs, making action potential initiation easier. Accordingly, the purpose of this study was to quantify the time course of EPSPs in motoneurons of stroke survivors. The experimental protocol, which was based on parameters derived from simulation, involved sequential subthreshold electrical stimuli delivered to the median nerve of hemispheric stroke survivors. The resulting H-reflex responses were recorded in 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 was estimated based on the probability of evoking an H reflex from the second electrical stimulus in the absence of a reflex response to the first stimulus. The accuracy of this time-course estimate was quantified using a computer simulation that explored a range of feasible EPSP parameters. Our experimental results showed that in all five hemispheric stroke survivors the rate of decay of the population EPSP was consistently slower in spastic compared with the contralateral motoneuron pools. We propose that one potential mechanism for hyperexcitability of motoneurons in spastic stroke survivors may be linked to this prolongation of the Ia EPSP time course. Our subthreshold double-stimulation approach also provides a noninvasive tool for quantifying the time course of EPSPs in both healthy and pathological conditions.

Evidence for altered upper extremity muscle synergies in chronic stroke survivors with mild and moderate impairment

Previous studies indicate that motor coordination may be achieved by assembling task-dependent combinations of a few muscle synergies, defined here as fixed patterns of activation across a set of muscles. Our recent study of severely impaired chronic stroke survivors showed that some muscle synergies underlying isometric force generation at the hand are altered in the affected arm. However, whether similar alterations are evident in stroke survivors with lesser impairment remains unclear. Accordingly, we examined muscle synergies underlying spatial patterns of elbow and shoulder muscle activation recorded during an isometric force target matching protocol performed by 16 chronic stroke survivors, evenly divided across mild and moderate impairment levels. We applied non-negative matrix factorization to identify the muscle synergies and compared their structure across groups, including previously collected data from six age-matched control subjects and eight severely impaired stroke survivors. For all groups, EMG spatial patterns were well explained by task-dependent combinations of only a few (typically 4) muscle synergies. Broadly speaking, elbow-related synergies were conserved across stroke survivors, regardless of impairment level. In contrast, the shoulder-related synergies of some stroke survivors with mild and moderate impairment differed from controls, in a manner similar to severely impaired subjects. Cluster analysis of pooled synergies for the 30 subjects identified seven distinct clusters (synergies). Subsequent analysis confirmed that the incidences of three elbow-related synergies were independent of impairment level, while the incidences of four shoulder-related synergies were systematically correlated with impairment level. Overall, our results suggest that alterations in the shoulder muscle synergies underlying isometric force generation appear prominently in mild and moderate stroke, as in most cases of severe stroke, in an impairment level-dependent manner.

Statistics of inter-spike intervals as a routine measure of accuracy in automatic decomposition of surface electromyogram

Automated motor unit (MU) decomposition algorithms of surface electromyogram (EMG) have been developed recently. However, a routine estimate of the decomposition accuracy is still lacking. The objective of this preliminary study was to examine the statistics of the inter-spike intervals (ISIs) of the identified MUs as a measure of the decomposition accuracy, such that the ISI analysis can be used as a routine procedure to assess the accuracy of the surface identified MU spike timings. A surface EMG recording and decomposition system was used to record EMG signals and extract single MU activities from the first dorsal interosseous muscle of three healthy individuals. The estimated ISI statistics were cross-validated with decomposed MUs from simultaneous intramuscular EMG recordings. Our preliminary results reveal that the distribution of the ISIs, specifically the deviation from the Gaussian distribution as represented by secondary peaks at the short or long ISIs, can provide information regarding the spurious errors and missed firing errors in the decomposition. In addition, the variability (coefficient of variation) of the ISIs also correlated inversely with the decomposition accuracy. These findings show that the ISI statistics can be used to assess the spike timing accuracy of the identified MUs from surface EMG decomposition algorithms.

Contributions of motoneuron hyperexcitability to clinical spasticity in hemispheric stroke survivors

OBJECTIVE

Muscle spasticity is one of the major impairments that limits recovery in hemispheric stroke survivors. One potential contributing mechanism is hyperexcitability of motoneurons. Previously, the response latency of the surface electromyogram (EMG) record evoked by joint rotation has been used to characterize motoneuron excitability. Given the limitations of this method, the objective of the current study was to reexamine the excitability of motoneurons in chronic stroke survivors by estimating reflex latency using single motor unit discharge.

METHODS

We quantified the excitability of spastic motoneurons using the response latency of a single motor unit discharge elicited by a position controlled tap on the biceps brachii tendon. We applied tendon taps of different amplitudes on the biceps tendons of both arms of the stroke survivors. Unitary reflex responses were recorded using intramuscular EMG recordings.

RESULTS

Our results showed that the latency of unitary discharge was systematically shorter in the spastic muscle compared with the contralateral muscle, and this effect was consistent across multiple tap amplitudes.

CONCLUSIONS

This method allowed us to quantify latencies more accurately, potentially enabling a more rigorous analysis of contributing mechanisms.

SIGNIFICANCE

The findings provide evidence supporting a contribution of hyperexcitable motoneurons to muscle spasticity.

Suppression of stimulus artifact contaminating electrically evoked electromyography

BACKGROUND

Electrical stimulation of muscle or nerve is a very useful technique for understanding of muscle activity and its pathological changes for both diagnostic and therapeutic purposes. During electrical stimulation of a muscle, the recorded M wave is often contaminated by a stimulus artifact. The stimulus artifact must be removed for appropriate analysis and interpretation of M waves.

OBJECTIVES

The objective of this study was to develop a novel software based method to remove stimulus artifacts contaminating or superimposing with electrically evoked surface electromyography (EMG) or M wave signals.

METHODS

The multiple stage method uses a series of signal processing techniques, including highlighting and detection of stimulus artifacts using Savitzky-Golay filtering, estimation of the artifact contaminated region with Otsu thresholding, and reconstruction of such region using signal interpolation and smoothing. The developed method was tested using M wave signals recorded from biceps brachii muscles by a linear surface electrode array. To evaluate the performance, a series of semi-synthetic signals were constructed from clean M wave and stimulus artifact recordings with different degrees of overlap between them.

RESULTS

The effectiveness of the developed method was quantified by a significant increase in correlation coefficient and a significant decrease in root mean square error between the clean M wave and the reconstructed M wave, compared with those between the clean M wave and the originally contaminated signal. The validity of the developed method was also demonstrated when tested on each channel's M wave recording using a linear electrode array.

CONCLUSIONS

The developed method can suppress stimulus artifacts contaminating M wave recordings.

A clinically applicable approach for detecting spontaneous action potential spikes in amyotrophic lateral sclerosis with a linear electrode array

Examination of spontaneous muscle activity is an important part of the routine electromyogram (EMG) in assessing neuromuscular diseases. The EMG is specifically valuable as a diagnostic test in supporting the diagnosis of amyotrophic lateral sclerosis. High-density surface EMG is a relatively new technique that has until now been used in research but has the potential for clinical application. This study presents a simple high-density surface EMG method for automatic detection of spontaneous action potentials from surface electrode array recordings of patients with amyotrophic lateral sclerosis. To reduce computational complexity while maintaining useful information from the electrode array recording, the multichannel high-density surface EMG was transferred to single-dimensional data by calculating the maximum difference across all channels of the electrode array. A spike detection threshold was then set in the single-dimensional domain to identify the firing times of each spontaneous action potential spike, whereas a spike extraction threshold was used to define the onset and offset of the spontaneous spikes. These data were used to extract the spontaneous spike waveforms from the electrode array EMG. A database of detected spontaneous spikes was thus obtained, including their waveforms, on all channels along with their corresponding firing times. This newly developed method makes use of the information from different channels of the electrode array EMG recording. It also has the primary feature of being simple and fast in implementation, with convenient parameter adjustment and user-computer interaction. Hence, it has good possibilities for clinical application.

Anomalous EMG-force relations during low-force isometric tasks in hemiparetic stroke survivors

Hemispheric brain injury resulting from a stroke is often accompanied by muscle weakness in contralateral limbs. In neurologically intact subjects, appropriate motoneuronal recruitment and rate modulation are utilized to optimize muscle force production. In the present study, we sought to determine whether weakness in an affected hand muscle in stroke survivors is partially attributable to alterations in the control of muscle activation. Specifically, our goal was to characterize whether the surface EMG amplitude was systematically larger as a function of (low) force in paretic hand muscles as compared to contralateral muscles in the same subject. We tested a multifunctional muscle, the first dorsal interosseous (FDI), in multiple directions about the second metacarpophalangeal joint in ten hemiparetic and six neurologically intact subjects. In six of the ten stroke subjects, the EMG-force slope was significantly greater on the affected side as compared to the contralateral side, as well as compared to neurologically intact subjects. An unexpected set of results was a nonlinear relation between recorded EMG and generated force commonly observed in the paretic FDI, even at very low-force levels. We discuss possible experimental as well as physiological factors that may contribute to an increased EMG-force slope, concluding that changes in motor unit (MU) control are the most likely reasons for the observed changes.

Control of motor unit firing during step-like increases in voluntary force

In most skeletal muscles, force is generated by a combination of motor unit (MU) recruitment and increases in the firing rate of previously active MUs. Two contrasting patterns of firing rate organization have been reported. In the first pattern, the earliest recruited MUs reach the highest firing rates as force is increased, and later recruited MUs fire at lower rates. When firing rate of multiple MUs are superimposed, these rate trajectories form a concentric layered profile termed "onion skin." In the second pattern, called "reverse onion skin," later recruited MUs reach higher firing rates, and crossing of firing rate trajectories for recorded MUs is common (although such trajectories are assembled routinely from different trials). Our present study examined the firing rate organization of concurrently active MUs of the first dorsal interosseous muscle during serial, step-like increases in isometric abduction forces. We used a surface sensor array coupled with MU discrimination algorithms to characterize MU firing patterns. Our objective was to determine whether "onion skin" profiles are contingent upon the force trajectory of the motor task, examined here using step-like increases of force output, and also whether they are manifested at different force levels. Our results revealed that the overall "onion skin" firing rate profile was retained as the force level increased with each force step up to 15% MVC. However, the distribution of firing rates across MUs was compressed with increasing force, and overlapping firing rate of units were observed. This rate compression was largely due to rate saturation of the relatively high frequency discharging MUs. Our results reflect flexible firing patterns across MUs at different levels of excitation drive. It is also evident that many units did not follow all the step increases consistently. This failure to track firing rate increases at higher forces could be due to an intrinsically mediated saturation of firing rates for the low threshold MUs, or potentially to some form of inhibitory interactions between active MUs as the level of excitation of the MU pool is progressively increased.

Finger-thumb coupling contributes to exaggerated thumb flexion in stroke survivors

The purpose of this study was to investigate altered finger-thumb coupling in individuals with chronic hemiparesis poststroke. First, an external device stretched finger flexor muscles by passively rotating the metacarpophalangeal (MCP) joints. Subjects then performed isometric finger or thumb force generation. Forces/torques and electromyographic signals were recorded for both the thumb and finger muscles. Stroke survivors with moderate (n = 9) and severe (n = 9) chronic hand impairment participated, along with neurologically intact individuals (n = 9). Stroke survivors exhibited strong interactions between finger and thumb flexors. The stretch reflex evoked by stretch of the finger flexors of stroke survivors led to heteronymous reflex activity in the thumb, while attempts to produce isolated voluntary finger MCP flexion torque/thumb flexion force led to increased and undesired thumb force/finger MCP torque production poststroke with a striking asymmetry between voluntary flexion and extension. Coherence between the long finger and thumb flexors estimated using intermuscular electromyographic correlations, however, was small. Coactivation of thumb and finger flexor muscles was common in stroke survivors, whether activation was evoked by passive stretch or voluntary activation. The coupling appears to arise from subcortical or spinal sources. Flexor coupling between the thumb and fingers seems to contribute to undesired thumb flexor activity after stroke and may impact rehabilitation outcomes.

Motor unit firing rate patterns during voluntary muscle force generation: a simulation study

OBJECTIVE

Muscle force is generated by a combination of motor unit (MU) recruitment and changes in the discharge rate of active MUs. There have been two basic MU recruitment and firing rate paradigms reported in the literature, which describe the control of the MUs during force generation. The first (termed the reverse 'onion skin' profile), exhibits lower firing rates for lower threshold units, with higher firing rates occurring in higher threshold units. The second (termed the 'onion skin' profile), exhibits an inverse arrangement, with lower threshold units reaching higher firing rates.

APPROACH

Using a simulation of the MU activity in a hand muscle, this study examined the force generation capacity and the variability of the muscle force magnitude at different excitation levels of the MU pool under these two different MU control paradigms. We sought to determine which rate/recruitment scheme was more efficient for force generation, and which scheme gave rise to the lowest force variability.

MAIN RESULTS

We found that the force output of both firing patterns leads to graded force output at low excitation levels, and that the force generation capacity of the two different paradigms diverged around 50% excitation. In the reverse 'onion skin' pattern, at 100% excitation, the force output reached up to 88% of maximum force, whereas for the 'onion skin' pattern, the force output only reached up to 54% of maximum force at 100% excitation. The force variability was lower at the low to moderate force levels under the 'onion skin' paradigm than with the reverse 'onion skin' firing patterns, but this effect was reversed at high force levels.

SIGNIFICANCE

This study captures the influence of MU recruitment and firing rate organization on muscle force properties, and our results suggest that the different firing organizations can be beneficial at different levels of voluntary muscle force generation and perhaps for different tasks.

Asymmetries in vestibular evoked myogenic potentials in chronic stroke survivors with spastic hypertonia: evidence for a vestibulospinal role

OBJECTIVE

Indirect evidence suggests that lateralized changes in motoneuron behavior post-stroke are potentially due to a depolarizing supraspinal drive to the motoneuron pool, but the pathways responsible are unknown. In this study, we assessed vestibular evoked myogenic potentials (VEMPs) in the neck muscles of hemispheric stroke survivors with contralesional spasticity to quantify the relative levels of vestibular drive to the spastic-paretic and contralateral motoneuron pools.

METHODS

VEMPs were recorded from each sternocleidomastoid muscle in chronic stroke survivors. Side-to-side differences in cVEMP amplitude were calculated and expressed as an asymmetry ratio, a proxy for the relative amount of vestibular drive to each side.

RESULTS

Spastic-paretic VEMPs were larger than contralateral VEMPs in 13/16 subjects. There was a strong positive relationship between the degree of asymmetry and the severity of spasticity in this subset of subjects. Remaining subjects had larger contralateral responses.

CONCLUSION

Vestibular drive to cervical motoneurons is asymmetric in spastic stroke survivors, supporting our hypothesis that there is an imbalance in descending vestibular drive to motoneuron pools post-stroke. We speculate this imbalance is a consequence of the unilateral disruption of inhibitory corticobulbar projections to the vestibular nuclei.

SIGNIFICANCE

This study sheds new light on the underlying mechanisms of post-stroke spasticity.

Disturbances of motor unit rate modulation are prevalent in muscles of spastic-paretic stroke survivors

Stroke survivors often exhibit abnormally low motor unit firing rates during voluntary muscle activation. Our purpose was to assess the prevalence of saturation in motor unit firing rates in the spastic-paretic biceps brachii muscle of stroke survivors. To achieve this objective, we recorded the incidence and duration of impaired lower- and higher-threshold motor unit firing rate modulation in spastic-paretic, contralateral, and healthy control muscle during increases in isometric force generated by the elbow flexor muscles. Impaired firing was considered to have occurred when firing rate became constant (i.e., saturated), despite increasing force. The duration of impaired firing rate modulation in the lower-threshold unit was longer for spastic-paretic (3.9 ± 2.2 s) than for contralateral (1.4 ± 0.9 s; P < 0.001) and control (1.1 ± 1.0 s; P = 0.005) muscles. The duration of impaired firing rate modulation in the higher-threshold unit was also longer for the spastic-paretic (1.7 ± 1.6 s) than contralateral (0.3 ± 0.3 s; P = 0.007) and control (0.1 ± 0.2 s; P = 0.009) muscles. This impaired firing rate of the lower-threshold unit arose, despite an increase in the overall descending command, as shown by the recruitment of the higher-threshold unit during the time that the lower-threshold unit was saturating, and by the continuous increase in averages of the rectified EMG of the biceps brachii muscle throughout the rising phase of the contraction. These results suggest that impairments in firing rate modulation are prevalent in motor units of spastic-paretic muscle, even when the overall descending command to the muscle is increasing.

Accuracy assessment of a surface electromyogram decomposition system in human first dorsal interosseus muscle

OBJECTIVE

The aim of this study is to assess the accuracy of a surface electromyogram (sEMG) motor unit (MU) decomposition algorithm during low levels of muscle contraction.

APPROACH

A two-source method was used to verify the accuracy of the sEMG decomposition system, by utilizing simultaneous intramuscular and surface EMG recordings from the human first dorsal interosseous muscle recorded during isometric trapezoidal force contractions. Spike trains from each recording type were decomposed independently utilizing two different algorithms, EMGlab and dEMG decomposition algorithms. The degree of agreement of the decomposed spike timings was assessed for three different segments of the EMG signals, corresponding to specified regions in the force task. A regression analysis was performed to examine whether certain properties of the sEMG and force signal can predict the decomposition accuracy.

MAIN RESULTS

The average accuracy of successful decomposition among the 119 MUs that were common to both intramuscular and surface records was approximately 95%, and the accuracy was comparable between the different segments of the sEMG signals (i.e., force ramp-up versus steady state force versus combined). The regression function between the accuracy and properties of sEMG and force signals revealed that the signal-to-noise ratio of the action potential and stability in the action potential records were significant predictors of the surface decomposition accuracy.

SIGNIFICANCE

The outcomes of our study confirm the accuracy of the sEMG decomposition algorithm during low muscle contraction levels and provide confidence in the overall validity of the surface dEMG decomposition algorithm.

Anodal transcranial direct current stimulation alters elbow flexor muscle recruitment strategies

BACKGROUND

Transcranial direct current stimulation (tDCS) is known to reliably alter motor cortical excitability in a polarity dependent fashion such that anodal stimulation increases cortical excitability and cathodal stimulation inhibits cortical excitability. However, the effect of tDCS on agonist and antagonist volitional muscle activation is currently not known.

OBJECTIVE

This study investigated the effect of motor cortical anodal tDCS on EMG/force relationships of biceps brachii (agonist) and triceps brachii (antagonist) using surface electromyography (EMG).

METHODS

Eighteen neurologically intact adults (9 tDCS and 9 controls) participated in this study. EMG/force relationships were established by having subjects perform submaximal isometric contractions at several force levels (12.5%, 25%, 37.5%, and 50% of maximum).

RESULTS

Results showed that anodal tDCS significantly affected the EMG/force relationship of the biceps brachii muscle. Specifically, anodal tDCS increased the magnitude of biceps brachii activation at 37.5% and 50% of maximum. Anodal tDCS also resulted in an increase in the peak force and EMG values during maximal contractions as compared to the control condition. EMG analyses of other elbow muscles indicated that the increase in biceps brachii activation after anodal tDCS was not related to alterations in synergistic or antagonistic muscle activity.

CONCLUSIONS

Our results indicate that anodal tDCS significantly affects the voluntary EMG/force relationship of the agonist muscles without altering the coactivation of the antagonistic muscles. The most likely explanation for the observed greater EMG per unit force after anodal tDCS appears to be related to alterations in motor unit recruitment strategies.

Characterizations of reflex and nonreflex changes in spastic multiple sclerosis

BACKGROUND

Spasticity, an increased resistance of a limb to movement, is associated with functional limitations and a major source of disability in neurological disorders, including multiple sclerosis (MS) and stroke. Despite the clinical significance of spasticity in brain and spinal cord injuries, it is often not clear whether the spasticity is due to reflex or non-reflex changes.

NEW METHOD

Reflex and nonreflex properties of the human knee joint were studied in eight MS patients with spasticity and ten healthy subjects. A digitally controlled joint driving device was used to apply small-amplitude, and band-limited white-noise perturbations to the knee to manifest the reflex and nonreflex properties. The subjects were asked to maintain a steady level of background muscle torque during the perturbation. A nonlinear delay differential equation model was used to characterize the reflex and intrinsic properties of the knee in terms of phasic stretch reflex gain, tonic stretch reflex gain, joint elastic stiffness, and coefficient of viscosity.

RESULTS

It was found that joint coefficient of viscosity and tonic stretch reflex gain of the spastic MS patients were significantly lower than those of normal controls. On the other hand, spastic MS patients showed higher phasic stretch reflex gains than normal controls and a trend of increased joint stiffness.

CONCLUSIONS

Simultaneous characterizations of changes in tonic and phasic reflexes and nonreflex changes in joint elastic stiffness and viscosity in neurological disorders may help us gain insight into mechanisms underlying spasticity and develop impairment-specific treatment.

Daily intermittent hypoxia enhances walking after chronic spinal cord injury: a randomized trial

Objectives:

To test the hypothesis that daily acute intermittent hypoxia (dAIH) and dAIH combined with overground walking improve walking speed and endurance in persons with chronic incomplete spinal cord injury (iSCI).

Methods:

Nineteen subjects completed the randomized, double-blind, placebo-controlled, crossover study. Participants received 15, 90-second hypoxic exposures (dAIH, fraction of inspired oxygen [Fio₂] = 0.09) or daily normoxia (dSHAM, Fio₂ = 0.21) at 60-second normoxic intervals on 5 consecutive days; dAIH was given alone or combined with 30 minutes of overground walking 1 hour later. Walking speed and endurance were quantified using 10-Meter and 6-Minute Walk Tests. The trial is registered at ClinicalTrials.gov (NCT01272349).

Results:

dAIH improved walking speed and endurance. Ten-Meter Walk time improved with dAIH vs dSHAM after 1 day (mean difference [MD] 3.8 seconds, 95% confidence interval [CI] 1.1–6.5 seconds, p = 0.006) and 2 weeks (MD 3.8 seconds, 95% CI 0.9–6.7 seconds, p = 0.010). Six-Minute Walk distance increased with combined dAIH + walking vs dSHAM + walking after 5 days (MD 94.4 m, 95% CI 17.5–171.3 m, p = 0.017) and 1-week follow-up (MD 97.0 m, 95% CI 20.1–173.9 m, p = 0.014). dAIH + walking increased walking distance more than dAIH after 1 day (MD 67.7 m, 95% CI 1.3–134.1 m, p = 0.046), 5 days (MD 107.0 m, 95% CI 40.6–173.4 m, p = 0.002), and 1-week follow-up (MD 136.0 m, 95% CI 65.3–206.6 m, p < 0.001).

Conclusions:

dAIH ± walking improved walking speed and distance in persons with chronic iSCI. The impact of dAIH is enhanced by combination with walking, demonstrating that combinatorial therapies may promote greater functional benefits in persons with iSCI.

Classification of evidence:

This study provides Class I evidence that transient hypoxia (through measured breathing treatments), along with overground walking training, improves walking speed and endurance after iSCI.

A pilot study on the feasibility of robot-aided leg motor training to facilitate active participation

Robot-aided gait therapy offers a promising approach towards improving gait function in individuals with neurological disorders such as stroke or spinal cord injury. However, incorporation of appropriate control strategies is essential for actively engaging the patient in the therapeutic process. Although several control algorithms (such as assist-as-needed and error augmentation) have been proposed to improve active patient participation, we hypothesize that the therapeutic benefits of these control algorithms can be greatly enhanced if combined with a motor learning task to facilitate neural reorganization and motor recovery. Here, we describe an active robotic training approach (patient-cooperative robotic gait training combined with a motor learning task) using the Lokomat and pilot-tested whether this approach can enhance active patient participation during training. Six neurologically intact adults and three chronic stroke survivors participated in this pilot feasibility study. Participants walked in a Lokomat while simultaneously performing a foot target-tracking task that necessitated greater hip and knee flexion during the swing phase of the gait. We computed the changes in tracking error as a measure of motor performance and changes in muscle activation as a measure of active subject participation. Repeated practice of the motor-learning task resulted in significant reductions in target-tracking error in all subjects. Muscle activation was also significantly higher during active robotic training compared to simply walking in the robot. The data from stroke participants also showed a trend similar to neurologically intact participants. These findings provide a proof-of-concept demonstration that combining robotic gait training with a motor learning task enhances active participation.

Assessment of validity of a high-yield surface electromyogram decomposition

BACKGROUND

Using recordings from a five-pin surface sensor array, a template-based surface electromyogram (sEMG) decomposition system has been developed to identify single motor unit discharge properties. However, the reliability of such template based decomposition results has not been thoroughly examined except by the developers. The focus of this study was to assess the validity of the motor unit decomposition technique, using EMG recordings from the first dorsal interosseous muscle of able-bodied human subjects.

METHODS

Two tests were utilized. In the first test, a spike triggered averaging (STA) analysis was used to derive motor unit action potential (MUAP) parameters. We examined these STA derived MUAP shapes after firing times were perturbed by added timing noise. In the second test, a cross-correlation analysis was performed between the sEMG signal and MUAP trains constructed using STA estimates and their firing times.

RESULTS

In the first test, we found that MUAP shape features deteriorated significantly when rather small (0.6-2 ms) timing errors were added, affirming that the decomposed firing times are presumptively valid. The results of the second test reveal that the cross-correlation index between the EMG and MUAP trains increased monotonically up to 0.71 when the identified MUs were progressively added to reconstructed MUAP trains; however, this increment disappeared when the firing times or the MUAP templates were shifted randomly.

CONCLUSIONS

Based on our STA selection criteria, our results suggest that the firing times and estimated MUAP shapes for each MU generated by the decomposition algorithms are presumptively valid.

Reliability of spike triggered averaging of the surface electromyogram for motor unit action potential estimation

INTRODUCTION

The reliability of estimated motor unit parameters using spike triggered averaging (STA) of the surface electromyogram (sEMG) has not been tested thoroughly. We investigated factors that may induce amplitude bias in estimated motor unit action potentials (MUAPs) and shape variations.

METHODS

An sEMG record was simulated. MUAPs were then estimated from the STA of the simulated EMG.

RESULTS

Variations in MUAP duration led to under-estimation of real MUAP amplitude, while synchronized firing led to over-estimation of amplitude. Spurious firing resulted in over-estimation of the amplitude of small motor units but under-estimation of the amplitude of large ones. Variability in amplitude and high firing rates had minimal influence on amplitude estimation. High firing rates and variation in MUAP duration led to large variations in MUAP shape. Estimation errors also correlated with shape variations.

CONCLUSIONS

Recommendations to enhance the accuracy of the STA estimates have been proposed.

A novel spatiotemporal muscle activity imaging approach based on the Extended Kalman Filter

A novel spatiotemporal muscle activity imaging (sMAI) approach has been developed using the Extended Kalman Filter (EKF) to reconstruct internal muscle activities from non-invasive multi-channel surface electromyogram (sEMG) recordings. A distributed bioelectric dipole source model is employed to describe the internal muscle activity space, and a linear relationship between the muscle activity space and the sEMG measurement space is then established. The EKF is employed to recursively solve the ill-posed inverse problem in the sMAI approach, in which the weighted minimum norm (WMN) method is utilized to calculate the initial state and a new nonlinear method is developed based on the propagating features of muscle activities to predict the recursive state. A series of computer simulations was conducted to test the performance of the proposed sMAI approach. Results show that the localization error rapidly decreases over 35% and the overlap ratio rapidly increases over 45% compared to the results achieved using the WMN method only. The present promising results demonstrate the feasibility of utilizing the proposed EKF-based sMAI approach to accurately reconstruct internal muscle activities from non-invasive sEMG recordings.

Motor unit pool organization examined via spike-triggered averaging of the surface electromyogram

Voluntary muscle force control is accomplished both by recruitment of motor units (MUs) and by firing rate modulation of active MUs. Typically, MU recruitment and firing rate organization is assessed using piecemeal intramuscular recordings drawn from different experiments, or even from different subjects. As a consequence, it is often difficult to assemble a systematic description of the relations between the different MU properties relevant to the control of muscle force. To address this gap, the objective of our current study was to characterize recruitment and firing rate organization of multiple MUs of differing action potential size, recorded simultaneously from the first dorsal interosseous muscle of intact human subjects, using a recently developed surface electromyogram (EMG) sensor array recording and decomposition system (Delsys). We sought to assess the relation between putative MU size and the recruitment and firing properties for these MUs, recorded at different muscle contraction levels. Spike-triggered averaging (STA) of the surface EMG was performed to estimate the action potential sizes using the firing times of discriminated MUs as the event triggers. The results show that the size principle, which relates MU size to recruitment rank order, was clearly evident during individual force contractions. In addition, the mean firing rate across MUs decreased with increasing size of the MU action potential and was also inversely proportional to the recruitment threshold force. We propose that surface EMG recordings together with advanced decomposition systems, combined with STA methods, may provide an efficient way to systematically examine MU pool organizational properties.

Motor assessment using the NIH Toolbox

Motor function involves complex physiologic processes and requires the integration of multiple systems, including neuromuscular, musculoskeletal, and cardiopulmonary, and neural motor and sensory-perceptual systems. Motor-functional status is indicative of current physical health status, burden of disease, and long-term health outcomes, and is integrally related to daily functioning and quality of life. Given its importance to overall neurologic health and function, motor function was identified as a key domain for inclusion in the NIH Toolbox for Assessment of Neurological and Behavioral Function (NIH Toolbox). We engaged in a 3-stage developmental process to: 1) identify key subdomains and candidate measures for inclusion in the NIH Toolbox, 2) pretest candidate measures for feasibility across the age span of people aged 3 to 85 years, and 3) validate candidate measures against criterion measures in a sample of healthy individuals aged 3 to 85 years (n = 340). Based on extensive literature review and input from content experts, the 5 subdomains of dexterity, strength, balance, locomotion, and endurance were recommended for inclusion in the NIH Toolbox motor battery. Based on our validation testing, valid and reliable measures that are simultaneously low-cost and portable have been recommended to assess each subdomain, including the 9-hole peg board for dexterity, grip dynamometry for upper-extremity strength, standing balance test, 4-m walk test for gait speed, and a 2-minute walk test for endurance.

Reducing robotic guidance during robot-assisted gait training improves gait function: a case report on a stroke survivor

OBJECTIVE

To test the feasibility of patient-cooperative robotic gait training for improving locomotor function of a chronic stroke survivor with severe lower-extremity motor impairments.

DESIGN

Single-subject crossover design.

SETTING

Performed in a controlled laboratory setting.

PARTICIPANT

A 62-year-old man with right temporal lobe ischemic stroke was recruited for this study. The baseline lower-extremity Fugl-Meyer score of the subject was 10 on a scale of 34, which represented severe impairment in the paretic leg. However, the subject had a good ambulation level (community walker with the aid of a stick cane and ankle-foot orthosis) and showed no signs of sensory or cognitive impairments.

INTERVENTIONS

The subject underwent 12 sessions (3 times per week for 4wk) of conventional robotic training with the Lokomat, where the robot provided full assistance to leg movements while walking, followed by 12 sessions (3 times per week for 4wk) of patient-cooperative robotic control training, where the robot provided minimal guidance to leg movements during walking.

MAIN OUTCOME MEASURES

Clinical outcomes were evaluated before the start of the intervention, immediately after 4 weeks of conventional robotic training, and immediately after 4 weeks of cooperative control robotic training. These included: (1) self-selected and fast walking speed, (2) 6-minute walk test, (3) Timed Up & Go test, and (4) lower-extremity Fugl-Meyer score.

RESULTS

Results showed that clinical outcomes changed minimally after full guidance robotic training, but improved considerably after 4 weeks of reduced guidance robotic training.

CONCLUSIONS

The findings from this case study suggest that cooperative control robotic training is superior to conventional robotic training and is a feasible option to restoring locomotor function in ambulatory stroke survivors with severe motor impairments. A larger trial is needed to verify the efficacy of this advanced robotic control strategy in facilitating gait recovery after stroke.

Alterations in upper limb muscle synergy structure in chronic stroke survivors

Previous studies in neurologically intact subjects have shown that motor coordination can be described by task-dependent combinations of a few muscle synergies, defined here as a fixed pattern of activation across a set of muscles. Arm function in severely impaired stroke survivors is characterized by stereotypical postural and movement patterns involving the shoulder and elbow. Accordingly, we hypothesized that muscle synergy composition is altered in severely impaired stroke survivors. Using an isometric force matching protocol, we examined the spatial activation patterns of elbow and shoulder muscles in the affected arm of 10 stroke survivors (Fugl-Meyer <25/66) and in both arms of six age-matched controls. Underlying muscle synergies were identified using non-negative matrix factorization. In both groups, muscle activation patterns could be reconstructed by combinations of a few muscle synergies (typically 4). We did not find abnormal coupling of shoulder and elbow muscles within individual muscle synergies. In stroke survivors, as in controls, two of the synergies were comprised of isolated activation of the elbow flexors and extensors. However, muscle synergies involving proximal muscles exhibited consistent alterations following stroke. Unlike controls, the anterior deltoid was coactivated with medial and posterior deltoids within the shoulder abductor/extensor synergy and the shoulder adductor/flexor synergy in stroke was dominated by activation of pectoralis major, with limited anterior deltoid activation. Recruitment of the altered shoulder muscle synergies was strongly associated with abnormal task performance. Overall, our results suggest that an impaired control of the individual deltoid heads may contribute to poststroke deficits in arm function.

Changes of Reflex, Non-reflex and Torque Generation Properties of Spastic Ankle Plantar Flexors Induced by Intelligent Stretching

Spasticity, contracture, and muscle weakness are major sources of disability in stroke. Changes of torque-generating capacity as well as reflex and non-reflex properties of ankle plantar flexors induced by strenuous stretching in chronic hemiplegia were investigated. Twelve subjects with a unilateral stroke and 10 healthy controls underwent 30 minutes of strenuous intelligent stretching treatment. Reflex and non-reflex components of spastic hypertonia and force-generating capacity of ankle plantar flexors were investigated. Dorsiflexion (DF) range of motion (ROM) was increased (p=0.002) and passive stiffness and passive resistant torque of the spastic muscles were decreased (p=0.004 and 0.007, respectively), while reflex hyper-excitability diminished slightly but with no statistical significance. The maximal voluntary contraction (MVC) torque of the spastic ankle plantar flexors was increased after the forceful stretching treatment (p=0.041). In contrast, the stretching treatment of the healthy plantar flexors did not change any of the variables measured before and after stretching. The stroke subjects who gained more DF ROM or larger decrement of stiffness achieved greater increment of the peak torque generation after the stretching (r=0.597 with p=0.040 and r=-0.746 with p=0.005, respectively). These results suggest that the strenuous dynamic stretching could improve the force-generating capacity of spastic muscles as well as reduce the passive stiffness and increase ROM.

Active robotic training improves locomotor function in a stroke survivor

Background

Clinical outcomes after robotic training are often not superior to conventional therapy. One key factor responsible for this is the use of control strategies that provide substantial guidance. This strategy not only leads to a reduction in volitional physical effort, but also interferes with motor relearning.

Methods

We tested the feasibility of a novel training approach (active robotic training) using a powered gait orthosis (Lokomat) in mitigating post-stroke gait impairments of a 52-year-old male stroke survivor. This gait training paradigm combined patient-cooperative robot-aided walking with a target-tracking task. The training lasted for 4-weeks (12 visits, 3 × per week). The subject’s neuromotor performance and recovery were evaluated using biomechanical, neuromuscular and clinical measures recorded at various time-points (pre-training, post-training, and 6-weeks after training).

Results

Active robotic training resulted in considerable increase in target-tracking accuracy and reduction in the kinematic variability of ankle trajectory during robot-aided treadmill walking. These improvements also transferred to overground walking as characterized by larger propulsive forces and more symmetric ground reaction forces (GRFs). Training also resulted in improvements in muscle coordination, which resembled patterns observed in healthy controls. These changes were accompanied by a reduction in motor cortical excitability (MCE) of the vastus medialis, medial hamstrings, and gluteus medius muscles during treadmill walking. Importantly, active robotic training resulted in substantial improvements in several standard clinical and functional parameters. These improvements persisted during the follow-up evaluation at 6 weeks.

Conclusions

The results indicate that active robotic training appears to be a promising way of facilitating gait and physical function in moderately impaired stroke survivors.

The effects of Robotic-Assisted Locomotor training on spasticity and volitional control

We studied the effects of Robotic-Assisted Locomotor (LOKOMAT) Training on spasticity and volitional control of the spastic ankle in persons with incomplete Spinal Cord Injury (SCI). LOKOMAT training was performed 3 days/week during a 1-hr period including set-up time with up to 30 minutes of training during a single session. The training was provided for 4 weeks and subjects were evaluated before and after 1, 2, and 4 weeks of training. Spasticity was charterized in terms of neuromuscular abnormalities associated with the spastic joint. A system identification technique was used to quantify the effects of LOKOMAT training on these neuromuscular abnormalities. The effect of LOKOMAT training on volitional control was determined by measuring isometric maximum voluntary contraction (MVC) of ankle extensor and flexor muscles. Our results indicated that the reflex stiffness, abnormally increases in SCI, was significantly reduced (up to 65%) following 4-weeks of LOKOMAT training. Similarly, intrinsic (muscular) stiffness, which also abnormally increases in SCI, decreased significantly (up to 60%). MVCs were increased substantially (up to 93% in extensors and 180% in flexors) following 4-week training. These findings demonstrate that LOKOMAT training is effective in reducing spasticity and improving volitional control in SCI.

Robustness of muscle synergies underlying three-dimensional force generation at the hand in healthy humans

Previous studies using advanced matrix factorization techniques have shown that the coordination of human voluntary limb movements may be accomplished using combinations of a small number of intermuscular coordination patterns, or muscle synergies. However, the potential use of muscle synergies for isometric force generation has been evaluated mostly using correlational methods. The results of such studies suggest that fixed relationships between the activations of pairs of muscles are relatively rare. There is also emerging evidence that the nervous system uses independent strategies to control movement and force generation, which suggests that one cannot conclude a priori that isometric force generation is accomplished by combining muscle synergies, as shown in movement control. In this study, we used non-negative matrix factorization to evaluate the ability of a few muscle synergies to reconstruct the activation patterns of human arm muscles underlying the generation of three-dimensional (3-D) isometric forces at the hand. Surface electromyographic (EMG) data were recorded from eight key elbow and shoulder muscles during 3-D force target-matching protocols performed across a range of load levels and hand positions. Four synergies were sufficient to explain, on average, 95% of the variance in EMG datasets. Furthermore, we found that muscle synergy composition was conserved across biomechanical task conditions, experimental protocols, and subjects. Our findings are consistent with the view that the nervous system can generate isometric forces by assembling a combination of a small number of muscle synergies, differentially weighted according to task constraints.

The effects of notch filtering on electrically evoked myoelectric signals and associated motor unit index estimates

Background

Notch filtering is the most commonly used technique for suppression of power line and harmonic interference that often contaminate surface electromyogram (EMG) signals. Notch filters are routinely included in EMG recording instrumentation, and are used very often during clinical recording sessions. The objective of this study was to quantitatively assess the effects of notch filtering on electrically evoked myoelectric signals and on the related motor unit index measurements.

Methods

The study was primarily based on an experimental comparison of M wave recordings and index estimates of motor unit number and size, with the notch filter function of the EMG machine (Sierra Wave EMG system, Cadwell Lab Inc, Kennewick, WA, USA) turned on and off, respectively. The comparison was implemented in the first dorsal interosseous (FDI) muscle from the dominant hand of 15 neurologically intact subjects and bilaterally in 15 hemiparetic stroke subjects.

Results

On average, for intact subjects, the maximum M wave amplitude and the motor unit number index (MUNIX) estimate were reduced by approximately 22% and 18%, respectively, with application of the built-in notch filter function in the EMG machine. This trend held true when examining the paretic and contralateral muscles of the stroke subjects. With the notch filter on vs. off, across stroke subjects, we observed a significant decrease in both maximum M wave amplitude and MUNIX values in the paretic muscles, as compared with the contralateral muscles. However, similar reduction ratios were obtained for both maximum M wave amplitude and MUNIX estimate. Across muscles of both intact and stroke subjects, it was observed that notch filtering does not have significant effects on motor unit size index (MUSIX) estimate. No significant difference was found in MUSIX values between the paretic and contralateral muscles of the stroke subjects.

Conclusions

The notch filter function built in the EMG machine may significantly reduce the M wave amplitude and the MUNIX measurement. However, the notch filtering does not jeopardize the evaluation of the reduction ratio in maximum M wave amplitude and MUNIX estimate of the paretic muscles of stroke subjects when compared with the contralateral muscles.

Assessing dexterity function: a comparison of two alternatives for the NIH Toolbox

STUDY DESIGN

Clinical measurement.

INTRODUCTION

Manual dexterity is an important aspect of motor function across the age span.

PURPOSE OF THE STUDY

To identify a single measure of manual dexterity for inclusion in the National Institutes of Health (NIH) Toolbox Assessment of Neurological and Behavioral Function.

METHODS

A total of 340 subjects participated in our study. Two alternatives, Rolyan® 9-Hole Peg Test (9-HPT) and Grooved Pegboard test, were compared by assessing their score range across age groups (3-85 yr) and their test-retest reliability, concurrent, and known groups validity.

RESULTS

The 9-HPT was a simple, efficient, and low-cost measure of manual dexterity appropriate for administration across the age range. Test-retest reliability coefficients were 0.95 and 0.92 for right and left hands, respectively. The 9-HPT correlated with Bruininks-Oseretsky Test (BOT) of Motor Proficiency, dexterity subscale, at -0.87 to -0.89 and with Purdue Pegboard at -0.74 to -0.75. The Grooved Pegboard had good test-retest reliability (0.91 and 0.85 for right and left hands, respectively). The Grooved Pegboard correlated with BOT at -0.50 to -0.63 and with Purdue Pegboard at -0.73 to -0.78. However, the Grooved Pegboard required longer administration time and was challenging for the youngest children and oldest adults.

CONCLUSIONS

Based on its feasibility and measurement properties, the 9-HPT was recommended for inclusion in the motor battery of the NIH Toolbox.

LEVEL OF EVIDENCE

NA.

Exposure to acute intermittent hypoxia augments somatic motor function in humans with incomplete spinal cord injury

BACKGROUND

Neural plasticity may contribute to motor recovery following spinal cord injury (SCI). In rat models of SCI with respiratory impairment, acute intermittent hypoxia (AIH) strengthens synaptic inputs to phrenic motor neurons, thereby improving respiratory function by a mechanism known as respiratory long-term facilitation. Similar intermittent hypoxia-induced facilitation may be feasible in somatic motor pathways in humans.

OBJECTIVE

Using a randomized crossover design, the authors tested the hypothesis that AIH increases ankle strength in people with incomplete SCI.

METHODS

Ankle strength was measured in 13 individuals with chronic, incomplete SCI before and after AIH. Voluntary ankle strength was estimated using changes in maximum isometric ankle plantar flexion torque generation and plantar flexor electromyogram activity following 15 low oxygen exposures (Fio(2) = 0.09, 1-minute intervals). Results were compared with trials where subjects received sham exposure to room air.

RESULTS

AIH increased plantar flexion torque by 82 ± 33% (P < .003) immediately following AIH and was sustained above baseline for more than 90 minutes (P < .007). Increased ankle plantar flexor electromyogram activity (P = .01) correlated with increased torque (r(2) = .5; P < .001). No differences in plantar flexion strength or electromyogram activity were observed in sham experiments.

CONCLUSIONS

AIH elicits sustained increases in volitional somatic motor output in persons with chronic SCI. Thus, AIH has promise as a therapeutic tool to induce plasticity and enhance motor function in SCI patients.

Diminished capacity to modulate motor activation patterns according to task contributes to thumb deficits following stroke

The objective of this study was to explore motor impairment of the thumb following stroke. More specifically, we quantitatively examined kinetic deficits of the thumb. We anticipated that force deficits would be nonuniformly distributed across the kinetic workspace, due in part to varying levels of difficulty in altering the motor activation pattern to meet the task. Eighteen stroke survivors with chronic hemiparesis participated in the trials, along with nine age-matched controls. Of the stroke-survivor group, nine subjects had moderate hand impairment, and the other nine subjects had severe hand impairment. Subjects were instructed to generate maximal isometric thumb-tip force, as measured with a load cell, in each of six orthogonal directions with respect to the thumb tip. Activity of three representative thumb muscles was monitored through intramuscular and surface electrodes. Univariate split-plot analysis of variance revealed that clinical impairment level had a significant effect on measured force (P < 0.001), with the severely impaired group producing only 13% of the control forces, and the moderately impaired group generating 32% of control forces, on average. Weakness in the moderately impaired group exhibited a dependence on force direction (P = 0.015), with the least-relative weakness in the medial direction. Electromyographic recordings revealed that stroke survivors exhibited limited modulation of thumb-muscle activity with intended force direction. The difference in activation presented by the control group for a given muscle was equal to 40% of its full activation range across force directions, whereas this difference was only 26% for the moderately impaired group and 15% for the severely impaired group. This diminished ability to modify voluntary activation patterns, which we observed previously in index-finger muscles as well, appears to be a primary factor in hand impairment following stroke.

An evaluation of passive properties of spastic muscles in hemiparetic stroke survivors

We have reported earlier [1] a new method for estimating reflex threshold in spastic muscles of stroke survivors, using controlled amplitude taps superimposed on progressive and controlled muscle indentation of the bicipital tendon in the bicipital fossa. This muscle indentation is done with a linear actuator positioned over the biceps muscle tendon at the elbow. In the course of testing for increased stretch reflex responses, (a cardinal feature of spasticity), we have also observed that the intrinsic or passive stiffness of the muscle is often increased. This assessment is derived from recordings of the force generated by the tendon during progressive loading, and by the instantaneous force response to the tendon tap. Thus, it appears that passive properties of muscle are often also changed in parallel with the reflex abnormalities. While some of these mechanical features have been described in earlier studies of torque-angle relations of spastic joints, it appears that these features can also be recognized readily using a small actuator that loads the tendon progressively. These findings may help clinicians recognize early changes in muscle mechanical properties, and may help them prevent large-scale adverse changes in muscle function.

Origins of spontaneous firing of motor units in the spastic-paretic biceps brachii muscle of stroke survivors

One potential expression of altered motoneuron excitability following a hemispheric stroke is the spontaneous unit firing (SUF) of motor units at rest. The elements contributing to this altered excitability could be spinal descending pathways, spinal interneuronal networks, afferent feedback, or intrinsic motoneuron properties. Our purpose was to examine the characteristics of spontaneous discharge in spastic-paretic and contralateral muscles of hemiparetic stroke survivors, to determine which of these mechanisms might contribute. To achieve this objective, we examined the statistics of spontaneous discharge of individual motor units and we conducted a coherence analyses on spontaneously firing motor unit pairs. The presence of significant coherence between units might indicate a common driving source of excitation to multiple motoneurons from descending pathways or regional interneurons, whereas a consistent lack of coherence might favor an intrinsic cellular mechanism of hyperexcitability. Spontaneous firing of motor units (i.e., ongoing discharge in the absence of an ongoing stimulus) was observed to a greater degree in spastic-paretic muscles (following 83.2 ± 16.7% of ramp contractions) than that in contralateral muscles (following just 14.1 ± 10.5% of ramp contractions; P < 0.001) and was not observed at all in healthy control muscle. The average firing rates of the spontaneously firing units were 8.4 ± 1.8 pulses/s (pps) in spastic-paretic muscle and 9.6 ± 2.2 pps in contralateral muscle (P < 0.001). In 37 instances (n = 63 pairs), we observed spontaneous discharge of two or more motor units simultaneously in spastic-paretic muscle. Seventy percent of the dually firing motor unit pairs exhibited significant coherence (P < 0.001) in the 0- to 4-Hz bandwidth (average peak coherence: 0.14 ± 0.13; range: 0.01-0.75) and 22% of pairs exhibited significant coherence (P < 0.001) in the 15- to 30-Hz bandwidth (average peak coherence: 0.07 ± 0.06; range: 0.01-0.31). We suggest that the spontaneous firing was likely not attributable solely to enhanced intrinsic motoneuron activation, but attributable, at least in part, to a low-level excitatory synaptic input to the resting spastic-paretic motoneuron pool, possibly from regional or supraspinal centers.

Lack of hypertonia in thumb muscles after stroke

Despite the importance of the thumb to hand function, little is known about the origins of thumb impairment poststroke. Accordingly, the primary purpose of this study was to assess whether thumb flexors have heightened stretch reflexes (SRs) following stroke-induced hand impairment. The secondary purpose was to compare SR characteristics of thumb flexors in relation to those of finger flexors since it is unclear whether SR properties of both muscle groups are similarly affected poststroke. Stretch reflexes in thumb and finger flexors were assessed at rest on the paretic side in each of 12 individuals with chronic, severe, stroke-induced hand impairment and in the dominant thumb in each of eight control subjects also at rest. Muscle activity and passive joint flexion torques were measured during imposed slow (SS) and fast stretches (FS) of the flexors that span the metacarpophalangeal joints. Putative spasticity was then quantified in terms of the peak difference between FS and SS joint torques and electromyographic changes. For both the hemiparetic and control groups, the mean normalized peak torque differences (PTDs) measured in thumb flexors were statistically indistinguishable (P = 0.57). In both groups, flexor muscles were primarily unresponsive to rapid stretching. For 10 of 12 hemiparetic subjects, PTDs in thumb flexors were less than those in finger flexors (P = 0.03). Paretic finger flexor muscle reflex activity was consistently elicited during rapid stretching. These results may reflect an important difference between thumb and finger flexors relating to properties of the involved muscle afferents and spinal motoneurons.

Extraction of individual muscle mechanical action from endpoint force

Most motor tasks require the simultaneous coordination of multiple muscles. That coordination is poorly understood in part because there is no noninvasive means of isolating a single muscle's contribution to the resultant endpoint force. The contribution of a single motor unit to isometric tasks can, however, be characterized using the spike-triggered averaging (STA) technique, applied to a single motor unit's spike train. We propose that a technique analogous to STA, which we call electromyogram (EMG)-weighted averaging (EWA), can be applied to surface EMGs to extract muscle mechanical action from the natural endpoint force fluctuations generated during steady isometric contraction. We demonstrate this technique on simultaneous recordings of fingertip force and surface EMG from the first dorsal interosseous (FDI) and extensor indicis (EI) of humans. The EWA direction was approximately the same across a wide range of fingertip force directions, and the average EWA direction was consistent with mechanical action direction of these muscles estimated from cadaveric and imaging data: the EWA directions were 193 +/- 2 degrees for the FDI and 71 +/- 5 degrees for the EI (95% confidence). EWA transient behavior also appears to capture temporal characteristics of muscle force fluctuations with peak force time and general waveform shape similar to that of the associated spike-triggered averages from single motor units. The EWA may provide a means of empirically characterizing the complex transformation between muscle force and endpoint force without the need for invasive electrode recordings or complex anatomical measurements of musculoskeletal geometry.

Predication of reflex recovery after stroke using quantitative assessments of motor impairment at 1 month

The objective of this study was to characterize the time-course of changes reflex stiffness after stroke, and to use the Fugl-Meyer Assessment (FMA) at 1 month to predict the ensuing recovery patterns over 1 year. We quantified the modulation of reflex stiffness as a function of elbow joint angles at 1, 2, 3, 6, and 12 months after stroke, using a parallel cascade system identification technique. We then used the "growth mixture" and logistic regression models to characterize recovery patterns over 1 year and to predict these patterns, based on the FMA score at 1 month. We observed two major distinct recovery classes for the relationship between reflex stiffness and elbow angle. The FMA at 1 month was a significant predictor of reflex stiffness as a function of elbow angle at different time points in the first year. The logistical regression class membership may enable us to accurately predict reflex behavior during the first year, information of great potential value for planning targeted therapeutic interventions. Finally, the findings suggest that abnormal reflex function could contribute to functional motor impairment.

Natural history of neuromuscular properties after stroke: a longitudinal study

BACKGROUND

A rigorous description of the time course of changes in neuromuscular properties after stroke may help us to understand the mechanisms underlying major motor impairments, and it will also help us track the efficacy of rehabilitation treatments. Such time course data have not been collected to date, primarily because of the lack of accurate tools for separating muscular and neural functional measures.

OBJECTIVE

To characterise the time course of changes in elbow neuromuscular properties in hemiparetic stroke survivors over a 1 year period.

METHODS

Using a system identification technique based on mechanical perturbations of elbow angle, we estimated intrinsic mechanical properties of muscles and stretch reflex parameters at 1, 2, 3, 6 and 12 months after stroke, at different mean elbow joint angles.

RESULTS

There were substantial and progressive changes in intrinsic and reflex stiffness in paretic elbow muscles, at all five selected time points, and over a range of mean joint angles. Two temporal patterns of change in these neuromuscular properties were identified. In the first, intrinsic and reflex stiffness increased continuously after the stroke while in the second, intrinsic stiffness decreased continuously over this 12 month interval.

CONCLUSIONS

These different recovery patterns may reflect the emergence of two simultaneous but potentially opposing mechanisms; brain recovery and changes in peripheral neuromuscular properties. One consequence is that global joint stiffness measures may be misleading as opposing contributions from intrinsic and reflex neuromuscular subcomponents may confound our interpretation of the mean joint stiffness estimates.

Origins of abnormal excitability in biceps brachii motoneurons of spastic-paretic stroke survivors

Stroke survivors often exhibit abnormal motoneuron excitability, manifested clinically as spasticity with exaggerated stretch reflexes in resting muscles. We examined whether this abnormal excitability is a result of increased activation of intrinsic voltage-dependent persistent inward currents (PICs) or whether it is a result of enhanced synaptic inputs to the motoneuron. This distinction was made by recording firing rate profiles of pairs of motor units during isometric contractions of elbow flexor muscles. To estimate PIC amplitude, the discharge of the lower-threshold (reporter) motor unit of the pair was used to estimate the synaptic input to the higher-threshold (test) motor unit. The estimated synaptic input required to recruit the test unit was compared with the synaptic input when the test unit was derecruited (DeltaF) and this served as an estimate of the intrinsic (PIC) contribution to motoneuron firing. We found that PIC estimates were not larger in spastic-paretic motoneurons (DeltaF = 4.0 +/- 1.6 pps) compared with contralateral (4.6 +/- 1.4 pps) and age-matched healthy control motoneurons (3.8 +/- 1.7, all P > 0.1). Instead, following the voluntary contractions, the majority of lower-threshold motor units in spastic-paretic muscles (83%) exhibited spontaneous discharge, compared with 14% of contralateral and 0% of control motor units. Furthermore, there was strong co-modulation of simultaneously active units in spastic muscle. The presence of ongoing, correlated unit activity at "rest," coupled with firing behavior at recruitment unique to lower-threshold motor units in spastic muscles, suggested that firing changes are likely a result of a low-level depolarizing synaptic drive to the resting motoneuron pool.

Modulation of stretch reflexes of the finger flexors by sensory feedback from the proximal upper limb poststroke

Neural coupling of proximal and distal upper limb segments may have functional implications in the recovery of hemiparesis after stroke. The goal of the present study was to investigate whether the stretch reflex response magnitude of spastic finger flexor muscles poststroke is influenced by sensory input from the shoulder and the elbow and whether reflex coupling of muscles throughout the upper limb is altered in spastic stroke survivors. Through imposed extension of the metacarpophalangeal (MCP) joints, stretch of the relaxed finger flexors of the four fingers was imposed in 10 relaxed stroke subjects under different conditions of proximal sensory input, namely static arm posture (3 different shoulder/elbow postures) and electrical stimulation (surface stimulation of biceps brachii or triceps brachii, or none). Fast (300 degrees/s) imposed stretch elicited stretch reflex flexion torque at the MCP joints and reflex electromyographic (EMG) activity in flexor digitorum superficialis. Both measures were greatest in an arm posture of 90 degrees of elbow flexion and neutral shoulder position. Biceps stimulation resulted in greater MCP stretch reflex flexion torque. Fast imposed stretch also elicited reflex EMG activity in nonstretched heteronymous upper limb muscles, both proximal and distal. These results suggest that in the spastic hemiparetic upper limb poststroke, sensorimotor coupling of proximal and distal upper limb segments is involved in both the increased stretch reflex response of the finger flexors and an increased reflex coupling of heteronymous muscles. Both phenomena may be mediated through changes poststroke in the spinal reflex circuits and/or in the descending influence of supraspinal pathways.

Delays in Grip Initiation and Termination in Persons With Stroke: Effects of Arm Support and Active Muscle Stretch Exercise

Stroke survivors' difficulty in releasing grasped objects may be attributable not only to impaired finger extension but also to delays in terminating activity in the gripping flexor muscles. This study was undertaken 1) to quantify the time needed to initiate and terminate grip muscular activity following stroke and 2) to examine effects of arm support, grip location, and active muscle stretch on the delays recorded in the paretic hand. Delays in initiation and termination of finger flexor muscle activity in response to an auditory stimulus were measured for both paretic and nonparetic hands of ten stroke survivors with chronic hemiparesis and the dominant hand of five neurologically intact subjects. Additionally, the delays for the paretic hand were recorded while an external arm support was used and after 30 min of active muscle stretch. We found that delays in grip initiation and termination were greatest for the paretic hand (1.9 and 5.0 s), followed by the nonparetic hand (0.5 and 1.6 s), and least for the control hand (0.2 and 0.4 s). Arm support reduced delay in grip termination 37% for the paretic hand. Repeated active muscle stretch resulted in 24% reduced delay in grip initiation and 32% increased delay in grip termination for the paretic hand. Therapies and interventions reducing these delays may improve the ability to grasp and release objects and thus increase functional independence for stroke survivors.

Time course of changes in neuromuscular properties following stroke

To characterize the natural history of stroke effects on neuromuscular properties in elbow muscles, we tracked changes in elbow mechanical properties in hemiparetic stroke survivors after stroke. Using a parallel cascade system identification technique, we estimated intrinsic and reflex mechanical properties at 1, 2, 3, 6 and 12 months post stroke. At each time point, we examined neuromuscular changes during variations in mean elbow joint angle. Modulation of intrinsic and reflex properties was assessed using small amplitude pseudorandom positional perturbations at different mean elbow angles, over the entire range of motion. We identified two patterns of stroke effects on neuromuscular properties. In Group 1, intrinsic stiffness increased continuously after the stroke. In Group 2, it decreased continuously over this interval. Analogous results were recorded for reflex stiffness. These different recovery patterns may reflect the simultaneous emergence of two opposing mechanisms; i.e. brain recovery and secondary effects on neuromuscular properties. It follows that the progress of recovery may not reflect a single mechanism, and could depend on which mechanism is dominant at each time point.

Endpoint force fluctuations reveal flexible rather than synergistic patterns of muscle cooperation

We developed a new approach to investigate how the nervous system activates multiple redundant muscles by studying the endpoint force fluctuations during isometric force generation at a multi-degree-of-freedom joint. We hypothesized that, due to signal-dependent muscle force noise, endpoint force fluctuations would depend on the target direction of index finger force and that this dependence could be used to distinguish flexible from synergistic activation of the musculature. We made high-gain measurements of isometric forces generated to different target magnitudes and directions, in the plane of index finger metacarpophalangeal joint abduction-adduction/flexion-extension. Force fluctuations from each target were used to calculate a covariance ellipse, the shape of which varied as a function of target direction. Directions with narrow ellipses were approximately aligned with the estimated mechanical actions of key muscles. For example, targets directed along the mechanical action of the first dorsal interosseous (FDI) yielded narrow ellipses, with 88% of the variance directed along those target directions. It follows the FDI is likely a prime mover in this target direction and that, at most, 12% of the force variance could be explained by synergistic coupling with other muscles. In contrast, other target directions exhibited broader covariance ellipses with as little as 30% of force variance directed along those target directions. This is the result of cooperation among multiple muscles, based on independent electromyographic recordings. However, the pattern of cooperation across target directions indicates that muscles are recruited flexibly in accordance with their mechanical action, rather than in fixed groupings.

The relation between Ashworth scores and neuromechanical measurements of spasticity following stroke

Background

Spasticity is a common impairment that follows stroke, and it results typically in functional loss. For this reason, accurate quantification of spasticity has both diagnostic and therapeutic significance. The most widely used clinical assessment of spasticity is the modified Ashworth scale (MAS), an ordinal scale, but its validity, reliability and sensitivity have often been challenged. The present study addresses this deficit by examining whether quantitative measures of neural and muscular components of spasticity are valid, and whether they are strongly correlated with the MAS.

Methods

We applied abrupt small amplitude joint stretches and Pseudorandom Binary Sequence (PRBS) perturbations to both paretic and non-paretic elbow and ankle joints of stroke survivors. Using advanced system identification techniques, we quantified the dynamic stiffness of these joints, and separated its muscular (intrinsic) and reflex components. The correlations between these quantitative measures and the MAS were investigated.

Results

We showed that our system identification technique is valid in characterizing the intrinsic and reflex stiffness and predicting the overall net torque. Conversely, our results reveal that there is no significant correlation between muscular and reflex torque/stiffness and the MAS magnitude. We also demonstrate that the slope and intercept of reflex and intrinsic stiffnesses plotted against the joint angle are not correlated with the MAS.

Conclusion

Lack of significant correlation between our quantitative measures of stroke effects on spastic joints and the clinical assessment of muscle tone, as reflected in the MAS suggests that the MAS does not provide reliable information about the origins of the torque change associated with spasticity, or about its contributing components.

Finite element analysis for evaluation of pressure ulcer on the buttock: development and validation

The interface pressure is currently the only clinical tool to estimate the risk of sitting-related pressure ulcers. However, it provides little information on the loading condition in deep tissues. We present a comprehensive 3-D finite element (FE) model for human buttocks with the consideration of the joint configuration and realistic boundary conditions in a sitting posture. Sitting induced soft tissue deformation, internal pressure, and von-Mises stress were computed. The FE model was well validated qualitatively using actual displacement obtained from magnetic resonance imaging (MRI) images. FE analysis demonstrated that the deformation induced by sitting pressure was substantially different among muscle, fat, and skin. The deformation of the muscle varied with location and the maximum was seen in the regions underneath the bony prominence of ischial tuberosity. In these regions, the range of compressive pressure was 65-80 kPa, 50-60 kPa, and 55-65 kPa, for skin, fat, and muscle, respectively. The von-Mises stress distribution had a similar pattern. In conclusion, this study suggests a new methodology for the development and validation of FE models for investigating the risk of sitting-related soft tissue injury. The proposed model may provide researchers and therapists with a powerful technique for evaluating the effectiveness of various postural modulations in preventing deep tissue ulcers.

Muscle and reflex changes with varying joint angle in hemiparetic stroke

Background

Despite intensive investigation, the origins of the neuromuscular abnormalities associated with spasticity are not well understood. In particular, the mechanical properties induced by stretch reflex activity have been especially difficult to study because of a lack of accurate tools separating reflex torque from torque generated by musculo-tendinous structures. The present study addresses this deficit by characterizing the contribution of neural and muscular components to the abnormally high stiffness of the spastic joint.

Methods

Using system identification techniques, we characterized the neuromuscular abnormalities associated with spasticity of ankle muscles in chronic hemiparetic stroke survivors. In particular, we systematically tracked changes in muscle mechanical properties and in stretch reflex activity during changes in ankle joint angle. Modulation of mechanical properties was assessed by applying perturbations at different initial angles, over the entire range of motion (ROM). Experiments were performed on both paretic and non-paretic sides of stroke survivors, and in healthy controls.

Results

Both reflex and intrinsic muscle stiffnesses were significantly greater in the spastic/paretic ankle than on the non-paretic side, and these changes were strongly position dependent. The major reflex contributions were observed over the central portion of the angular range, while the intrinsic contributions were most pronounced with the ankle in the dorsiflexed position.

Conclusion

In spastic ankle muscles, the abnormalities in intrinsic and reflex components of joint torque varied systematically with changing position over the full angular range of motion, indicating that clinical perceptions of increased tone may have quite different origins depending upon the angle where the tests are initiated.

Furthermore, reflex stiffness was considerably larger in the non-paretic limb of stroke patients than in healthy control subjects, suggesting that the non-paretic limb may not be a suitable control for studying neuromuscular properties of the ankle joint.

Our findings will help elucidate the origins of the neuromuscular abnormalities associated with stroke-induced spasticity.

Investigation of soft-tissue stiffness alteration in denervated human tissue using an ultrasound indentation system

BACKGROUND/OBJECTIVE

Differences in soft-tissue stiffness may provide for a quantitative assessment and detection technique for pressure ulcers or deep-tissue injury. An ultrasound indentation system may provide a relatively convenient, simple, and noninvasive method for quantitative measurement of changes in soft-tissue stiffness in vivo.

METHODS

The Tissue Ultrasound Palpation System (TUPS) was used to quantitatively measure changes in soft-tissue stiffness at different anatomical locations within and between able-bodied persons and individuals with chronic spinal cord injury (SCI). The stiffness of soft tissue was measured at the ischial tuberosity, greater trochanter, posterior midthigh, and biceps brachii. Additionally, soft-tissue thickness and soft-tissue deformation were also measured.

RESULTS

Significant differences in soft-tissue stiffness were observed within the various anatomical locations tested, in both the able-bodied and SCI groups. Differences in soft-tissue stiffness were also observed between the 2 groups. Participants with SCI had significantly softer tissue in their buttock-thigh area.

CONCLUSIONS

TUPS is a clinically feasible technology that can reliably and effectively detect changes in soft-tissue stiffness. The study has provided a better understanding of the tissue mechanical response to external loading, specifically in the SCI population, suggesting the use of tissue stiffness as a parameter to detect and assess pressure-related soft-tissue injury.