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.

Stretch reflex excitability in contralateral limbs of stroke survivors is higher than in matched controls.

BACKGROUND: Spasticity, characterized by hyperreflexia, is a motor impairment that can arise following a hemispheric stroke. While the neural mechanisms underlying spasticity in chronic stroke survivors are unknown, one probable cause of hyperreflexia is increased motoneuron (MN) excitability. Potential sources of increased spinal MN excitability after a stroke include increased vestibulospinal (VS) and/or reticulospinal (RS) drive. Spasticity, as clinically assessed in stroke survivors, is highly lateralized, thus RS contributions to stroke-induced spasticity are more difficult to reconcile, as RS nuclei routinely project bilaterally to the spinal cord. Yet studies in stroke survivors suggest that there may also be changes in neuromodulation at the spinal level, indicative of RS tract influence. We hypothesize that after hemispheric stroke, alterations in the excitability of the RS nuclei affect both sides of the spinal cord, and thereby contribute to increased MN excitability on both paretic/spastic and contralateral sides of stroke survivors, as compared to neurologically intact subjects. METHODS: We estimated stretch reflex thresholds of the biceps brachii (BB) muscle using a position-feedback controlled linear motor to progressively indent the BB distal tendon in both spastic and contralateral limbs of hemispheric stroke survivors and in age-matched intact subjects. RESULTS: Our previously reported results show a significant difference between reflex thresholds of spastic and contralateral limbs of stroke survivors recorded from BB-medial (p < 0.005) and BB-lateral (p < 0.001). For this study, we report that there is also a significant difference between the reflex thresholds in the contralateral limb of stroke subjects and the dominant arm of intact subjects, again measured from both BB-medial (p < 0.05) and BB-lateral (p < 0.05). CONCLUSION: The reduction in stretch reflex thresholds in the contralateral limb of stroke survivors, based here on comparisons with thresholds of intact subjects, suggests an increased MN excitability on contralateral sides of stroke survivors as compared to intact subjects. This in turn supports our contention that RS tract activation, which has bilateral descending influences, is at least partially responsible for increased stretch reflex excitability, post-stroke, as both contralateral and affected sides show increased MN excitability as compared to intact subjects. Still, spasticity, presently diagnosed only on the affected side, with increased MN excitability on the affected side as compared to the contralateral side (our previous study), may be due to a different strongly lateralized pathway, such as the VS tract, which has not been directly tested here. Currently available clinical methods of spasticity assessment, such as the Modified Ashworth Scale, lack the resolution to quantify this phenomenon of a bilateral increase in MN excitability.

Organization of the motor-unit pool for different directions of isometric contraction of the first dorsal interosseous muscle.

INTRODUCTION: Muscle force generation involves recruitment and firing rate modulation of motor units (MUs). The control of MUs in producing multidirectional forces remains unclear. METHODS: We studied MU recruitment and firing properties, recorded from the first dorsal interosseous muscle, for 3 different directions of contraction: abduction; abduction/flexion combination; and flexion. RESULTS: MUs were recruited systematically at higher threshold force during flexion. Larger MUs were recruited and firing rates of MUs were lower during abduction. There was an orderly recruitment of MUs according to MU size regardless of contraction direction, obeying the "size principle." Firing rates of earlier-recruited MUs were consistently higher than later-recruited MUs, affirming the "onion-skin" property. DISCUSSION: Our findings suggest that the size principle and onion-skin organization together provide a general description of MU recruitment patterns and firing properties. The directional alternations of MU control properties likely reflect changes in neural drive to the muscle. Muscle Nerve 57: E85-E93, 2018.

Muscle fatigue increases beta-band coherence between the firing times of simultaneously active motor units in the first dorsal interosseous muscle.

Synchronization between the firing times of simultaneously active motor units (MUs) is generally assumed to increase during fatiguing contractions. To date, however, estimates of MU synchronization have relied on indirect measures, derived from surface electromyographic (EMG) interference signals. This study used intramuscular coherence to investigate the correlation between MU discharges in the first dorsal interosseous muscle during and immediately following a submaximal fatiguing contraction, and after rest. Coherence between composite MU spike trains, derived from decomposed surface EMG, were examined in the delta (1-4 Hz), alpha (8-12 Hz), beta (15-30 Hz), and gamma (30-60 Hz) frequency band ranges. A significant increase in MU coherence was observed in the delta, alpha, and beta frequency bands postfatigue. In addition, wavelet coherence revealed a tendency for delta-, alpha-, and beta-band coherence to increase during the fatiguing contraction, with subjects exhibiting low initial coherence values displaying the greatest relative increase. This was accompanied by an increase in MU short-term synchronization and a decline in mean firing rate of the majority of MUs detected during the sustained contraction. A model of the motoneuron pool and surface EMG was used to investigate factors influencing the coherence estimate. Simulation results indicated that changes in motoneuron inhibition and firing rates alone could not directly account for increased beta-band coherence postfatigue. The observed increase is, therefore, more likely to arise from an increase in the strength of correlated inputs to MUs as the muscle fatigues.

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.

Changes in motor unit behavior following isometric fatigue of the first dorsal interosseous muscle.

The neuromuscular strategies employed to compensate for fatigue-induced muscle force deficits are not clearly understood. This study utilizes surface electromyography (sEMG) together with recordings of a population of individual motor unit action potentials (MUAPs) to investigate potential compensatory alterations in motor unit (MU) behavior immediately following a sustained fatiguing contraction and after a recovery period. EMG activity was recorded during abduction of the first dorsal interosseous in 12 subjects at 20% maximum voluntary contraction (MVC), before and directly after a 30% MVC fatiguing contraction to task failure, with additional 20% MVC contractions following a 10-min rest. The amplitude, duration and mean firing rate (MFR) of MUAPs extracted with a sEMG decomposition system were analyzed, together with sEMG root-mean-square (RMS) amplitude and median frequency (MPF). MUAP duration and amplitude increased immediately postfatigue and were correlated with changes to sEMG MPF and RMS, respectively. After 10 min, MUAP duration and sEMG MPF recovered to prefatigue values but MUAP amplitude and sEMG RMS remained elevated. MU MFR and recruitment thresholds decreased postfatigue and recovered following rest. The increase in MUAP and sEMG amplitude likely reflects recruitment of larger MUs, while recruitment compression is an additional compensatory strategy directly postfatigue. Recovery of MU MFR in parallel with MUAP duration suggests a possible role for metabolically sensitive afferents in MFR depression postfatigue. This study provides insight into fatigue-induced neuromuscular changes by examining the properties of a large population of concurrently recorded single MUs and outlines possible compensatory strategies involving alterations in MU recruitment and MFR.

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.

Changes in motoneuron afterhyperpolarization duration in stroke survivors.

Hemispheric brain injury resulting from a stroke is often accompanied by muscle weakness in limbs contralateral to the lesion. In the present study, we investigated whether weakness in contralesional hand muscle in stroke survivors is partially attributable to alterations in motor unit activation, including alterations in firing rate modulation range. The afterhyperpolarization (AHP) potential of a motoneuron is a primary determinant of motoneuron firing rate. We examined differences in AHP duration in motoneurons innervating paretic and less impaired (contralateral) limb muscles of hemiparetic stroke survivors as well as in control subjects. A novel surface EMG (sEMG) electrode was used to record motor units from the first dorsal interosseous muscle. The sEMG data were subsequently decomposed to derive single-motor unit events, which were then utilized to produce interval (ISI) histograms of the motoneuron discharges. A modified version of interval death rate (IDR) analysis was used to estimate AHP duration. Results from data analyses performed on both arms of 11 stroke subjects and in 7 age-matched control subjects suggest that AHP duration is significantly longer for motor units innervating paretic muscle compared with units in contralateral muscles and in units of intact subjects. These results were supported by a coefficient of variation (CV) analysis showing that paretic motor unit discharges have a lower CV than either contralateral or control units. This study suggests that after stroke biophysical changes occur at the motoneuron level, potentially contributing to lower firing rates and potentially leading to less efficient force production in paretic muscles.

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.

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.

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.

Anomalies of motor unit amplitude and territory after botulinum toxin injection.

Objective. Botulinum toxin (BT) induced cholinergic denervation of hyperactive motor units (MUs) is a clinically accepted and extensively practiced way of managing focal spasticity after stroke. The denervation potentially initiates a temporary reorganization of the MU activation patterns and structures by inducing the emergence of a large number of newly innervated muscle fibers. In this study, we quantify the effect of the BT on MU action potential (MUAP) amplitudes and on the MU territory areas (MUTAs) as seen on the surface of the skin over the biceps brachii (BB) muscle.Approach. We have used a 128-channel high-density surface electromyography (HDsEMG) grid on the spastic and contralateral BB muscle and recorded the myoelectric activity along with the contraction force during isometric contraction of the elbow muscles. We have decomposed the recorded EMG signal into individual MU potentials and estimated the MUAP amplitudes and territory areas before and two weeks after a BT injection.Main result. There were significantly larger median (47 ± 9%) MUAP amplitudes as well as reduction of MUTA (20 ± 2%) two weeks after the injection compared to the respective pre-injection recording.Significance. The observed covariation of the amplitude and the territory area indicates that the large amplitude MUs that appeared after the BT injection have a relatively smaller territory area. These results provide a rare insight into the BT-induced changes of MU characteristics and have the potential to improve spasticity treatment. We discuss the potential contributing factors to these changes subsequent to the injection in the context of the investigated subject cohort.

Characteristic Variation of Electromechanical Delay After the Botulinum Toxin Injection in Spastic Biceps Brachii Muscles.

The objective of this study was to characterize the effects of intramuscular botulinum toxin (BT) injections on the electromechanical delay (EMD) in spastic human biceps muscles. The EMD is calculated as the time lag between the muscle activation onset, as recorded from the surface electromyogram (sEMG), and the onset of recorded force. In a cohort of chronic stroke survivors, we compared the computed EMD derived from the spastic (injected) biceps brachii with that from the contralateral muscle. Eight participants were tested before and up to 3 months after a BT injection. At each session, participants followed an isometric trapezoidal force trajectory at 50 and 30%, respectively, of the tested maximum voluntary contraction (MVC). Joint force and sEMG signals were recorded as well. The number of zero crossings (ZC) of the sEMG during the steady-state portion of the task was also computed. The EMD post-BT was found to increase by 64 ± 10% (at 50% MVC) and 93 ± 18% (at 30% MVC) when compared to pre-BT values, while the number of sEMG-ZC, the mean MVC values, and the force-EMD slope exhibited striking reductions. These parameters, calculated on the contralateral side, remained relatively constant across sessions, with the EMD significantly lower and the MVC values much higher. We discuss potential contributing factors to an increase in EMD values on the affected side, both pre- and post-BT. The observed co-variation across sessions of the increased EMD values with the decreased ZC estimates, a surrogate of motor outflow, and, potentially, more compliant muscle fascicles suggests that the altered motor unit (MU) behavior contributes, at least in part, to the delayed force production.

Performance Evaluation of a Wearable Tattoo Electrode Suitable for High-Resolution Surface Electromyogram Recording.

OBJECTIVE: High-density surface electromyography (HD-sEMG) has been utilized extensively in neuromuscular research. Despite its potential advantages, limitations in electrode design have largely prevented widespread acceptance of the technology. Commercial electrodes have limited spatial fidelity, because of a lack of sharpness of the signal, and variable signal stability. We demonstrate here a novel tattoo electrode that addresses these issues. Our dry HD electrode grid exhibits remarkable deformability which ensures superior conformity with the skin surface, while faithfully recording signals during different levels of muscle contraction. METHOD: We fabricated a 4 cm×3 cm tattoo HD electrode grid on a stretchable electronics membrane for sEMG applications. The grid was placed on the skin overlying the biceps brachii of healthy subjects, and was used to record signals for several hours while tracking different isometric contractions. RESULTS: The sEMG signals were recorded successfully from all 64 electrodes across the grid. These electrodes were able to faithfully record sEMG signals during repeated contractions while maintaining a stable baseline at rest. During voluntary contractions, broad EMG frequency content was preserved, with accurate reproduction of the EMG spectrum across the full signal bandwidth. CONCLUSION: The tattoo grid electrode can potentially be used for recording high-density sEMG from skin overlying major limb muscles. Layout programmability, good signal quality, excellent baseline stability, and easy wearability make this electrode a potentially valuable component of future HD electrode grid applications. SIGNIFICANCE: The tattoo electrode can facilitate high fidelity recording in clinical applications such as tracking the evolution and time-course of challenging neuromuscular degenerative disorders.

Quantifying the Peak Amplitude Distributions of Electromyogram in Bicep Brachii muscle after Stroke.

The objective of this study was to quantify the differences in surface electromyogram (EMG) signal characteristics between affected and contralateral arm muscles of hemispheric stroke survivors. EMG signals were recorded from the biceps brachii muscles using single differential electrodes. Four chronic stroke subjects performed isometric elbow flexions at sub-maximal voluntary contraction levels on both the affected and contralateral limbs. The force generated on the contralateral side was matched to the force generated on the affected side. We observed different types of EMG activation on the affected side compared to the contralateral side.Specifically, two subjects showed lower RMS EMG activity on the affected side whereas two subjects showed greater EMG activity on the affected side compared to the contralateral side. Analysis of the peak amplitudes of the EMG activity showed greater number of peaks in the EMG on affected side compared to the contralateral side in all subjects. The histogram of the peak amplitudes showed greater number of smaller peak amplitudes in subjects with lower EMG activity on the affected side suggesting a reliance on smaller motor units. Our combined EMG signal analysis techniques on one set of recorded signals provides insight regarding potential mechanisms of weakness.Clinical Relevance- Decoding neural information from surface EMG signals without decomposition into individual motor units could provide clinicians with quick insight about disease progress and potential treatment.

Characterization of Differences in the Time Course of Reflex and Voluntary Responses Following Botulinum Toxin Injections in Chronic Stroke Survivors.

Spasticity is a major impairment that can occur following a hemispheric stroke and is often treated with injections of botulinum toxin, a neurotoxin that impairs transmission at the neuromuscular junction. Hyperreflexia is a defining feature of spasticity. Our main objective here was to quantify the time course of changes in the deep tendon reflex (DTR) responses and voluntary activation capacity following BT injection as well as to track changes in a clinical assessment of spasticity. Four chronic stroke survivors, scheduled to receive BT in their Biceps Brachii(BB) as part of their clinical care plan, were recruited for repeated testing sessions over the course of 4 months post injection. Both surface BB EMG reflex response to bicipital tendon taps as well as signals of applied tendon tap forces were recorded before and up to 18 weeks post-BT. Voluntary force and biceps EMG signals were also recorded during maximum voluntary (isometric) contractions (MVC) at each testing session. Our results show major reductions (up to 75%) in voluntary sEMG and force arising between 11 to 35 days post-BT-injection. The stretch reflex gain declined two weeks after the maximal reductions in voluntary EMG and force. Paradoxically, there was a short-term increase in stretch reflex gain, in three out of four participants, approximately 11-35 days post BT. The time course of recovery of voluntary MVC and reflex responses varied considerably with a longer recovery time for the reflex responses.

In-Vivo Study of Passive Musculotendon Mechanics in Chronic Hemispheric Stroke Survivors.

We characterized the passive mechanical properties of the affected and contralateral musculotendon units in 9 chronic stroke survivors as well as in 6 neurologically-intact controls. Using a position-controlled motor, we precisely indented the distal tendon of the biceps brachii to a 20 mm depth from skin, recording both its sagittal motion using ultrasound movies and the compression force at the tip of the indenter. Length changes of 8 equally-spaced features along the aponeurosis axis were quantified using a pixel-tracking protocol. We report that, on the aggregate and with respect to contralateral and control, respectively, the affected side initiates feature motion at a shorter indentation distance by 61% and 50%, travels further by 15% and 9%, at a lower rate of 28% and 15%, and is stiffer by 40% and 57%. In an extended analysis including the spatial location of the 8 designated features, we report that in contrast to the contralateral and control muscles, the affected musculotendon unit does not strain measurably within the imaging window. These results confirm that chronic stroke-induced spasticity changes musculotendon unit passive mechanics, causing it to not strain under stretch. The mechanisms responsible for altered passive mechanics may lie within extracellular matrix fibrosis.

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.

Increase in Muscle Activation during Physiotherapy with Electromyography Biofeedback for Patients with Acute Cervical Spinal Cord Injuries.

Surface electromyography (sEMG) can be used as a biofeedback (BFB) parameter to provide information to participants regarding muscle activation in a variety of settings. The objective of our study is to assess whether an sEMG BFB display during physical therapy sessions for acute spinal cord injured inpatients would assist in increasing muscle use and patient engagement during therapy. In addition, we sought to assess whether the therapist and patients would find the sEMG BFB relevant and useful. To achieve this objective, we examined the effect of visual sEMG BFB system in improving muscle activation and therapeutic outcomes during experimental sessions that mimicked conventional slings therapy sessions with a research therapist providing the therapy. We recruited two inpatients with recent spinal cord injuries (SCI) that had been prescribed (clinical) slings therapy as part of their clinical standard of care at our acute rehabilitation hospital. During each experimental session there were two portions. One was the control period which required the participants to repeat elbow flexion 10 times under conventional clinical slings therapy protocols. The other was the BFB period, where the therapist guided the participant in the same movement but both therapist and the participant were provided with visual feedback of real-time sEMG signals recorded from participant's biceps brachii muscles. Our preliminary results show that both participants demonstrated statistically significant improvement of muscle activation level with the sEMG BFB system compared with conventional clinical slings therapy protocols.

A Tendon Indentation Method to Quantify Velocity-Dependent Reflex Responses after Hemispheric Stroke.

Stretch reflex responses in passive muscle can be utilized to assess spasticity in chronic stroke survivors. In this study, we present a different method of eliciting the reflex response by imposing tendon indentation using a linear motor. Specifically, we test a "Ramp-and-hold" protocol, utilizing a linear motor controlled by a position-controlled feedback loop (Linmot, Inc), to indent the biceps brachii distal tendon at different velocities. The protocol was tested on the affected arm of three stroke subjects. We also utilized a tendon indentation combined with tendon-tapping method to quantify the reflex threshold. Our results indicate that the reflex response was elicited at velocities equal to or above 50 mm/s in 2/3 subjects. No reflex response was detected in one subject. All subjects showed a distinct reflex threshold using the indentation/tapping method. Furthermore, the presence of a reflex response during tendon-tapping was not necessarily accompanied by the elicitation of a reflex response during the ramp-and-hold. However, our data suggests that the indentation threshold during tapping is correlated to the presence of a reflex response at the velocities tested during the ramp-and-hold. Though more time consuming, tendon indentation using ramp-and-hold could provide greater resolution of the reflex response to quantify spasticity than the current clinically employed ballistic tapping method using a reflex hammer.