Human spinal cord injury: motor unit properties and behaviour

Changes in thoracic erector spinae regional activation during postural adjustments and functional reaching tasks after spinal cord injury

Many individuals with incomplete spinal cord injury (SCI) exhibit reduced volitional control of trunk muscles, such as impaired voluntary contractions of the erector spinae (ES), due to damage to the neural pathways regulating sensorimotor function. Studies using conventional bipolar electromyography (EMG) showed alterations in the overall, or global, activation of the trunk muscles in people with SCI. However, how activation varied across specific regions within the ES, referred to as regional activation, remains unknown. The aim of the study was to investigate the regional distribution of the ES activity below the level of injury in individuals with incomplete SCI during postural tasks and multidirectional reaching tasks using high-density EMG. Twenty-one individuals with incomplete SCI and age-matched controls were recruited. The EMG amplitude of the thoracic ES and displacement of the arm, trunk, and center of pressure were recorded during the tasks. Activation was more in the lower region of the ES in individuals with SCI than in the controls during the postural tasks. In addition, activation was limited to a small area of the ES during the reaching tasks. The EMG amplitude was greater during reaching forward than returning to the upright posture in the controls; however, this phase-dependent difference in the EMG amplitude was not present in individuals with SCI. Our findings demonstrate changes in regional activation of the thoracic ES during postural and reaching tasks, likely reflecting injury-induced changes in selective neural control to activate residual muscle fibers of the ES for postural control and function after SCI.NEW & NOTEWORTHY We demonstrate that individuals with chronic incomplete spinal cord injury (SCI) recruit lower part of the thoracic erector spinae (ES) for postural control of the trunk. We also show that activation was restricted in a smaller part of the ES, and the discrete control of the ES was lost during functional reaching movements in individuals with SCI. Our study provides evidence of alterations in neural control between vertebral levels in individuals with SCI.

The discharge characteristics of motor units innervating functionally paralyzed muscles

For individuals with motor complete spinal cord injury (SCI), previous works have shown that spared motor neurons below the injury level can still be voluntarily controlled. In this study, we investigated the behavior of these neurons after SCI by analyzing neural and spatial properties of individual motor units using high-density surface electromyography (HDsEMG) and ultrasound imaging. The dataset for this study is based on motor unit data from our previous work (Oliveira et al. Brain 147: 3583-3595, 2024). Eight participants with chronic motor complete SCI and twelve uninjured controls attempted multiple hand movements, guided by a virtual hand, while we recorded forearm muscle activity. We analyzed the common synaptic input to motor neurons with a factorization method and found two dominant motor unit modes in both the SCI and control groups. Each mode was strongly correlated with the virtual hand's flexion or extension movements. The delay between flexion and extension movements and the motor unit modes was similar between groups, suggesting preserved common input to motor neurons after SCI. We classified motor units into task-modulated or nonmodulated (i.e., tonic or irregularly firing) based on their discharge patterns and phase difference with virtual hand kinematics and found a higher proportion of nonmodulated motor units in the SCI group. At the motor unit action potential level, we found larger motor unit territories after SCI. Finally, we observed distinct movements of paralyzed muscles with concurrent HDsEMG and ultrasound imaging, indicating the presence of highly functional motor units with distinct spared territories after SCI.NEW & NOTEWORTHY Here, we observed a similar pattern of motor unit activation during attempted hand movements in individuals with complete SCI, who cannot move their fingers, and in a control group, who performed the prescribed movements. Despite differences in individual motor unit behavior between these groups, such as a higher proportion of nonmodulated motor units in SCI, movement intention can still be decoded from paralyzed muscles.

Improving localization and measurements of M-waves using high-density surface electromyography

Surface electromyography (sEMG) is useful for studying muscle function and controlling prosthetics, but cross talk from nearby muscles often limits its effectiveness. High-density surface EMG (HD-sEMG) improves spatial resolution, allowing for the isolation of M-waves in the densely packed forearm muscles. This study assessed HD-sEMG for localizing M-waves and evaluated the impact of spatial filters on cross talk reduction. We administered peripheral nerve stimulation to activate forearm muscles in five participants. We analyzed cross talk by correlating the shape of M-waves between electrodes and used ultrasound to confirm muscle identity and location. At low-stimulation intensities, we successfully isolated M-waves with minimal cross talk without spatial filtering. Higher recruitment levels produced significant cross talk, which was reduced by applying bipolar or tripolar spatial filters. M-waves from the monopolar HD-sEMG montage showed high correlations between electrodes (r = 0.97 transversely; r = 0.95 longitudinally), while bipolar and tripolar montages showed lower correlations (bipolar: r = 0.41 transversely; r = 0.19 longitudinally; tripolar: r = 0.17 transversely; r = 0.01 longitudinally). The tripolar filter significantly reduced cross talk (51.10% amplitude decay one electrode away) compared with no filter (10.32% amplitude decay one electrode away), effectively reducing cross talk to negligible levels at distances ≥2.55 cm. Ultrasound was crucial for distinguishing true activation from artifacts caused by converging signals along muscle boundaries. Spatially filtered HD-sEMG accurately detects and isolates M-waves in the forearm, and ultrasound imaging is useful for verifying the location and identity of the muscles underlying the HD-sEMG grids.NEW & NOTEWORTHY This study introduces an innovative approach to enhancing evoked potential measurements using high-density surface electromyography (HD-sEMG). The precision and localization of evoked potentials are significantly improved by spatial filters and ultrasound imaging, offering a novel method for better assessing motor pathway integrity. These advancements could lead to more accurate tools for detecting and treating neurological deficits, making it a significant contribution to neurophysiological research.

Acute intermittent hypoxia increases maximal motor unit discharge rates in people with chronic incomplete spinal cord injury

Acute intermittent hypoxia (AIH) is an emerging technique for enhancing neuroplasticity and motor function in respiratory and limb musculature. Thus far, AIH-induced improvements in strength have been reported for upper and lower limb muscles after chronic incomplete cervical spinal cord injury (iSCI), but the underlying mechanisms have been elusive. We used high-density surface EMG (HDsEMG) to determine if motor unit discharge behaviour is altered after 15 × 60 s exposures to 9% inspired oxygen, interspersed with 21% inspired oxygen (AIH), compared to breathing only 21% air (SHAM). We recorded HDsEMG from the biceps and triceps brachii of seven individuals with iSCI during maximal elbow flexion and extension contractions, and motor unit spike trains were identified using convolutive blind source separation. After AIH, elbow flexion and extension torque increased by 54% and 59% from baseline (P = 0.003), respectively, whereas there was no change after SHAM. Across muscles, motor unit discharge rates increased by ∼4 pulses per second (P = 0.002) during maximal efforts, from before to after AIH. These results suggest that excitability and/or activation of spinal motoneurons is augmented after AIH, providing a mechanism to explain AIH-induced increases in voluntary strength. Pending validation, AIH may be helpful in conjunction with other therapies to enhance rehabilitation outcomes after incomplete spinal cord injury, due to these enhancements in motor unit function and strength. KEY POINTS: Acute intermittent hypoxia (AIH) causes increases in muscular strength and neuroplasticity in people living with chronic incomplete spinal cord injury (SCI), but how it affects motor unit discharge rates is unknown. Motor unit spike times were identified from high-density surface electromyograms during maximal voluntary contractions and tracked from before to after AIH. Motor unit discharge rates were increased following AIH. These findings suggest that AIH can facilitate motoneuron function in people with incomplete SCI.

Neuromuscular consequences of spinal cord injury: New mechanistic insights and clinical considerations

The spinal cord facilitates communication between the brain and the body, containing intrinsic systems that work with lower motor neurons (LMNs) to manage movement. Spinal cord injuries (SCIs) can lead to partial paralysis and dysfunctions in muscles below the injury. While traditionally this paralysis has been attributed to disruptions in the corticospinal tract, a growing body of work demonstrates LMN damage is a factor. Motor units, comprising the LMN and the muscle fibers with which they connect, are essential for voluntary movement. Our understanding of their changes post-SCI is still emerging, but the health of motor units is vital, especially when considering innovative SCI treatments like nerve transfer surgery. This review seeks to collate current literature on how SCI impact motor units and explore neuromuscular clinical implications and treatment avenues. SCI reduced motor unit number estimates, and surviving motor units had impaired signal transmission at the neuromuscular junction, force-generating capacity, and excitability, which have the potential to recover chronically, yet the underlaying mechanisms are unclear. Furthermore, electrodiagnostic evaluations can aid in assessing the health lower and upper motor neurons, identify suitable targets for nerve transfer surgeries, and detect patients with time sensitive injuries. Lastly, many electrodiagnostic abnormalities occur in both chronic and acute SCI, yet factors contributing to these abnormalities are unknown. Future studies are required to determine how motor units adapt following SCI and the clinical implications of these adaptations.

Clustering index analysis on EMG-Torque relation-based representation of complex neuromuscular changes after spinal cord injury

Spinal cord injury (SCI) resulting in complex neuromuscular pathology is not sufficiently well understood. To better quantify neuromuscular changes after SCI, this study uses a clustering index (CI) method for surface electromyography (sEMG) clustering representation to investigate the relation between sEMG and torque in SCI survivors. The sEMG signals were recorded from 13 subjects with SCI and 13 gender-age matched able-bodied subjects during isometric contraction of the biceps brachii muscle at different torque levels using a linear electrode array. Two torque representations, maximum voluntary contraction (MVC%) and absolute torque, were used. CI values were calculated for sEMG. Regression analyses were performed on CI values and torque levels of elbow flexion, revealing a strong linear relationship. The slopes of regressions between SCI survivors and control subjects were compared. The findings indicated that the range of distribution of CI values and slopes was greater in subjects with SCI than in control subjects (p < 0.05). The increase or decrease in slope was also observed at the individual level. This suggests that the CI and its sEMG clustering-torque relation may serve as valuable quantitative indicators for determining neuromuscular lesions after SCI, contributing to the development of effective rehabilitation strategies for improving motor performance.

Estimation of the number of motor units in the human extensor digitorum brevis using MScanFit

OBJECTIVE

Our aim was to determine the number and size parameters of EDB motor units in healthy young adults using MScanFit, a novel approach to motor unit number estimation (MUNE). Since variability in MUNE is related to compound muscle action potential (CMAP) size, we employed a procedure to document the optimal EDB electromyographic (EMG) electrode position prior to recording MUNE, a neglected practice in MUNE.

METHODS

Subjects were 21 adults 21-44 y. Maximum CMAPs were recorded from 9 sites in a 4 cm2 region centered over the EDB and the site with the largest amplitude was used in the MUNE experiment. For MUNE, the peroneal nerve was stimulated at the fibular head to produce a detailed EDB stimulus-response curve or "MScan". Motor unit number and size parameters underlying the MScan were simulated using the MScanFit mathematical model.

RESULTS

In 19 persons, the optimal recording site was superior, superior and proximal, or superior and distal to the EDB mid-belly, whereas in 3 persons it was proximal to the mid-belly. Ranges of key MScanFit parameters were as follows: maximum CMAP amplitude (3.1-8.5 mV), mean SMUP amplitude (34.4-106.7 μV), mean normalized SMUP amplitude (%CMAP max, 0.95-2.3%), largest SMUP amplitude (82.7-348 μV), and MUNE (43-103). MUNE was not related to maximum CMAP amplitude (R2 = 0.09), but was related to mean SMUP amplitude (R2 = -0.19, P = 0.05).

CONCLUSION

The EDB CMAP was highly sensitive to electrode position, and the optimal position differed between subjects. Individual differences in EDB MUNE were not related to CMAP amplitude. Inter-subject variability of EDB MUNE (coefficient of variation) was much less than previously reported, possibly explained by better optimization of the EMG electrode and the unique approach of MScanFit MUNE.

Disruption of Neuromuscular Junction Following Spinal Cord Injury and Motor Neuron Diseases

The neuromuscular junction (NMJ) is a crucial structure that connects the cholinergic motor neurons to the muscle fibers and allows for muscle contraction and movement. Despite the interruption of the supraspinal pathways that occurs in spinal cord injury (SCI), the NMJ, innervated by motor neurons below the injury site, has been found to remain intact. This highlights the importance of studying the NMJ in rodent models of various nervous system disorders, such as amyotrophic lateral sclerosis (ALS), Charcot-Marie-Tooth disease (CMT), spinal muscular atrophy (SMA), and spinal and bulbar muscular atrophy (SBMA). The NMJ is also involved in myasthenic disorders, such as myasthenia gravis (MG), and is vulnerable to neurotoxin damage. Thus, it is important to analyze the integrity of the NMJ in rodent models during the early stages of the disease, as this may allow for a better understanding of the condition and potential treatment options. The spinal cord also plays a crucial role in the functioning of the NMJ, as the junction relays information from the spinal cord to the muscle fibers, and the integrity of the NMJ could be disrupted by SCI. Therefore, it is vital to study SCI and muscle function when studying NMJ disorders. This review discusses the formation and function of the NMJ after SCI and potential interventions that may reverse or improve NMJ dysfunction, such as exercise, nutrition, and trophic factors.

Quantifying unsupported sitting posture impairments in humans with cervical spinal cord injury using a head-mounted IMU sensor

STUDY DESIGN

Cross-sectional study.

OBJECTIVES

To evaluate unsupported sitting posture impairments and identify postural regulatory strategies in cervical spinal cord injury (cSCI) participants via a head-mounted IMU sensor.

SETTING

A research lab in the United States of America.

METHODS

cSCI participants and controls maintained postural stability during unsupported sitting with eyes either open or closed. The head-mounted IMU sensor recorded accelerometer data to calculate cumulative sway motion. The postural regulatory strategy was analyzed by assessing the normalized power spectral density (PSD) in four frequency bands: 0-0.1 Hz (visual regulation), 0.1-0.5 Hz (vestibular regulation), 0.5-1 Hz (cerebellar regulation), and >1 Hz (proprioception and muscle control).

RESULTS

Significant increases in postural sway were observed in cSCI participants compared to controls during unsupported sitting. For cSCI participants, normalized PSD significantly increased in the low-frequency bands (0-0.1 Hz and 0.1-0.5 Hz) but decreased in the high-frequency band (>1 Hz) compared to controls.

CONCLUSIONS

cSCI participants were more reliant on visual and vestibular systems for sitting balance, while depending less on proprioception and muscle control compared to controls. These findings suggest that the altered postural regulatory strategy is ineffective in maintaining postural stability during unsupported sitting, emphasizing the importance of proprioception and muscle control for seated postural stability in cSCI participants.

Motor unit activity and synaptic inputs to motoneurons in the caudal part of the injured spinal cord

Spinal cord injury (SCI) disrupts neuronal function below the lesion epicenter, causing disuse muscle atrophy. We investigated motor unit (MU) activity and synaptic inputs to motoneurons in the caudal region of the injured spinal cord. Participants with C4-C7 cervical injuries were studied. The extensor digitorum communis (EDC) muscle, which is mainly innervated by C8, was assessed for disuse muscle atrophy. Using advanced electromyography and signal-processing techniques, we examined the concurrent activation of a substantial population of MUs during force-tracking tasks. We found that in participants with SCI (n = 9), both MU discharge rates and the amplitudes of MU action potentials were significantly lower than in controls (n = 9). After SCI, MUs were recruited in a limited force range as the strength of muscle contractions increased, implying a disruption in the orderly MU recruitment pattern. Coherence analysis revealed reduced synaptic inputs to motoneurons in the delta band (0.5-5 Hz) for participants with SCI, suggesting diminished common synaptic inputs to the EDC muscle. In addition, participants with SCI exhibited greater muscle force variability. Using principal component analysis on low-frequency MU discharge rates, we found that the first common component (FCC) captured the most discharge variability in participants with SCI. The coefficients of variation (CV) of the FCC correlated with force signal CVs, suggesting force variability mainly results from common synaptic inputs to the EDC muscle after SCI. These results advance our understanding of the neurophysiology of disuse muscle atrophy in human SCI, paving the way for therapeutic interventions to restore muscle function.NEW & NOTEWORTHY This study analyzed motor unit (MU) function below the lesion epicenter in patients with spinal cord injury (SCI). We found reduced MU discharge rates and action potential amplitudes in participants with SCI compared with controls. The strength of common synaptic inputs to motoneurons was reduced in patients with SCI, with increased force variability primarily due to low-frequency oscillations of common inputs. This study enhances understanding of neurophysiological and behavioral changes in disuse muscle atrophy post-SCI.

Different Thumb Positions in the Tetraplegic Hand

OBJECTIVE

To analyze factors associated with malposition that affects function of the thumb in individuals with tetraplegia.

DESIGN

Retrospective cross-sectional study.

SETTING

Rehabilitation Center for Spinal Cord Injury.

PARTICIPANTS

Anonymized data from 82 individuals (68 men), mean age 52.9±20.2 (SD) with acute/subacute cervical spinal cord injury C2-C8 AIS A-D recorded during 2018-2020.

INTERVENTIONS

Not applicable.

MAIN OUTCOME MEASURES

Motor point (MP) mapping and manual muscle test (MRC) of 3 extrinsic thumb muscles (flexor pollicis longus (FPL), extensor pollicis longus (EPL), and abductor pollicis longus (APL)).

RESULTS

159 hands in 82 patients with tetraplegia C2-C8 AIS A-D were analyzed and assigned to "key pinch" (40.3%), "slack thumb" (26.4%), and "thumb-in-palm" (7.5%) positions. There was a significant (P<.0001) difference between the 3 thumb positions depicted in lower motor neuron (LMN) integrity tested by MP mapping and muscle strength of the 3 muscles examined. All studied muscles showed a significantly different expression of MP and the MRC values (P<.0001) between the "slack thumb" and "key pinch" position. MRC of FPL was significantly greater in the group "thumb-in-palm" compared with "key pinch" position (P<.0001).

CONCLUSIONS

Malposition of the thumb due to tetraplegia seems to be related to the integrity of LMN and voluntary muscle activity of the extrinsic thumb muscles. Assessments such as MP mapping and MRC of the 3 thumb muscles enable the identification of potential risk factors for the development of thumb malposition in individuals with tetraplegia.

Deletion of Sod1 in Motor Neurons Exacerbates Age-Related Changes in Axons and Neuromuscular Junctions in Mice

Whole-body knock-out of Cu,Zn superoxide dismutase (Sod1KO) results in accelerated, age-related loss of muscle mass and function associated with neuromuscular junction (NMJ) breakdown similar to sarcopenia. In order to determine whether altered redox in motor neurons underlies this phenotype, an inducible neuron-specific deletion of Sod1 (i-mnSod1KO) was compared with wild-type (WT) mice of different ages (adult, mid-age, and old) and whole-body Sod1KO mice. Nerve oxidative damage, motor neuron numbers and structural changes to neurons and NMJ were examined. Tamoxifen-induced deletion of neuronal Sod1 from two months of age. No specific effect of a lack of neuronal Sod1 was seen on markers of nerve oxidation (electron paramagnetic resonance of an in vivo spin probe, protein carbonyl, or protein 3-nitrotyrosine contents). i-mnSod1KO mice showed increased denervated NMJ, reduced numbers of large axons and increased number of small axons compared with old WT mice. A large proportion of the innervated NMJs in old i-mnSod1KO mice displayed a simpler structure than that seen in adult or old WT mice. Thus, previous work showed that neuronal deletion of Sod1 induced exaggerated loss of muscle in old mice, and we report that this deletion leads to a specific nerve phenotype including reduced axonal area, increased proportion of denervated NMJ, and reduced acetyl choline receptor complexity. Other changes in nerve and NMJ structure seen in the old i-mnSod1KO mice reflect aging of the mice.

Electromyography-Force Relation and Muscle Fiber Conduction Velocity Affected by Spinal Cord Injury

A surface electromyography (EMG) analysis was performed in this study to examine central neural and peripheral muscle changes after a spinal cord injury (SCI). A linear electrode array was used to record surface EMG signals from the biceps brachii (BB) in 15 SCI subjects and 14 matched healthy control subjects as they performed elbow flexor isometric contractions from 10% to 80% maximum voluntary contraction. Muscle fiber conduction velocity (MFCV) and BB EMG-force relation were examined. MFCV was found to be significantly slower in the SCI group than the control group, evident at all force levels. The BB EMG-force relation was well fit by quadratic functions in both groups. All healthy control EMG-force relations were best fit with positive quadratic coefficients. In contrast, the EMG-force relation in eight SCI subjects was best fit with negative quadratic coefficients, suggesting impaired EMG modulation at high forces. The alterations in MFCV and EMG-force relation after SCI suggest complex neuromuscular changes after SCI, including alterations in central neural drive and muscle properties.

Riluzole for treating spasticity in patients with chronic traumatic spinal cord injury: Study protocol in the phase ib/iib adaptive multicenter randomized controlled RILUSCI trial

BACKGROUND

Satisfactory treatment is often lacking for spasticity, a highly prevalent motor disorder in patients with spinal cord injury (SCI). Low concentrations of riluzole potently reduce the persistent sodium current, the post-SCI increase in which contributes to spasticity. The repurposing of this drug may therefore constitute a useful potential therapeutic option for relieving SCI patients suffering from chronic traumatic spasticity.

OBJECTIVE

RILUSCI is a phase 1b-2b trial designed to assess whether riluzole is a safe and biologically effective means of managing spasticity in adult patients with traumatic chronic SCI.

METHODS

In this multicenter double-blind trial, adults (aged 18-65 years) suffering from spasticity after SCI (target enrollment: 90 participants) will be randomly assigned to be given either a placebo or a recommended daily oral dose of riluzole for two weeks. The latter dose will be previously determined in phase 1b of the study by performing double-blind dose-finding tests using a Bayesian continuous reassessment method. The primary endpoint of the trial will be an improvement in the Modified Ashworth Score (MAS) or the Numerical Rating Score (NRS) quantifying spasticity. The secondary outcomes will be based on the safety and pharmacokinetics of riluzole as well as its impact on muscle spasms, pain, bladder dysfunction and quality of life. Analyses will be performed before, during and after the treatment and the placebo-controlled period.

CONCLUSION

To the best of our knowledge, this clinical trial will be the first to document the safety and efficacy of riluzole as a means of reducing spasticity in patients with chronic SCI.

TRIAL REGISTRATION

The clinical trial, which is already in progress, was registered on the ClinicalTrials.gov website on August 9, 2016 under the registration number NCT02859792.

TRIAL SPONSOR

Assistance Publique-Hôpitaux de Marseille.

Motor performance, motor impairments, and quality of life after eccentric resistance training in neurological populations: A systematic review and meta-analyses

BACKGROUND

Many overlapping factors impair motor performance and quality of life in neurological patients. Eccentric resistance training (ET) has potential benefits for improving motor performance and treating motor impairments better than some traditional rehabilitation approaches.

OBJECTIVE

To estimate the effect of ET in neurological settings.

METHODS

Seven databases were reviewed up to May 2022 according to PRSIMA guidelines to find randomized clinical trials involving adults with a neurological condition, who underwent ET as set by the American College of Sports Medicine. Motor performance (main outcome) was assessed as strength, power and capacities during activity. Secondary outcomes (impairments) were muscle structure, flexibility, muscle activity, tone, tremor, balance and fatigue. Tertiary outcomes were risk of fall, and self-reports of quality of life.

RESULTS

Ten trials were included, assessed using Risk of Bias 2.0 tool, and used to compute meta-analyses. Effective effects in favour of ET were found for strength and power, but not for capacities during activity. Mixed results were found for secondary and tertiary outcomes.

CONCLUSION

ET may be a promising intervention to better improve strength/power in neurological patients. More studies are needed to improve the quality of evidence underlying changes responsible for these results.

Mechanism of skeletal muscle atrophy after spinal cord injury: A narrative review

Spinal cord injury leads to loss of innervation of skeletal muscle, decreased motor function, and significantly reduced load on skeletal muscle, resulting in atrophy. Factors such as braking, hormone level fluctuation, inflammation, and oxidative stress damage accelerate skeletal muscle atrophy. The atrophy process can result in skeletal muscle cell apoptosis, protein degradation, fat deposition, and other pathophysiological changes. Skeletal muscle atrophy not only hinders the recovery of motor function but is also closely related to many systemic dysfunctions, affecting the prognosis of patients with spinal cord injury. Extensive research on the mechanism of skeletal muscle atrophy and intervention at the molecular level has shown that inflammation and oxidative stress injury are the main mechanisms of skeletal muscle atrophy after spinal cord injury and that multiple pathways are involved. These may become targets of future clinical intervention. However, most of the experimental studies are still at the basic research stage and still have some limitations in clinical application, and most of the clinical treatments are focused on rehabilitation training, so how to develop more efficient interventions in clinical treatment still needs to be further explored. Therefore, this review focuses mainly on the mechanisms of skeletal muscle atrophy after spinal cord injury and summarizes the cytokines and signaling pathways associated with skeletal muscle atrophy in recent studies, hoping to provide new therapeutic ideas for future clinical work.

Paired pulse transcranial magnetic stimulation in the assessment of biceps voluntary activation in individuals with tetraplegia

After spinal cord injury (SCI), motoneuron death occurs at and around the level of injury which induces changes in function and organization throughout the nervous system, including cortical changes. Muscle affected by SCI may consist of both innervated (accessible to voluntary drive) and denervated (inaccessible to voluntary drive) muscle fibers. Voluntary activation measured with transcranial magnetic stimulation (VATMS) can quantify voluntary cortical/subcortical drive to muscle but is limited by technical challenges including suboptimal stimulation of target muscle relative to its antagonist. The motor evoked potential (MEP) in the biceps compared to the triceps (i.e., MEP ratio) may be a key parameter in the measurement of biceps VATMS after SCI. We used paired pulse TMS, which can inhibit or facilitate MEPs, to determine whether the MEP ratio affects VATMS in individuals with tetraplegia. Ten individuals with tetraplegia following cervical SCI and ten non-impaired individuals completed single pulse and paired pulse VATMS protocols. Paired pulse stimulation was delivered at 1.5, 10, and 30 ms inter-stimulus intervals (ISI). In both the SCI and non-impaired groups, the main effect of the stimulation pulse (paired pulse compared to single pulse) on VATMS was not significant in the linear mixed-effects models. In both groups for the stimulation parameters we tested, the MEP ratio was not modulated across all effort levels and did not affect VATMS. Linearity of the voluntary moment and superimposed twitch moment relation was lower in SCI participants compared to non-impaired. Poor linearity in the SCI group limits interpretation of VATMS. Future work is needed to address methodological issues that limit clinical application of VATMS.

Lower motoneuron dysfunction impacts spontaneous motor recovery in acute cervical spinal cord injury

Paresis after spinal cord injury is caused by damage to upper and lower motoneurons and may differentially impact neurological recovery. This prospective monocentric longitudinal observational study investigated the extent and severity of lower motoneuron dysfunction and its impact on upper extremity motor recovery after acute cervical spinal cord injury. Pathological spontaneous activity at rest and/or increased discharge rates of motor unit action potentials recorded by needle electromyography (EMG) were taken as parameters for lower motoneuron dysfunction and its relation to the extent of myelopathy in the first available spine MRI was determined. Motor recovery was assessed by standardized neurological examination within the first 4 weeks (acute stage) and up to 1 year (chronic stage) after injury. Eighty-five muscles of 17 individuals with cervical spinal cord injury (neurological level of injury from C1 to C7) and a median age of 54 (28-59) were examined. The results showed that muscles with signs of lower motoneuron dysfunction peaked at the lesion center (Χ²[2,n=85]=6.6, p=0.04) and that the severity of lower motoneuron dysfunction correlated with T2-weighted hyperintense MRI signal changes in routine spine MRI at the lesion site (spearman ρ=0.31, p=0.01). Muscles exhibiting signs of lower motoneuron dysfunction, as indicated by pathological spontaneous activity at rest and/or increased discharge rates of motor unit action potentials, were associated with more severe paresis in both the acute and chronic stages after spinal cord injury (spearman ρ acute=-0.22, p=0.04 and chronic=-0.31, p=0.004). Moreover, the severity of lower motoneuron dysfunction in the acute stage was also associated with a greater degree of paresis (spearman ρ acute=-0.24, p=0.03 and chronic=-0.35, p=0.001). While both muscles with and without signs of lower motoneuron dysfunction were capable of regaining strength over time, those without lower motoneuron dysfunctions had a higher potential to reach full strength. Muscles with signs of lower motoneuron dysfunction in the acute stage displayed increased amplitudes of motor unit action potentials with chronic-stage needle EMG, indicating reinnervation through peripheral collateral sprouting as compensatory mechanism (Χ²[1,n=72]=4.3, p=0.04). Thus, lower motoneuron dysfunction represents a relevant factor contributing to motor impairment and recovery in acute cervical spinal cord injury. Defined recovery mechanisms (peripheral reinnervation) may at least partially underlie spontaneous recovery in respective muscles. Therefore, assessment of lower motoneuron dysfunction could help refine prediction of motor recovery following spinal cord injury.

The assessment of biceps voluntary activation with transcranial magnetic stimulation in individuals with tetraplegia

BACKGROUND: Assessment of voluntary activation is useful in the study of neuromuscular impairments, particularly after spinal cord injury (SCI). Measurement of voluntary activation with transcranial magnetic stimulation (VATMS) is limited by technical challenges, including the difficulty in preferential stimulation of cortical neurons projecting to the target muscle and minimal stimulation of antagonists. Thus, the motor evoked potential (MEP) response to TMS in the target muscle compared to its antagonist may be an important parameter in the assessment of VATMS. OBJECTIVE: The purpose of this study was to evaluate the effect of isometric elbow flexion angle on two metrics in individuals with tetraplegia following SCI: 1) the ratio of biceps/triceps MEP amplitude across a range of voluntary efforts, and 2) VATMS. METHODS: Ten individuals with tetraplegia and ten nonimpaired individuals were recruited to participate in three sessions wherein VATMS was assessed at 45°, 90°, and 120° of isometric elbow flexion. RESULTS: In SCI participants, the biceps/triceps MEP ratio was not modulated by elbow angle. In nonimpaired participants, the biceps/triceps MEP ratio was greater in the more flexed elbow angle (120° flexion) compared to 90° during contractions of 50% and 75% MVC, but VATMS was not different. VATMS assessed in the more extended elbow angle (45° flexion) was lower relative to 90° elbow flexion; this effect was dependent on the biceps/triceps MEP ratio. In both groups, VATMS was sensitive to the linearity of the voluntary moment and superimposed twitch relationship, regardless of elbow angle. Linearity was lower in SCI relative to nonimpaired participants. CONCLUSIONS: Increasing the MEP ratio via elbow angle did not enable estimation of VATMS in SCI participants. VATMS may not be a viable approach to assess neuromuscular function in individuals with tetraplegia.

A Prediction Model for Various Treatment Pathways of Upper Extremity in Tetraplegia

Upper extremity function is essential for the autonomy in patients with cervical spinal cord injuries and consequently a focus of the rehabilitation and treatment efforts. Routinely, an individualized treatment plan is proposed to the patient by an interprofessional team. It dichotomizes into a conservative and a surgical treatment pathway. To select an optimal pathway, it is important to define predictors that substantiate the treatment strategy. Apart from standard assessments (Standards for Neurological Classification of Spinal Cord Injury (ISNCSCI), the manual muscle test (MRC), and lower motoneuron integrity of key actuators for hand function performed by motor point (MP) mapping might serve as a possible predictor. Type of damage (upper motor neuron (UMN) or lower motor neuron (LMN) lesion) influences hand posture and thus treatment strategy as positioning and splinting of fingers, hands, arms, and surgical reconstructive procedures (muscle-tendon or nerve transfers) in choice and timing of intervention. For this purpose, an analysis of a database comprising 220 patients with cervical spinal cord injury is used. It includes ISNCSCI, MRC, and MP mapping of defined muscles at selected time points after injury. The ordinal regression analysis performed indicates that MP and ASIA impairment scale (AIS) act as predictors of muscle strength acquisition. In accordance with the innervation status defined by MP, electrical stimulation (ES) is executed either via nerve or direct muscle stimulation as a supplementary therapy to the traditional occupational and physiotherapeutic treatment methods. Depending on the objective, ES is applied for motor learning, strengthening, or maintenance of muscle contractile properties. By employing ES, hand and arm function can be predicted by MP and AIS and used as the basis for providing an individualized treatment plan.

Investigation of neural and biomechanical impairments leading to pathological toe and heel gaits using neuromusculoskeletal modelling

KEY POINTS

Pathological toe and heel gaits are commonly present in various conditions such as spinal cord injury, stroke or cerebral palsy. These conditions present various neural and biomechanical impairments and the cause-effect relationships between these impairments and pathological gaits are hard to establish clinically. Based on neuromechanical simulation, this study focuses on the plantarflexor muscles and builds a new reflex circuit controller to model and evaluate the potential effect of both neural and biomechanical impairments on gait. Our results suggest an important contribution of active reflex mechanisms in pathological toe gait. This "what if" based on neuromechanical modelling is thus deemed of great interest to target potential pathological gait causes.

ABSTRACT

This study investigates the pathological toe and heel gaits in human locomotion using neuromusculoskeletal modelling and simulation. In particular, it aims at investigating potential cause-effect relationships between biomechanical or neural impairments and pathological gaits. Toe and heel gaits are commonly present in spinal cord injury, stroke or cerebral palsy. Toe walking is mainly attributed to spasticity and contracture at plantarflexor muscles, whereas heel walking can be attributed to muscle weakness from biomechanical or neural origin. To investigate the effect of these impairments on gait, this study focuses on the soleus and gastrocnemius muscles as they contribute to ankle plantarflexion. We built a reflex circuit model on top of Geyer and Herr's work (2010) with additional pathways affecting the plantarflexor muscles. The SCONE software, which provides optimisation tools for 2D neuromechanical simulation of human locomotion, is used to optimise the corresponding reflex parameters and simulate healthy gait. We then modelled various bilateral plantarflexors biomechanical and neural impairments, and individually introduced them in the healthy model. We characterised the resulting simulated gaits as pathological or not by comparing ankle kinematics and ankle moment with the healthy optimised gait based on metrics used in clinical studies. Our simulations suggest that toe walking can be generated by hyperreflexia, whereas muscle and neural weaknesses induce partially heel gait. Thus, this "what if" approach is deemed of great interest as it allows the investigation of the effect of various impairments on gait and suggests an important contribution of active reflex mechanisms in pathological toe gait. Abstract figure legend Various biomechanical and neural impairments are individually modelled at the level of the plantarflexor muscles in a musculoskeletal model and a complex reflex circuit-based gait controller. For instance, as shown on the left, the plantarflexors spindle reflex gain (KS) is increased to mimic hyperreflexia. The gait controller is then optimised for each of the impaired condition and the resulting gaits are characterised as pathological gait based on ankle kinematics and ankle moment metrics used in clinical studies. Thus, this "what if" approach allows the investigation of the effect of various impairments on gait presented in the table on the right. This article is protected by copyright. All rights reserved.

Increased intensity of unintended mirror muscle contractions after cervical spinal cord injury is associated with changes in interhemispheric and corticomuscular coherences

Mirror contractions refer to unintended contractions of the contralateral homologous muscles during voluntary unilateral contractions or movements. Exaggerated mirror contractions have been found in several neurological diseases and indicate dysfunction or lesion of the cortico-spinal pathway. The present study investigates mirror contractions and the associated interhemispheric and corticomuscular interactions in adults with spinal cord injury (SCI) - who present a lesion of the cortico-spinal tract - compared to able-bodied participants (AB). Eight right-handed adults with chronic cervical SCI and ten age-matched right-handed able-bodied volunteers performed sets of right elbow extensions at 20% of maximal voluntary contraction. Electromyographic activity (EMG) of the right and left elbow extensors, interhemispheric coherence over cerebral sensorimotor regions evaluated by electroencephalography (EEG) and corticomuscular coherence between signals over the cerebral sensorimotor regions and each extensor were quantified. Overall, results revealed that participants with SCI exhibited (1) increased EMG activity of both active and unintended active limbs, suggesting more mirror contractions, (2) reduced corticomuscular coherence between signals over the left sensorimotor region and the right active limb and increased corticomuscular coherence between the right sensorimotor region and the left unintended active limb, (3) decreased interhemispheric coherence between signals over the two sensorimotor regions. The increased corticomuscular communication and decreased interhemispheric communication may reflect a reduced inhibition leading to increased communication with the unintended active limb, possibly resulting to exacerbated mirror contractions in SCI. Finally, mirror contractions could represent changes of neural and neuromuscular communication after SCI.

Sensing and decoding the neural drive to paralyzed muscles during attempted movements of a person with tetraplegia using a sleeve array

Motor neurons convey information about motor intent that can be extracted and interpreted to control assistive devices. However, most methods for measuring the firing activity of single neurons rely on implanted microelectrodes. Although intracortical brain-computer interfaces (BCIs) have been shown to be safe and effective, the requirement for surgery poses a barrier to widespread use that can be mitigated by instead using noninvasive interfaces. The objective of this study was to evaluate the feasibility of deriving motor control signals from a wearable sensor that can detect residual motor unit activity in paralyzed muscles after chronic cervical spinal cord injury (SCI). Despite generating no observable hand movement, volitional recruitment of motor units below the level of injury was observed across attempted movements of individual fingers and overt wrist and elbow movements. Subgroups of motor units were coactive during flexion or extension phases of the task. Single digit movement intentions were classified offline from the EMG power (RMS) or motor unit firing rates with median classification accuracies >75% in both cases. Simulated online control of a virtual hand was performed with a binary classifier to test feasibility of real-time extraction and decoding of motor units. The online decomposition algorithm extracted motor units in 1.2 ms, and the firing rates predicted the correct digit motion 88 ± 24% of the time. This study provides the first demonstration of a wearable interface for recording and decoding firing rates of motor units below the level of injury in a person with motor complete SCI.

Maturation of persistent and hyperpolarization-activated inward currents shapes the differential activation of motoneuron subtypes during postnatal development

The size principle underlies the orderly recruitment of motor units; however, motoneuron size is a poor predictor of recruitment amongst functionally defined motoneuron subtypes. Whilst intrinsic properties are key regulators of motoneuron recruitment, the underlying currents involved are not well defined. Whole-cell patch-clamp electrophysiology was deployed to study intrinsic properties, and the underlying currents, that contribute to the differential activation of delayed and immediate firing motoneuron subtypes. Motoneurons were studied during the first three postnatal weeks in mice to identify key properties that contribute to rheobase and may be important to establish orderly recruitment. We find that delayed and immediate firing motoneurons are functionally homogeneous during the first postnatal week and are activated based on size, irrespective of subtype. The rheobase of motoneuron subtypes becomes staggered during the second postnatal week, which coincides with the differential maturation of passive and active properties, particularly persistent inward currents. Rheobase of delayed firing motoneurons increases further in the third postnatal week due to the development of a prominent resting hyperpolarization-activated inward current. Our results suggest that motoneuron recruitment is multifactorial, with recruitment order established during postnatal development through the differential maturation of passive properties and sequential integration of persistent and hyperpolarization-activated inward currents.

Increased Ipsilateral M1 Activation after Incomplete Spinal Cord Injury Facilitates Motor Performance

Incomplete spinal cord injury (SCI) may result in muscle weakness and difficulties with force gradation. Although these impairments arise from the injury and subsequent changes at spinal levels, changes have also been demonstrated in the brain. Blood-oxygen-level dependent (BOLD) imaging was used to investigate these changes in brain activation in the context of unimanual contractions with the first dorsal interosseous muscle. BOLD- and force data were obtained in 19 individuals with SCI (AISA Impairment Scale [AIS] C/D, level C4-C8) and 24 able-bodied controls during maximal voluntary contractions (MVCs). To assess force modulation, participants performed 12 submaximal contractions with each hand (at 10, 30, 50, and 70% MVC) by matching their force level to a visual target. MVCs were weaker in the SCI group (both hands p < 0.001), but BOLD activation did not differ between SCI and control groups. For the submaximal contractions, force (as %MVC) was similar across groups. However, SCI participants showed increased activity of the ipsilateral motor cortex and contralateral cerebellum across all contractions, with no differential effect of force level. Activity of ipsilateral M1 was best explained by force of the target hand (vs. the non-target hand). In conclusion, the data suggest that after incomplete cervical SCI, individuals remain capable of producing maximal supraspinal drive and are able to modulate this drive adequately. Activity of the ipsilateral motor network appears to be task related, although it remains uncertain how this activity contributes to task performance and whether this effect could potentially be harnessed to improve motor functioning.

Respiratory axon regeneration in the chronically injured spinal cord

Promoting the combination of robust regeneration of damaged axons and synaptic reconnection of these growing axon populations with appropriate neuronal targets represents a major therapeutic goal following spinal cord injury (SCI). A key impediment to achieving this important aim includes an intrinsic inability of neurons to extend axons in adult CNS, particularly in the context of the chronically-injured spinal cord. We tested whether an inhibitory peptide directed against phosphatase and tensin homolog (PTEN: a central inhibitor of neuron-intrinsic axon growth potential) could restore inspiratory diaphragm function by reconnecting critical respiratory neural circuitry in a rat model of chronic cervical level 2 (C2) hemisection SCI. We found that systemic delivery of PTEN antagonist peptide 4 (PAP4) starting at 8 weeks after C2 hemisection promoted substantial, long-distance regeneration of injured bulbospinal rostral Ventral Respiratory Group (rVRG) axons into and through the lesion and back toward phrenic motor neurons (PhMNs) located in intact caudal C3-C5 spinal cord. Despite this robust rVRG axon regeneration, PAP4 stimulated only minimal recovery of diaphragm function. Furthermore, re-lesion through the hemisection site completely removed PAP4-induced functional improvement, demonstrating that axon regeneration through the lesion was responsible for this partial functional recovery. Interestingly, there was minimal formation of putative excitatory monosynaptic connections between regrowing rVRG axons and PhMN targets, suggesting that (1) limited rVRG-PhMN synaptic reconnectivity was responsible at least in part for the lack of a significant functional effect, (2) chronically-injured spinal cord presents an obstacle to achieving synaptogenesis between regenerating axons and post-synaptic targets, and (3) addressing this challenge is a potentially-powerful strategy to enhance therapeutic efficacy in the chronic SCI setting. In conclusion, our study demonstrates a non-invasive and transient pharmacological approach in chronic SCI to repair the critically-important neural circuitry controlling diaphragmatic respiratory function, but also sheds light on obstacles to circuit plasticity presented by the chronically-injured spinal cord.

Assessing redistribution of muscle innervation zones after spinal cord injuries

This study aimed to examine the redistribution of neuromuscular junctions or innervation zones (IZs) after spinal cord injuries (SCI). Fifteen able-bodied subjects and 15 subjects with SCI (American Spinal Injury Association Impairment Scale A to D), participated in the study. Surface electromyography (EMG) signals were collected from the biceps brachii muscle by a customized linear electrode array when subjects generated maximal isometric voluntary contractions. The Radon transform was applied to detect the IZ locations in the multiple channel surface EMG signals which were differentiated between consecutive channels. The distribution of IZs was compared between the SCI and control groups using the student-t test. Statistical analysis disclosed a significantly wider range of IZs in the SCI group compared with the control group (SCI: 3.83 ± 1.32 IED, control: 2.83 ± 0. 87 IED, IED: inter-electrode distance, p < 0.05). No remarkable shifts of the center of the distribution were observed between the two groups (SCI: 9.23 ± 2.35 IED, control: 8.53 ± 2.33 IED, p = 0.42). Changes of IZ distribution in the paralyzed muscles could be associated with the complex neuromuscular reorganization after the SCI.

Neuromuscular efficiency of men with high and low spinal cord injury levels compared with non-disabled participants

BACKGROUND: The neuromuscular efficiency index (NME) is defined as the individual ability to generate force in relation to the muscle activation level and might be useful to the assessment of individuals with spinal cord injury (SCI) and might elucidate the modifications in strength after an SCI compared to non-disabled subjects (CG). OBJECTIVE: Verify if the NME of fully and partially preserved muscles discriminate men with low and high levels of SCI and a matched non-disabled CG. METHODS: Fifty-four men with SCI were stratified into the high (HP), and low (LP) paraplegia groups and twenty-seven non-disabled individuals were selected (CG). All subjects performed maximum strength tests in the isokinetic dynamometer for shoulder abduction/adduction (isokinetic) and trunk flexion/extension (isometric). Surface electromyography was measured to calculate the NME, and discriminant analysis was carried out to identify which NME variables would be able to discriminate HP, LP, and CG. RESULTS: There were no NME significant differences between groups for the primary muscles of the shoulder abduction/adduction. All NME data failed at discriminant tolerance test to compare HP from LP. The latissimus dorsi NME during trunk extension discriminated CG from HP and LP. CONCLUSIONS: The latissimus dorsi NME during trunk extension might be used as an assessment tool to compare SCI individuals and the non-disabled-matched controls. The authors recommend using the NME index for the analysis or comparisons between the same SCI levels.

Lower Motor Neuron Abnormality in Chronic Cervical Spinal Cord Injury: Implications for Nerve Transfer Surgery

Nerve transfer surgery (NT) constitutes an exciting option to improve upper limb functions in chronic spinal cord injury (SCI), but requires intact sublesional lower motor neuron (LMN) health. The purpose of this study was to characterize patterns of LMN abnormality in nerve-muscle groups that are the potential recipients of NT, using a standardized electrodiagnostic examination, in individuals with chronic SCI (injury duration >2 years, injury levels C4-T1). The LMN abnormality was determined using a semihierarchical approach, combining the amplitude compound muscle action potential (CMAP) and abnormal spontaneous activity on needle electromyography (EMG). Ten participants (46 potential recipient muscles) were included (median age, 42.5 years; six males and four females; median duration from injury, 15.5 years). A high frequency of LMN abnormality was observed (87%), although there was substantial variation within and between individuals. No statistically significant discordance was observed between LMN abnormality on CMAP and EMG (p = 0.24), however, 50% of muscles with normal CMAP demonstrated abnormal spontaneous activity. The high frequency of LMN abnormality in recipient nerve-muscle groups has implications to candidate selection for NT surgery in chronic SCI and supports the important role of the pre-operative electrodiagnostic examination. Our results further support the inclusion of both CMAP and needle EMG parameters for characterization of LMN health. Although the number of nerve-muscle groups with normal LMN health was small (13%), this underscores the neurophysiological potential of some patients with chronic injuries to benefit from NT surgery.

Spatial filtering for enhanced high-density surface electromyographic examination of neuromuscular changes and its application to spinal cord injury

Background

Spatial filtering of multi-channel signals is considered to be an effective pre-processing approach for improving signal-to-noise ratio. The use of spatial filtering for preprocessing high-density (HD) surface electromyogram (sEMG) helps to extract critical spatial information, but its application to non-invasive examination of neuromuscular changes have not been well investigated.

Methods

Aimed at evaluating how spatial filtering can facilitate examination of muscle paralysis, three different spatial filtering methods are presented using principle component analysis (PCA) algorithm, non-negative matrix factorization (NMF) algorithm, and both combination, respectively. Their performance was evaluated in terms of diagnostic power, through HD-sEMG clustering index (CI) analysis of neuromuscular changes in paralyzed muscles following spinal cord injury (SCI).

Results

The experimental results showed that: (1) The CI analysis of conventional single-channel sEMG can reveal complex neuromuscular changes in paralyzed muscles following SCI, and its diagnostic power has been confirmed to be characterized by the variance of Z scores; (2) the diagnostic power was highly dependent on the location of sEMG recording channel. Directly averaging the CI diagnostic indicators over channels just reached a medium level of the diagnostic power; (3) the use of either PCA-based or NMF-based filtering method yielded a greater diagnostic power, and their combination could even enhance the diagnostic power significantly.

Conclusions

This study not only presents an essential preprocessing approach for improving diagnostic power of HD-sEMG, but also helps to develop a standard sEMG preprocessing pipeline, thus promoting its widespread application.

Interfacing With Alpha Motor Neurons in Spinal Cord Injury Patients Receiving Trans-spinal Electrical Stimulation

Trans-spinal direct current stimulation (tsDCS) provides a non-invasive, clinically viable approach to potentially restore physiological neuromuscular function after neurological impairment, e.g., spinal cord injury (SCI). Use of tsDCS has been hampered by the inability of delivering stimulation patterns based on the activity of neural targets responsible to motor function, i.e., α-motor neurons (α-MNs). State of the art modeling and experimental techniques do not provide information about how individual α-MNs respond to electrical fields. This is a major element hindering the development of neuro-modulative technologies highly tailored to an individual patient. For the first time, we propose the use of a signal-based approach to infer tsDCS effects on large α-MNs pools in four incomplete SCI individuals. We employ leg muscles spatial sampling and deconvolution of high-density fiber electrical activity to decode accurate α-MNs discharges across multiple lumbosacral segments during isometric plantar flexion sub-maximal contractions. This is done before, immediately after and 30 min after sub-threshold cathodal stimulation. We deliver sham tsDCS as a control measure. First, we propose a new algorithm for removing compromised information from decomposed α-MNs spike trains, thereby enabling robust decomposition and frequency-domain analysis. Second, we propose the analysis of α-MNs spike trains coherence (i.e., frequency-domain) as an indicator of spinal response to tsDCS. Results showed that α-MNs spike trains coherence analysis sensibly varied across stimulation phases. Coherence analyses results suggested that the common synaptic input to α-MNs pools decreased immediately after cathodal tsDCS with a persistent effect after 30 min. Our proposed non-invasive decoding of individual α-MNs behavior may open up new avenues for the design of real-time closed-loop control applications including both transcutaneous and epidural spinal electrical stimulation where stimulation parameters are adjusted on-the-fly.

Calpain fosters the hyperexcitability of motoneurons after spinal cord injury and leads to spasticity

Up-regulation of the persistent sodium current (INaP) and down-regulation of the potassium/chloride extruder KCC2 lead to spasticity after spinal cord injury (SCI). We here identified calpain as the driver of the up- and down-regulation of INaP and KCC2, respectively, in neonatal rat lumbar motoneurons. Few days after SCI, neonatal rats developed behavioral signs of spasticity with the emergence of both hyperreflexia and abnormal involuntary muscle contractions on hindlimbs. At the same time, in vitro isolated lumbar spinal cords became hyperreflexive and displayed numerous spontaneous motor outputs. Calpain-I expression paralleled with a proteolysis of voltage-gated sodium (Nav) channels and KCC2. Acute inhibition of calpains reduced this proteolysis, restored the motoneuronal expression of Nav and KCC2, normalized INaP and KCC2 function, and curtailed spasticity. In sum, by up- and down-regulating INaP and KCC2, the calpain-mediated proteolysis of Nav and KCC2 drives the hyperexcitability of motoneurons which leads to spasticity after SCI.

Assessing Rat Forelimb and Hindlimb Motor Unit Connectivity as Objective and Robust Biomarkers of Spinal Motor Neuron Function

Sensitive and objective biomarkers of neuronal injury, degeneration, and regeneration can help facilitate translation of experimental findings into clinical testing. Whereas measures of upper motor neuron connectivity have been readily established, functional assessments of lower motor neuron (LMN) innervation of forelimb muscles are lacking. Compound muscle action potential (CMAP) and motor unit (MU) number estimation (MUNE) are well-established methods that allow longitudinal MU integrity monitoring in patients. In analogy we refined CMAP and MUNE methods for assessing spinal MU input in the rat forelimb and hindlimb. Repeated CMAP and MUNE recordings are robust (coefficients of variability: 4.5-11.3%), and MUNE measurements from forelimb wrist flexor muscles (415 ± 8 [SEM]) align with back-traced anatomical LMN counts (336 ± 16 [SEM]). For disease validation, cross-sectional blinded electrophysiological and muscle contractility measurements were obtained in a cohort of G93A SOD1 mutant overexpressing rats and compared with controls. Longitudinal assessment of mutant animals demonstrated progressive motor unit decline in the hindlimb to a greater extent than the forelimb. Hindlimb CMAP and MUNE demonstrated strong correlations with plantarflexion muscle contractility. Cross-species assessment of upper/fore- limb and lower/hind- limb motor units using objective electrophysiological CMAP and MUNE values as biomarkers will guide and improve bi-directional translation.

Spasms after spinal cord injury show low-frequency intermuscular coherence

Intermuscular coherence allows the investigation of common input to muscle groups. Although beta-band (15–30 Hz) intermuscular coherence is well understood as originating from the cortex, the source of intermuscular coherence at lower frequencies is still unclear. We used a wearable device that recorded electromyographic (EMG) signals during a 24-h period in four lower limb muscles of seven spinal cord injury patients (American Spinal Cord Injury Association impairment scale: A, 6 subjects; B, 1 subject) while they went about their normal daily life activities. We detected natural spasms occurring during these long-lasting recordings and calculated intermuscular coherence between all six possible combinations of muscle pairs. There was significant intermuscular coherence at low frequencies, between 2 and 13 Hz. The most likely source for this was the spinal cord and its peripheral feedback loops, because the spinal lesions in these patients had interrupted connections to supraspinal structures. This is the first report to demonstrate that the spinal cord is capable of producing low-frequency intermuscular coherence with severely reduced or abolished descending drive.

NEW & NOTEWORTHY This is the first report to demonstrate that intermuscular coherence between lower limb muscles at low frequencies can be produced by the spinal cord with severely reduced or abolished descending drive.

Activity-Based Physical Rehabilitation with Adjuvant Testosterone to Promote Neuromuscular Recovery after Spinal Cord Injury

Neuromuscular impairment and reduced musculoskeletal integrity are hallmarks of spinal cord injury (SCI) that hinder locomotor recovery. These impairments are precipitated by the neurological insult and resulting disuse, which has stimulated interest in activity-based physical rehabilitation therapies (ABTs) that promote neuromuscular plasticity after SCI. However, ABT efficacy declines as SCI severity increases. Additionally, many men with SCI exhibit low testosterone, which may exacerbate neuromusculoskeletal impairment. Incorporating testosterone adjuvant to ABTs may improve musculoskeletal recovery and neuroplasticity because androgens attenuate muscle loss and the slow-to-fast muscle fiber-type transition after SCI, in a manner independent from mechanical strain, and promote motoneuron survival. These neuromusculoskeletal benefits are promising, although testosterone alone produces only limited functional improvement in rodent SCI models. In this review, we discuss the (1) molecular deficits underlying muscle loss after SCI; (2) independent influences of testosterone and locomotor training on neuromuscular function and musculoskeletal integrity post-SCI; (3) hormonal and molecular mechanisms underlying the therapeutic efficacy of these strategies; and (4) evidence supporting a multimodal strategy involving ABT with adjuvant testosterone, as a potential means to promote more comprehensive neuromusculoskeletal recovery than either strategy alone.

Strategies to augment volitional and reflex function may improve locomotor capacity following incomplete spinal cord injury

Many studies highlight the remarkable plasticity demonstrated by spinal circuits following an incomplete spinal cord injury (SCI). Such plasticity can contribute to improvements in volitional motor recovery, such as walking function, although similar mechanisms underlying this recovery may also contribute to the manifestation of exaggerated responses to afferent input, or spastic behaviors. Rehabilitation interventions directed toward augmenting spinal excitability have shown some initial success in improving locomotor function. However, the potential effects of these strategies on involuntary motor behaviors may be of concern. In this article, we provide a brief review of the mechanisms underlying recovery of volitional function and exaggerated reflexes, and the potential overlap between these changes. We then highlight findings from studies that explore changes in spinal excitability during volitional movement in controlled conditions, as well as altered kinematic and behavioral performance during functional tasks. The initial focus will be directed toward recovery of reflex and volitional behaviors following incomplete SCI, followed by recent work elucidating neurophysiological mechanisms underlying patterns of static and dynamic muscle activation following chronic incomplete SCI during primarily single-joint movements. We will then transition to studies of locomotor function and the role of altered spinal integration following incomplete SCI, including enhanced excitability of specific spinal circuits with physical and pharmacological interventions that can modulate locomotor output. The effects of previous and newly developed strategies will need to focus on changes in both volitional function and involuntary spastic reflexes for the successful translation of effective therapies to the clinical setting.

Constraints on Stance-Phase Force Production during Overground Walking in Persons with Chronic Incomplete Spinal Cord Injury

Persons with incomplete spinal cord injury (iSCI) face ongoing struggles with walking, including reduced speed and increased reliance on assistive devices (ADs). The forces underlying body weight support and gait, as measured by ground reaction forces (GRFs), are likely altered after iSCI because of weakness and AD dependence but have not been studied. The purpose of this study was to examine GRF production during overground walking after iSCI, because greater insight into GRF constraints is important for refining therapeutic interventions. Because of reduced and discoordinated motor output after iSCI, we hypothesized that persons with iSCI would exert smaller GRFs and altered GRF modifications to increased cadence compared with able-bodied (AB) persons, especially when using an AD. Fifteen persons with chronic iSCI, stratified into no AD (n = 7) and AD (n = 8) groups, walked across an instrumented walkway at self-selected and fast (115% self-selected) cadences. Fifteen age-matched AB controls walked at their own cadences and iSCI-matched conditions (cadence and AD). Results showed fore-aft GRFs are reduced in persons with iSCI compared with AB controls, with reductions greatest in persons dependent on an AD. When controlling for cadence and AD, propulsive forces were still lower in persons with iSCI. Compared with AB controls, persons with iSCI demonstrated altered GRF modifications to increased cadence. Persons with iSCI exhibit different stance-phase forces compared with AB controls, which are impacted further by AD use and slower walking speed. Minimizing AD use and/or providing propulsive biofeedback during walking could enhance GRF production after iSCI.

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

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

Real-Time Control of an Exoskeleton Hand Robot with Myoelectric Pattern Recognition

Robot-assisted training provides an effective approach to neurological injury rehabilitation. To meet the challenge of hand rehabilitation after neurological injuries, this study presents an advanced myoelectric pattern recognition scheme for real-time intention-driven control of a hand exoskeleton. The developed scheme detects and recognizes user’s intention of six different hand motions using four channels of surface electromyography (EMG) signals acquired from the forearm and hand muscles, and then drives the exoskeleton to assist the user accomplish the intended motion. The system was tested with eight neurologically intact subjects and two individuals with spinal cord injury (SCI). The overall control accuracy was [Formula: see text] for the neurologically intact subjects and [Formula: see text] for the SCI subjects. The total lag of the system was approximately 250[Formula: see text]ms including data acquisition, transmission and processing. One SCI subject also participated in training sessions in his second and third visits. Both the control accuracy and efficiency tended to improve. These results show great potential for applying the advanced myoelectric pattern recognition control of the wearable robotic hand system toward improving hand function after neurological injuries.

Increases in human motoneuron excitability after cervical spinal cord injury depend on the level of injury

After human spinal cord injury (SCI), motoneuron recruitment and firing rate during voluntary and involuntary contractions may be altered by changes in motoneuron excitability. Our aim was to compare F waves in single thenar motor units paralyzed by cervical SCI to those in uninjured controls because at the single-unit level F waves primarily reflect the intrinsic properties of the motoneuron and its initial segment. With intraneural motor axon stimulation, F waves were evident in all 4 participants with C₄-level SCI, absent in 8 with C₅ or C₆ injury, and present in 6 of 12 Uninjured participants (P < 0.001). The percentage of units that generated F waves differed across groups (C₄: 30%, C₅ or C₆: 0%, Uninjured: 16%; P < 0.001). Mean (±SD) proximal axon conduction velocity was slower after C₄ SCI [64 ± 4 m/s (n = 6 units), Uninjured: 73 ± 8 m/s (n = 7 units); P = 0.037]. Mean distal axon conduction velocity differed by group [C₄: 40 ± 8 m/s (n = 20 units), C₅ or C₆: 49 ± 9 m/s (n = 28), Uninjured: 60 ± 7 m/s (n = 45); P < 0.001]. Motor unit properties (EMG amplitude, twitch force) only differed after SCI (P ≤ 0.004), not by injury level. Motor units with F waves had distal conduction velocities, M-wave amplitudes, and twitch forces that spanned the respective group range, indicating that units with heterogeneous properties produced F waves. Recording unitary F waves has shown that thenar motoneurons closer to the SCI (C₅ or C₆) have reduced excitability whereas those further away (C₄) have increased excitability, which may exacerbate muscle spasms. This difference in motoneuron excitability may be related to the extent of membrane depolarization following SCI.

NEW & NOTEWORTHY

Unitary F waves were common in paralyzed thenar muscles of people who had a chronic spinal cord injury (SCI) at the C₄ level compared with uninjured people, but F waves did not occur in people that had SCI at the C₅ or C₆ level. These results highlight that intrinsic motoneuron excitability depends, in part, on how close the motoneurons are to the site of the spinal injury, which could alter the generation and strength of voluntary and involuntary muscle contractions.

Taking a bite out of spinal cord injury: do dental stem cells have the teeth for it?

Dental stem cells are an emerging star on a stage that is already quite populated. Recently, there has been a lot of hype concerning these cells in dental therapies, especially in regenerative endodontics. It is fitting that most research is concentrated on dental regeneration, although other uses for these cells need to be explored in more detail. Being a true mesenchymal stem cell, their capacities could also prove beneficial in areas outside their natural environment. One such field is the central nervous system, and in particular, repairing the injured spinal cord. One of the most formidable challenges in regenerative medicine is to restore function to the injured spinal cord, and as yet, a cure for paralysis remains to be discovered. A variety of approaches have already been tested, with graft-based strategies utilising cells harbouring appropriate properties for neural regeneration showing encouraging results. Here we present a review focusing on properties of dental stem cells that endorse their use in regenerative medicine, with particular emphasis on repairing the damaged spinal cord.

Reduced voluntary drive during sustained but not during brief maximal voluntary contractions in the first dorsal interosseous weakened by spinal cord injury

In able-bodied (AB) individuals, voluntary muscle activation progressively declines during sustained contractions. However, few data are available on voluntary muscle activation during sustained contractions in muscles weakened by spinal cord injury (SCI), where greater force declines may limit task performance. SCI-related impairment of muscle activation complicates interpretation of the interpolated twitch technique commonly used to assess muscle activation. We attempted to estimate and correct for the SCI-related-superimposed twitch. Seventeen participants, both AB and with SCI (American Spinal Injury Association Impairment Scale C/D) produced brief and sustained (2-min) maximal voluntary contractions (MVCs) with the first dorsal interosseous. Force and electromyography were recorded together with superimposed (doublet) twitches. MVCs of participants with SCI were weaker than those of AB participants (20.3 N, SD 7.1 vs. 37.9 N, SD 9.5; P < 0.001); MVC-superimposed twitches were larger in participants with SCI (SCI median 10.1%, range 2.0-63.2%; AB median 4.7%, range 0.0-18.4% rest twitch; P = 0.007). No difference was found after correction for the SCI-related-superimposed twitch (median 6.7%, 0.0-17.5% rest twitch, P = 0.402). Thus during brief contractions, the maximal corticofugal output that participants with SCI could exert was similar to that of AB participants. During the sustained contraction, force decline (SCI, 58.0%, SD 15.1; AB, 57.2% SD 13.3) was similar (P = 0.887) because participants with SCI developed less peripheral (P = 0.048) but more central fatigue than AB participants. The largest change occurred at the start of the sustained contraction when the (corrected) superimposed twitches increased more in participants with SCI (SCI, 16.3% rest twitch, SD 20.8; AB, 2.7%, SD 4.7; P = 0.01). The greater reduction in muscle activation after SCI may relate to a reduced capacity to overcome fast fatigue-related excitability changes at the spinal level.

Alterations in multidimensional motor unit number index of hand muscles after incomplete cervical spinal cord injury

The objective of this study was to apply a novel multidimensional motor unit number index (MD-MUNIX) technique to examine hand muscles in patients with incomplete cervical spinal cord injury (SCI). The MD-MUNIX was estimated from the compound muscle action potential (CMAP) and different levels of surface interference pattern electromyogram (EMG) at multiple directions of voluntary isometric muscle contraction. The MD-MUNIX was applied in the first dorsal interosseous (FDI), thenar and hypothenar muscles of SCI (n = 12) and healthy control (n = 12) subjects. The results showed that the SCI subjects had significantly smaller CMAP and MD-MUNIX in all the three examined muscles, compared to those derived from the healthy control subjects. The multidimensional motor unit size index (MD-MUSIX) demonstrated significantly larger values for the FDI and hypothenar muscles in SCI subjects than those from healthy control subjects, whereas the MD-MUSIX enlargement was marginally significant for the thenar muscles. The findings from the MD-MUNIX analyses provide an evidence of motor unit loss in hand muscles of cervical SCI patients, contributing to hand function deterioration.

Impaired Organization of Paired-Pulse TMS-Induced I-Waves After Human Spinal Cord Injury

Paired-pulse transcranial magnetic stimulation (TMS) of the human motor cortex results in consecutive facilitatory motor-evoked potential (MEP) peaks in surface electromyography in intact humans. Here, we tested the effect of an incomplete cervical spinal cord injury (SCI) on early (first) and late (second and third) MEP peaks in a resting intrinsic finger muscle. We found that all peaks had decreased amplitude in SCI subjects compared with controls. The second and third peaks were delayed with the third peak also showing an increased duration. The delay of the third peak was smaller than that seen in controls at lower stimulation intensity, suggesting lesser influence of decreased corticospinal inputs. A mathematical model showed that after SCI the third peak aberrantly contributed to spinal motoneurone recruitment, regardless on the motor unit threshold tested. Temporal and spatial aspects of the late peaks correlated with MEP size and hand motor output. Thus, early and late TMS-induced MEP peaks undergo distinct modulation after SCI, with the third peak likely reflecting a decreased ability to summate descending volleys at the spinal level. We argue that the later corticospinal inputs on the spinal cord might be crucial for recruitment of motoneurones after human SCI.

Motor unit firing rates during spasms in thenar muscles of spinal cord injured subjects

Involuntary contractions of paralyzed muscles (spasms) commonly disrupt daily activities and rehabilitation after human spinal cord injury (SCI). Our aim was to examine the recruitment, firing rate modulation, and derecruitment of motor units that underlie spasms of thenar muscles after cervical SCI. Intramuscular electromyographic activity (EMG), surface EMG, and force were recorded during thenar muscle spasms that occurred spontaneously or that were triggered by movement of a shoulder or leg. Most spasms were submaximal (mean: 39%, SD: 33 of the force evoked by median nerve stimulation at 50 Hz) with strong relationships between EMG and force (R (2) > 0.69). Unit recruitment occurred over a wide force range (0.2-103% of 50 Hz force). Significant unit rate modulation occurred during spasms (frequency at 25% maximal force: 8.8 Hz, 3.3 SD; at maximal force: 16.1 Hz, 4.1 SD). Mean recruitment frequency (7.1 Hz, 3.2 SD) was significantly higher than derecruitment frequency (5.4 Hz, 2.4 SD). Coactive unit pairs that fired for more than 4 s showed high (R (2) > 0.7, n = 4) or low (R (2):0.3-0.7, n = 12) rate-rate correlations, and derecruitment reversals (21 pairs, 29%). Later recruited units had higher or lower maximal firing rates than lower threshold units. These discrepant data show that coactive motoneurons are drive both by common inputs and by synaptic inputs from different sources during muscle spasms. Further, thenar motoneurons can still fire at high rates in response to various peripheral inputs after SCI, supporting the idea that low maximal voluntary firing rates and forces in thenar muscles result from reduced descending drive.

Contributions to muscle force and EMG by combined neural excitation and electrical stimulation

OBJECTIVE

Stimulation of muscle for research or clinical interventions is often superimposed on ongoing physiological activity without a quantitative understanding of the impact of the stimulation on the net muscle activity and the physiological response. Experimental studies show that total force during stimulation is less than the sum of the isolated voluntary and stimulated forces, but the occlusion mechanism is not understood.

APPROACH

We develop a model of efferent motor activity elicited by superimposing stimulation during a physiologically activated contraction. The model combines action potential interactions due to collision block, source resetting, and refractory periods with previously published models of physiological motor unit recruitment, rate modulation, force production, and EMG generation in human first dorsal interosseous muscle to investigate the mechanisms and effectiveness of stimulation on the net muscle force and EMG.

MAIN RESULTS

Stimulation during a physiological contraction demonstrates partial occlusion of force and the neural component of the EMG, due to action potential interactions in motor units activated by both sources. Depending on neural and stimulation firing rates as well as on force-frequency properties, individual motor unit forces can be greater, smaller, or unchanged by the stimulation. In contrast, voluntary motor unit EMG potentials in simultaneously stimulated motor units show progressive occlusion with increasing stimulus rate. The simulations predict that occlusion would be decreased by a reverse stimulation recruitment order.

SIGNIFICANCE

The results are consistent with and provide a mechanistic interpretation of previously published experimental evidence of force occlusion. The models also predict two effects that have not been reported previously--voluntary EMG occlusion and the advantages of a proximal stimulation site. This study provides a basis for the rational design of both future experiments and clinical neuroprosthetic interventions involving either motor or sensory stimulation.