Temporal facilitation of spastic stretch reflexes following human spinal cord injury

Preservation of functional descending input to paralyzed upper extremity muscles in motor complete cervical spinal cord injury

OBJECTIVE

Spinal cord injury (SCI) is classified as complete or incomplete depending on the extent of sensorimotor preservation below the injury level. However, individuals with complete SCIs can voluntarily activate paralyzed lower limb muscles alone or by engaging non-paralyzed muscles during neurophysiological assessments, indicating presence of residual pathways across the injury. However, similar phenomena have not been explored for the upper extremity (UE) muscles following cervical SCIs.

METHODS

Eighteen individuals with motor complete cervical SCI (AIS A or B) and five age-matched non-injured (NI) individuals performed various UE events against manual resistance during functional neurophysiological assessment (FNPA), and electromyographic (EMG) activity was recorded from UE muscles.

RESULTS

Our findings demonstrated i) voluntary activation of clinically paralyzed muscles as evident from EMG readouts, ii) increased activity in these muscles during events engaging muscles above the injury level, iii) reduced spectral properties of paralyzed muscles in SCI compared to NI participants.

CONCLUSIONS

Functional EMG activity in clinically paralyzed muscles indicate presence of residual pathways across the injury establishing supralesional control over the sublesional neural circuitry.

SIGNIFICANCE

The findings may help explain the neurophysiological basis for UE recovery and can be exploited in designing rehabilitation techniques to facilitate UE recovery following cervical SCIs.

Altered cutaneous reflexes to non-noxious stimuli in the triceps surae of people with chronic incomplete spinal cord injury

Following spinal cord injury (SCI) task-dependent modulation of spinal reflexes are often impaired. To gain insight into the state of the spinal interneuronal pathways following injury, we studied the amplitude modulation of triceps surae cutaneous reflexes to non-noxious stimuli during standing and early-to-mid stance phase of walking in participants with and without chronic incomplete SCI. Reflex eliciting nerve stimulation was delivered to the superficial peroneal, sural, and distal tibial nerves about the ankle. Reflexes were analyzed in the short (SLR, 50-80 ms post stimulation onset) and the medium (MLR, 80-120 ms) latency response windows. Further, the relation between cutaneous and H-reflexes was also examined during standing. In participants without injuries the soleus SLR was modulated task-dependently with nerve specificity, and the soleus and medial gastrocnemius MLRs were modulated task-dependently. In contrast, participants with SCI, no task-dependent or nerve-specific modulation of triceps cutaneous reflexes was observed. The triceps surae cutaneous and H-reflexes were not correlated in either group (r = 0.01-0.37). The presence of cutaneous reflexes but the absence of significant amplitude modulation may suggest impaired function of spinal interneuronal pathways in this population. The lack of correlation between the cutaneous and H-reflexes may suggest that interneurons that are involved in H-reflex modulation and cutaneous reflex modulation do not receive common input, or the impact of the common input is outweighed by other input. Present findings highlight the importance of examining multiple spinal reflexes to better understanding spinal interneuronal pathways that affect motor control in people after SCI.

Surface EMG in Subacute and Chronic Care after Traumatic Spinal Cord Injuries

Background: Traumatic spinal cord injury (SCI) is a devastating condition commonly originating from motor vehicle accidents or falls. Trauma care after SCI is challenging; after decompression surgery and spine stabilization, the first step is to assess the location and severity of the traumatic lesion. For this, clinical outcome measures are used to quantify the residual sensation and volitional control of muscles below the level of injury. These clinical assessments are important for decision-making, including the prediction of the recovery potential of individuals after the SCI. In clinical care, this quantification is usually performed using sensation and motor scores, a semi-quantitative measurement, alongside the binary classification of the sacral sparing (yes/no). Objective: In this perspective article, I review the use of surface EMG (sEMG) as a quantitative outcome measurement in subacute and chronic trauma care after SCI. Methods: Here, I revisit the main findings of two comprehensive scoping reviews recently published by our team on this topic. I offer a perspective on the combined findings of these scoping reviews, which integrate the changes in sEMG with SCI and the use of sEMG in neurorehabilitation after SCI. Results: sEMG provides a complimentary assessment to quantify the residual control of muscles with great sensitivity and detail compared to the traditional clinical assessments. Our scoping reviews unveiled the ability of the sEMG assessment to detect discomplete lesions (muscles with absent motor scores but present sEMG). Moreover, sEMG is able to measure the spontaneous activity of motor units at rest, and during passive maneuvers, the evoked responses with sensory or motor stimulation, and the integrity of the spinal cord and descending tracts with motor evoked potentials. This greatly complements the diagnostics of the SCI in the subacute phase of trauma care and deepens our understanding of neurorehabilitation strategies during the chronic phase of the traumatic injury. Conclusions: sEMG offers important insights into the neurophysiological factors underlying sensorimotor impairment and recovery after SCIs. Although several qualitative or semi-quantitative outcome measures determine the level of injury and the natural recovery after SCIs, using quantitative measures such as sEMG is promising. Nonetheless, there are still several barriers limiting the use of sEMG in the clinical environment and a need to advance high-density sEMG technology.

Properties of the surface electromyogram following traumatic spinal cord injury: a scoping review

Traumatic spinal cord injury (SCI) disrupts spinal and supraspinal pathways, and this process is reflected in changes in surface electromyography (sEMG). sEMG is an informative complement to current clinical testing and can capture the residual motor command in great detail-including in muscles below the level of injury with seemingly absent motor activities. In this comprehensive review, we sought to describe how the sEMG properties are changed after SCI. We conducted a systematic literature search followed by a narrative review focusing on sEMG analysis techniques and signal properties post-SCI. We found that early reports were mostly focused on the qualitative analysis of sEMG patterns and evolved to semi-quantitative scores and a more detailed amplitude-based quantification. Nonetheless, recent studies are still constrained to an amplitude-based analysis of the sEMG, and there are opportunities to more broadly characterize the time- and frequency-domain properties of the signal as well as to take fuller advantage of high-density EMG techniques. We recommend the incorporation of a broader range of signal properties into the neurophysiological assessment post-SCI and the development of a greater understanding of the relation between these sEMG properties and underlying physiology. Enhanced sEMG analysis could contribute to a more complete description of the effects of SCI on upper and lower motor neuron function and their interactions, and also assist in understanding the mechanisms of change following neuromodulation or exercise therapy.

Modulation of soleus stretch reflexes during walking in people with chronic incomplete spinal cord injury

In people with spasticity due to chronic incomplete spinal cord injury (SCI), it has been presumed that the abnormal stretch reflex activity impairs gait. However, locomotor stretch reflexes across all phases of walking have not been investigated in people with SCI. Thus, to understand modulation of stretch reflex excitability during spastic gait, we investigated soleus stretch reflexes across the entire gait cycle in nine neurologically normal participants and nine participants with spasticity due to chronic incomplete SCI (2.5–11 year post-injury). While the participant walked on the treadmill at his/her preferred speed, unexpected ankle dorsiflexion perturbations (6° at 250°/s) were imposed every 4–6 steps. The soleus H-reflex was also examined. In participants without SCI, spinal short-latency “M1”, spinal medium latency “M2”, and long-latency “M3” were clearly modulated throughout the step cycle; the responses were largest in the mid-stance and almost completely suppressed during the stance-swing transition and swing phases. In participants with SCI, M1 and M2 were abnormally large in the mid–late-swing phase, while M3 modulation was similar to that in participants without SCI. The H-reflex was also large in the mid–late-swing phase. Elicitation of H-reflex and stretch reflexes in the late swing often triggered clonus and affected the soleus activity in the following stance. In individuals without SCI, moderate positive correlation was found between H-reflex and stretch reflex sizes across the step cycle, whereas in participants with SCI, such correlation was weak to non-existing, suggesting that H-reflex investigation would not substitute for stretch reflex investigation in individuals after SCI.

Influence of Visual Dependence on Inter-Segmental Coordination during Upright Stance in Cerebral Palsy

The presence of visual dependence as an influential factor on the development of functional stability in ambulatory individuals with cerebral palsy (CP) was studied in 22 adults with spastic bilateral CP, 11 of whom were considered visually dependent, and 18 healthy adults. Participants stood upright during pitch plane disturbances of the visual field and support surface. Intersegmental coordination behaviors were assessed by fitting trajectories of adjacent body segments to an ellipse. Mixed-model repeated measures ANOVAs were performed on ellipse orientation angle and area. Dissimilar stabilizing strategies adopted by the two groups with CP imply that visual dependence impacts postural control. Postural reorganization in response to visual flow in all groups indicates that we cannot ignore perceptual aspects of postural control when designing therapeutic interventions.

Temporal Indices of Ankle Clonus and Relationship to Electrophysiologic and Clinical Measures in Persons With Spinal Cord Injury

BACKGROUND AND PURPOSE

Clonus arising from plantar flexor hyperreflexia is a phenomenon that is commonly observed in persons with spastic hypertonia. We assessed the temporal components of a biomechanical measure to quantify ankle clonus, and validated these in persons with spasticity due to spinal cord injury.

METHODS

In 40 individuals with chronic (>1 year) spinal cord injury, we elicited ankle clonus using a standardized mechanical perturbation (drop test). We examined reliability and construct validity of 2 components of the drop test: clonus duration (timed with a stopwatch) and number of oscillations in the first 10-second interval (measured via optical motion capture). We compared these measures to the Spinal Cord Assessment Tool for Spastic reflexes (SCATS) clonus score and H-reflex/M-wave (H/M) ratio, a clinical and electrophysiologic measure, respectively.

RESULTS

Intra- and interrater reliability of clonus duration measurement was good [intraclass correlation coefficient, ICC (2, 1) = 1.00]; test-retest reliability was good both at 1 hour [ICC (2, 2) = 0.99] and at 1 week [ICC (2, 2) = 0.99]. Clonus duration was moderately correlated with SCATS clonus score (r = 0.58). Number of oscillations had good within-session test-retest reliability [ICC (2, 1) > 0.90] and strong correlations with SCATS clonus score (r = 0.86) and soleus H/M ratio (r = 0.77).

DISCUSSION AND CONCLUSIONS

Clonus duration and number of oscillations as measured with a standardized test are reliable and valid measures of plantar flexor hyperreflexia that are accessible for clinical use. Tools for objective measurement of ankle clonus are valuable for assessing effectiveness of interventions directed at normalizing reflex activity associated with spasticity.Video Abstract available for more insights from the authors (see Supplemental Digital Content 1, http://links.lww.com/JNPT/A179).

Impaired regulation post-stroke of motor unit firing behavior during volitional relaxation of knee extensor torque assessed using high density surface EMG decomposition

The purpose of this study was to use high density surface EMG recordings to quantify stroke-related abnormalities in motor unit firing behavior during repeated sub-maximal knee extensor contractions. A high density surface EMG system (sEMG) was used to record and extract single motor unit firing behavior in the vastus lateralis muscle of 6 individuals with chronic stroke and 8 controls during repeated sub-maximal isometric knee extension contractions. Paretic motor unit firing rates were increased with subsequent contractions (6.19±0.35 pps vs 7.89±0.66 pps, P <; 0.05) during task phases of torque decline as compared to controls (6.95±0.40 pps vs 6.68±0.41 pps). In addition, corresponding rates of torque decline were decreased for the paretic leg as compared to the non-paretic leg. These results suggest that regulation of declining forces may be impaired post stroke due to prolonged firing of paretic motor units.

Prochlorperazine Increases KCC2 Function and Reduces Spasticity after Spinal Cord Injury

In mature neurons, low intracellular chloride level required for inhibition is maintained by the potassium-chloride cotransporter, KCC2. Impairment of Cl- extrusion after KCC2 dysfunction has been involved in many central nervous system disorders, such as seizures, neuropathic pain, or spasticity, after a spinal cord injury (SCI). This makes KCC2 an appealing drug target for restoring Cl- homeostasis and inhibition in pathological conditions. In the present study, we screen the Prestwick Chemical Library® and identify conventional antipsychotics phenothiazine derivatives as enhancers of KCC2 activity. Among them, prochlorperazine hyperpolarizes the Cl- equilibrium potential in motoneurons of neonatal rats and restores the reciprocal inhibition post-SCI. The compound alleviates spasticity in chronic adult SCI rats with an efficacy equivalent to the antispastic agent, baclofen, and rescues the SCI-induced downregulation of KCC2 in motoneurons below the lesion. These pre-clinical data support prochlorperazine for a new therapeutic indication in the treatment of spasticity post-SCI and neurological disorders involving a KCC2 dysfunction.

Reflex wind-up in early chronic spinal injury: plasticity of motor outputs

In this study we evaluate temporal summation (wind-up) of reflexes in select distal and proximal hindlimb muscles in response to repeated stimuli of the distal tibial or superficial peroneal nerves in cats 1 mo after complete spinal transection. This report is a continuation of our previous paper on reflex wind-up in the intact and acutely spinalized cat. To evaluate reflex wind-up in both studies, we recorded electromyographic signals from the following left hindlimb muscles: lateral gastrocnemius (LG), tibialis anterior (TA), semitendinosus (ST), and sartorius (Srt), in response to 10 electrical pulses to the tibial or superficial peroneal nerves. Two distinct components of the reflex responses were considered, a short-latency compound action potential (CAP) and a longer duration bout of sustained activity (SA). These two response types were shown to be differentially modified by acute spinal injury in our previous work (Frigon A, Johnson MD, Heckman CJ. J Physiol 590: 973-989, 2012). We show that these responses exhibit continued plasticity during the 1-mo recovery period following acute spinalization. During this early chronic phase, wind-up of SA responses returned to preinjury levels in one muscle, the ST, but remained depressed in all other muscles tested. In contrast, CAP response amplitudes, which were initially potentiated following acute transection, returned to preinjury levels in all muscles except for Srt, which continued to show marked increase. These findings illustrate that spinal elements exhibit considerable plasticity during the recovery process following spinal injury and highlight the importance of considering SA and CAP responses as distinct phenomena with unique underlying neural mechanisms.NEW & NOTEWORTHY This research is the first to assess temporal summation, also called wind-up, of muscle reflexes during the 1-mo recovery period following spinal injury. Our results show that two types of muscle reflex activity are differentially modulated 1 mo after spinal cord injury (SCI) and that spinal reflexes are altered in a muscle-specific manner during this critical period. This postinjury plasticity likely plays an important role in spasticity experienced by individuals with SCI.

Measurement of Resistive Plantar Flexion Torque of the Ankle during Passive Stretch in Healthy Subjects and Patients with Poststroke Hemiplegia

BACKGROUND

Quantification of increased muscle tone for patients with spasticity has been performed to date using various devices to replace the manual scales, such as the modified Ashworth scale or the Tardieu scale. We developed a device that could measure resistive plantar flexion (PF) torque of the ankle during passive dorsiflexion (DF) as an indicator of muscle tone of ankle plantar flexors.

METHODS

The primary objective was to explore the test-retest intrarater reliability of a custom-built device. Participants were 11 healthy subjects (7 men, 4 women; mean age 47.0 years) and 22 patients with poststroke hemiplegia (11 hemorrhagic, 11 ischemic; 14 men, 8 women; mean age 57.2 years). The device was affixed to the ankle. Subjects were seated with knees either flexed or extended. The ankle was passively dorsiflexed from 20° of PF to more than 10° of DF at 5°/second (slow stretch) or 90°/second (fast stretch). Angle and torque were measured twice during the stretches. The intraclass correlation coefficients (ICCs) of torque at 10° of DF (T10) in the 4 conditions-slow and fast stretches with knee flexed or extended-were calculated.

RESULTS

The T10 ICCs of the 4 conditions were .95-.99 in both groups. The healthy subjects showed significantly higher T10 of knee extension than of knee flexion during slow and fast stretches. The patients showed increased velocity-dependent torque during fast stretches.

CONCLUSIONS

Excellent reliability was observed. The device is suitable for measuring resistive PF torque during passive stretch in a flexed knee condition.

Increased Lower Limb Spasticity but Not Strength or Function Following a Single-Dose Serotonin Reuptake Inhibitor in Chronic Stroke

OBJECTIVE

To investigate the effects of single doses of a selective serotonin reuptake inhibitor (SSRI) on lower limb voluntary and reflex function in individuals with chronic stroke.

DESIGN

Double-blind, randomized, placebo-controlled crossover trial.

SETTING

Outpatient research setting.

PARTICIPANTS

Individuals (N=10; 7 men; mean age ± SD, 57±10y) with poststroke hemiplegia of >1 year duration who completed all assessments.

INTERVENTIONS

Patients were assessed before and 5 hours after single-dose, overencapsulated 10-mg doses of escitalopram (SSRI) or placebo, with 1 week between conditions.

MAIN OUTCOME MEASURES

Primary assessments included maximal ankle and knee isometric strength, and velocity-dependent (30°/s-120°/s) plantarflexor stretch reflexes under passive conditions, and separately during and after 3 superimposed maximal volitional drive to simulate conditions of increased serotonin release. Secondary measures included clinical measures of lower limb coordination and locomotion.

RESULTS

SSRI administration significantly increased stretch reflex torques at higher stretch velocities (eg, 90°/s; P=.03), with reflexes at lower velocities enhanced by superimposed voluntary drive (P=.02). No significant improvements were seen in volitional peak torques or in clinical measures of lower limb function (lowest P=.10).

CONCLUSIONS

Increases in spasticity but not strength or lower limb function were observed with single-dose SSRI administration in individuals with chronic stroke. Further studies should evaluate whether repeated dosing of SSRIs, or as combined with specific interventions, is required to elicit significant benefit of these agents on lower limb function poststroke.

Targeted neuroplasticity for rehabilitation

An operant-conditioning protocol that bases reward on the electromyographic response produced by a specific CNS pathway can change that pathway. For example, in both animals and people, an operant-conditioning protocol can increase or decrease the spinal stretch reflex or its electrical analog, the H-reflex. Reflex change is associated with plasticity in the pathway of the reflex as well as elsewhere in the spinal cord and brain. Because these pathways serve many different behaviors, the plasticity produced by this conditioning can change other behaviors. Thus, in animals or people with partial spinal cord injuries, appropriate reflex conditioning can improve locomotion. Furthermore, in people with spinal cord injuries, appropriate reflex conditioning can trigger widespread beneficial plasticity. This wider plasticity appears to reflect an iterative process through which the multiple behaviors in the individual's repertoire negotiate the properties of the spinal neurons and synapses that they all use. Operant-conditioning protocols are a promising new therapeutic method that could complement other rehabilitation methods and enhance functional recovery. Their successful use requires strict adherence to appropriately designed procedures, as well as close attention to accommodating and engaging the individual subject in the conditioning process.

Restoring walking after spinal cord injury: operant conditioning of spinal reflexes can help

People with incomplete spinal cord injury (SCI) frequently suffer motor disabilities due to spasticity and poor muscle control, even after conventional therapy. Abnormal spinal reflex activity often contributes to these problems. Operant conditioning of spinal reflexes, which can target plasticity to specific reflex pathways, can enhance recovery. In rats in which a right lateral column lesion had weakened right stance and produced an asymmetrical gait, up-conditioning of the right soleus H-reflex, which increased muscle spindle afferent excitation of soleus, strengthened right stance and eliminated the asymmetry. In people with hyperreflexia due to incomplete SCI, down-conditioning of the soleus H-reflex improved walking speed and symmetry. Furthermore, modulation of electromyographic activity during walking improved bilaterally, indicating that a protocol that targets plasticity to a specific pathway can trigger widespread plasticity that improves recovery far beyond that attributable to the change in the targeted pathway. These improvements were apparent to people in their daily lives. They reported walking faster and farther, and noted less spasticity and better balance. Operant conditioning protocols could be developed to modify other spinal reflexes or corticospinal connections; and could be combined with other therapies to enhance recovery in people with SCI or other neuromuscular disorders.

Synchronous and asynchronous electrically evoked motor activities during wind-up stimulation are differentially modulated following an acute spinal transection

In this study, we used a novel technique to study reflex wind-up when the spinal cord is intact and following an acute spinal transection. Specifically, we evaluated reflex responses evoked by a series of 10 electrical pulses to the tibial or superficial peroneal nerves in 9 decerebrate adult cats, before and after an acute spinal transection. Electromyograms were recorded in four hindlimb muscles (lateral gastrocnemius, tibialis anterior, semitendinosus, and sartorius) to evaluate reflex amplitude, duration, and the temporal summation of reflex responses, so-called wind-up. We identified two distinct reflex responses evoked by electrical stimulation of the tibial or superficial peroneal nerves on the basis of their pattern of change following acute spinal transection, a short-latency (∼10 ms) compound action potential (CAP) that was followed by a burst of sustained activity (SA). Wind-up of CAP and SA amplitudes was clearly present when the spinal cord was intact but was drastically reduced after acute spinalization in some muscles. Moreover, CAP and SA reflex responses were differentially modified by the acute spinalization. When the effects of acute spinal transection were significant, CAP responses were increased after acute spinalization, whereas SA responses were reduced, suggesting that the two signals are regulated by different neuronal mechanisms. The present results provide the first assessment of reflex wind-up before and after an acute spinal transection in the same animals and indicate that different reflex components must be considered separately when evaluating changes in neuronal excitability following SCI.

Bilateral oscillatory hip movements induce windup of multijoint lower extremity spastic reflexes in chronic spinal cord injury

After spinal cord injury (SCI), alterations in intrinsic motoneuron properties have been shown to be partly responsible for spastic reflex behaviors in human SCI. In particular, a dysregulation of voltage-dependent depolarizing persistent inward currents (PICs) may permit sustained muscle contraction after the removal of a brief excitatory stimulus. Windup, in which the motor response increases with repeated activation, is an indicator of PICs. Although windup of homonymous stretch reflexes has been shown, multijoint muscle activity is often observed following imposed limb movements and may exhibit a similar windup phenomenon. The purpose of this study was to identify and quantify windup of multijoint reflex responses to repeated imposed hip oscillations. Ten chronic SCI subjects participated in this study. A custom-built servomotor apparatus was used to oscillate the legs about the hip joint bilaterally and unilaterally from 10° of extension to 40° flexion for 10 consecutive cycles. Surface electromyograms (EMGs) and joint torques were recorded from both legs. Consistent with a windup response, hip and knee flexion/extension and ankle plantarflexion torque and EMG responses varied according to movement cycle number. The temporal patterns of windup depended on the muscle groups that were activated, which may suggest a difference in the response of neurons in different spinal pathways. Furthermore, because windup was seen in muscles that were not being stretched, these results imply that changes in interneuronal properties are also likely to be associated with windup of spastic reflexes in human SCI.

Riluzole decreases flexion withdrawal reflex but not voluntary ankle torque in human chronic spinal cord injury

The objectives of this study were to probe the contribution of spinal neuron persistent sodium conductances to reflex hyperexcitability in human chronic spinal cord injury. The intrinsic excitability of spinal neurons provides a novel target for medical intervention. Studies in animal models have shown that persistent inward currents, such as persistent sodium currents, profoundly influence neuronal excitability, and recovery of persistent inward currents in spinal neurons of animals with spinal cord injury routinely coincides with the appearance of spastic reflexes. Pharmacologically, this neuronal excitability can be decreased by agents that reduce persistent inward currents, such as the selective persistent sodium current inhibitor riluzole. We were able to recruit seven subjects with chronic incomplete spinal cord injury who were not concurrently taking antispasticity medications into the study. Reflex responses (flexion withdrawal and H-reflexes) and volitional strength (isometric maximum voluntary contractions) were tested at the ankle before and after placebo-controlled, double-blinded oral administration of riluzole (50 mg). Riluzole significantly decreased the peak ankle dorsiflexion torque component of the flexion withdrawal reflex. Peak maximum voluntary torque in both dorsiflexion and plantarflexion directions was not significantly changed. Average dorsiflexion torque sustained during the 5-s isometric maximum voluntary contraction, however, increased significantly. There was no effect, however, on the monosynaptic plantar and dorsiflexor H-reflex responses. Overall, these results demonstrate a contribution of persistent sodium conductances to polysynaptic reflex excitability in human chronic spinal cord injury without a significant role in maximum strength production. These results suggest that intrinsic spinal cellular excitability could be a target for managing chronic spinal cord injury hyperreflexia impairments without causing a significant loss in volitional strength.

Central excitability contributes to supramaximal volitional contractions in human incomplete spinal cord injury

Despite greater muscle fatigue in individuals with spinal cord injury (SCI) when compared to neurologically intact subjects using neuromuscular electrical stimulation (NMES)protocols, few studies have investigated the extent of volitional fatigue in motor incomplete SCI. Using an established protocol of 20 repeated, intermittent, maximal volitional effort (MVE) contractions, we previously demonstrated that subjects with incomplete SCI unexpectedly demonstrated a 15% increase in peak knee extensor torques within the first five MVEs with minimal evidence of fatigue after 20 contraction. In the present study, we investigated potential segmental mechanisms underlying this supramaximal torque generation. Changes in twitch properties and maximum compound muscle action potentials (M-waves) were assessed prior to and following one, three and five MVEs, revealing a significant 17% increase only in maximum twitch torques after a single MVE. Despite this post-activation potentiation of the muscle, use of conventional NMES protocols to elicit repeated muscular contractions resulted in a significant decrease in evoked torque generation, suggesting limited the muscular contributions to the observed phenomenon. To evaluate potential central mechanisms underlying the augmented torques, non-linear responses to wide-pulse width (1 ms), low-intensity, variable-frequency (25–100 Hz) NMES were also tested prior to and following repeated MVEs.When variable-frequency NMES was applied following the repeated MVEs, augmented and prolonged torques were observed and accompanied by sustained quadriceps electromyographic activity often lasting > 2s after stimulus termination. Such data suggest a potential contribution of elevated spinal excitability to the reserve in volitional force generation in incomplete SCI.

Activity-dependent increase in neurotrophic factors is associated with an enhanced modulation of spinal reflexes after spinal cord injury

Activity-based therapies such as passive bicycling and step-training on a treadmill contribute to motor recovery after spinal cord injury (SCI), leading to a greater number of steps performed, improved gait kinematics, recovery of phase-dependent modulation of spinal reflexes, and prevention of decrease in muscle mass. Both tasks consist of alternating movements that rhythmically stretch and shorten hindlimb muscles. However, the paralyzed hindlimbs are passively moved by a motorized apparatus during bike-training, whereas locomotor movements during step-training are generated by spinal networks triggered by afferent feedback. Our objective was to compare the task-dependent effect of bike- and step-training after SCI on physiological measures of spinal cord plasticity in relation to changes in levels of neurotrophic factors. Thirty adult female Sprague-Dawley rats underwent complete spinal transection at a low thoracic level (T12). The rats were assigned to one of three groups: bike-training, step-training, or no training. The exercise regimen consisted of 15 min/d, 5 days/week, for 4 weeks, beginning 5 days after SCI. During a terminal experiment, H-reflexes were recorded from interosseus foot muscles following stimulation of the tibial nerve at 0.3, 5, or 10 Hz. The animals were sacrificed and the spinal cords were harvested for Western blot analysis of the expression of neurotrophic factors in the lumbar spinal cord. We provide evidence that bike- and step-training significantly increase the levels of brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), and NT-4 in the lumbar enlargement of SCI rats, whereas only step-training increased glial cell-derived neurotrophic factor (GDNF) levels. An increase in neurotrophic factor protein levels that positively correlated with the recovery of H-reflex frequency-dependent depression suggests a role for neurotrophic factors in reflex normalization.

Wind-up of stretch reflexes as a measure of spasticity in chronic spinalized rats: The effects of passive exercise and modafinil

Spasticity is a common disorder following spinal cord injury that can impair function and quality of life. While a number of mechanisms are thought to play a role in spasticity, the role of motoneuron persistent inward currents (PICs) is emerging as pivotal. The presence of PICs can be evidenced by temporal summation or wind-up of reflex responses to brief afferent inputs. In this study, a combined neurophysiological and novel biomechanical approach was used to assess the effects of passive exercise and modafinil administration on hyper-reflexia and spasticity following complete T-10 transection in the rat. Animals were divided into 3 groups (n=8) and provided daily passive cycling exercise, oral modafinil, or no intervention. After 6weeks, animals were tested for wind-up of the stretch reflex (SR) during repeated dorsiflexion stretches of the ankle. H-reflexes were tested in a subset of animals. Both torque and gastrocnemius electromyography showed evidence of SR wind-up in the transection only group that was significantly different from both treatment groups (p<0.05). H-reflex frequency dependent depression was also restored to normal levels in both treatment groups. The results provide support for the use of passive cycling exercise and modafinil in the treatment of spasticity and provide insight into the possible contribution of PICs.

Chapter 11--novel mechanism for hyperreflexia and spasticity

We established that hyperreflexia is delayed after spinal transection in the adult rat and that passive exercise could normalize low frequency-dependent depression of the H-reflex. We were also able to show that such passive exercise will normalize hyperreflexia in patients with spinal cord injury (SCI). Recent results demonstrate that spinal transection results in changes in the neuronal gap junction protein connexin 36 below the level of the lesion. Moreover, a drug known to increase electrical coupling was found to normalize hyperreflexia in the absence of passive exercise, suggesting that changes in electrical coupling may be involved in hyperreflexia. We also present results showing that a measure of spasticity, the stretch reflex, is rendered abnormal by transection and normalized by the same drug. These data suggest that electrical coupling may be dysregulated in SCI, leading to some of the symptoms observed. A novel therapy for hyperreflexia and spasticity may require modulation of electrical coupling.

Effects of multijoint spastic reflexes of the legs during assisted bilateral hip oscillations in human spinal cord injury

OBJECTIVE

To investigate the timing and magnitude of muscle activation during an active-assist bilateral hip motor task in human spinal cord injury (SCI).

DESIGN

A single test session using a novel robotic system to alternately flex and extend the hips from 40 degrees of hip flexion to 10 degrees of hip extension at 1 of 3 frequencies (.25, .50, .75Hz). Subjects were asked either to actively assist the movements or to remain relaxed during the imposed oscillations.

SETTING

All data were collected in a research laboratory.

PARTICIPANTS

Ten subjects with motor incomplete (American Spinal Injury Association grade C or D) SCI and 10 individuals without neurologic injury participated in this study.

INTERVENTIONS

Not applicable.

MAIN OUTCOME MEASURES

Electromyograms and joint torques were recorded from the lower extremities of SCI subjects and compared with electromyograms and joint torque patterns recorded from 10 neurologically healthy individuals completing the same tasks.

RESULTS

In trials involving active assistance of the imposed hip oscillations, SCI subjects produced muscle activation patterns that were phased differently from muscle activity of neurologically intact subjects. SCI subjects generated peak torque at the end ranges of movement (ie, 40 degrees hip flexion, 10 degrees extension), whereas control subjects generated the greatest torque midway through the movements. Moreover, the phasing of active-assist hip torque in SCI subjects was similar to the phasing of reflexive hip torques produced during the unassisted condition (ie, SCI subjects instructed to relax), while control subjects produced no reflexive torques during unassisted trials.

CONCLUSIONS

The differences in the timing of muscle activity during the active-assist task in controls and SCI subjects highlights problems in generating appropriately timed muscle activity during ongoing movements. The similarity in muscle activity patterns for the active-assist and unassisted trials in SCI subjects further suggests that reflex feedback from hip afferents contributes substantially to muscle activation during active-assist movements. These findings demonstrate the disruptions in reflex regulation of movement in people with incomplete SCI and suggest that spastic reflexes might disrupt motor control.

Abnormal volitional hip torque phasing and hip impairments in gait post stroke

The purpose of this study was to quantify how volitional control of hip torque relates to walking function poststroke. Volitional phasing of hip flexion and extension torques was assessed using a load-cell-instrumented servomotor drive system in 11 chronic stroke subjects and 5 age-matched controls. Hips were oscillated from approximately 40 degrees of hip flexion to 10 degrees of hip extension at a frequency of 0.50 Hz during three movement conditions [hips in phase (IP), 180 degrees out of phase (OP), and unilateral hip movement (UN)] while the knees and ankles were held stationary. The magnitude and phasing of hip, knee, and ankle torques were measured during each movement condition. Surface electromyography was measured throughout the legs. Over ground gait analysis was done for all stroke subjects. During robotic-assisted movement conditions, the paretic limb produced peak hip torques when agonist hip musculature was stretched instead of midway through the movement as seen in the nonparetic and control limbs (P < 0.012). However, mean torque magnitudes of the paretic and nonparetic limbs were not significantly different. Abnormalities of paretic hip torque phasing were more pronounced during bilateral movement conditions and were associated with quadriceps overactivity. The magnitude of flexion torque produced during maximal hip extension was correlated with the Fugl Meyer Score, self-selected walking speed, and maximal hip extension during over ground walking. These results suggest that hyperexcitable stretch reflexes in the paretic limb impair coordinated hip torque phasing and likely interfere with walking function post stroke.

Therapeutic approaches for spinal cord injury induced spasticity

Spasticity is evident in both humans and animals following spinal cord injury (SCI) and can contribute to significant functional limitation and disruption in quality of life of patients with this disorder. This mini-review describes a number of preclinical and clinical studies that promise to improve outcomes for, especially in terms of spasticity and hyper-reflexia, patients with SCI. A gold standard for the quantification of spasticity has proved elusive, but the combination of H-reflex frequency dependent depression and a novel stretch reflex (SR) windup protocol have the potential to provide new insights. As the pathophysiology of hyper-reflexia and spasticity continue to be investigated, the documented onset in the animal model of SCI provides critical time points for further study into these complex mechanisms. The positive effects of a passive exercise protocol and several potential pharmacological interventions are reviewed as well as a novel potential mechanism of action. Further work is needed to determine additional mechanisms that are involved in SCI, and how to optimize multiple therapies to overcome some of the deficits induced by SCI.

Repeated maximal volitional effort contractions in human spinal cord injury: initial torque increases and reduced fatigue

BACKGROUND

Substantial data indicate greater muscle fatigue in individuals with spinal cord injury (SCI) compared with healthy control subjects when tested by using electrical stimulation protocols. Few studies have investigated the extent of volitional fatigue in motor incomplete SCI.

METHODS

Repeated, maximal volitional effort (MVE) isometric contractions of the knee extensors (KE) were performed in 14 subjects with a motor incomplete SCI and in 10 intact subjects. Subjects performed 20 repeated, intermittent MVEs (5 seconds contraction/5 seconds rest) with KE torques and thigh electromyographic (EMG) activity recorded.

RESULTS

Peak KE torques declined to 64% of baseline MVEs with repeated efforts in control subjects. Conversely, subjects with SCI increased peak torques during the first 5 contractions by 15%, with little evidence of fatigue after 20 repeated efforts. Increases in peak KE torques and the rate of torque increase during the first 5 contractions were attributed primarily to increases in quadriceps EMG activity, but not to decreased knee flexor co-activation. The observed initial increases in peak torque were dependent on the subject's volitional activation and were consistent on the same or different days, indicating little contribution of learning or accommodation to the testing conditions. Sustained MVEs did not elicit substantial increases in peak KE torques as compared to repeated intermittent efforts.

CONCLUSIONS

These data revealed a marked divergence from expected results of increased fatigability in subjects with SCI, and may be a result of complex interactions between mechanisms underlying spastic motor activity and changes in intrinsic motoneuron properties.

Rebound responses to prolonged flexor reflex stimuli in human spinal cord injury

The purpose of this study was to examine the reflex effects of electrical stimulation applied to the thigh using skin electrodes, targeting the sensory fibers of the rectus femoris and sartorius, in people with spinal cord injury (SCI). Thirteen individuals with SCI were recruited to participate in experiments using prolonged electrical stimuli on the right medial thigh over the regions of the sartorius and rectus femoris muscles. Three stimuli, spaced 20 s apart, were applied at 30 Hz for 1 s at four different intensities (15-60 mA) while subjects rested in a seated position. Isometric joint torques of the hip, knee and ankle, and electromyograms (EMGs) from six muscles of the leg were recorded during the stimulation. Early in the stimulation, a flexion response was observed at the hip and ankle, analogous to a flexor reflex; however, this response was usually followed by a "rebound" response consisting of hip extension, knee flexion and ankle plantarflexion, occurring in 10/13 subjects. Stimuli applied in a more lateral (mid thigh) electrode position (i.e. over the rectus femoris) were less effective in producing the response than medial placement, despite vigorous quadriceps activation. This complex reflex response is consistent with activation of a coordinating spinal circuit that could play a role in motor function. The reversal of the reflex pattern emphasizes the potential connection between skin/muscle afferents of the thigh, possibly including sartorius muscle afferents and locomotor reflex centers. This knowledge may be helpful in identifying rehabilitation strategies for enhancing gait training in human SCI.

Spasticity measurement based on tonic stretch reflex threshold in stroke using a portable device

OBJECTIVES

We investigated intra- and inter-evaluator reliability to quantify spasticity based on the tonic stretch reflex threshold (TSRT) and the correlation between TSRT and resistance to stretch.

METHODS

Spasticity was evaluated in 20 subjects with chronic stroke-related spasticity using a portable device and the Modified Ashworth Scale (MAS). Evaluations were done on 2 days, by three evaluators. Biceps brachii EMG signals and elbow displacement were recorded during 20 elbow stretches applied at different velocities for each evaluation. Velocity-dependent dynamic stretch reflex thresholds (angle where EMG signal increased in the biceps for a given velocity of stretch) were recorded. These values were used to compute TSRT (excitability of motoneurons at 0 degrees /s). Spasticity was also measured with MAS.

RESULTS

Reliability was moderately good for subjects with moderate to high spasticity (intra-evaluator: 0.46-0.68, and inter-evaluator: 0.53-0.68). The TSRT measure of spasticity did not correlate with resistance to stretch (MAS).

CONCLUSIONS

TSRT may be a more representative measure for subjects with moderate to high spasticity. Further improvements are suggested for the portable device in order to quantify all the levels of spasticity.

SIGNIFICANCE

TSRT may be an alternative clinical measure to current clinical scales.

Prolonged quadriceps activity following imposed hip extension: a neurophysiological mechanism for stiff-knee gait?

The biomechanical characteristics of stiff knee gait following neurological injury include decreased knee flexion velocity at toe-off, which may be due to exaggerated quadriceps activity. The neuromuscular mechanism underlying this abnormal activity is unclear, although hyperexcitable heteronymous reflexes may be a source of impaired coordination. The present study examines the contribution of reflex activity from hip flexors on knee extensors following stroke and its association with reduced swing-phase knee flexion during walking. Twelve individuals poststroke and six control subjects were positioned in supine on a Biodex dynamometer with the ankle and knee held in a static position. Isolated hip extension movements were imposed at 60, 90, and 120 degrees /s through a 50 degrees excursion to end-range hip extension. Reflexive responses of the rectus femoris (RF), vastus lateralis (VL), and vastus medialis (VM) were quantified during and after the imposed hip rotation. Gait analysis was also performed for all subjects in the stroke group. In subjects with stroke, imposed hip extension evoked a brief reflexive response in the quadriceps, followed by a heightened level of sustained activity. The initial response was velocity dependent and was larger in the stroke group than in the control group. In contrast, the prolonged response was not velocity dependent, was significantly greater in the VL and RF in subjects with stroke, and, importantly, was correlated to decreased swing-phase knee flexion. Hyperexcitable heteronymous connections from hip flexors to knee extensors appear to elicit prolonged quadriceps activity and may contribute to altered swing-phase knee kinematics following stroke.

Plasticity of inwardly rectifying conductances following a corticospinal lesion in human subjects

This study investigated whether there are changes in the excitability of motor axons in peripheral nerves of patients with corticospinal lesions, reflecting plasticity of the motoneuron due to altered descending drives and/or changes in afferent feedback. The excitability of motor and sensory axons in peripheral nerves of the affected limb of 11 patients with unilateral hemiparesis due to stroke was compared with that for the unaffected limbs and with data for 12 age-matched controls. There was significantly less accommodation to hyperpolarizing currents in motor axons on the affected side. There were small differences between the data for the unaffected side and that of the control subjects but these were not statistically significant. Other findings indicate that there was no change in resting membrane potential. There was no comparable alteration in the excitability of sensory axons. The changes in response of motor axons to hyperpolarizing currents could be reproduced in a computer model of the human motor axon by reducing the hyperpolarization-activated conductance, IH, by 30% and the quantitatively small leak conductance by 77%. The data for the uninvolved side matched the data for control subjects best when IH was increased. These findings are consistent with modulation of IH by activity. They demonstrate a change in the biophysical properties of motor axons not directly affected by the pathology and synaptically remote from the lesion, and have implications for 'trans-synaptic' changes in central nervous system pathways. In human subjects studies of motor axon properties may allow insight into processes affecting the motoneuron.

Fibre type-specific increase in passive muscle tension in spinal cord-injured subjects with spasticity

Patients with spasticity typically present with an increased muscle tone that is at least partly caused by an exaggerated stretch reflex. However, intrinsic changes in the skeletal muscles, such as altered mechanical properties of the extracellular matrix or the cytoskeleton, have been reported in response to spasticity and could contribute to hypertonia, although the underlying mechanisms are poorly understood. Here we examined the vastus lateralis muscles from spinal cord-injured patients with spasticity (n = 7) for their passive mechanical properties at three different levels of structural organization, in comparison to healthy controls (n = 7). We also assessed spasticity-related alterations in muscle protein expression and muscle ultrastructure. At the whole-muscle level in vivo, we observed increased passive tension (PT) in some spasticity patients particularly at long muscle lengths, unrelated to stretch reflex activation. At the single-fibre level, elevated PT was found in cells expressing fast myosin heavy chain (MyHC) isoforms, especially MyHC-IIx, but not in those expressing slow MyHC. Type IIx fibres were present in higher than normal proportions in spastic muscles, whereas type I fibres were proportionately reduced. At the level of the isolated myofibril, however, there were no differences in PT between patients and controls. The molecular size of the giant protein titin, a main contributor to PT, was unchanged in spasticity, as was the titin : MyHC ratio and the relative desmin content. Electron microscopy revealed extensive ultrastructural changes in spastic muscles, especially expanded connective tissue, but also decreased mitochondrial volume fraction and appearance of intracellular amorphous material. Results strongly suggest that the global passive muscle stiffening in spasticity patients is caused to some degree by elevated PT of the skeletal muscles themselves. We conclude that this increased PT component arises not only from extracellular matrix remodelling, but also from structural and functional adaptations inside the muscle cells, which alter their passive mechanical properties in response to spasticity in a fibre type-dependent manner.