Impaired modulation of quadriceps tendon jerk reflex during spastic gait: differences between spinal and cerebral lesions

Comparison of systemic and localized carrier-mediated delivery of methylprednisolone succinate for treatment of acute spinal cord injury

Localized carrier-mediated administration of drugs is a promising approach to treatment of acute phase of spinal cord injury (SCI) as it allows enhanced and/or sustained drug delivery to damaged tissues along with minimization of systemic side effects. We studied the effect of locally applied self-assembling micellar formulation of methylprednisolone succinate (MPS) with trifunctional block copolymer of ethylene oxide and propylene oxide (TBC) on functional recovery and tissue drug content after SCI in rats in comparison with local and systemic administration of MPS alone. Variations in the amplitude of motor evoked responses in the hindlimb muscles induced by epidural stimulation during acute phase of SCI and restoration of movements during chronic period after local vs. systemic application of MPS were evaluated in this study. Results demonstrate that local delivery of MPS in combination with TBC facilitates spinal cord sensorimotor circuitry, increasing the excitability. In addition, this formulation was found to be more effective in improvement of locomotion after SCI compared to systemic administration. LC-MS/MS data shows that the use of TBC carrier increases the glucocorticoid content in treated spinal cord by more than four times over other modes of treatment. The results of this study demonstrate that the local treatment of acute SCI with MPS in the form of mixed micelles with TBC can provide improved therapeutic outcome by promoting drug accumulation and functional restoration of the spinal cord.

Physics-Based Simulations to Predict the Differential Effects of Motor Control and Musculoskeletal Deficits on Gait Dysfunction in Cerebral Palsy: A Retrospective Case Study

Physics-based simulations of walking have the theoretical potential to support clinical decision-making by predicting the functional outcome of treatments in terms of walking performance. Yet before using such simulations in clinical practice, their ability to identify the main treatment targets in specific patients needs to be demonstrated. In this study, we generated predictive simulations of walking with a medical imaging based neuro-musculoskeletal model of a child with cerebral palsy presenting crouch gait. We explored the influence of altered muscle-tendon properties, reduced neuromuscular control complexity, and spasticity on gait dysfunction in terms of joint kinematics, kinetics, muscle activity, and metabolic cost of transport. We modeled altered muscle-tendon properties by personalizing Hill-type muscle-tendon parameters based on data collected during functional movements, simpler neuromuscular control by reducing the number of independent muscle synergies, and spasticity through delayed muscle activity feedback from muscle force and force rate. Our simulations revealed that, in the presence of aberrant musculoskeletal geometries, altered muscle-tendon properties rather than reduced neuromuscular control complexity and spasticity were the primary cause of the crouch gait pattern observed for this child, which is in agreement with the clinical examination. These results suggest that muscle-tendon properties should be the primary target of interventions aiming to restore an upright gait pattern for this child. This suggestion is in line with the gait analysis following muscle-tendon property and bone deformity corrections. Future work should extend this single case analysis to more patients in order to validate the ability of our physics-based simulations to capture the gait patterns of individual patients pre- and post-treatment. Such validation would open the door for identifying targeted treatment strategies with the aim of designing optimized interventions for neuro-musculoskeletal disorders.

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.

Repeated anodal trans-spinal direct current stimulation results in long-term reduction of spasticity in mice with spinal cord injury

KEY POINTS

Spasticity is a disorder of muscle tone that is associated with lesions of the motor system. This condition involves an overactive spinal reflex loop that resists the passive lengthening of muscles. Previously, we established that application of anodal trans-spinal direct current stimulation (a-tsDCS) for short periods of time to anaesthetized mice sustaining a spinal cord injury leads to an instantaneous reduction of spasticity. However, the long-term effects of repeated a-tsDCS and its mechanism of action remained unknown. In the present study, a-tsDCS was performed for 7 days and this was found to cause long-term reduction in spasticity, increased rate-dependent depression in spinal reflexes, and improved ground and skill locomotion. Pharmacological, molecular and cellular evidence further suggest that a novel mechanism involving Na-K-Cl cotransporter isoform 1 mediates the observed long-term effects of repeated a-tsDCS.

ABSTRACT

Spasticity can cause pain, fatigue and sleep disturbances; restrict daily activities such as walking, sitting and bathing; and complicate rehabilitation efforts. Thus, spasticity negatively influences an individual's quality of life and novel therapeutic interventions are needed. We previously demonstrated in anaesthetized mice that a short period of trans-spinal subthreshold direct current stimulation (tsDCS) reduces spasticity. In the present study, the long-term effects of repeated tsDCS to attenuate abnormal muscle tone in awake female mice with spinal cord injuries were investigated. A motorized system was used to test velocity-dependent ankle resistance and associated electromyographical activity. Analysis of ground and skill locomotion was also performed, with electrophysiological, molecular and cellular studies being conducted to reveal a potential underlying mechanism of action. A 4 week reduction in spasticity was associated with an increase in rate-dependent depression of spinal reflexes, and ground and skill locomotion were improved following 7 days of anodal-tsDCS (a-tsDCS). Secondary molecular, cellular and pharmacological experiments further demonstrated that the expression of K-Cl co-transporter isoform 2 (KCC2) was not changed in animals with spasticity. However, Na-K-Cl cotransporter isoform 1 (NKCC1) was significantly up-regulated in mice that exhibited spasticity. When mice were treated with a-tsDCS, down regulation of NKCC1 was detected, and this level did not significantly differ from that in the non-injured control mice. Thus, long lasting reduction of spasticity by a-tsDCS via downregulation of NKCC1 may constitute a novel therapy for spasticity following spinal cord injury.

A spasticity model based on feedback from muscle force explains muscle activity during passive stretches and gait in children with cerebral palsy

Muscle spasticity is characterized by exaggerated stretch reflexes and affects about 85% of the children with cerebral palsy. However, the mechanisms underlying spasticity and its influence on gait are not well understood. Here, we first aimed to model the response of spastic hamstrings and gastrocnemii in children with cerebral palsy to fast passive stretches. Then, we evaluated how the model applied to gait. We developed three models based on exaggerated proprioceptive feedback. The first model relied on feedback from muscle fiber length and velocity (velocity-related model), the second model relied on feedback from muscle fiber length, velocity, and acceleration (acceleration-related model), and the third model relied on feedback from muscle force and its first time derivative (force-related model). The force-related model better reproduced measured hamstrings and gastrocnemii activity during fast passive stretches (coefficients of determination (R2): 0.73 ± 0.10 and 0.60 ± 0.13, respectively, and root mean square errors (RMSE): 0.034 ± 0.031 and 0.009 ± 0.007, respectively) than the velocity-related model (R2: 0.46 ± 0.15 and 0.07 ± 0.13, and RMSE: 0.053 ± 0.051 and 0.015 ± 0.009), and the acceleration-related model (R2: 0.47 ± 0.15 and 0.09 ± 0.14, and RMSE: 0.052 ± 0.050 and 0.015 ± 0.008). Additionally, the force-related model predicted hamstrings and gastrocnemii activity that better correlated with measured activity during gait (cross correlations: 0.82 ± 0.09 and 0.85 ± 0.06, respectively) than the activity predicted by the velocity-related model (cross correlations: 0.49 ± 0.17 and 0.71 ± 0.22) and the acceleration-related model (cross correlations: 0.51 ± 0.16 and 0.67 ± 0.20). Our results therefore suggest that force encoding in muscle spindles in combination with altered feedback gains and thresholds underlie activity of spastic muscles during passive stretches and gait. Our model of spasticity opens new perspectives for studying movement impairments due to spasticity through simulation.

Human cervical spinal cord circuitry activated by tonic input can generate rhythmic arm movements

The coordination between arms and legs during human locomotion shares many features with that in quadrupeds, yet there is limited evidence for the central pattern generator for the upper limbs in humans. Here we investigated whether different types of tonic stimulation, previously used for eliciting stepping-like leg movements, may evoke nonvoluntary rhythmic arm movements. Twenty healthy subjects participated in this study. The subject was lying on the side, the trunk was fixed, and all four limbs were suspended in a gravity neutral position, allowing unrestricted low-friction limb movements in the horizontal plane. The results showed that peripheral sensory stimulation (continuous muscle vibration) and central tonic activation (postcontraction state of neuronal networks following a long-lasting isometric voluntary effort, Kohnstamm phenomenon) could evoke nonvoluntary rhythmic arm movements in most subjects. In ∼40% of subjects, tonic stimulation elicited nonvoluntary rhythmic arm movements together with rhythmic movements of suspended legs. The fact that not all participants exhibited nonvoluntary limb oscillations may reflect interindividual differences in responsiveness of spinal pattern generation circuitry to its activation. The occurrence and the characteristics of induced movements highlight the rhythmogenesis capacity of cervical neuronal circuitries, complementing the growing body of work on the quadrupedal nature of human gait.

Can Treadmill Perturbations Evoke Stretch Reflexes in the Calf Muscles?

Disinhibition of reflexes is a problem amongst spastic patients, for it limits a smooth and efficient execution of motor functions during gait. Treadmill belt accelerations may potentially be used to measure reflexes during walking, i.e. by dorsal flexing the ankle and stretching the calf muscles, while decelerations show the modulation of reflexes during a reduction of sensory feedback. The aim of the current study was to examine if belt accelerations and decelerations of different intensities applied during the stance phase of treadmill walking can evoke reflexes in the gastrocnemius, soleus and tibialis anterior in healthy subjects. Muscle electromyography and joint kinematics were measured in 10 subjects. To determine whether stretch reflexes occurred, we assessed modelled musculo-tendon length and stretch velocity, the amount of muscle activity, as well as the incidence of bursts or depressions in muscle activity with their time delays, and co-contraction between agonist and antagonist muscle. Although the effect on the ankle angle was small with 2.8±1.0°, the perturbations caused clear changes in muscle length and stretch velocity relative to unperturbed walking. Stretched muscles showed an increasing incidence of bursts in muscle activity, which occurred after a reasonable electrophysiological time delay (163-191 ms). Their amplitude was related to the muscle stretch velocity and not related to co-contraction of the antagonist muscle. These effects increased with perturbation intensity. Shortened muscles showed opposite effects, with a depression in muscle activity of the calf muscles. The perturbations only slightly affected the spatio-temporal parameters, indicating that normal walking was retained. Thus, our findings showed that treadmill perturbations can evoke reflexes in the calf muscles and tibialis anterior. This comprehensive study could form the basis for clinical implementation of treadmill perturbations to functionally measure reflexes during treadmill-based clinical gait analysis.

Implementation of an iPod wireless accelerometer application using machine learning to classify disparity of hemiplegic and healthy patellar tendon reflex pair

The characteristics of the patellar tendon reflex provide fundamental insight regarding the diagnosis of neurological status. Based on the features of the tendon reflex response, a clinician may establish preliminary perspective regarding the global condition of the nervous system. Current techniques for quantifying the observations of the reflex response involve the application of ordinal scales, requiring the expertise of a highly skilled clinician. However, the reliability of the ordinal scale approach is debatable. Highly skilled clinicians have even disputed the presence of asymmetric reflex pairs. An alternative strategy was the implementation of an iPod wireless accelerometer application to quantify the reflex response acceleration waveform. An application enabled the recording of the acceleration waveform and later wireless transmission as an email attachment by connectivity to the Internet. A potential energy impact pendulum enabled the patellar tendon reflex to be evoked in a predetermined and targeted manner. Three feature categories of the reflex response acceleration waveform (global parameters, temporal organization, and spectral features) were incorporated into machine learning to distinguish a subject's hemiplegic and healthy reflex pair. Machine learning attained perfect classification of the hemiplegic and healthy reflex pair. The research findings implicate the promise of machine learning for providing increased diagnostic acuity regarding the acceleration waveform of the tendon reflex response.

Implementation of a smartphone as a wireless gyroscope application for the quantification of reflex response

The patellar tendon reflex constitutes a fundamental aspect of the conventional neurological evaluation. Dysfunctional characteristics of the reflex response can augment the diagnostic acuity of a clinician for subsequent referral to more advanced medical resources. The capacity to quantify the reflex response while alleviating the growing strain on specialized medical resources is a topic of interest. The quantification of the tendon reflex response has been successfully demonstrated with considerable accuracy and consistency through using a potential energy impact pendulum attached to a reflex hammer for evoking the tendon reflex with a smartphone, such as an iPhone, application representing a wireless accelerometer platform to quantify reflex response. Another sensor integrated into the smartphone, such as an iPhone, is the gyroscope, which measures rate of angular rotation. A smartphone application enables wireless transmission through Internet connectivity of the gyroscope signal recording of the reflex response as an email attachment. The smartphone wireless gyroscope application demonstrates considerable accuracy and consistency for the quantification of the tendon reflex response.

Quantifying the deep tendon reflex using varying tendon indentation depths: applications to spasticity

The deep tendon reflex (DTR) is often utilized to characterize the neuromuscular health of individuals because it is cheap, quick to implement, and requires limited equipment. However, DTR assessment is unreliable and assessor-dependent improve the reliability of the DTR assessment, we devised a novel standardization procedure. Our approach is based on the hypothesis that the neuromuscular state of a muscle changes systematically with respect to the indentation depth of its tendon. We tested the hypothesis by progressively indenting the biceps tendons on each side of nine hemiplegic stroke survivors to different depths, and then superimposing a series of brief controlled taps at each indentation depth to elicit a reflex response. Our results show that there exists a unique indentation depth at which reflex responses are consistently recorded (termed the Reflex Threshold) with increasing amplitude along increasing indentation depth. We further show that the reflex threshold depth is systematically smaller on the affected side of stroke survivors and that it is negatively correlated with the Modified Ashworth Score (VAF 70%). Our procedure also enables measurement of passive mechanical properties at the indentation location. In conclusion, our study shows that controlling for the indentation depth of the tendon of a muscle alters its reflex response predictably. Our novel device and method could be used to estimate neuromuscular changes in muscle (e.g., spasticity). Although some refinement is needed, this method opens the door to more reliable quantification of the DTR.

Altering length and velocity feedback during a neuro-musculoskeletal simulation of normal gait contributes to hemiparetic gait characteristics

Background

Spasticity is an important complication after stroke, especially in the anti-gravity muscles, i.e. lower limb extensors. However the contribution of hyperexcitable muscle spindle reflex loops to gait impairments after stroke is often disputed. In this study a neuro-musculoskeletal model was developed to investigate the contribution of an increased length and velocity feedback and altered reflex modulation patterns to hemiparetic gait deficits.

Methods

A musculoskeletal model was extended with a muscle spindle model providing real-time length and velocity feedback of gastrocnemius, soleus, vasti and rectus femoris during a forward dynamic simulation (neural control model). By using a healthy subject’s base muscle excitations, in combination with increased feedback gains and altered reflex modulation patterns, the effect on kinematics was simulated. A foot-ground contact model was added to account for the interaction effect between the changed kinematics and the ground. The qualitative effect i.e. the directional effect and the specific gait phases where the effect is present, on the joint kinematics was then compared with hemiparetic gait deviations reported in the literature.

Results

Our results show that increased feedback in combination with altered reflex modulation patterns of soleus, vasti and rectus femoris muscle can contribute to excessive ankle plantarflexion/inadequate dorsiflexion, knee hyperextension/inadequate flexion and increased hip extension/inadequate flexion during dedicated gait cycle phases. Increased feedback of gastrocnemius can also contribute to excessive plantarflexion/inadequate dorsiflexion, however in combination with excessive knee and hip flexion. Increased length/velocity feedback can therefore contribute to two types of gait deviations, which are both in accordance with previously reported gait deviations in hemiparetic patients. Furthermore altered modulation patterns, in particular the reduced suppression of the muscle spindle feedback during swing, can contribute largely to an increased plantarflexion and knee extension during the swing phase and consequently to hampered toe clearance.

Conclusions

Our results support the idea that hyperexcitability of length and velocity feedback pathways, especially in combination with altered reflex modulation patterns, can contribute to deviations in hemiparetic gait. Surprisingly, our results showed only subtle temporal differences between length and velocity feedback. Therefore, we cannot attribute the effects seen in kinematics to one specific type of feedback.

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.

Spasticity

Antispastic medications that are directed to reduce clinical signs of spasticity, such as exaggerated reflexes and muscle tone, do not improve the movement disorder. Medication can even increase weakness which might interfere with functional movements, such as walking. In this chapter we address how spasticity affects mobility and how this should be taken into account in the treatment of spasticity. In clinical practice, signs of exaggerated tendon tap reflexes associated with muscle hypertonia are the consequence of spinal cord injury (SCI). They are generally thought to be responsible for spastic movement disorders. Most antispastic treatments are, therefore, directed at the reduction of reflex activity. In recent years, a discrepancy between spasticity as measured in the clinic and functional spastic movement disorder was noticed, which is primarily due to the different roles of reflexes in passive and active states, respectively. We now know that central motor lesions are associated with loss of supraspinal drive and defective use of afferent input with impaired behavior of short-latency and long-latency reflexes. These changes lead to paresis and maladaptation of the movement pattern. Secondary changes in mechanical muscle fiber, collagen tissue, and tendon properties (e.g., loss of sarcomeres, subclinical contractures) result in spastic muscle tone, which in part compensates for paresis and allows functional movements on a simpler level of organization. Antispastic drugs should primarily be applied in complete SCI. In mobile patients they can accentuate paresis and therefore should be applied with caution.

Implementation of an iPhone wireless accelerometer application for the quantification of reflex response

The patellar tendon reflex represents an inherent aspect of the standard neurological evaluation. The features of the reflex response provide initial perspective regarding the status of the nervous system. An iPhone wireless accelerometer application integrated with a potential energy impact pendulum attached to a reflex hammer has been successfully developed, tested, and evaluated for quantifying the patellar tendon reflex. The iPhone functions as a wireless accelerometer platform. The wide coverage range of the iPhone enables the quantification of reflex response samples in rural and remote settings. The iPhone has the capacity to transmit the reflex response acceleration waveform by wireless transmission through email. Automated post-processing of the acceleration waveform provides feature extraction of the maximum acceleration of the reflex response ascertained after evoking the patellar tendon reflex. The iPhone wireless accelerometer application demonstrated the utility of the smartphone as a biomedical device, while providing accurate and consistent quantification of the reflex response.

[Pathophysiology of spasticity]

The term "spasticity" describes the velocity-dependent increase in tonic stretch reflexes. The symptom is commonly seen in patients with injury to the central nervous system. It is rarely isolated but, instead, part of a set of symptoms that is sometimes confusing. However, the pathophysiology of the symptom has evolved over the past three decades, and it is now considered part of a global process that includes not only spinal reflex loop modifications, but also changes in the biomechanical properties of muscle fibers. Finally, recent studies of changes in the membrane properties of motor neurons and the occurrence of plateau potential have opened new perspectives. This review aims to describe these new pathophysiological models.

Plasticity of corticospinal neural control after locomotor training in human spinal cord injury

Spinal lesions substantially impair ambulation, occur generally in young and otherwise healthy individuals, and result in devastating effects on quality of life. Restoration of locomotion after damage to the spinal cord is challenging because axons of the damaged neurons do not regenerate spontaneously. Body-weight-supported treadmill training (BWSTT) is a therapeutic approach in which a person with a spinal cord injury (SCI) steps on a motorized treadmill while some body weight is removed through an upper body harness. BWSTT improves temporal gait parameters, muscle activation patterns, and clinical outcome measures in persons with SCI. These changes are likely the result of reorganization that occurs simultaneously in supraspinal and spinal cord neural circuits. This paper will focus on the cortical control of human locomotion and motor output, spinal reflex circuits, and spinal interneuronal circuits and how corticospinal control is reorganized after locomotor training in people with SCI. Based on neurophysiological studies, it is apparent that corticospinal plasticity is involved in restoration of locomotion after training. However, the neural mechanisms underlying restoration of lost voluntary motor function are not well understood and translational neuroscience research is needed so patient-orientated rehabilitation protocols to be developed.

WIRELESS THREE DIMENSIONAL ACCELEROMETER REFLEX QUANTIFICATION DEVICE WITH ARTIFICIAL REFLEX SYSTEM

Fundamental to the neurological examination is the deep tendon reflex. Two important tendon reflex parameters are response and latency. Response can be quantified by the NINDS Myotatic Reflex Scale; however, controversy exists with respect to the accuracy of the scale. Electrodiagnostic testing may derive parameters, similar to the validity of the reflex latency; however, such tests require highly specialized resources. Attempts have been made to develop quantified reflex devices. Two wireless three-dimensional (3D) accelerometers incorporating MEMS technology have been integrated into a device for quantifying reflex response and latency. The device is tested and evaluated using an artificial reflex system. The reflex quantification device obtained reflex response and latency parameters based on the artificial reflex device, which were bounded by a 98% confidence level with a 2% margin of error about the mean.

Stance-phase force on the opposite limb dictates swing-phase afferent presynaptic inhibition during locomotion

Presynaptic inhibition is a powerful mechanism for selectively and dynamically gating sensory inputs entering the spinal cord. We investigated how hindlimb mechanics influence presynaptic inhibition during locomotion using pioneering approaches in an in vitro spinal cord-hindlimb preparation. We recorded lumbar dorsal root potentials to measure primary afferent depolarization-mediated presynaptic inhibition and compared their dependence on hindlimb endpoint forces, motor output, and joint kinematics. We found that stance-phase force on the opposite limb, particularly at toe contact, strongly influenced the magnitude and timing of afferent presynaptic inhibition in the swinging limb. Presynaptic inhibition increased in proportion to opposite limb force, as well as locomotor frequency. This form of presynaptic inhibition binds the sensorimotor states of the two limbs, adjusting sensory inflow to the swing limb based on forces generated by the stance limb. Functionally, it may serve to adjust swing-phase sensory transmission based on locomotor task, speed, and step-to-step environmental perturbations.

Quantified reflex strategy using an iPod as a wireless accelerometer application

A primary aspect of a neurological evaluation is the deep tendon reflex, frequently observed through the patellar tendon reflex. The reflex response provides preliminary insight as to the status of the nervous system. A quantified reflex strategy has been developed, tested, and evaluated though the use of an iPod as a wireless accelerometer application integrated with a potential energy device to evoke the patellar tendon reflex. The iPod functions as a wireless accelerometer equipped with robust software, data storage, and the capacity to transmit the recorded accelerometer waveform of the reflex response wirelessly through email for post-processing. The primary feature of the reflex response acceleration waveform is the maximum acceleration achieved subsequent to evoking the patellar tendon reflex. The quantified reflex strategy using an iPod as a wireless accelerometer application yields accurate and consistent quantification of the reflex response.

TENDON REFLEX AND STRATEGIES FOR QUANTIFICATION, WITH NOVEL METHODS INCORPORATING WIRELESS ACCELEROMETER REFLEX QUANTIFICATION DEVICES, A PERSPECTIVE REVIEW

The deep tendon reflex is a fundamental aspect of a neurological examination. The two major parameters of the tendon reflex are response and latency, which are presently evaluated qualitatively during a neurological examination. The reflex loop is capable of providing insight into the status and therapy response of both upper and lower motor neuron syndromes. Attempts have been made to ascertain reflex response and latency; however, these systems are relatively complex, resource intensive, with issues of consistent and reliable accuracy. The solution presented is a wireless quantified reflex device using tandem three-dimensional (3D) wireless accelerometers to obtain response based on acceleration waveform amplitude and latency derived from temporal acceleration waveform disparity. Three specific aims have been established for the proposed wireless quantified reflex device:

(1) Demonstrate the wireless quantified reflex device is reliably capable of ascertaining quantified reflex response and latency using a quantified input.

(2) Evaluate the precision of the device using an artificial reflex system.

(3) Conduct a longitudinal study respective of subjects with healthy patellar tendon reflexes, using the wireless quantified reflex evaluation device to obtain quantified reflex response and latency.

Aim 1 has led to a steady evolution of the wireless quantified reflex device from a singular 2D wireless accelerometer capable of measuring reflex response to a tandem 3D wireless accelerometer capable of reliably measuring reflex response and latency. The hypothesis for aim 1 is that a reflex quantification device can be established for reliably measuring reflex response and latency for the patellar tendon reflex, comprised of an integrated system of wireless 3D MEMS accelerometers. Aim 2 further emphasized the reliability of the wireless quantified reflex device by evaluating an artificial reflex system. The hypothesis for aim 2 is that the wireless quantified reflex device can obtain reliable reflex parameters (response and latency) from an artificial reflex device. Aim 3 synthesizes the findings relevant to aim 1 and 2, while applying the wireless accelerometer reflex quantification device to a longitudinal study of healthy patellar tendon reflexes. The hypothesis for aim 3 is that during a longitudinal evaluation of the deep tendon reflex the parameters for reflex response and latency can be measured with a considerable degree of accuracy, reliability, and reproducibility. Enclosed is a detailed description of a wireless quantified reflex device with research findings and potential utility of the system, inclusive of a comprehensive description of tendon reflexes, prior reflex quantification systems, and correlated applications.

ACCELEROMETERS FOR QUANTIFICATION OF GAIT AND MOVEMENT DISORDERS: A PERSPECTIVE REVIEW

Accelerometers have become increasingly integrated in the biomedical field, as they are highly portable and capable of objectively and reliably quantifying motion. Two specific applications for accelerometers are the quantification of gait and movement disorders, such as Parkinson's disease and essential tremor. The evolution of accelerometers to their present status is discussed. Accelerometry is contrasted with more traditional means for accessing gait and movement disorders. Advances in the research validation of accelerometers for the characterization of gait and movement disorders, such as essential tremor and Parkinson's disease, are addressed. The review concludes with the advancement of three-dimensional (3D) wireless accelerometers and pertinent future implications.

QUANTIFIED DEEP TENDON REFLEX DEVICE FOR RESPONSE AND LATENCY, THIRD GENERATION

Deep tendon reflex is fundamental for a neurological examination. A hyperactive reflex response is correlated with spasticity, which can also be associated with the degree of damage to the supraspinal input, essentially assessing the severity of traumatic brain injury. Clinical evaluation of the myotatic stretch reflex is provided by the National Institute of Neurological Disorders and Stroke (NINDS) Myotatic Reflex Scale (0 to 4); however, the results of the NINDS Myotatic Reflex Scale vary in terms of interpretation and lack temporal data. Deep tendon reflex can assess the severity and degree of peripheral neuropathy. Subsequent to the neurological examination, suspect patients are often referred to a specialist for definitive electrodiagnostic testing. A study by Cocito found that 28% of the prescriptions for testing were considered to be inappropriate. Therefore, the solution is a fully quantified tendon reflex evaluation system. The input force of the reflex hammer is derived from a predetermined potential energy setting. Tandem wireless three-dimensional (3D) microelectromechanical systems (MEMS) accelerometers quantify the output and latency time of the reflex. The wireless 3D MEMS accelerometers are positioned to a standard anchor point near the ankle and reflex hammer swing arm. Reflex response is quantified by the maximum and minimum components of the acceleration profile. The temporal disparity between hammer strike and response defines the latency of the reflex loop. The quantified data collected from wireless 3D MEMS accelerometers are conveyed to a portable computer. Enclosed are the initial test and evaluation and the description of such a device, which quantitatively evaluates the reflex response and latency using wireless 3D MEMS accelerometers, while demonstrating precision for reproducibility.

FOURTH GENERATION WIRELESS REFLEX QUANTIFICATION SYSTEM FOR ACQUIRING TENDON REFLEX RESPONSE AND LATENCY

An intrinsic aspect of the standard neurological examination is the deep tendon reflex. A clinician is tasked with qualitatively evaluating reflex parameters, such as reflex response and latency. The tendon reflex is capable of providing preliminary insight with respect to dysfunction of the central and peripheral nervous systems. The qualitative assessment of the tendon reflex can be classified through the implementation of an ordinal scale, such as the NINDS scale which spans five ordinal components from 0 to 4. The reliability and accuracy of the ordinal-scale method for classifying reflex characteristics have been demonstrated to be an issue of controversy. Ordinal scales lack the capacity to properly classify the temporal features of the tendon reflex. Electrodiagnostic testing traditionally provides higher fidelity evaluation of peripheral neuropathy; however, a study by Cocito et al., has discovered 28% of the prescriptions were inappropriate. The fourth-generation wireless reflex quantification system provides a less resource intensive, highly accurate, reliable, and reproducible alternative. The patellar tendon reflex is evoked through a predetermined potential energy derived swing arm attached to a standard reflex hammer. Tandem wireless 3D MEMS accelerometers quantify reflex response and latency. The reflex response maximum and minimum are acquired from the wireless 3D MEMS accelerometer positioned above the ankle joint. The latencies derived from the maximum and minimum of the reflex responses are derived from the temporal disparity relative to the acceleration waveforms of the reflex response and swing arm evoking the tendon reflex. The fourth-generation wireless reflex quantification system has been evolved with a more user-convenient wirelessly activated datalogger mode, which is subsequently downloaded to a local PC wirelessly. The wireless datalogger mode enables sampling at a greater rate relative to the real-time streaming data mode. An automated MATLAB software program is implemented for acquiring reflex parameters. Enclosed is the longitudinal study of the fourth-generation wireless reflex quantification system that demonstrates considerable precision for accuracy, reliability, and reproducibility. As a supplement to the research, a brief reflex modulation study is amended to the longitudinal study.

QUANTIFIED DEEP TENDON REFLEX DEVICE, SECOND GENERATION

The deep tendon reflex is a fundamental aspect of neurological examinations. The severity of and degree of recovery from a traumatic brain injury can be assessed by the myotatic stretch reflex. A hyperactive reflex response is correlated with spasticity, which can also be correlated with the degree of damage to the supraspinal input, in essence assessing the severity of traumatic brain injury. The myotatic stretch reflex is clinically evaluated by the National Institute of Neurological Disorders and Stroke (NINDS) reflex scale (0–4); however, this scale lacks temporal data and may also vary in interpretation. The solution is a fully quantified evaluation system of the myotatic stretch reflex, whereby a patellar hammer's force input is based on original potential energy and a microelectromechanical system (MEMS) accelerometer quantifies the output. The MEMS accelerometer is attached to a set anchor point near the ankle. The reflex amplitude is based on the maximum acceleration of the reflex response. The quantified data collected from MEMS accelerometers are transmitted by a portable computer (i.e. a Pocket PC). This paper describes a device that quantitatively evaluates the reflex response using accelerometers and that demonstrates precision for reproducibility.

Quadrupedal coordination of bipedal gait: implications for movement disorders

During recent years, evidence has come up that bipedal locomotion is based on a quadrupedal limb coordination. A task-dependent neuronal coupling of upper and lower limbs allows one to involve the arms during gait but to uncouple this connection during voluntarily guided arm/hand movements. Hence, despite the evolution of a strong cortico-spinal control of hand/arm movements in humans, a quadrupedal limb coordination persists during locomotion. This has consequences for the limb coordination in movement disorders such as in Parkinson's disease (PD) and after stroke. In patients suffering PD, the quadrupedal coordination of gait is basically preserved. The activation of upper limb muscles during locomotion is strong, similar as in age-matched healthy subjects although arm swing is reduced. This suggests a contribution of biomechanical constraints to immobility. In post-stroke subjects a close interactions between unaffected and affected sides with an impaired processing of afferent input takes place. An afferent volley applied to a leg nerve of the unaffected leg leads to a normal reflex activation of proximal arm muscles of both sides. In contrast, when the nerve of the affected leg was stimulated, neither on the affected nor in the unaffected arm muscles EMG responses appear. Muscle activation on the affected arm becomes normalized by influences of the unaffected side during locomotion. These observations have consequences for the rehabilitation of patients suffering movement disorders.

Locomotion in stroke subjects: interactions between unaffected and affected sides

The aim of this study was to evaluate the sensorimotor interactions between unaffected and affected sides of post-stroke subjects during locomotion. In healthy subjects, stimulation of the tibial nerve during the mid-stance phase is followed by electromyography responses not only in the ipsilateral tibialis anterior, but also in the proximal arm muscles of both sides, with larger amplitudes prior to swing over an obstacle compared with normal swing. In post-stroke subjects, the electromyography responses were stronger on both sides when the tibial nerve of the unaffected leg was stimulated compared with stimulation of the affected leg. This difference was more pronounced when stimuli were applied prior to swing over an obstacle than prior to normal swing. This indicates an impaired processing of afferent input from the affected leg resulting in attenuated and little task-modulated reflex responses in the arm muscles on both sides. In contrast, an afferent volley from the unaffected leg resulted in larger electromyography responses, even in the muscles of the affected arm. Arm muscle activations were stronger during swing over an obstacle than during normal swing, with no difference in electromyography amplitudes between the unaffected and affected sides. It is concluded that the deficits of the affected arm are compensated for by influences from the unaffected side. These observations indicate strong mutual influences between unaffected and affected sides during locomotion of post-stroke subjects, which might be used to optimize rehabilitation approaches.

Plantar Cutaneous Afferents Normalize the Reflex Modulation Patterns During Stepping in Chronic Human Spinal Cord Injury

Plantar cutaneous afferent transmission is critical for recovery of locomotion in spinalized animals, whereas a phase-dependent reflex modulation is apparent during fictive or real locomotion. In nine people with a chronic spinal cord injury (SCI) the effects of foot sole stimulation on the soleus H-reflex and tibialis anterior (TA) flexion reflex modulation patterns during assisted stepping were established on different days. The soleus H-reflex was elicited by posterior tibial nerve stimulation followed by a supramaximal stimulus 100 ms after the test H-reflex to control for movement of recording electrodes. The flexion reflex was evoked by sural nerve stimulation with a 30-ms pulse train, recorded from the ipsilateral TA muscle, and elicited at 1.2- to twofold the reflex threshold. During assisted stepping, spinal reflexes were conditioned by percutaneous stimulation of the ipsilateral metatarsals at threefold perceptual threshold with a 20-ms pulse train delivered at 9- to 11-ms conditioning-test intervals. Stimuli were randomly dispersed across the step cycle, which was divided into 16 equal bins. The conditioned soleus H-reflex was significantly facilitated at midstance and depressed during midswing when compared with the unconditioned soleus H-reflex recorded during stepping. Foot sole stimulation induced a significant facilitation of the long-latency TA flexion reflex before, during, and after stance-to-swing transition when compared with the unconditioned long-latency TA flexion reflex during stepping. This study provides evidence that plantar cutaneous afferents remarkably influence the soleus H-reflex and TA flexion reflex modulation patterns during stepping and support that actions of plantar cutaneous afferents onto spinal interneuronal circuits engaged in locomotion are manifested in a phase-dependent manner in chronic SCI subjects.

Contractures and involuntary muscle overactivity in severe brain injury

PRIMARY OBJECTIVE

The aim of the present study was to evaluate the association of contractures with an increase or reduction of non-spastic muscle overactivity due to severe cerebral damage.

METHODS AND PROCEDURES

Forty-five patients with tetraparesis after severe cerebral damage were investigated. Three groups were defined based on the presence of spasticity (revealed as resistance to passive stretch (= hypertonia)), and the presence of contracture of the relevant knee joint: Group(s) (17 patients with hypertonia without contracture), Group(s+c) (20 patients with hypertonia and contracture), and Group(c) (eight patients without hypertonia and with contracture). In all groups spontaneous involuntary muscle activity was assessed continuously over a 12-hour period through isometric measurement of knee joint flexion torque. A mathematical algorithm differentiated an hourly muscle activity spectrum (PI(h)). The frequency of peaks (peaks(h)) from the activity spectrum was determined.

MAIN OUTCOMES AND RESULTS

We revealed that Group(s) had higher PI(h) and more frequent peaks(h) compared with Group(s+c) and Group(c) (p<0.05). Group(c) had comparable PI(h) and peaks(h) compared with Group(s+c) (p>0.05).

CONCLUSION

The presence of contractures was associated with lower involuntary muscle overactivity in terms of lower PI(h) and less frequent peaks(h), indicating that contractures may be associated with reduced non-spastic positive features of the upper motor neurone syndrome in patients with severe brain damage.

Transmission in heteronymous spinal pathways is modified after stroke and related to motor incoordination

Changes in reflex spinal pathways after stroke have been shown to affect motor activity in agonist and antagonist muscles acting at the same joint. However, only a few studies have evaluated the heteronymous reflex pathways modulating motoneuronal activity at different joints. This study investigates whether there are changes in the spinal facilitatory and inhibitory pathways linking knee to ankle extensors and if such changes may be related to motor deficits after stroke. The early facilitation and later inhibition of soleus H reflex evoked by the stimulation of femoral nerve at 2 times the motor threshold of the quadriceps were assessed in 15 healthy participants and on the paretic and the non-paretic sides of 15 stroke participants. The relationships between this reflex modulation and the levels of motor recovery, coordination and spasticity were then studied. Results show a significant (Mann-Whitney U; P<0.05) increase in both the peak amplitude (mean±SEM: 80±22% enhancement of the control H reflex) and duration (4.2±0.5 ms) of the facilitation on the paretic side of the stroke individuals compared to their non-paretic side (36±6% and 2.9±0.4 ms) and to the values of the control subjects (33±4% and 2.8±0.4 ms, respectively). Moreover, the later strong inhibition observed in all control subjects was decreased in the stroke subjects. Both the peak amplitude and the duration of the increased facilitation were inversely correlated (Spearman r = −0.65; P = 0.009 and r = −0.67; P = 0.007, respectively) with the level of coordination (LEMOCOT) of the paretic leg. Duration of this facilitation was also correlated (r = −0.58, P = 0.024) with the level of motor recovery (CMSA). These results confirm changes in transmission in heteronymous spinal pathways that are related to motor deficits after stroke.

Wireless accelerometer reflex quantification system characterizing response and latency

The evaluation of the deep tendon reflex is a standard aspect of a neurological evaluation, which is frequently evoked through the patellar tendon reflex. Important features of the reflex are response and latency, providing insight to status for peripheral neuropathy and upper motor neuron syndrome. A wireless accelerometer reflex quantification system has been developed, tested, and evaluated. The reflex input is derived from a potential energy setting. Wireless accelerometers characterize the reflex hammer strike and reflex response acceleration waveforms, enabling the quantification of reflex response and latency. Spectral analysis of the reflex response acceleration waveform elucidates the frequency domain, opening the potential for new reflex classification metrics. The wireless accelerometer reflex quantification system yields accurate and consistent quantification of reflex response and latency.

Soleus H-reflex modulation during body weight support treadmill walking in spinal cord intact and injured subjects

The soleus H-reflex modulation pattern was investigated in ten spinal cord intact subjects during treadmill walking at varying levels of body weight support (BWS), and nine spinal cord injured (SCI) subjects at a BWS level that promoted the best stepping pattern. The soleus H-reflex was elicited by tibial nerve stimulation with a single 1-ms pulse at an intensity that the M-waves ranged from 4 to 8% of the maximal M-wave (M(max)). During treadmill walking, the H-reflex was elicited every four steps, and stimuli were randomly dispersed across the gait cycle which was divided into 16 equal bins. EMGs were recorded with surface electrodes from major left and right hip, knee, and ankle muscles. M-waves and H-reflexes at each bin were normalized to the M(max) elicited at 60-100 ms after the test reflex stimulus. For every subject, the integrated EMG area of each muscle was established and plotted as a function of the step cycle phase. The H-reflex gain was determined as the slope of the relationship between H-reflex and soleus EMG amplitudes at 60 ms before H-reflex elicitation for each bin. In spinal cord intact subjects, the phase-dependent H-reflex modulation, reflex gain, and EMG modulation pattern were constant across all BWS (0, 25, and 50) levels, while tibialis anterior muscle activity increased with less body loading. In three out of nine SCI subjects, a phase-dependent H-reflex modulation pattern was evident during treadmill walking at BWS that ranged from 35 to 60%. In the remaining SCI subjects, the most striking difference was an absent H-reflex depression during the swing phase. The reflex gain was similar for both subject groups, but the y-intercept was increased in SCI subjects. We conclude that the mechanisms underlying cyclic H-reflex modulation during walking are preserved in some individuals after SCI.

Changes in soleus H-reflex modulation after treadmill training in children with cerebral palsy

In healthy children, short latency leg muscle reflexes are profoundly modulated throughout the step cycle in a functionally meaningful way and contribute to the electromyographic (EMG) pattern observed during gait. With maturation of the corticospinal tract, the reflex amplitudes are depressed via supraspinal inhibitory mechanisms. In the soleus muscle the rhythmic part of the modulation pattern is present in children with cerebral palsy (CP), but the development of tonic depression with increasing age, as seen in healthy children, is disturbed. Treadmill training clinically improves the walking pattern in children with CP. Presuming that short latency reflexes contribute significantly to the walking pattern, a change in the modulation may occur after training. The aim of this study was to assess whether treadmill training also improves the soleus reflex modulation during gait in children with CP. Seven children with CP underwent brief treadmill training for 10 min a day over 10 consecutive days; all of them were functional walkers. Soleus Hoffmann (H-) reflexes were investigated during walking on a treadmill before the first, and one day after the last, training session. Treadmill training led to a considerable clinical improvement in gait velocity. After 10 days of training, soleus H-reflexes during gait were almost completely depressed during the swing phase. The complete suppression of the soleus H-reflex during the swing phase, which is also exhibited by healthy subjects, could reflect an improvement towards a functionally more useful pattern. In conclusion, treadmill training can induce changes in the modulation of short latency reflexes during gait.

Modulation of soleus H-reflexes during gait in children with cerebral palsy

In healthy adults, soleus H-reflexes are rhythmically modulated and generally depressed during gait compared with rest. From ages 6 to 13 yr, there is a progressive increase in the tonic inhibition of H-reflexes during walking, especially during the stance phase of the step cycle. In adults, rhythmic modulation and tonic depression are severely disturbed after bilateral spinal lesions but remain partly preserved after unilateral cerebral lesions. Children with diplegic cerebral palsy (CP) suffer from a bilateral supraspinal lesion of the corticospinal tract that occurs before the maturation of the CNS is complete. If supraspinal structures are involved in the tonic, but not rhythmic, age-dependent reflex depression, it could be hypothesized that the tonic reflex depression with age is disturbed in CP, whereas the rhythmic part of the modulation remains unaffected. To test this hypothesis, soleus H-reflexes were assessed during gait in 16 CP children aged 5-11 and 15-16 and compared with 25 age-matched healthy children walking at similar velocities. Although the rhythmic part of the modulation pattern was present in CP, there was no significant tonic reflex depression with age, thus reflecting a lack of maturation of the corticospinal tract. It is argued the rhythmic part of the modulation may be generated on a spinal or brain stem level and is therefore not affected by the bilateral supraspinal lesion, whereas the tonic depression that occurs with maturation of the CNS is under supraspinal control. In conclusion, the supraspinal structures affected in CP are therefore likely involved in this age-dependent tonic depression.

Spastic movement disorder: impaired reflex function and altered muscle mechanics

In clinical practice, signs of exaggerated tendon tap reflexes associated with muscle hypertonia are generally thought to be responsible for spastic movement disorders. Most antispastic treatments are, therefore, directed at the reduction of reflex activity. In recent years, however, researchers have noticed a discrepancy between spasticity as measured in the clinic and functional spastic movement disorders, which is primarily due to the different roles of reflexes in passive and active states, respectively. We now know that central motor lesions are associated with loss of supraspinal drive and defective use of afferent input with impaired behaviour of short-latency and long-latency reflexes. These changes lead to paresis and maladaptation of the movement pattern. Secondary changes in mechanical muscle fibre, collagen tissue, and tendon properties (eg, loss of sarcomeres, subclinical contractures) result in spastic muscle tone, which in part compensates for paresis and allows functional movements on a simpler level of organisation. Antispastic drugs can accentuate paresis and therefore should be applied with caution in mobile patients.

Single joint perturbation during gait: preserved compensatory response pattern in spinal cord injured subjects

OBJECTIVE

Responses to afferent input during locomotion are organized at the spinal level but modulated by supraspinal centers. The study aim was to examine whether supraspinal influences affect the behavior of complex electromyographic (EMG) responses to single limb perturbations during walking.

METHODS

Subjects with motor-complete (MCSCI), motor-incomplete spinal cord injury (MISCI), and non-disabled (ND) subjects participated. Hip or knee joint trajectory was briefly arrested by a robotic device at early or late swing phase. EMG responses from muscles of both legs were analyzed.

RESULTS

Perturbation-induced EMG responses of spinal cord injured and ND individuals were similar in basic structure, with the exception that tibialis anterior onset times were delayed for SCI subjects. Across all groups, perturbations in late swing (i.e., near the swing-to-stance transition) were associated with shorter muscle onset times and higher EMG amplitudes. Knee perturbations were associated with shorter muscle response onset times, while hip perturbations elicited higher response amplitudes. EMG responses were also evoked in muscles contralateral to the perturbation.

CONCLUSIONS

These data indicate that neuronal circuits within the spinal cord deprived of normal supraspinal input respond to swing phase perturbations in a manner that is similar to that of the intact spinal cord.

SIGNIFICANCE

The adult human spinal cord is capable of generating complex, phase-appropriate responses much as has been observed in studies of human infants and in spinal animals.

Physiological evaluation of gait disturbances post stroke

A large proportion of stroke survivors have to deal with problems in mobility. Proper evaluations must be undertaken to understand the sensorimotor impairments underlying locomotor disorders post stroke, so that evidence-based interventions can be developed. The current electrophysiological, biomechanical, and imagery evaluations that provide insight into locomotor dysfunction post stroke, as well as their advantages and limitations, are reviewed in this paper. In particular, electrophysiological evaluations focus on the contrast of electromyographic patterns and integrity of spinal reflex pathways during perturbed and unperturbed locomotion between persons with stroke and healthy individuals. At a behavioral level, biomechanical evaluations that include temporal distance factors, kinematic and kinetic analyses, as well as the mechanical energy and metabolic cost, are useful when combined with electrophysiological measures for the interpretation of gait disturbances that are related to the control of the central nervous system or secondary to biomechanical constraints. Finally, current methods in imaging and transcranial magnetic stimulation can provide further insight into cortical control of locomotion and the integrity of the corticospinal pathways.

Modulation of soleus H-reflexes during gait in healthy children

During locomotion spinal short latency reflexes are rhythmically modulated and depressed compared to rest. In adults this modulation is severely disturbed after bilateral spinal lesions indicating a role for supra-spinal control. Soleus reflex amplitudes are large in the stance phase and suppressed in the swing phase contributing to the reciprocal muscle activation pattern required for walking. In early childhood the EMG pattern during gait underlies an age-dependent process changing from co-contraction of agonists and antagonists to a reciprocal pattern at the age of 5-7 years. It is unknown whether at this stage apart from the EMG also reflexes are modulated, and if so, whether the reflex modulation is fully mature or still underlies an age-dependent development. This may give important information about the maturation of CNS structures involved in gait control. Soleus Hoffmann H-reflexes were investigated in 36 healthy children aged 7-16 years during treadmill walking at 1.2 km/h and 3.0 km/h. At 7 years old a rhythmic modulation similar to adults was observed. The H-reflex size during the stance phase decreased significantly with age while the maximum H-reflex (H (max)) at rest remained unchanged. At 3.0 km/h H-reflexes were significantly larger during the stance phase and smaller during the swing phase as compared to 1.2 km/h but the age-dependent suppression was observed at both walking velocities. In conclusion H-reflex modulation during gait is already present in young children but still underlies an age-dependent process independent of the walking velocity. The finding that the rhythmic part of the modulation is already present at the age of 7 years may indicate that the supra-spinal structures involved mature earlier than those involved in the tonic reflex depression. This may reflect an increasing supra-spinal control of spinal reflexes under functional conditions with maturation.

Influence of posture and stimulus parameters on post-activation depression of the soleus H-reflex in individuals with chronic spinal cord injury

In non-disabled (ND) individuals, reflexes are modulated by influences related to physiologic state (e.g., posture, joint position, load) and activation history. Repeated activation of the H-reflex results in post-activation depression (PAD) of the response amplitude. The modulation associated with physiologic state and activation history is suppressed or abolished in individuals with spinal cord injury (SCI). While posture is known to affect H-reflex amplitude and PAD in non-disabled individuals, the effect of posture on PAD in SCI individuals is not known. Further, while the amount of PAD is also known to be influenced by the stimulus rate and by the amplitude of the evoked reflex, the interaction of posture with stimulus parameters has not been previously investigated in either group. We investigated differences in PAD of the soleus H-reflex between SCI subjects and ND subjects during sitting versus supported standing. Subjects were tested using paired conditioning-test stimulus pulses of 2.5s and 5s interpulse intervals (ISI) and with stimulus intensity adjusted to evoke reflex responses of 20% and 40% of the maximum motor response. We found standing posture to be associated with significantly less PAD in SCI subjects compared to ND subjects. In both groups, shorter ISIs and smaller reflex amplitudes were associated with greater PAD of the H-reflex. These results indicate that postural influences on post-activation modulation, while present, are impaired in individuals with chronic incomplete SCI.

Plasticity of spinal cord reflexes after a complete transection in adult rats: relationship to stepping ability

Changes in epidurally induced (S1) spinal cord reflexes were studied as a function of the level of restoration of stepping ability after spinal cord transection (ST). Three types of responses were observed. The early response (ER) had a latency of 2.5 to 3 ms and resulted from direct stimulation of motor fibers or motoneurons. The middle response (MR) had a latency of 5 to 7 ms and was monosynaptic. The late response (LR) had a latency of 9 to 11 ms and was polysynaptic. After a complete midthoracic ST, the LR was abolished, whereas the MR was facilitated and progressively increased. The LR reappeared about 3 wk after ST and increased during the following weeks. Restoration of stepping induced by epidural stimulation at 40 Hz coincided with changes in the LR. During the first 2 wk post-ST, rats were unable to step and electrophysiological assessment failed to show any LR. Three weeks post-ST, epidural stimulation resulted in a few steps and these coincided with reappearance of the LR. The ability of rats to step progressively improved from wk 3 to wk 6 post-ST. There was a continuously improved modulation of rhythmic EMG bursts that was correlated with restoration of the LR. These results suggest that restoration of polysynaptic spinal cord reflexes after complete ST coincides with restoration of stepping function when facilitated by epidural stimulation. Combined, these findings support the view that restoration of polysynaptic spinal cord reflexes induced epidurally may provide a measure of functional restoration of spinal cord locomotor networks after ST.

The tonic stretch reflex and spastic hypertonia after spinal cord injury

The operational definition of spasticity is focused on increased resistance of joints to passive rotation and the possible origin of this increased resistance in the induced tonic stretch reflex (TSR). This term is applied in the context of both cerebral and spinal injury, implying that a similar reflex mechanism underlies the two disorders. From recent studies it is clear that increased passive joint resistance in resting limbs following stroke is highly correlated with the induced TSR, but this evidence is lacking in spinal injury. The contribution of the TSR to hypertonia in spinal cord injury (SCI) is unclear and it is possible that hypertonia has a different origin in SCI. The contribution of resting and activated TSR activity to joint stiffness was compared in SCI and normal subjects. The magnitude of the TSR in ankle dorsiflexors (DF) and plantarflexors (PF) and mechanical ankle resistive torque were measured at rest and over a range of contraction levels in normal subjects. Similar measures were made in 13 subjects with SCI to the limits of their range of voluntary contraction. Normals and SCI received a pseudo-sinusoidal stretch perturbation of maximum amplitude +/- 20 degrees and frequency band 0.1-3.5 Hz that was comparable to that used in manual clinical testing of muscle tone. Elastic resistance and resonant frequency of the ankle joint, after normalization for limb volume, were significantly lower in complete and incomplete SCI than normal subjects. No reflex response related to stretch velocity was observed. Resting DF and PF TSR gain, when averaged over the tested band of frequencies, were significantly lower in complete SCI than in resting normal subjects (<0.5 microV/deg). Linear regression analysis found no significant relationship between TSR gain and resting joint stiffness in SCI. Mean TSR gain of DFs and PFs at rest was not correlated with the subject variables: age, time since SCI, level of injury, Frankel score, number of spasms per day, Ashworth score or anti-spastic medication. DF and PF reflex gain were linearly related to voluntary contraction level and regression analysis produced similar slopes in incomplete SCI and normal subjects. Hence TSR loop gain was not significantly increased in SCI at any equivalent contraction level. Extrapolation of the regression lines to zero contraction level predicted that reflex threshold was not reduced in SCI. Low frequency passive stretches did not induce significant TSR activity in the resting limbs of any member of this SCI group. The TSR thus did not contribute to their clinical hypertonia. Other reflex mechanisms must contribute to hypertonia as assessed clinically. This result contrasts with our similar study of cerebral spasticity after stroke, where a comparable low frequency stretch perturbation produced clear evidence of increased TSR gain that was correlated with the hypertonia at rest. We conclude that a low frequency stretch perturbation clearly distinguished between spasticity after stroke and SCI. Spasticity in the two conditions is not equivalent and care should be taken in generalizing results between them.

Suppression of cutaneous reflexes by a conditioning pulse during human walking

There are two ways in which responses to successive unexpected stimuli are attenuated, namely through habituation and conditioning. For the latter, it suffices that the unexpected stimulus is preceded by another just perceivable stimulus. In spinal cord reflexes this is termed conditioning, while in brainstem reflexes this is usually referred to as prepulse inhibition. Cutaneous reflexes in Tibialis Anterior (TA) are particularly strong during gait and they are thought to involve a transcortical loop. Can these reflexes be suppressed by giving a brief pulse prior to a reflex-evoking pulse given to the same nerve? To examine this question, electromyographic signals were recorded in healthy humans during walking. Sural nerve stimulation (train of five pulses (1 ms duration)) at 200 Hz were applied at two times perception threshold during different phases of the step cycle. The preceding pulse (single pulse of 1 ms at same intensity) was applied to the same nerve 150 ms before the reflex-evoking pulse train. Conditioning stimulation with a single pulse lowered significantly the following reflex response in the ipsilateral TA but much less in other muscles such as biceps femoris. The preceding pulse did not disturb the phase-dependent modulation or the typical reflex reversal. The finding that TA is selectively involved indicates that the suppressing mechanism may involve the motor cortex, which is known to be involved in the control of TA. The conditioning pulse did not cause a reduction in background activity. Therefore, the suppression of the reflex responses points to a premotoneuronal source such as presynaptic inhibition.

The amplitude modulation of the quadriceps H-reflex in relation to the knee joint action during walking

Previously the modulation of the quadriceps H-reflex has only been investigated in the initial part of the gait cycle, and it was suggested that the quadriceps H-reflex modulates with relative high reflex gain at heel contact and decreases during the subsequent part of stance (Dietz et al. 1990b). The objectives of the present study was to elaborate on the previous results by increasing the measurement resolution around heel contact and include additional measures in order to relate the H-reflex modulation to the mechanical function of the knee extensors throughout the gait cycle. EMG profiles were measured in quadriceps and the antagonistic hamstring muscles simultaneously with the knee joint kinematics in ten subjects during treadmill walking at preferred speed. H-reflex excitability was measured in vastus lateralis (VL) and rectus femoris (RF) at 11 selected positions during the gait cycle. The resulting excitability curves showed a significant modulation of the quadriceps H-reflex during the gait cycle. The H-reflex amplitude increases shortly after heel contact and reflex inhibition is present in the remaining part of stance and most of the swing phase. The modulation of the quadriceps H-reflex during walking does not follow the classical pattern of reciprocal inhibition between antagonistic muscles. It is suggested that at least during the stance phase the modulation of the quadriceps H-reflex is controlled by presynaptic inhibition. The present results confirm the idea that the excitability of the quadriceps H-reflex is controlled to comply with the different mechanical demands on the muscle during the gait cycle in humans.

Identification of intrinsic and reflex ankle stiffness components in stroke patients

Reflex and intrinsic contributions to ankle stiffness were examined in 11 stroke patients with clinical evidence of ankle spasticity and nine gender-matched and age-matched controls. Subjects lay supine with one foot placed in a custom-fitted boot attached to an electro-hydraulic actuator. They were instructed to relax while pseudo-random binary sequence perturbations were applied to their ankle joint. The ankle position and torque, as well as EMG from the ankle dorsiflexors and plantarflexors were recorded. These were used to identify reflex and intrinsic components of ankle stiffness, using a non-linear, parallel-cascade, system identification method. Results demonstrated that the majority of stroke patients (7/11) had ankle stiffness similar to that of control subjects. In contrast, a minority of stroke patients (4/11) had an abnormal increase in ankle stiffness, most of which could be attributed to an increased reflex gain. Reflex stiffness increased as the ankle was dorsiflexed in all subjects. These results differ from a previous study showing that reflex gain and intrinsic stiffness were increased in all patients with spinal cord injury. This difference may reflect the different topography of the lesions in the two neurological conditions.