Horizontal foot orientation affects the distribution of neural drive between gastrocnemii during plantarflexion, without changing neural excitability

The distribution of activation among muscles from the same anatomical group can be affected by the mechanical constraints of the task, such as limb orientation. For example, the distribution of activation between the gastrocnemius medialis (GM) and lateralis (GL) muscles during submaximal plantarflexion depends on the orientation of the foot in the horizontal plane. The neural mechanisms behind these modulations are not known. The overall aim of this study was to determine whether the excitability of the two gastrocnemius muscles is differentially affected by changes in foot orientation. Nineteen males performed isometric plantarflexions with their foot internally (toes-in) or externally (toes-out) rotated. GM and GL motor unit discharge characteristics were estimated from high-density surface electromyography to estimate neural drive. GM and GL corticospinal excitability and intracortical activity were assessed using transcranial magnetic stimulation through motor-evoked potentials. The efficacy of synaptic transmission between Ia-afferent fibers and α-motoneurons of the GM and GL was evaluated through the Hoffmann reflex. We observed a differential change in neural drive between GM (toes-out > toes-in) and GL (toes-out < toes-in). However, there was no foot orientation-related modulation in corticospinal excitability of the GM or GL, either at the cortical level or through modulation of the efficacy of Ia-α-motoneuron transmission. These results demonstrate that change in the motor pathway excitability is not the mechanism controlling the different distribution of neural drive between GM and GL with foot orientation.NEW & NOTEWORTHY Horizontal foot orientation affects the distribution of neural drive between the gastrocnemii during plantarflexion. There is no foot orientation-related modulation in the corticospinal excitability of the gastrocnemii, either at the cortical level or through modulation of the efficacy of Ia-α-motoneuron transmission. Change in motor pathway excitability is not the mechanism controlling the different distribution of neural drive between gastrocnemius medialis and lateralis with foot orientation.

Segmental motor neuron dysfunction in amyotrophic lateral sclerosis: Insights from H reflex paradigms

INTRODUCTION/AIMS

In amyotrophic lateral sclerosis (ALS), the role of spinal interneurons in ALS is underrecognized. We aimed to investigate pre- and post-synaptic modulation of spinal motor neuron excitability by studying the H reflex, to understand spinal interneuron function in ALS.

METHODS

We evaluated the soleus H reflex, and three different modulation paradigms, to study segmental spinal inhibitory mechanisms. Homonymous recurrent inhibition (H'RI ) was assessed using the paired H reflex technique. Presynaptic inhibition of Ia afferents (H'Pre ) was evaluated using D1 inhibition after stimulation of the common peroneal nerve. We also studied inhibition of the H reflex after cutaneous stimulation of the sural nerve (H'Pos ).

RESULTS

Fifteen ALS patients (median age 57.0 years), with minimal signs of lower motor neuron involvement and good functional status, and a control group of 10 healthy people (median age 57.0 years) were studied. ALS patients showed reduced inhibition, compared to controls, in all paradigms (H'RI 0.35 vs. 0.11, p = .036; H'Pre 1.0 vs. 5.0, p = .001; H'Pos 0.0 vs. 2.5, p = .031). The clinical UMN score was a significant predictor of the amount of recurrent and presynaptic inhibition.

DISCUSSION

Spinal inhibitory mechanisms are impaired in ALS. We argue that hyperreflexia could be associated with dysfunction of spinal inhibitory interneurons. In this case, an interneuronopathy could be deemed a major feature of ALS.

Effects of Dry Needling on Spasticity in Multiple Sclerosis Evaluated Through the Rate-Dependent Depression of the H Reflex: A Case Report

Background

Spasticity is a common symptom of multiple sclerosis (MS) which affects mobility. Dry Needling (DN) has shown a reduction in spasticity in neuromuscular conditions such as stroke and spinal cord injury although the mechanism of action is still unclear. In spastic individuals, the Rate-Dependent Depression (RDD) of the H reflex is decreased as compared to controls and analyzing the effects of DN in the RDD may help to understand its mechanism of action.

Objective

To evaluate the effect of Dry Needling on spasticity measured by the Rate-dependent Depression (RDD) of the H reflex in an MS patient.

Methods

Three time points were evaluated: Pre-intervention (T1), Post-intervention assessments were carried out in the seventh week at two-time points: Before DN (T2) and After DN (T3). Main outcomes included the RDD and latency of the H reflex in the lower limbs at stimulation frequencies of 0.1, 1, 2, and 5 Hz in a five consecutive pulses protocol.

Results

An impairment of the RDD of the H reflex at frequencies ≥1 Hz was found. Statistically significant differences were found when comparing the mean RDD of the H reflex in Pre-intervention compared to Post-intervention at 1, 2, and 5 Hz stimulation frequencies. Mean latencies were statistically lower when comparing Pre- vs Post-intervention.

Conclusion

Results suggest a partial reduction in spasticity represented by decrease of the excitability of the neural elements involved in the RDD of the H reflex following DN. The RDD of the H reflex could be implemented as an objective tool to monitor changes in spasticity in larger DN trials.

AGE-RELATED CHANGES IN SENSORY AND MOTOR COMPONENTS OF THE HOFFMANN-REFLEX PATHWAY OF THE FLEXOR CARPI RADIALIS

Ageing is accompanied by numerous changes within the sensory and motor components of the muscle spindle pathway. To further document these age-related changes, this study compared the characteristics of the Hoffmann (H) reflex and M wave, evoked with several pulse durations, between young and old adults. The H-reflex and M-wave recruitment curves were recorded at rest in the flexor carpi radialis of 12 young (21-36 yrs) and 12 older adults (62-80 yrs). For each pulse duration (0.05-ms, 0.2-ms, and 1-ms), the maximal M-wave (M_MAX ) and H-reflex (H_MAX ) amplitude, the M-wave amplitude associated with H_MAX (M_Hmax ), and the H-reflex amplitude for a stimulus intensity evoking an M-wave of 5% M_MAX (H_M5% ) were measured. The strength-duration time constant and response threshold were estimated from the charge/stimulus-duration relation for the H reflex and M wave. Results indicate that varying pulse duration mainly induces a similar effect on H-reflex and M-wave recruitment curves between young and older adults. Regardless of pulse duration, old adults had lesser H_MAX (p = 0.029) and H_M5% (p<0.001) but greater M_Hmax (p<0.001). The H-reflex and M-wave response thresholds were greater in old than young adults (p=0.003), but the strength-duration time constant was lesser in old than young adults for the H reflex (p=0.048) but not the M wave (p=0.21). These results suggest greater age-related changes in the sensory than the motor component of the H-reflex pathway, which may be indicative of a greater loss of sensory than motor axons or alterations of synapses between Ia afferents and motor neurones.

Revisiting the use of Hoffmann reflex in motor control research on humans

Research in movement science aims at unravelling mechanisms and designing methods for restoring and maximizing human functional capacity, and many techniques provide access to neural adjustments (acute changes) or long-term adaptations (chronic changes) underlying changes in movement capabilities. First described by Paul Hoffmann over a century ago, when an electrical stimulus is applied to a peripheral nerve, this causes action potentials in afferent axons, primarily the Ia afferents of the muscle spindles, which recruit homonymous motor neurons, thereby causing an electromyographic response known as the Hoffmann (H) reflex. This technique is a valuable tool in the study of the neuromuscular function in humans and has provided relevant information in the neural control of movement. The large use of the H reflex in motor control research on humans relies in part to its relative simplicity. However, such simplicity masks subtleties that require rigorous experimental protocols and careful data interpretation. After highlighting basic properties and methodological aspects that should be considered for the correct use of the H-reflex technique, this brief narrative review discusses the purpose of the H reflex and emphasizes its use as a tool to assess the effectiveness of Ia afferents in discharging motor neurones. The review also aims to reconsider the link between H-reflex modulation and Ia presynaptic inhibition, the use of the H-reflex technique in motor control studies, and the effects of ageing. These aspects are summarized as recommendations for the use of the H reflex in motor control research on humans.

Regulation of primary afferent depolarization and homosynaptic post-activation depression during passive and active lengthening, shortening and isometric conditions

PURPOSE

This study aimed to determine whether the modulation of primary afferent depolarization (PAD) and homosynaptic post-activation depression (HPAD) are involved in the lower efficacy of Ia-afferent-α-motoneuron transmission commonly observed during lengthening compared to isometric and shortening conditions.

METHODS

15 healthy young individuals participated in two experimental sessions dedicated to measurement in passive and active muscle states, respectively. In each session, PAD, HPAD and the efficacy of Ia-afferent-α-motoneuron transmission were evaluated during lengthening, shortening and isometric conditions. PAD was evaluated with D1 inhibition technique. Posterior tibial nerve stimulation was used to study HPAD and the efficacy of the Ia-afferent-α-motoneuron transmission through the recording of the soleus Hoffmann reflex (H reflex).

RESULTS

PAD was increased in lengthening than shortening (11.2%) and isometric (12.3%) conditions regardless of muscle state (P < 0.001). HPAD was increased in lengthening than shortening (5.1%) and isometric (4.2%) conditions in the passive muscle state (P < 0.05), while no difference was observed in the active muscle state. H reflex was lower in lengthening than shortening (- 13.2%) and isometric (- 9.4%) conditions in both muscle states (P < 0.001).

CONCLUSION

These results highlight the specific regulation of PAD and HPAD during lengthening conditions. However, the differences observed during passive lengthening compared to shortening and isometric conditions seem to result from an increase in Ia-afferent discharge, while the variations highlighted during active lengthening might come from polysynaptic descending pathways involving supraspinal centres that could regulate PAD mechanism.

[Very-early and early neuroelectrophysiological features of childhood Guillain-Barré syndrome]

OBJECTIVES

To study the very-early and early neuroelectrophysiological features of childhood Guillain-Barré syndrome (GBS) and their association with clinical diagnosis.

METHODS

A retrospective analysis was performed on the neuroelectrophysiological data of 43 children with GBS. According to the interval from onset to neuroelectrophysiological examination, the children were divided into a very-early examination group with 18 children (an interval from onset to the examination of ≤7 days) and an early examination group with 25 children (an interval from onset to the examination of 7 to ≤14 days). The children with acute flaccid paralysis, matched for the examination time of GBS children, were enrolled as the control group. The abnormal rates of neuroelectrophysiological parameters were compared between the above groups. According to the results of the H reflex test, the GBS children were divided into an abnormal H reflex group and a normal H reflex group, and related clinical data were compared between the two groups.

RESULTS

Compared with the control group, the very-early and early examination groups had a significantly higher abnormal rate of H reflex (P<0.05), while there was no significant difference in the abnormal rates of F wave, motor nerve conduction, and sensory nerve conduction (P>0.05). Compared with the normal H reflex group, the abnormal H reflex group had a significantly shorter interval from onset to the time of confirmed diagnosis (P<0.05).

CONCLUSIONS

Absence of the H reflex is a valuable parameter of neuroelectrophysiological abnormalities in the early stage of GBS and can help with the diagnosis of GBS.

The effect of acute intermittent hypoxia on human limb motoneurone output

NEW FINDINGS

What is the central question of this study? Does a single session of repeated bouts of acute intermittent hypoxic breathing enhance the motoneuronal output of the limb muscles of healthy able-bodied participants? What is the main finding and its importance? Compared to breathing room air, there were some increases in motoneuronal output following acute intermittent hypoxia, but the increases were variable across participants, in time after the intervention and depended on which neurophysiological measure was checked.

ABSTRACT

Acute intermittent hypoxia (AIH) induces persistent increases in output from rat phrenic motoneurones. Studies in people with spinal cord injury suggest AIH improves limb performance, perhaps via postsynaptic changes at cortico-motoneuronal synapses. We assessed whether limb motoneurone output in response to reflex and descending synaptic activation is facilitated after one session of AIH in healthy able-bodied volunteers. Fourteen participants completed two experimental days, either AIH or a sham intervention (randomised crossover design). We measured H-reflex recruitment curves and homosynaptic post-activation depression (HPAD) of the H reflex in soleus, and motor evoked potentials (MEPs) evoked by transcranial magnetic stimulation (TMS) and their recruitment curves, in first dorsal interosseous. All measurements were performed at rest and occurred at baseline, 0, 20, 40, and 60 minutes post-intervention. The intervention was 30 minutes of either normoxia (sham, FiO₂ ≈ 0.21) or AIH (alternate 1-minute hypoxia [FiO₂ ≈ 0.09], 1-minute normoxia). After AIH the H-reflex recruitment curve shifted leftward. Lower stimulation intensities were needed to evoke 5%, 50%, and 99% of the maximal H reflex at 40 and 60 minutes after AIH (P<0.04). The maximal H reflex, recruitment slope and HPAD, were unchanged after AIH. MEPs evoked by constant intensity TMS were larger 40 minutes after AIH (P = 0.027). There was no change in MEP recruitment or the maximal MEP. In conclusion, some measures of the evoked responses from limb motoneurones increased after a single AIH session, but only at discrete time points. It is unclear to what extent these changes alter functional performance. This article is protected by copyright. All rights reserved.

Influence of Spine Curvature on the Efficacy of Transcutaneous Lumbar Spinal Cord Stimulation

Transcutaneous spinal cord stimulation is a non-invasive method for neuromodulation of sensorimotor function. Its main mechanism of action results from the activation of afferent fibers in the posterior roots-the same structures as targeted by epidural stimulation. Here, we investigated the influence of sagittal spine alignment on the capacity of the surface-electrode-based stimulation to activate these neural structures. We evaluated electromyographic responses evoked in the lower limbs of ten healthy individuals during extension, flexion, and neutral alignment of the thoracolumbar spine. To control for position-specific effects, stimulation in these spine alignment conditions was performed in four different body positions. In comparison to neutral and extended spine alignment, flexion of the spine resulted in a strong reduction of the response amplitudes. There was no such effect on tibial-nerve evoked H reflexes. Further, there was a reduction of post-activation depression of the responses to transcutaneous spinal cord stimulation evoked in spinal flexion. Thus, afferent fibers were reliably activated with neutral and extended spine alignment. Spinal flexion, however, reduced the capacity of the stimulation to activate afferent fibers and led to the co-activation of motor fibers in the anterior roots. This change of action was due to biophysical rather than neurophysiological influences. We recommend applying transcutaneous spinal cord stimulation in body positions that allow individuals to maintain a neutral or extended spine.

Vibration decreases the responsiveness of Ia afferents and spinal motoneurons in humans

After vibration, Hoffmann reflex (H reflex) amplitude is depressed; however, the mechanisms underlying these phenomena remain unknown. This study investigated the influence of frequency and duration of vibration on the H reflex amplitude, heteronymous facilitation of the tendon jerk (T wave) mediated by group Ia afferents, and cervicomedullary motor evoked potential (CMEP) amplitude in 18 healthy human subjects. The H reflex of the flexor carpi radialis (FCR) was induced by median nerve stimulation at the elbow, and the conditioning FCR stimulation enhanced the T wave of the biceps brachii (BB). After vibration was applied to the FCR muscle belly, the amplitudes of the H reflex and heteronymous facilitation of the T wave were depressed; these influences persisted after the removal of vibration in all subjects. For the H reflex, there was no difference in the amount of depression among the frequencies of vibration used (57, 77, and 100 Hz). Higher frequencies of vibration were associated with longer recovery times of postvibration depression, and a longer duration of vibration was associated with a longer recovery time of the depression. Similar results were observed for heteronymous facilitation of the T wave, suggesting that the depression is caused by a decrease in postsynaptic potentials evoked by Ia afferents in spinal motoneurons; it was probably due to reduction in the number of Ia afferents recruited by the median nerve stimulation. Moreover, because the FCR CMEP amplitude was depressed after vibration, vibration should affect the responsiveness of spinal motoneurons. These mechanisms are considered to contribute to the H reflex depression after vibration.NEW & NOTEWORTHY Vibration decreased the responsiveness of Ia afferents from the muscle exposed to vibration, and the duration of depressive effect was modulated by the duration and frequency of the vibration: a longer duration and a higher frequency of vibration led to a longer recovery time of the depression. In addition to this presynaptic effect, it also depressed the responsiveness of spinal motoneurons, indicating postsynaptic inhibition through specific circuits triggered by Ia impulses.

Contralateral training effects of low-intensity blood-flow restricted and high-intensity unilateral resistance training

PURPOSE

Determine whether unilateral low-intensity blood-flow restricted (LIBFR) exercise is as effective as high-intensity (HI) resistance training for improving contralateral muscle strength.

METHODS

Thirty healthy adults (20-30 years) were randomly allocated to the following dynamic plantar-flexion training interventions: HI [75% of one-repetition maximum (1RM), 4 sets, 10 reps] and LIBFR [20% of 1RM, 4 sets, 30 + 15 + 15 + 15 reps]. Evoked V-wave and H-reflex recruitment curves, as well as maximal voluntary contraction (MVC) and panoramic ultrasound assessments of the trained and untrained soleus muscles were obtained pre-training, post-4 weeks of training and post-4 weeks of detraining.

RESULTS

Both interventions failed to increase contralateral MVC and muscle cross-sectional area (CSA). Yet, contralateral rate of torque development (RTD) was enhanced by both regimens (12-26%) and this was accompanied by heightened soleus EMG within the first milliseconds of the rising torque-time curve (14-22%; p < 0.05). These improvements were dissipated after detraining. Contralateral adaptations were not accompanied by changes in V-wave or H-reflex excitability. Conversely, LIBFR and HI elicited a similar magnitude of ipsilateral increase in MVC, RTD and CSA post-training (10-18%). Improvements in V-wave amplitude and soleus EMG were limited to the trained leg assigned to LIBFR training (p < 0.05). While gains in strength and CSA remained preserved post-4 weeks of detraining, this did not occur with RTD.

CONCLUSION

Since gains in RTD were similar between interventions, our findings indicate that both training regimens can be used interchangeably for improving contralateral rapid torque production. Ultimately, this may be beneficial in circumstances of limb immobilization after injury or surgery.

A Re-evaluation of Whether Non-monosynaptic Homonymous H Reflex Facilitation Tests Propriospinal Circuits

The C3-C4 propriospinal system is an important pathway mediating movement in cats; it contributes to movements in primates (including humans), and may have a role in recovery after lesion. Validated clinical tests of this system would find many applications, therefore we sought to test whether non-monosynaptic homonymous facilitation of the forearm flexor H reflex is mediated solely via a C3-C4 propriospinal pathway. In one anesthetized macaque monkey, median nerve stimulation elicited an H reflex in the flexor carpi radialis (FCR). Median nerve conditioning stimuli at sub-threshold intensities facilitated the H reflex, for inter-stimulus intervals up to 30 ms. Successive spinal surgical hemisections were then made. C2 lesion left the homonymous facilitation intact, suggesting mediation by spinal, not supraspinal pathways. Facilitation also remained after a second lesion at C5, indicating a major role for segmental (C7-C8) rather than propriospinal (C3-C4) interneurons. In separate experiments in five healthy human subjects, a threshold tracking approach assessed changes in peripheral axon excitability after conditioning stimulation. This was found to be enhanced up to 20 ms after the conditioning stimulus, and could partly, although not completely, underlie the H reflex facilitation seen. We conclude that homonymous facilitation of the H reflex in FCR can be produced by segmental spinal mechanisms, as well as by a supranormal period of nerve excitability. Unfortunately, this straightforward test cannot therefore be used for selective assessment of propriospinal circuits.

A waves in electroneurography: differential diagnosis with other late responses

Neurographic studies are an extension of clinical examination and are performed for the functional assessment of peripheral nerves. The study of motor and sensory conduction velocity and the presence, amplitude, morphology and symmetry of the response to electrical stimulation are crucial for the diagnosis and management of peripheral neuromuscular disorders. Neurography also plays an important role in the search for so-called late responses comprising the F wave, H reflex, axonal response and A wave. By analysing the parameters of each late wave, this paper addresses the pathophysiological features and the most common conditions impairing the physiology of late responses, with a special focus on A waves.

[Recurrent inhibition during Jendrassik maneuver]

Objective - Conflicting theoretical models exist regarding the mechanism related to the ability of the Jendrassik maneuver to reinforce reflex parameters. Our objective was to investigate if vigorous handgrip would induce changes in recurrent inhibition of soleus motoneurons. Method - Soleus H reflex was evoked by stimulating the tibial nerve at rest and during bilateral vigorous handgrip, alternating single (H1) and paired stimulation (H2). At paired stimulation we used interstimulus intervals of 10, 15, 20 and 25 ms and supramaximal test stimulus. H1- and H2-wave amplitudes were expressed as percentage of maximal M-wave amplitude. Conditioned H2 wave maximal (H2max) and minimal (H2) amplitudes evoked at rest and expressed as a percentage of the unconditioned H1max amplitude were compared with the corresponding values obtained during handgrip by means of paired Student test and Bonferroni correction. Subjects - At the study participated 28 healthy volunteers. Results - The H1max/Mmax × 100 values obtained during handgrip (37.5±10.1) were significantly higher than those obtained at rest (27.1±7.4). The H2max/H1max × 100-va-lues obtained at paired stimulation were significantly higher during handgrip than at rest, while no significant diffe-rence was found between the H2/H1max × 100-values obtained during handgrip and at rest respectively. Discussion - The H2max/H1max is determined by both the excitability of the motoneurons and the recurrent inhibition elicited by the conditioning stimulus, while H2/H1max indicates only the level of recurrent inhibition. According to our results the Renshaw cells retain their inhibitory effect on the soleus alpha motoneurons during remote muscle contraction. Conclusion - Soleus H reflex enhancement during Jendrassik maneuver is not due to decrease of recurrent inhibition.

Lack of effect on ambulation of dalfampridine-ER (4-AP) treatment in adult SMA patients

SMA is a genetically determined motor system disorder that results in muscle weakness, selective motor neuron death, muscle atrophy, and impaired functional mobility. In SMA model systems, long-term treatment with 4-aminopyridine (4-AP) has been shown to improve motor function. To assess tolerability and preliminary efficacy of 4-AP on walking ability, endurance and EMG in adult ambulatory SMA patients, we conducted a double blind, placebo control, crossover pilot study with dalfampridine (4-AP, 10 mg BID). The study is comprised of a short-term (2 weeks) treatment arm with 1-week washout and a long-term (6 weeks) treatment arm with a 2-week washout. The primary outcome measure, for which the study was powered, was the 6 min walk test (6MWT, distance and percent fatigue); secondary outcome measures were the Hammersmith Functional Motor Scale Expanded (HFMSE), Manual Muscle Testing (MMT), Myometry with Hand held Dynamometry, HHD) and Quantitative Gait Analyses. We performed electrophysiology, including CMAP and H-reflex, during the short-term treatment trial. The mean age of the 11 participants enrolled was 37.7 ± 11.9 years; 54.5% were male. Dalfampridine was safe and well tolerated and no patient suffered a serious adverse event related to treatment. We observed no statistically significant positive effects of dalfampridine treatment on our primary functional motor outcome (6MWT distance, fatigue). Dalfampridine had a positive effects on H-reflex and H/M ratio but not on CMAP amplitude. The effect on the H-reflex is of interest, as it suggests dalfampridine may enhance neuronal activity, an effect observed in SMA Drosophila and mouse models at doses (mg/kg) not recommended for clinical use. Larger studies with dalfampridine in SMA patients are needed to confirm our findings, especially in light of studies in other populations showing drug effects in only a subset of patients.

[Anal tone: Physiology, clinical and instrumental characteristics]

INTRODUCTION

The anal tone allows the maintenance of anorectal continence. Its regulation depends on spinal segmental mechanisms under supra-sacral control.

MATERIAL AND METHODS

A systematic review was performed using Medline database, according to PRISMA methodology, using following keywords anal tone ; anal sphincter ; anorectal function ; reflex ; digital rectal examination.

RESULTS

Anal hypertonia is an increase in the muscle's resistance to passive stretching. Muscular hypotonia is a decrease in muscle tone. It is associated with a decrease in resistance to passive mobilization. It is not possible to quantify the prevalence of anal tone alterations in the general population and in specific pathological conditions (urinary disorders, neurogenic or non-neurogenic anorectal disorders). In case of hypotonia, most often due to a lower motor neuron lesion, fecal incontinence may occur. Hypertonia (anal sphincter overactivity) is not always due to perineal spasticity. Indeed, in the majority of the cases, the cause of this anal hypertonia in a neurologic context, can be secondary to an upper motor neuron disease due to spinal or encephalic lesion, leading to recto-anal dyssynergia, giving distal constipation. In another way, this anal hypertonia can be purely behavioral, with no direct pathological significance. The evaluation of anal tone is clinical with validated scores but whose sensitivity is not absolute, and instrumental with, on the one hand, the measurement of anal pressure in manometry and, on the other hand, electrophysiological testing which still require validation in this indication.

CONCLUSION

Anal tone assessment is of interest in clinical practice because it gives diagnostic arguments for the neurological lesion and its level, in the presence of urinary or anorectal symptoms.

Bilateral and asymmetrical contributions of passive and active ankle plantar flexors stiffness to spasticity in humans with spinal cord injury

Spasticity is one of the most common symptoms present in humans with spinal cord injury (SCI); however, its clinical assessment remains underdeveloped. The purpose of the study was to examine the contribution of passive muscle stiffness and active spinal reflex mechanisms to clinical outcomes of spasticity after SCI. It is important that passive and active contributions to increased muscle stiffness are distinguished to make appropriate decisions about antispastic treatments and to monitor its effectiveness. To address this question, we combined biomechanical and electrophysiological assessments of ankle plantarflexor muscles bilaterally in individuals with and without chronic SCI. Spasticity was assessed using the Modified Ashworth Scale (MAS) and a self-reported questionnaire. We performed slow and fast dorsiflexion stretches of the ankle joint to measure passive muscle stiffness and reflex-induced torque using a dynamometer and the soleus H reflex using electrical stimulation over the posterior tibial nerve. All SCI participants reported the presence of spasticity. While 96% of them reported higher spasticity on one side compared with the other, the MAS detected differences across sides in only 25% of the them. Passive muscle stiffness and the reflex-induced torque were larger in SCI compared with controls more on one side compared with the other. The soleus stretch reflex, but not the H reflex, was larger in SCI compared with controls and showed differences across sides, with a larger reflex in the side showing a higher reflex-induced torque. MAS scores were not correlated with biomechanical and electrophysiological outcomes. These findings provide evidence for bilateral and asymmetric contributions of passive and active ankle plantar flexors stiffness to spasticity in humans with chronic SCI and highlight a poor agreement between a self-reported questionnaire and the MAS for detecting asymmetries in spasticity across sides.NEW & NOTEWORTHY Spasticity affects a number of people with spinal cord injury (SCI). Using biomechanical, electrophysiological, and clinical assessments, we found that passive muscle properties and active spinal reflex mechanisms contribute bilaterally and asymmetrically to spasticity in ankle plantarflexor muscles in humans with chronic SCI. A self-reported questionnaire had poor agreement with the Modified Ashworth Scale in detecting asymmetries in spasticity. The nature of these changes might contribute to the poor sensitivity of clinical exams.

Interrogating interneurone function using threshold tracking of the H reflex in healthy subjects and patients with motor neurone disease

OBJECTIVE

The excitability of the lower motoneurone pool is traditionally tested using the H reflex and a constant-stimulus paradigm, which measures changes in the amplitude of the reflex response. This technique has limitations because reflex responses of different size must involve the recruitment or inhibition of different motoneurones. The threshold-tracking technique ensures that the changes in excitability occur for an identical population of motoneurones. We aimed to assess this technique and then apply it in patients with motor neurone disease (MND).

METHODS

The threshold-tracking approach was assessed in 17 healthy subjects and 11 patients with MND. The soleus H reflex was conditioned by deep peroneal nerve stimulation producing reciprocal Ia and so-called D1 and D2 inhibitions, which are believed to reflect presynaptic inhibition of soleus Ia afferents.

RESULTS

Threshold tracking was quicker than the constant-stimulus technique and reliable, properties that may be advantageous for clinical studies. D1 inhibition was significantly reduced in patients with MND.

CONCLUSIONS

Threshold tracking is useful and may be preferable under some conditions for studying the excitability of the motoneurone pool. The decreased D1 inhibition in the patients suggests that presynaptic inhibition may be reduced in MND.

SIGNIFICANCE

Reduced presynaptic inhibition could be evidence of an interneuronopathy in MND. It is possible that the hyperreflexia is a spinal pre-motoneuronal disorder, and not definitive evidence of corticospinal involvement in MND.

Heteronymous H reflex in temporal muscle as sign of hyperexcitability in ALS patients

OBJECTIVE

The stimulation of the masseteric nerve elicits a homonymous and a heteronymous H reflex in the masseter muscle and in the temporalis one. The presence of the H reflex may be considered a sign of upper motor neuron (UMN) involvement in amyotrophic lateral sclerosis (ALS) patients. The aim of this study was to evaluate the presence of the heteronymous H reflex in patients with ALS and compare it with normal subjects.

METHODS

We enrolled 36 ALS patients and 52 healthy subjects. We stimulated the masseteric nerve in the infratemporal fossa and recorded the muscle responses ipsilaterally to the stimulation.

RESULTS

The heteronymous temporalis H reflex was elicitable in 88.9% of ALS patients and in none of the controls.

CONCLUSION

The heteronymous H reflex does not disappear when the stimulation intensity is increased. It can be used as sign of UMN involvement and may prove useful in patients with suspected MND/ALS with purely lower motor neurons (LMN) signs and no signs of UMN involvement.

SIGNIFICANCE

The heteronymous H reflex is present far more often in ALS patients than in healthy people. It is a simple test that may be used to detect UMN involvement in patients in whom the only evident signs are LMN impairment, improving diagnosis of ALS.

Reduced corticospinal responses in older compared with younger adults during submaximal isometric, shortening, and lengthening contractions

The aim of this study was to assess differences in motor performance, as well as corticospinal and spinal responses to transcranial magnetic and percutaneous nerve stimulation, respectively, during submaximal isometric, shortening, and lengthening contractions between younger and older adults. Fifteen younger [26 yr (SD 4); 7 women, 8 men] and 14 older [64 yr (SD 3); 5 women, 9 men] adults performed isometric and shortening and lengthening dorsiflexion on an isokinetic dynamometer (5°/s) at 25% and 50% of contraction type-specific maximums. Motor evoked potentials (MEPs) and H reflexes were recorded at anatomical zero. Maximal dorsiflexor torque was greater during lengthening compared with shortening and isometric contractions ( P < 0.001) but was not age dependent ( P = 0.158). However, torque variability was greater in older compared with young adults ( P < 0.001). Background electromyographic (EMG) activity was greater in older compared with younger adults ( P < 0.005) and was contraction type dependent ( P < 0.001). As evoked responses are influenced by both the maximal level of excitation and background EMG activity, the responses were additionally normalized {[MEP/maximum M wave (M_max)]/root-mean-square EMG activity (RMS) and [H reflex (H)/M_max]/RMS}. (MEP/M_max)/RMS and (H/M_max)/RMS were similar across contraction types but were greater in young compared with older adults ( P < 0.001). Peripheral motor conduction times were prolonged in older adults ( P = 0.003), whereas peripheral sensory conduction times and central motor conduction times were not age dependent ( P ≥ 0.356). These data suggest that age-related changes throughout the central nervous system serve to accommodate contraction type-specific motor control. Moreover, a reduction in corticospinal responses and increased torque variability seem to occur without a significant reduction in maximal torque-producing capacity during older age. NEW & NOTEWORTHY This is the first study to have explored corticospinal and spinal responses with aging during submaximal contractions of different types (isometric, shortening, and lengthening) in lower limb musculature. It is demonstrated that despite preserved maximal torque production capacity corticospinal responses are reduced in older compared with younger adults across contraction types along with increased torque variability during dynamic contractions. This suggests that the age-related corticospinal changes serve to accommodate contraction type-specific motor control.

Modulation of the Hoffmann reflex in soleus and medial gastrocnemius during stair ascent and descent in young and older adults

BACKGROUND

The Hoffmann (H) reflex can provide relevant information on spinal control of leg muscles during locomotor tasks in young and older adults.

RESEARCH QUESTION

Is the H reflex in the leg muscles differently modulated during stair gait in young and older adults?

METHOD

The H reflex in soleus (SOL) and medial gastrocnemius (MG) (normalized to the maximal M-wave amplitude obtained during upright standing; M_max) was recorded in 19 young and 18 older adults during upright standing, and stair ascent and descent of a 3-step staircase.

RESULTS

H-reflex amplitude during upright standing was greater in young than older adults for SOL (48% vs. 26% M_max; p = 0.001) and MG (23% vs. 14% M_max; p = 0.02). When data were averaged across groups during stair ascent, H-reflex amplitude in SOL increased from 15% M_max at the beginning of the stance phase to 29% M_max at mid-stance, then decreased to be 4% M_max in the swing phase. During stair descent, H-reflex amplitude was maximal (20% M_max) at the beginning of the stance phase, decreased to 5% M_max at the end of stance, and increased to 11% M_max in the swing phase. Similar adjustments were observed for the H reflex in MG for both ascent and descent. H-reflex modulation during gait cycle (relative to upright standing) is less pronounced in older adults (p < 0.05). However, no difference was observed between subgroups of young and older adults matched for H-reflex amplitude in upright standing. In both groups, H-reflex modulation was not associated with changes in background electromyographic activity.

SIGNIFICANCE

This study indicates that the H reflex is modulated within the stair gait cycle during ascent and descent. Although its magnitude was slightly reduced, the overall modulation of the H reflex is not affected in healthy older adults.

F wave, A wave, H reflex, and blink reflex

Late responses include F waves, A waves, H reflex, and the blink reflex. These responses help enhance routine nerve conduction studies. Despite the use of F waves in multiple clinical applications, their studies can technically challenge even the most experienced electromyographers. They vary in latency, amplitude, and configuration, whereas A waves show no change in latency or morphology. Electrical stimulation of the supraorbital branch of the trigeminal nerve on one side results in a reflexive activation of the facial nucleus causing contraction of the orbicularis oculi muscle, short latency R1 ipsilaterally, and long latency R2 bilaterally. F waves can help determine the presence of a polyneuropathy. A waves can reflect axonal damage. H reflexes provide nerve conduction measurements along the entire length of the nerve, demonstrating abnormalities in neuropathies and radiculopathies. Abnormalities in the blink reflex can suggest the presence of an acoustic neuroma or a demyelinating polyneuropathy, which can affect the cranial nerves. This reflex, which also needs appropriate technical expertise, helps to assess cranial nerves V and VII along with their connections in the pons and medulla. The blink reflex, the electrical version of the corneal reflex, represents a polysynaptic reflex.

Reversible conduction failure on the deep tendon reflex response recording in early Guillain-Barré syndrome

Objective

To describe the case of a patient with Guillain-Barré syndrome (GBS) showing early reversible conduction failure (RCF) detected by means of serial deep tendon reflex response (T-reflex) study.

Methods

A 36-year-old woman had a 5-day history of foot and hand paresthesias ascending to thighs and arms, throbbing interscapular and neck pain, mild to moderate tetraparesis, and areflexia. Nerve conduction studies (NCS) were performed on days 7 and 33 after onset.

Results

NCS showed an equivocal electrophysiologic pattern, just an isolated distal RCF being detected on the right radial nerve at initial examination. Motor latency on deltoid muscle after Erb's point stimulation was preserved. Sensory conduction velocities were normal or slightly slowed. Somatosensory evoked potentials from median and tibial nerves were normal. Initially, F-wave study demonstrated reversible abnormalities, consisting of multiple A waves and low F-wave persistence, minimal F-wave latencies being preserved. Biceps brachii T-reflex was normal, whereas Achilles T-reflex was absent bilaterally, appearing on the second study with normal T-wave morphology and latency, thus conforming to the requirements for RCF diagnosis. Soleus H-reflex was also initially absent.

Conclusions

Serial T-reflex study is a useful technique for detecting early RCF of proximal nerve trunks in early GBS.

Significance

T-reflex is useful tool for GBS in association with NCS.

Age-related reversal of spinal excitability during anticipatory postural control

Introduction

An internal perturbation of standing balance activates muscles critical for maintaining balance and is preceded by anticipatory postural adjustments (APAs). In healthy younger adults, a measure of spinal excitability in the form of the Hoffmann (H) reflex becomes depressed during APAs but how aging affects the reflex control of APAs is unknown.

Methods

We compared H reflex excitability profiles in the right soleus muscle, indirectly indicating APA, between younger (n = 11, age 19–24 years), middle-aged (n = 10, age 37–56 years), and older healthy adults (n = 11, age 63–78 years). Subjects rapidly raised the right-dominant arm in response to an auditory cue. The H reflex was evoked 120 ms, 100 ms, 80 ms, 60 ms, 40 ms, 20 ms, and 0 ms before as well as 20 ms after the onset of the right anterior deltoid muscle activation. For data processing, each trial was controlled for the corresponding background EMG activity before normalizing the standing data to the data in sitting in the 8 time bins.

Results

All subjects showed a silent period in the soleus background electromyographic activity, suggesting the presence of APA. We found that the stereotypical H reflex depression associated with APAs in younger adults was reduced in middle-aged adults and reversed to facilitation in older adults. The depression occurred in 10 out of 11 younger adults, whereas all 11 older adults exhibited facilitation.

Conclusion

Because APAs are organized at the supraspinal level, we speculate a supraspinal origin of the age-related reflex facilitation during APAs.

Lipopolysaccharide-induced inflammation does not alter muscle spindle afferent mechanosensation or sensory integration in the spinal cord of adult mice

Inflammation is known to alter nervous system function, but its effect on muscle spindle afferent mechanosensation and sensory integration in the spinal cord has not been well studied. We tested the hypothesis that systemic inflammation induced by an intraperitoneal injection of the endotoxin lipopolysaccharide (LPS; 7.5 × 105 endotoxin units/kg 18 h before experiment) would alter muscle spindle afferent mechanosensation and spinal cord excitability to Group Ia input in male and female adult C57Bl/6 mice. LPS injection caused a systemic immune response, evidenced by decreased white blood cell, monocyte, and lymphocyte concentrations in the blood, increased blood granulocyte concentration, and body weight loss. The immune response in both sexes was qualitatively similar. We used an in vitro muscle-nerve preparation to assay muscle spindle afferent response to stretch and vibration. LPS injection did not significantly change the response to stretch or vibration, with the exception of small decreases in the ability to entrain to high-frequency vibration in male mice. Similarly, LPS injection did not alter spinal cord excitability to Group Ia muscle spindle afferent input as measured by the Hoffman's reflex test in anesthetized mice (100 mg/kg ketamine, 10 mg/kg xylazine). Specifically, there were no changes in M or H wave latencies nor in the percentage of motor neurons excited by electrical afferent stimulation (Hmax /Mmax ). Overall, we found no major alterations in muscle proprioceptor function or sensory integration following exposure to LPS at a dose and time course that causes changes in nociceptor function and central processing.

Region-specific modulation of tendon reflex along human rectus femoris muscle

INTRODUCTION

We investigated regional differences in amplitude modulation of the spinal reflex along the human rectus femoris (RF) muscle to test the hypothesis that this muscle is regionally regulated at the spinal cord or a higher level.

METHODS

Surface electromyography was conducted at six different sites along the RF muscle during the conditioned patellar tendon reflex in eight healthy young men.

RESULTS

A significant difference in the reflex amplitude among the channels was observed during 20% of the maximal voluntary contraction (MVC) and there was a significant difference in normalized reflex amplitude between 10 and 20% of the MVC at most proximal channel (p < 0.05), but not at the other channels (p > 0.05), during knee flexion of the ipsilateral leg.

DISCUSSION

From the results in the present study, we infer that the amplitude modulation of the tendon reflex within the RF muscle is regionally regulated, and that this regulation is dependent on the performed tasks.

Normal tendon reflexes despite absent sensory nerve action potentials in CANVAS: a neurophysiological study

CANVAS is a recently defined progressive ataxic syndrome with impairment of vestibular, somatosensory and cerebellar function due to atrophic degeneration of dorsal root ganglia and dorsal columns, of cranial nerve somatosensory ganglia, of vestibular ganglia and vestibular nerves and of cerebellar Purkinje cells. While all patients eventually develop sensory impairment in a non-length dependent pattern and lose sensory nerve action potentials, some retain their tendon reflexes. Here we study 5 CANVAS patients with absent sensory nerve action potentials but intact, even brisk Achilles tendon reflexes and, in 4, preserved H reflexes in the upper and lower limbs. These findings imply that dorsal root ganglion neurons subserving cutaneous afferents more vulnerable than those subserving muscle afferents. Our findings have a clinical message: preservation of the Achilles tendon jerk does not exclude a large fibre peripheral neuronopathy.

Spinal and corticospinal pathways are differently modulated when standing at the bottom and the top of a three-step staircase in young and older adults

PURPOSE

This study investigated the modulation of spinal (group Ia afferents) and corticospinal pathways when young (22.7 ± 1.3 years) and older adults (72.2 ± 7.9 years) stood at the bottom and at the top of a three-step staircase equipped with force platforms.

METHOD

Changes in submaximal H-reflex amplitude (H ₅₀) and slope of the H-reflex input-output relation (spinal pathway), and in amplitude of motor-evoked potentials (MEP) triggered by transcranial magnetic stimulation (corticospinal pathway) at two intensities (1.1× and 1.2× motor threshold) were recorded in soleus when subjects stood as steady as possible downstairs and upstairs. The centre of pressure (CoP) excursion was analyzed in the time and frequency domains in both conditions.

RESULTS

Regardless of age, the mean CoP velocity was greater when standing upstairs (11.1 ± 3.5 mm s^-1) than downstairs (9.0 ± 2.3 mm s^-1; p = 0.002). The CoP power spectral density (PSD) in the 0-0.5 Hz band was greater upstairs than downstairs (+18.4%; p = 0.03) whereas PSD in the 2-20Hz frequency band was lesser (-41%) upstairs than downstairs (p < 0.001), regardless of age. In both groups, the H ₅₀ amplitude (-30.6%; p < 0.001) and slope of H-reflex input-output relation (-10.2%; p = 0.002) were lesser when standing upstairs than downstairs, whereas no significant difference was observed in MEP amplitude and silent period between balance conditions (p > 0.05).

CONCLUSION

These results indicate a lower dependence on spinal pathway to control soleus motor neurones when standing upstairs than downstairs accompanied by a change in postural control. This suggests that healthy older adults preserved their ability to adjust postural control to environmental demands.

Optimization of surface electrodes location for H-reflex recordings in soleus muscle

The Hoffmann reflex (H reflex) is extensively used to investigate the spinal motor neuron excitability in healthy and pathological subjects. Obtaining a representative and robust amplitude estimation of the H reflex is of marked relevance in clinical as well as in research applications. As for the motor responses, this issue is strictly related to the electrode positioning, especially for large, pinnate muscles such as the triceps surae. In this study we investigated the effect of electrode position on soleus H-reflex amplitude. A grid of 96 electrodes was used to identify maximal H reflexes (Hmax) across the whole soleus region available for surface recording. The spatial distribution of Hmax amplitude detected in monopolar and single-differential derivations was used to determine where greatest reflex responses were detected from soleus. For both derivations and for all participants, largest Hmax were detected consistently over the central soleus region, in correspondence of the muscle superficial aponeurosis. Indeed, the amplitude of Hmax provided by conventional electrodes (1cm² area, 2cm apart) located centrally was significantly greater (median: 35% for monopolar and 79% for single-differential derivations) than that obtained medially, where surface electromyograms are typically recorded from soleus. Computer simulations, used to assist in the interpretation of results, suggest the soleus pinnate architecture was the key determinant of the medio-lateral variability observed for the experimental Hmax. The presented results provide a clear indication for electrode positioning, of crucial relevance in applied studies aimed at eliciting H reflexes.

Clinical uses of H reflexes of upper and lower limb muscles

H reflexes can be recorded from virtually all muscles that have muscle spindles, but reflex reinforcement may be required for the reflex response to be demonstrable. This can allow conduction across proximal nerve segments and most nerve root segments commonly involved by pathology. Stimulus rate is critical in subjects who are at rest. However the reflex attenuation with higher rates is greatly reduced during a background contraction of the test muscle, with only minor changes in latency if any. In addition the contraction ensures that the reflex response occurs in the desired muscle. Reflex latencies should be corrected for height (or limb length) and age. Because the reflex discharge requires a synchronised volley in group Ia afferents, large increases in reflex latency occur rarely with purely sensory lesions. If the H reflex of soleus, quadriceps femoris or flexor carpi radialis is absent at rest but appears during a voluntary contraction at near-normal latency, there is either low central excitability or a predominantly sensory abnormality. With the former H reflexes will be difficult to elicit throughout the body. If H reflexes can be recorded at rest from muscles for which no reflex can normally be demonstrated, there is good evidence for hyperreflexia. In the context of possible ALS, this is an important finding when there is EMG evidence of chronic partial denervation in that muscle.

After-effects of pedaling exercise on spinal excitability and spinal reciprocal inhibition in patients with chronic stroke

PURPOSE OF THE STUDY

To evaluate the after-effects of pedaling on spinal excitability and spinal reciprocal inhibition in patients with post-stroke spastic hemiparesis.

MATERIALS AND METHODS

Twenty stroke patients with severe hemiparesis participated in this study and were instructed to perform 7 min of active pedaling and 7 min of passive pedaling with a recumbent ergometer at a comfortable speed. H reflexes and M waves of paretic soleus muscles were recorded at rest before, immediately after and 30 min after active and passive pedaling. The Hmax/Mmax ratio and H recruitment curve were measured. Reciprocal inhibition was assessed using the soleus H reflex conditioning test paradigm.

RESULTS

The Hmax/Mmax ratio was significantly decreased after active and passive pedaling exercise. The decreased Hmax/Mmax ratio after active pedaling lasted at least for 30 min. The H recruitment curve and reciprocal inhibition did not change significantly after active or passive pedaling exercise.

CONCLUSIONS

Pedaling exercise decreased spinal excitability in patients with severe hemiparesis. Pedaling may be effective in rehabilitation following stroke.

Delayed grip relaxation and altered modulation of intracortical inhibition with aging

Grip relaxation is a voluntary action that requires an increase in short-interval intracortical inhibition (SICI) in healthy young adults, rather than a simple termination of excitatory drive. The way aging affects this voluntary inhibitory action and timing of grip relaxation is currently unknown. The objective of this study was to examine aging-related delays in grip relaxation and SICI modulation for the flexor digitorum superficialis muscle during grip relaxation. The main finding was that young adults increased SICI to relax their grips, whereas older adults did not increase SICI with a prolonged grip relaxation time (p < 0.05 for both SICI modulation and grip relaxation time). A secondary experiment showed that both young and older adults did not change H reflex excitability during grip relaxation. Our data suggest that grip relaxation is mediated by increased cortical inhibitory output in young adults, and aging-related impairment in increasing cortical inhibitory output may hamper timely cessation of muscle activity. Our data also suggest a lesser role of the spinal circuits in grip muscle relaxation. This knowledge may contribute to understanding of aging-related movement deterioration and development of interventions for improving modulation of SICI to improve muscle relaxation and movement coordination.

Intradetrusorial Botulinum Toxin in Patients with Multiple Sclerosis: A Neurophysiological Study

Patients with multiple sclerosis (MS) often complain of urinary disturbances characterized by overactive bladder syndrome and difficulties in bladder emptying. The aim of the study was to investigate the pathophysiology of bladder dysfunction and the neurophysiological effects of intradetrusorial incobotulinum toxin A (BoNT/A) in patients with MS having both brain and spinal MS-related lesions. Twenty-five MS patients with neurogenic detrusor overactivity (NDO) underwent clinical evaluation and soleus Hoffmann reflex (H reflex) study during urodynamics. Of the 25 patients, 14 underwent a further session one month after intradetrusorial BoNT/A injection. Eighteen healthy subjects acted as the control. In healthy subjects, the H reflex size significantly decreased at maximum cystometric capacity (MCC), whereas in MS patients with NDO, the H reflex remained unchanged. In the patients who received intradetrusorial BoNT/A, clinical and urodynamic investigations showed that NDO improved significantly. Volumes at the first, normal and strong desire to void and MCC increased significantly. Despite its efficacy in improving bladder symptoms and in increasing volumes for first desire, normal and strong desire to void, BoNT/A left the H reflex modulation during bladder filling unchanged. In the MS patients we studied having both brain and spinal MS-related lesions, the H reflex size remained unchanged at maximum bladder filling. Since this neurophysiological pattern has been previously found in patients with spinal cord injury, we suggest that bladder dysfunction arises from the MS-related spinal lesions. BoNT/A improves bladder dysfunction by changing bladder afferent input, as shown by urodynamic findings on bladder filling sensations, but its effects on H reflex modulation remain undetectable.

Reduced postactivation depression of soleus H reflex and root evoked potential after transcranial magnetic stimulation

Postactivation depression of the Hoffmann (H) reflex is associated with a transient period of suppression following activation of the reflex pathway. In soleus, the depression lasts for 100-200 ms during voluntary contraction and up to 10 s at rest. A reflex root evoked potential (REP), elicited after a single pulse of transcutaneous stimulation to the thoracolumbar spine, has been shown to exhibit similar suppression. The present study systematically characterized the effect of transcranial magnetic stimulation (TMS) on postactivation depression using double-pulse H reflexes and REPs. A TMS pulse reduced the period of depression to 10-15 ms for both reflexes. TMS could even produce postactivation facilitation of the H reflex, as the second reflex response was increased to 243 ± 51% of control values at the 75-ms interval. The time course was qualitatively similar for the REP, yet the overall increase was less. While recovery of the H reflex was slower in the relaxed muscle, the profile exhibited a distinct bimodal shape characterized by an early peak at the 25-ms interval, reaching 72 ± 23% of control values, followed by a trough at 50 ms, and then a gradual recovery at intervals > 50 ms. The rapid recovery of two successively depressed H reflexes, ∼ 25 ms apart, was also possible with double-pulse TMS. The effect of the TMS-induced corticospinal excitation on postactivation depression may be explained by a combination of pre- and postsynaptic mechanisms, although further investigation is required to distinguish between them.

Independent modulation of corticospinal and group I afferents pathways during upright standing

Balance control during upright standing is accompanied by an increased amplitude of motor-evoked potentials (MEP) induced by transcranial magnetic stimulation and a decreased amplitude of the Hoffmann (H) reflex in the soleus muscle. Nonetheless, whether these observations reflect reciprocal adjustments between corticospinal and group I afferents pathways during upright standing remains unknown. To further investigate this question, cathodal transcranial direct current stimulation (c-tDCS) applied over the motor cortex and vibration of Achilles tendons were used to modify the excitability of corticospinal and group I afferent pathways, respectively. MEPs and H reflexes were recorded in the soleus muscle during upright standing with or without bilateral Achilles tendon vibration, these recordings being performed before and after 20 min of c-tDCS (1.5 mA) or sham stimulation applied over the sensorimotor cortex. The results indicate that tendon vibration increased MEP amplitude (+28%) and decrease (-68%) the H-reflex amplitude (p<0.05). After c-tDCS, MEP amplitude was reduced by 13% and 26% without and with tendon vibration (p<0.05), respectively. In contrast, no significant change occurred in H-reflex amplitude after c-tDCS. Regardless of the conditions (c-tDCS and tendon vibration), no significant correlation was observed between changes in MEP and H-reflex amplitudes. The results failed to demonstrate close reciprocal changes in soleus MEP and H-reflex amplitudes during upright standing. These original findings suggest independent adjustments in corticospinal and group I afferents pathways during upright standing.

Locomotor training improves premotoneuronal control after chronic spinal cord injury

Spinal inhibition is significantly reduced after spinal cord injury (SCI) in humans. In this work, we examined if locomotor training can improve spinal inhibition exerted at a presynaptic level. Sixteen people with chronic SCI received an average of 45 training sessions, 5 days/wk, 1 h/day. The soleus H-reflex depression in response to low-frequency stimulation, presynaptic inhibition of soleus Ia afferent terminals following stimulation of the common peroneal nerve, and bilateral EMG recovery patterns were assessed before and after locomotor training. The soleus H reflexes evoked at 1.0, 0.33, 0.20, 0.14, and 0.11 Hz were normalized to the H reflex evoked at 0.09 Hz. Conditioned H reflexes were normalized to the associated unconditioned H reflex evoked with subjects seated, while during stepping both H reflexes were normalized to the maximal M wave evoked after the test H reflex at each bin of the step cycle. Locomotor training potentiated homosynaptic depression in all participants regardless the type of the SCI. Presynaptic facilitation of soleus Ia afferents remained unaltered in motor complete SCI patients. In motor incomplete SCIs, locomotor training either reduced presynaptic facilitation or replaced presynaptic facilitation with presynaptic inhibition at rest. During stepping, presynaptic inhibition was modulated in a phase-dependent manner. Locomotor training changed the amplitude of locomotor EMG excitability, promoted intralimb and interlimb coordination, and altered cocontraction between knee and ankle antagonistic muscles differently in the more impaired leg compared with the less impaired leg. The results provide strong evidence that locomotor training improves premotoneuronal control after SCI in humans at rest and during walking.

Input-output characteristics of soleus homonymous Ia afferents and corticospinal pathways during upright standing differ between young and elderly adults

AIM

This study investigated the effects of ageing on the excitability of soleus homonymous Ia afferents and corticospinal pathways during bipedal upright standing.

METHODS

The input-output relations for the Hoffmann (H) reflex and motor-evoked potential (MEP) were computed for the soleus in response to electrical nerve stimulation and transcranial magnetic stimulation, respectively, in young (n = 16) and elderly (n = 16) adults. In subsets of subjects, the input-output relations were compared between normal and supported upright standing, and corticomotoneuronal excitability was assessed during upright standing with an H-reflex conditioning method. For the H-reflex and MEP threshold, maximal amplitude (Hmax , MEPmax ) and the slope of the input-output relation (Hslope , MEPslope ) were measured and normalized to the corresponding M-wave value.

RESULTS

In normal standing, the Hmax /Mmax [mean (SD); young: 48.3 (14.2)%; elderly: 17.3 (6.4)%] and Hslope /Mslope were significantly (P < 0.05) lower in elderly than in young adults, whereas the MEPmax /Mmax [young: 13.6 (7.5)%; elderly: 24.5 (12.8)%] and MEPslope /Mslope were greater in elderly adults (P < 0.05). The Hslope /Mslope and MEPslope /Mslope decreased and increased, respectively, from supported to normal standing for both age groups but more so in elderly adults. Furthermore, the conditioned H reflex was greater (P < 0.05) in elderly [175.1 (34.3)%] than in young adults [141.8 (29.8)%] during normal standing.

CONCLUSION

This is the first study that clearly indicates lower efficacy of Ia afferents to discharge spinal motor neurones accompanied by greater corticospinal excitability in elderly adults, suggesting an increased contribution of the descending drive in controlling soleus activity during upright standing with ageing.

Postactivation depression changes after robotic-assisted gait training in hemiplegic stroke patients

Postactivation depression is decreased in patients with spasticity and partially restored by physical exercise in spinal cord injured patients. Up until now, the possibility to modulate postactivation depression with motor training has never been explored in subjects with spasticity following brain lesions. Postactivation depression, assessed as frequency related depression of soleus H-reflex, was investigated before and after robotic-assisted gait training in a group of seven subjects with spastic hemiparesis following hemispheric stroke. Patients received three sessions per week of robotic-assisted gait training for a period of 4 weeks (12 sessions in total). Postactivation depression was measured before the treatment (T0), after the first session (T1) and after the last session (T2). Postactivation depression was quantified as the ratio between H-reflex amplitude at 1 Hz and at 0.1 Hz. The greater the 1 Hz/0.1 Hz ratio, the smaller the postactivation depression. Following robotic-assisted gait training, the 1 Hz/0.1 Hz ratio decreased from 0.79±0.26 at T0 to 0.56±0.18 at T1 and 0.58±0.13 at T2. Post hoc analysis showed a significant difference between T0 and T1 and between T0 and T2, stating an increase of postactivation depression. No significant differences were found between T1 and T2. This study provides the first demonstration that physical exercise can determine a partial normalization of postactivation depression in hemiparetic patients with spasticity following unilateral hemispheric stroke.

The effects of cervical traction on the soleus h reflex in stroke patients

The purpose of this study was to investigate the effects of cervical traction on the soleus H reflex amplitude in stroke patients. Cervical traction with intensity of 3 kgw was performed on six stroke patients. The soleus H reflex was evoked before, during, and after the cervical traction. All subjects showed smaller H reflex amplitudes in the second and third minutes after traction than before the traction. The results suggested that cervical traction with intensity of 3 kgw inhibited the soleus H reflex amplitude several minutes after traction in stroke patients.