Clinical uses of H reflexes of upper and lower limb muscles

Early Electrophysiology in Suspected Acute Guillain-Barré Syndrome: A Prospective Study of Comprehensive Testing

PURPOSE

Electrophysiologic changes in early Guillain-Barré Syndrome (GBS) can be nondiagnostic. Improved testing paradigms may improve hyperacute treatment.

METHODS

This work prospectively evaluated consecutive patients admitted to a metropolitan teaching hospital in Melbourne, Australia, with suspected acute GBS. We performed extensive neurophysiology at three different time points. Novel tests, including cutaneous silent periods, long latency reflexes, and contraction-induced H reflexes, were assessed.

RESULTS

Twenty-three participants were studied, including 13 cases of acute GBS. In total, 69% of acute cases of GBS were accurately diagnosed on the first nerve conduction study using published neurophysiologic criteria, with serial studies rarely altering the GBS subtype classification. Antidromic and orthodromic upper limb sensory studies were diagnostically equivalent. A sural sparing pattern was seen in 77% of cases of GBS at the first test. Long latency reflexes and contraction-induced H reflexes testing were abnormal in most participants but were limited by muscle weakness in some. Cutaneous silent periods testing was unobtainable in approximately 50% of cases because of weakness and did not discriminate from mimic disorders.

CONCLUSIONS

Abnormalities of long latency reflexes and contraction-induced H reflexes may be helpful where initial electrophysiology is nondiagnostic but are nonspecific. Cutaneous silent periods testing seems of limited value. Comprehensive testing provides diagnostic certainty in most cases of GBS from the very first study.

Is Intraoperative Muscle Motor Evoked Potential Variability due to Fluctuating Lower Motor Neuron Background Excitability?

PURPOSE

This pilot study tests the contribution of fluctuating lower motor neuron excitability to motor evoked potential (MEP) variability.

METHODS

In six pediatric patients with idiopathic scoliosis and normal neurologic examination, cascades of 30 intraoperative H-reflexes (HRs) and MEPs were evoked in the soleus muscle using constant-current stimulators and recorded through surface electrodes with a 20-second interstimulus interval. First, HRs were obtained with an intensity capable of evoking the maximum response. Subsequently, MEPs were obtained with double trains and an intensity of 700 to 900 mA. Coefficients of variation (CVs) of amplitude and area under the curve from HRs and MEPs were compared using a paired two-tailed Student t test. Coefficients of correlation between the mean CVs of HR and MEP parameters were also assessed.

RESULTS

Pooling the results from the six patients, the mean CV of amplitude from the MEP (24.6 ± 3) was significantly higher than that from the HR (3.5 ± 4.4) ( P = 0.000091). The mean CV of the MEP area under the curve (21.8 ± 4.8) was also statistically significantly higher than that from the HR area under the curve (3.4 ± 4.5) ( P = 0.00091). The coefficients of correlation of the mean CV of the HR amplitude and area under the curve compared with the corresponding values of the MEP were low ( r = 0.29) and very low ( r = 0.03), respectively.

CONCLUSIONS

Our results suggest that fluctuations in lower motor neuron excitability may be less important than previously thought to explain the magnitude of MEP variability. The efficacy of corticospinal volleys to recruit a larger and more stable lower motor neuron population would be critical to obtain reproducible MEPs.

Spatial-Temporal Dynamics of Evoked Action in Finger Flexors: Implications for Optimizing Transcutaneous Nerve Stimulation

OBJECTIVE

Transcutaneous nerve stimulation (TNS) is a promising approach for the neurorehabilitation of hand function; however, its effects on muscle activation patterns remain poorly understood. To investigate the spatial and temporal distributions of H-reflexes and M-waves in finger flexor muscles using multichannel TNS and high-density electromyography.

METHODS

Fifteen healthy participants underwent stimulation of the median and ulnar nerves, and the muscle activity and finger forces were recorded. Recruitment curves and spatial activation maps were constructed for the H-reflexes and M-waves across stimulation intensities and locations.

RESULTS

Considerable inter-individual variability was observed in the recruitment patterns and spatial distributions. Higher spatial congruence between the H-reflex and M-wave activation patterns in extrinsic arm muscles than in intrinsic hand muscles was associated with more efficient force production. The relationship between spatial activation patterns and force outputs varied across fingers, with earlier recruitment of index finger muscles.

CONCLUSION

This study provides new insights into the complex interplay between the afferent and efferent pathways in hand motor control. The associations between spatial congruence, recruitment patterns, and force production efficiency enhance our understanding of the neuromuscular activation mechanisms.

SIGNIFICANCE

These findings have implications for optimizing TNS protocols in neurorehabilitation and developing personalized interventions for individuals with impaired hand function.

Changes in gait asymmetry may be caused by adaptation of spinal reflexes

In a recent human study, we found that adaptive changes in step length asymmetry (SLA) are correlated with similar changes in the H-reflex gains of the leg muscles during split-belt treadmill locomotion. Although this observation indicated a closer link between gait asymmetry and spinal reflex adaptation, it did not reveal their causal relationship. To better understand this relationship, here we use a neuromuscular model of human walking whose control relies primarily on spinal reflexes. Subjecting the model to split-belt treadmill locomotion with different combinations of belt speed and reflex gain patterns, we find that belt speed changes increase the variability in SLA but do not result in consistent SLA patterns as observed in human experiments, whereas reflex gain changes do. Furthermore, we find that the model produces SLA patterns similar to healthy adults when its reflex gains are adapted in a way similar to the H-reflex changes we observed in our previous study. The model also predicts SLA patterns similar to the ones observed for cerebellar degeneration patients when the reflexes do not adapt beyond a sudden dip at the time the ipsilateral belt speed is lowered. Our results suggest that SLA does not arise from imposing belt speed changes but requires the change of the reflex gains and that the dynamic adjustment of these gains may be an essential part of human gait control when encountering unexpected environmental changes such as uneven speed changes in split-belt treadmill locomotion.NEW & NOTEWORTHY This work uses computational modeling to investigate the role of spinal reflex tuning during locomotor adaptation. We show, in simulation, that tuning spinal reflex gains leads to gait asymmetry adaptation, not vice versa, and that patterns of gait adaptation on a split-belt treadmill are mostly driven by tuning of spinal reflexes, and not by biomechanical disturbances triggered by belt changes. The model further hints at the cerebellum as the source of spinal reflex modulation.

Priming with transcutaneous spinal direct current stimulation followed by robotic exoskeleton gait training in individuals with spinal cord injury: A case series

OBJECTIVES

To investigate the safety and feasibility of repeated transcutaneous spinal direct current stimulation (tsDCS) as priming strategy during exoskeleton-assisted locomotor training in individuals with SCI and evaluate potential neurophysiologic and functional gait changes.

STUDY DESIGN

Case series experimental design.

SETTING

Research laboratory at a post-acute rehabilitation hospital.

PARTICIPANTS

Four participants with chronic incomplete SCI.

INTERVENTIONS

Four participants with chronic incomplete SCI received three weeks of training consisting of two types of interventions after baseline (A): 20-minute tsDCS (anode or cathode) applied over the spinous processes of T10 followed by 20-minute exoskeleton-assisted locomotor training (B, B1, B2) and 20-minute exoskeleton-assisted locomotor training (C, C1, C2). Each phase consisted of five consecutive intervention sessions. Two participants underwent sequence A-B1-C-B2 and two sequence A-C1-B-C2. Soleus Hoffmann Reflex (H-reflex) and gait speed (10-m walk test) were assessed on the first and fifth days of training for each training type.

OUTCOME MEASURES

Adverse skin reactions or other events, H-reflex (Hmax/Mmax ratio), and gait speed (10-m walk test).

RESULTS

No adverse events occurred. All participants tolerated tsDCS with no negative effects on their skin. Participants demonstrated varied responses in their H/M ratios after tsDCS followed by exoskeleton-assisted locomotor training as well as after exoskeleton-assisted locomotor training. No consistent pattern can be identified in this case series. Three participants showed an increase in gait speed after tsDCS combined with exoskeleton-assisted locomotor training.

CONCLUSIONS

Noninvasive repeated spinal stimulation can safely be used in individuals with incomplete SCI. Further large-scale research is necessary to determine the efficacy of tsDCS for priming the spinal cord in facilitating recovery of gait in individuals with SCI.

Epineural stimulation on distal brachial plexus for functional restoration of the upper limb in a primate study

Restoring upper limb function is critical in individuals with central paralysis, and hand control is a priority in patients with neurological impairments. Functional electrical stimulation with implantable electrodes targeting the peripheral nervous system has the potential to selectively recruit hand muscles and generate multiple functional hand movements. However, the implantation of electrodes in the forearm or elbow areas requires multiple incisions for surgery, and elbow joint movements cannot be performed. In this study, we designed and implanted two epineural cuffs on the median and radial nerves in the distal brachial plexus of a single Japanese macaque (Macaca fuscata) monkey. The cuffs were successfully placed via an axillary approach using a single incision. Electrical stimuli were applied to innervate the contraction patterns of the hand, forearm, and triceps muscles relevant to the median and radial nerves. The evoked potentials of the target muscles electrically stimulated the distal brachial plexus to reliably and selectively innervate the upper limb muscles at the functional group level. Our results demonstrated that the distal brachial plexus can be a useful stimulation site for upper limb muscle contraction and that the axillary approach enables electrode placement to peripheral nerves required for upper limb control.

Modulation of Primary Afferent Nerve Fiber (Ia) Reciprocal Inhibition Under Voluntary and Electrically Stimulated Muscle Conditions: Within-Subject Study Design

Background: Reciprocal inhibition (RI) is a spinal reflex that controls posture and movement. The modulation of spinal RI represented by the H-reflex has been studied, before and after voluntary contraction and electrical nerve stimulation but not during voluntary, electrically induced muscle contraction or a combination of voluntary and electrically induced muscle contractions. This study investigates the effects of the ongoing voluntary isometric contraction, the electrically induced isometric contraction, and the combination of voluntary with electrically induced isometric contraction of the Tibialis Anterior (TA) muscle on spinal RI represented by Soleus H-reflex. Methods: Eighteen healthy adults participated. Soleus H-reflex and M-response were measured during four different conditions as follows: (1) at rest, (2) electrically induced isometric contraction of the TA, (3) voluntary isometric contraction of the TA with a 1 kg force, and (4) combined voluntary and electrically induced isometric contraction of the TA with a 1 kg force. Results: The ANOVA clearly demonstrated significant differences in Soleus H-reflex amplitude across the four recording conditions (F3,16, 17.28, p < 0.001). The amplitude at rest was significantly higher than during electrically induced isometric contraction, voluntary isometric contraction, and the combined contraction conditions (p < 0.05). Furthermore, the amplitude recorded during the electrically induced isometric contraction condition significantly surpassed that of voluntary isometric contraction and the combined contraction conditions (p < 0.05). Moreover, there was no significant difference between Soleus H-reflex amplitude recorded during voluntary isometric contraction and the combined voluntary isometric contraction and electrically induced isometric contraction (p < 0.87). The combined voluntary isometric contraction and electrically induced isometric contraction condition had a higher inhibitory effect on the Soleus H-reflex with no significant differences from voluntary isometric contraction. Moreover, both were significantly better than electrically induced isometric contraction (p = 0.05). In terms of Soleus H-reflex latency, there was no significant difference among all four conditions (p > 0.05), meaning Soleus H-reflex latency was not influenced by the conditions. Conclusions: RI can be best modulated by combining voluntary with electrically induced isometric muscle contractions.

Reflex Responses in Muscles of the Lower Extremities Elicited by Transcutaneous Stimulation of Cauda Equina: Part 1. Methodology and Normative Data

INTRODUCTION

Transcutaneous electrical stimulation is used to stimulate the dorsal roots of the cauda equina. Multiple elicited responses recorded in the lower extremity muscles are called posterior root muscle reflexes (PRMRs). Normal PRMR values in the muscles of healthy lower extremities have yet to be determined.

METHODS

Thirty subjects without known lumbosacral spinal root illness were included in this study. Subsequently, they were subjected to transcutaneous electrical stimulation of the cauda equina. Posterior root muscle reflex was recorded in the four muscle groups of both lower extremities. We elicited multiple PRMR and examined their characteristics in order to establish normal electrophysiological parameter values.

RESULTS

Posterior root muscle reflex was successfully elicited in the tibialis anterior (96.7%), gastrocnemius (100%), quadriceps femoris (93.3%), and hamstring (96.7%). No statistically significant differences were found in the intensity of stimulation, latencies, or area under the PRMR between the right and left leg muscles. The area under PRMR varied significantly among the participants. Higher body weight and abdominal girth showed a significant positive correlation with stimulation intensity for eliciting PRMR, and a significant negative correlation with the area under PRMR. Older age showed a significant negative correlation with the success of eliciting PRMR and the area under the PRMR.

CONCLUSIONS

Posterior root muscle reflex is a noninvasive and successful method for eliciting selective reflex responses of cauda equina posterior roots. Obtained values could be used in future studies to evaluate the utility of this methodology in clinical practice. This methodology could improve testing of the proximal lumbosacral nervous system functional integrity.

The calcium channel blocker nimodipine inhibits spinal reflex pathways in humans

Voltage-sensitive calcium channels contribute to depolarization of both motor neurons and interneurons in animal studies, but less is known of their contribution to human motor control and whether blocking them has potential in future antispasmodic treatment in humans. Therefore, this study investigated the acute effect of nimodipine on the transmission of human spinal reflex pathways involved in spasticity. In a double-blinded, crossover study, we measured soleus muscle stretch reflexes and H reflexes and tibialis anterior cutaneous reflexes in 19 healthy subjects before and after nimodipine (tablet 60 mg) or baclofen (tablet 25 mg). Baclofen was used as a control to compare nimodipine's effects with known antispastic treatment. Changes in the size of the maximum H reflex (Hmax)/maximum direct motor response in muscle (Mmax) ratio and stretch and cutaneous reflexes following intervention with nimodipine and baclofen, respectively, were analyzed with a one-way repeated-measures (RM) ANOVA. Nimodipine significantly reduced the Hmax/Mmax ratio [F(2.5,42) = 15; P < 0.0001] and the normalized soleus stretch reflex [F(2.6,47) = 4.8; P = 0.0073] after administration. A similar tendency was seen after baclofen [Hmax/Mmax ratio: F(2.1,39) = 4.0, P = 0.024; normalized stretch reflex: F(2.8,50) = 2.4; P = 0.083]. The Mmax response was unaffected by either intervention. Interestingly, during voluntary soleus activation, the stretch reflex remained unchanged with either treatment. For the cutaneous reflexes, there was a trend toward reduced early inhibition [F(1.6,9.3) = 4.5; P = 0.050] and subsequent facilitation [F(1.3,8.0) = 4.3; P = 0.065] after nimodipine. No severe adverse effects were reported after nimodipine. These findings suggest that nimodipine acutely reduced electrophysiological measures related to spasticity in healthy individuals. The effect seemed located at the spinal level, and voluntary contraction counterbalanced the reduction of the stretch reflex, highlighting its relevance for future studies on antispastic therapies.NEW & NOTEWORTHY The calcium channel antagonist nimodipine significantly reduces the size of the soleus H reflex and stretch reflex in healthy individuals without affecting maximum direct motor response (Mmax) or the stretch reflex during voluntary activation. This underscores the importance of exploring nimodipine as a potential antispastic medication in the future.

Contraction-Induced H Reflexes of the Upper and Lower Limbs in Healthy Adults

PURPOSE

Contraction-induced H reflexes are a late neurophysiologic response elicited with submaximal nerve stimulation during isometric muscle contraction. Mediated by spinal pathways, like other H reflexes, their use has remained somewhat limited despite a long history of development dating back to the original description by Hoffman. There is a paucity of data on normal reference ranges, which this article aims to add to.

METHODS

Contraction-induced H reflexes were elicited from the first dorsal interosseous, flexor carpi radialis, and tibialis anterior bilaterally in 100 healthy volunteers. Reference values, including side-to-side variation, were calculated. Pearson test and multiple regression were used to evaluate the relationship of H-reflex latency to height, age, and sex of participants.

RESULTS

The mean onset latencies of 28.00, 17.44, and 31.10 ms were seen for first dorsal interosseous, flexor carpi radialis, and tibialis anterior muscles, respectively. The calculated allowable side-to-side latency difference in individual participants was 3 to 4 ms. A correlation to participant height was seen.

CONCLUSIONS

This work provides normal reference values of contraction-induced H reflexes to three muscles, including allowable side-to-side variation. The latter suggests that bilateral testing evaluating for asymmetry within an individual is likely to be optimally sensitive. The relationship to height is also confirmed.

Between-day variability of soleus H reflex recruitment curve parameters: how many stimulations are required to maximise the reproducibility of the measurements?

PURPOSE

The H reflex recruitment curve represents the gold standard for quantifying changes in spinal circuitries. However, there is no agreement on how many stimulations should be applied for each parameter. Thus, we explored the impact of varying the number of stimulations (3, 6, 9, 12 and 15 stimuli per intensity) on between-day reliability of soleus H reflex.

MATERIALS AND METHODS

Twenty healthy participants (11 males, 9 females; age: 22.4 ± 2.3 years) visited the laboratory on two days for H-recruitment curves construction, using a 3 s inter-stimuli interval. To explore whether H reflex parameters differed between days, without varying the number of stimulations, paired-sample t tests were performed. Relative and absolute reliability were calculated using the intraclass correlation coefficients (ICCs) and the coefficients of variation (CVs), respectively.

RESULTS

Mmax, Hmax/Mmax, H slope, Hthresh, current at 50% of Hmax and current at Hmax were not significantly different between days (all p > 0.05). ICCs of Mmax, Hmax/Mmax and Hthresh were all good (0.79-0.89). H slope ICCs were moderate (0.56-0.73) between 3 and 12, but good (0.75) with 15 stimulations. Current at Hmax ICCs were moderate (0.55-0.0.73) with all stimulations, except with 9 (good: 0.76). CVs of the current at Hmax were all moderate (between 5-to-10%). Hthresh CV was poor (>10%) with three, but moderate for other stimulations. Mmax, Hmax/Mmax and H slope CVs were generally poor.

CONCLUSIONS

9 stimulations are required to ensure higher between-day reliability of H reflex parameters, except for H slope that requires 15 stimulations.

Effects of Acute Lateral Ankle Sprain on Spinal Reflex Excitability and Time-to-Boundary Postural Control in Single-Leg Stance

Background/Objectives: Acute lateral ankle sprain (ALAS) affects balance, often assessed by changes in traditional center of pressure (COP) parameters. Spatiotemporal measures of COP and time-to-boundary (TTB) analysis may offer improved sensitivity in detecting postural deviations associated with ALAS. However, the neurophysiological mechanism underlying these changes remains unknown. This study aimed to explore the effects of ALAS on spinal reflex excitability in the fibularis longus (FL) during single-leg balance and TTB parameters following ALAS. Methods: Fourteen participants with and without ALAS were recruited within 14 days from the onset of the injury. We assessed FL spinal reflex excitability and postural control during a single-leg stance. The primary outcomes included the H/M ratio, H-latency, and TTB parameters. For H-reflex testing, the peripheral electrical stimulation was delivered at the sciatic nerve before bifurcating into the tibial and common fibular nerve while participants maintained a single-leg balance position with the involved side of the limb. The TTB parameters of the medial-lateral (ML) and anterior-posterior (AP) directions of the mean, SD, and minimum were assessed, which indicate postural correction and strategies. Results: Patients with ALAS had a significantly lower AP-TTB minimum compared with healthy uninjured controls, with a moderate effect size (p = 0.039; d = -0.83). However, there was no significant difference in the H/M ratio (ALAS: 0.29 ± 0.16 vs. CON: 0.24 ± 0.10; p = 0.258) and H-reflex latency (ALAS: 34.6 ± 1.92 vs. CON: 33.8 ± 1.75 ms; p = 0.277); Conclusions: These results indicate that reflex control at the spinal level may have a minimal role in response to balance deficits following ALAS.

The H-reflex study of the flexor carpi radialis muscle in healthy individuals

Objective

This study aimed to investigate the physiological and anatomical factors influencing the flexor carpi radialis (FCR) H-reflex and to establish reference values for FCR H-reflex parameters in relation to these factors.

Methods

The FCR H-reflexes, elicited by median nerve stimulation, were assessed in 80 healthy individuals both at rest and during isometric voluntary contraction (IVC). Multiple linear regression analyses were performed with H-reflex parameters as the dependent variables, while age, gender, height, arm length, and weight were included as independent variables.

Results

The FCR H-reflex was recorded bilaterally in nearly all healthy individuals (76 out of 80) during IVC, while it could be obtained in only 35% (28 out of 80) of these individuals at rest. During IVC, the maximum H-reflex amplitude (Hmax) and its ratio to the maximum M-response amplitude (Hmax/Mmax ratio) were significantly increased (p < 0.001). However, there were no changes in H-reflex latency, latency difference, conduction velocity (HRCV), or amplitude ratio (p > 0.05). In both conditions, age and arm length were the most important factors affecting H-reflex latency (p < 0.001), while HRCV was influenced only by age (p < 0.01). Women exhibited shorter H-reflex latencies (p < 0.01), and both Hmax amplitude and Hmax/Mmax ratio were higher in women during IVC (p < 0.05). The H-reflex amplitude ratio during IVC showed a tendency to decrease with age (p < 0.05).

Conclusion

These findings suggest that FCR H-reflexes are more reliably elicited during IVC, and that both physiological and anatomical factors should be considered when assessing H-reflex abnormalities.

Frequency-dependent effects of superimposed NMES on spinal excitability in upper and lower limb muscles

Superimposing neuromuscular electrical stimulation (NMES) on voluntary contractions has proven to be highly effective for improving muscle strength and performance. These improvements might involve specific adaptations occurring at cortical and spinal level. The effects of NMES on corticospinal activation seem to be frequency dependent and differ between upper and lower limb muscles. The aim of this study was to investigate acute responses in spinal excitability, as measured by H-reflex amplitude of flexor carpi radialis (FCR) and soleus (SOL) muscles, after NMES superimposed on voluntary contractions (NMES + ISO) at two different pulse frequencies (40 and 80 Hz). Conditions involved fifteen intermittent contractions at submaximal level. Before and after each condition, H-reflexes were elicited in FCR and SOL muscles. H-reflex amplitudes increased in FCR and SOL following both NMES + ISO at 40 and 80 Hz. The potentiation of the H-reflex was greater following the 40 Hz condition compared to 80 Hz, although no differences between muscles emerged. These findings indicated that superimposing NMES has an excitatory effect on spinal motoneurons in both upper and lower limb muscles with an overall greater response after low frequency NMES. Such facilitation could be associated to enhanced somatosensory stimuli conjunctly with higher supraspinal downward commands.

Synaptic dynamics linked to widespread elevation of H-reflex before peripheral denervation in amyotrophic lateral sclerosis

Changes in Hoffmann reflex (H-reflex) exhibit heterogeneity among patients with amyotrophic lateral sclerosis (ALS), likely due to phenotype diversity. Current knowledge primarily focuses on soleus H-reflex, which may demonstrate an initial increase before subsequent decline throughout the disease course. The main objective was to investigate other muscles, to determine whether H-reflex changes could be associated with patient phenotype (onset site, functional disabilities). Additional experiments were performed to elucidate the neurophysiological mechanisms underlying H-reflex modifications. In age- and sex-matched groups of control subjects and patients, we compared H-reflex recruitment curves in soleus, quadriceps, and forearm flexors. Additionally, we examined H-reflex and motor evoked potential (MEP) recruitment curves in quadriceps. Last, to assess potential changes in monosynaptic excitatory postsynaptic potentials (EPSPs) of both peripheral and cortical origins, we analyzed peristimulus time histograms (PSTHs) and peristimulus frequencygrams (PSFs) of single motor units, along with H-reflex occurrence after paired-pulse stimuli. The ratio between maximal amplitudes of H-reflex and direct motor response increased in all muscles, irrespective of disease onset, and was found positively correlated with exaggerated osteotendinous reflexes and spasticity but depressed in patients on riluzole. This finding was accompanied by a reduction in MEP size and no changes in PSTH, PSF, and paired-pulse H-reflex probability. It is speculated that spinal interneurons may compensate for potential depression of monosynaptic EPSPs in ALS. From a clinical perspective, although the added value of H-reflex to osteotendinous reflex evaluation may be limited, it can serve as a valuable quantitative biomarker of pyramidal dysfunction in clinical trials.NEW & NOTEWORTHY Without significant evidence of peripheral denervation, H-reflex enhancement appears to be a widespread phenomenon, regardless of disease onset site. This increase is likely associated with a decrease in inhibitory control over presynaptic transmission of the synapse between muscle group Ia afferents and motoneurons. Although the link to exaggerated osteotendinous reflexes and spasticity implies a restricted role in identifying a pyramidal syndrome, its quantitative aspect positions the H-reflex as a valuable biomarker in clinical trials.

Soleus arthrogenic muscle inhibition following acute lateral ankle sprain correlates with symptoms and ankle disability but not with postural control

BACKGROUND

Acute lateral ankle sprains (ALAS) are associated with long-term impairments and instability tied to altered neural excitability. Arthrogenic muscle inhibition (AMI) has been observed in this population; however, relationships with injury-related impairments are unclear, potentially due to the resting, prone position in which AMI is typically measured. Assessing AMI during bipedal stance may provide a better understanding of this relationship.

METHODS

AMI was assessed in 38 young adults (19 ALAS within 72 h of injury: 10 males, 21.4 ± 2.7 years; 19 healthy controls: 10 males, 21.9 ± 2.2 years; mean ± SD) using the Hoffmann reflex (H-reflex) during bipedal stance. Electrical stimulation was administered to identify the maximal H-reflex (Hmax) and maximal motor response (Mmax) from the soleus, fibularis longus, and tibialis anterior muscles. The primary outcome measure was the Hmax/Mmax ratio. Secondary outcomes included acute symptoms (pain and swelling), postural control during bipedal stance, and self-reported function.

RESULTS

No significant group-by-limb interactions were observed for any muscle. However, a significant group main effect was observed in the soleus muscle (F(1,35) = 6.82, p = 0.013), indicating significantly lower Hmax/Mmax ratios following ALAS (0.38 ± 0.20) compared to healthy controls (0.53 ± 0.16). Furthermore, lower Hmax/Mmax ratios in the soleus significantly correlated with acute symptoms and self-reported function but not with postural control.

CONCLUSION

This study supports previous evidence of AMI in patients with ALAS, providing insight into neurophysiologic impacts of musculoskeletal injury. Our results suggest that assessing AMI in a standing position following acute injury may provide valuable insight into how AMI develops and guide potential therapeutic options to curb and offset the formation of joint instability.

Neurophysiological and imaging biomarkers of lower motor neuron dysfunction in motor neuron diseases/amyotrophic lateral sclerosis: IFCN handbook chapter

This chapter discusses comprehensive neurophysiological biomarkers utilised in motor neuron disease (MND) and, in particular, its commonest form, amyotrophic lateral sclerosis (ALS). These encompass the conventional techniques including nerve conduction studies (NCS), needle and high-density surface electromyography (EMG) and H-reflex studies as well as novel techniques. In the last two decades, new methods of assessing the loss of motor units in a muscle have been developed, that are more convenient than earlier methods of motor unit number estimation (MUNE),and may use either electrical stimulation (e.g. MScanFit MUNE) or voluntary activation (MUNIX). Electrical impedance myography (EIM) is another novel approach for the evaluation that relies upon the application and measurement of high-frequency, low-intensity electrical current. Nerve excitability techniques (NET) also provide insights into the function of an axon and reflect the changes in resting membrane potential, ion channel dysfunction and the structural integrity of the axon and myelin sheath. Furthermore, imaging ultrasound techniques as well as magnetic resonance imaging are capable of detecting the constituents of morphological changes in the nerve and muscle. The chapter provides a critical description of the ability of each technique to provide neurophysiological insight into the complex pathophysiology of MND/ALS. However, it is important to recognise the strengths and limitations of each approach in order to clarify utility. These neurophysiological biomarkers have demonstrated reliability, specificity and provide additional information to validate and assess lower motor neuron dysfunction. Their use has expanded the knowledge about MND/ALS and enhanced our understanding of the relationship between motor units, axons, reflexes and other neural circuits in relation to clinical features of patients with MND/ALS at different stages of the disease. Taken together, the ultimate goal is to aid early diagnosis, distinguish potential disease mimics, monitor and stage disease progression, quantify response to treatment and develop potential therapeutic interventions.

Forelimb movements contribute to hindlimb cutaneous reflexes during locomotion in cats

During quadrupedal locomotion, interactions between spinal and supraspinal circuits and somatosensory feedback coordinate forelimb and hindlimb movements. How this is achieved is not clear. To determine whether forelimb movements modulate hindlimb cutaneous reflexes involved in responding to an external perturbation, we stimulated the superficial peroneal nerve in six intact cats during quadrupedal locomotion and during hindlimb-only locomotion (with forelimbs standing on stationary platform) and in two cats with a low spinal transection (T12-T13) during hindlimb-only locomotion. We compared cutaneous reflexes evoked in six ipsilateral and four contralateral hindlimb muscles. Results showed similar occurrence and phase-dependent modulation of short-latency inhibitory and excitatory responses during quadrupedal and hindlimb-only locomotion in intact cats. However, the depth of modulation was reduced in the ipsilateral semitendinosus during hindlimb-only locomotion. Additionally, longer-latency responses occurred less frequently in extensor muscles bilaterally during hindlimb-only locomotion, whereas short-latency inhibitory and longer-latency excitatory responses occurred more frequently in the ipsilateral and contralateral sartorius anterior, respectively. After spinal transection, short-latency inhibitory and excitatory responses were similar to both intact conditions, whereas mid- or longer-latency excitatory responses were reduced or abolished. Our results in intact cats and the comparison with spinal-transected cats suggest that the absence of forelimb movements suppresses inputs from supraspinal structures and/or cervical cord that normally contribute to longer-latency reflex responses in hindlimb extensor muscles.NEW & NOTEWORTHY During quadrupedal locomotion, the coordination of forelimb and hindlimb movements involves central circuits and somatosensory feedback. To demonstrate how forelimb movement affects hindlimb cutaneous reflexes during locomotion, we stimulated the superficial peroneal nerve in intact cats during quadrupedal and hindlimb-only locomotion as well as in spinal-transected cats during hindlimb-only locomotion. We show that forelimb movement influences the modulation of hindlimb cutaneous reflexes, particularly the occurrence of long-latency reflex responses.

Longitudinal follow-up and prognostic factors in nitrous oxide-induced neuropathy

BACKGROUND AND AIM

Recreational use of nitrous oxide (N2O) has been associated with the development of severe nitrous oxide-induced neuropathy (N2On). Follow-up of these patients poses challenges, and their clinical progression remains largely unknown. The identification of prognostic factors is made difficult by the lack of standardized longitudinal assessments in most studies. The objective was to document the course of neuropathy through systematic follow-up assessments in N2On patients to identify prognostic factors for persistent disability after 6 months.

METHODS

We gathered demographic, clinical, biological, and electrophysiological data from N2On patients hospitalized in the Referral center in Marseille, both at baseline and during a standardized follow-up assessment at 6 months.

RESULTS

We retrospectively included 26 N2On patients (mean age 22.6 ± 4.4). Significant improvements were observed in all main clinical scores including Rankin, ONLS, and MRC testing (p < .01). Electrophysiological studies (EDX) revealed a predominantly motor neuropathy with marked reduction in CMAP in the lower limbs at baseline, and no significant improvement in motor parameters (p = .543). Rankin score at 6 months correlated with the initial weekly N2O consumption (r = .43, p = .03) and the CMAP sum score in the lower limbs at the first EDX (r = -.47, p = .02). Patients with and without myelitis showed similar Rankin and ONLS score after 6 months.

INTERPRETATION

The clinical course generally improved favorably at 6 months with notable amelioration in the primary disability scores, sensory deficits, and ataxia. However, distal motor impairment associated with peripheral neuropathy persisted, with distal axonal loss emerging as the main prognostic factor for long-term disability in these young patients.

Optimizing assessment of low frequency H-reflex depression in persons with spinal cord injury

Considering the growing interest in clinical applications of neuromodulation, assessing effects of various modulatory approaches is increasingly important. Monosynaptic spinal reflexes undergo depression following repeated activation, offering a means to quantify neuromodulatory influences. Following spinal cord injury (SCI), changes in reflex modulation are associated with spasticity and impaired motor control. To assess disrupted reflex modulation, low-frequency depression (LFD) of Hoffman (H)-reflex excitability is examined, wherein the amplitudes of conditioned reflexes are compared to an unconditioned control reflex. Alternatively, some studies utilize paired-pulse depression (PPD) in place of the extended LFD train. While both protocols induce similar amounts of H-reflex depression in neurologically intact individuals, this may not be the case for persons with neuropathology. We compared the H-reflex depression elicited by PPD and by trains of 3-10 pulses to an 11-pulse LFD protocol in persons with incomplete SCI. The amount of depression produced by PPD was less than an 11-pulse train (mean difference = 0.137). When compared to the 11-pulse train, the 5-pulse train had a Pearson's correlation coefficient (R) of 0.905 and a coefficient of determination (R2) of 0.818. Therefore, a 5-pulse train for assessing LFD elicits modulation similar to the 11-pulse train and thus we recommend its use in lieu of longer trains.

Soleus Muscle H-reflex: Reference Data of Adult Population From a Tertiary Care Center in India

Introduction The Hoffmann reflex (H reflex) is one of the most studied reflexes in human neurophysiological literature. Detection of the H reflex is useful in the diagnosis of early polyneuropathy, S1 radiculopathy, early GBS, tibial neuropathy and sciatica, and sacral plexopathy. The H reflex is also used as a tool to measure the excitability of the nervous components of the arc, regardless of the sensitivity of the sensory organs. The monosynaptic nature of reflex circuits makes H-reflex an attractive tool for clinical neurophysiology and research. Objective The objective is to create reference data of soleus H-reflex latency in an adult population from a tertiary care center in India. Materials and methods Seven hundred eighty-four healthy volunteers underwent a physical examination and brief electrophysiological examination before elicitation of the soleus H reflex of both lower extremities using standard techniques. Reference values ​​are expressed as mean ± standard deviation as well as the third and 97th percentiles for latency as the dependent variable. Results The study population included 346 (44.1%) women and 438 (55.9%) men. The men were aged 40.46 ± 14.76 years, and the women were aged 41.63 ± 13.49 years. The average weight of the men was 73.32 ± 10.28 kilograms, and the women were 62.91 ± 7.46 kilograms. The average height of the men was 172.06 ± 4.22 cm, and the women were 159.12 ± 2.42 cm. The third and 97th percentiles for H-reflex latency on the right side were 22.86 ms to 34.22 ms and on the left side were 22.86 ms to 35.39 ms. The average right tibial H latency and left tibial H latency were 28.18 ± 2.59 ms and 28.14 ± 2.70 ms, respectively. Conclusion A sizable subject population was used to provide reference data for this study. Because of the huge sample size and nearly appropriate coverage of different age groups, reference ranges have been established for various age, height, and BMI groups.

Advancing Intraoperative Neurophysiological Monitoring With Human Reflexes

Human reflexes are simple motor responses that are automatically elicited by various sensory inputs. These reflexes can provide valuable insights into the functioning of the nervous system, particularly the brainstem and spinal cord. Reflexes involving the brainstem, such as the blink reflex, laryngeal adductor reflex, trigeminal hypoglossal reflex, and masseter H reflex, offer immediate information about the cranial-nerve functionality and the overall state of the brainstem. Similarly, spinal reflexes such as the H reflex of the soleus muscle, posterior root muscle reflexes, and sacral reflexes provide crucial information about the functionality of the spinal cord and peripheral nerves. One of the critical benefits of reflex monitoring is that it can provide continuous feedback without disrupting the surgical process due to no movement being induced in the surgical field. These reflexes can be monitored in real time during surgical procedures to assess the integrity of the nervous system and detect potential neurological damage. It is particularly noteworthy that the reflexes provide motor and sensory information on the functional integrity of nerve fibers and nuclei. This article describes the current techniques used for monitoring various human reflexes and their clinical significance in surgery. We also address important methodological considerations and their impact on surgical safety and patient outcomes. Utilizing these methodologies has the potential to advance or even revolutionize the field of intraoperative continuous monitoring, ultimately leading to improved surgical outcomes and enhanced patient care.

Impaired Vibratory and Reciprocal Inhibition in Soleus H-Reflex Testing in Children With Spastic Cerebral Palsy

Introduction Cerebral palsy (CP) is a neurodevelopmental condition that results from an injury to a developing brain. Children with CP fail to execute precise, well-coordinated movements, and excessive muscular co-contraction or co-activation is a prominent attribute of CP. The normal reciprocal relationship between agonists and antagonists during voluntary movements is altered in patients with CP. H-reflex, which is often regarded as the electrical equivalent of the spinal stretch reflex, can be used to examine the overall reflex arc, including the Ia sensory afferent strength and the spinal motoneuron excitability state. Furthermore, neuromodulatory influence of vibration on H-reflex has been found, which has been increasingly investigated to ascertain its potential use as an intervention in patients with increased spinal reflex excitability. Our goal was to identify the brain mechanism underlying the motor deficits by studying Soleus H-reflex changes during voluntary movement (dorsiflexion) and also to determine the role of vibration in H-reflex modulation in children with spastic CP. Methods Soleus H-reflex was recorded in 12 children with spastic CP (10-16 years) and 15 age-matched controls. Recordings were obtained at rest, during dorsiflexion, and during vibratory stimulation for each subject. H-responses (Hmax amplitudes and Hmax-to-Mmax ratio) were compared among the controls and the cases (CP), for the experiments performed, by the Wilcoxon signed-rank test. The recruitment curves depicting the distribution of mean H-response amplitudes with stimulus intensity increment, for dorsiflexion and vibration were compared among controls and cases by the two-sample Kolmogorov-Smirnov (KS) test. p-value <0.05 was considered as statistically significant. Results Hmax amplitudes and the Hmax-to-Mmax ratio increased (15 % and 12.2 % increment, respectively) from the resting values in the children with CP (p<0.05), while controls exhibited a decrease (reduction of 62% and 57 %, respectively) during dorsiflexion (p<0.05). Vibratory stimulation produced a decreasing trend in H-response measures in both the groups. There was about 15 % and 16 % reduction respectively among children with CP while that of 24 % and 21 % respectively among the controls. The differences in the recruitment curves (distribution of average H-response amplitudes with stimulation intensity) recorded during dorsiflexion and vibration experiments among controls compared with those with CP were found to be statistically significant by the two-sample KS test (p<0.0001). Conclusion The failure of H-reflex suppression during voluntary antagonist muscle activation suggests the presence of impaired reciprocal inhibition in spastic CP. The relatively modest H-response reduction caused by vibratory stimulation in children with CP provides limited evidence of vibratory regulation of the H-reflex in CP. More research into the mechanisms driving motor abnormalities in children with CP is needed, which could aid in therapy planning.

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.

Local vibration induces changes in spinal and corticospinal excitability in vibrated and antagonist muscles

Local vibration (LV) applied over the muscle tendon constitutes a powerful stimulus to activate the muscle spindle primary (Ia) afferents that project to the spinal level and are conveyed to the cortical level. This study aimed to identify the neuromuscular changes induced by a 30-min LV-inducing illusions of hand extension on the vibrated flexor carpi radialis (FCR) and the antagonist extensor carpi radialis (ECR) muscles. We studied the change of the maximal voluntary isometric contraction (MVIC, experiment 1) for carpal flexion and extension, motor-evoked potentials (MEPs, experiment 2), cervicomedullary motor-evoked potentials (CMEPs, experiment 2), and Hoffmann's reflex (H-reflex, experiment 3) for both muscles at rest. Measurements were performed before (PRE) and at 0, 30, and 60 min after LV protocol. A lasting decrease in strength was only observed for the vibrated muscle. The reduction in CMEPs observed for both muscles seems to support a decrease in alpha motoneurons excitability. In contrast, a slight decrease in MEPs responses was observed only for the vibrated muscle. The MEP/CMEP ratio increase suggested greater cortical excitability after LV for both muscles. In addition, the H-reflex largely decreased for the vibrated and the antagonist muscles. The decrease in the H/CMEP ratio for the vibrated muscle supported both pre- and postsynaptic causes of the decrease in the H-reflex. Finally, LV-inducing illusions of movement reduced alpha motoneurons excitability for both muscles with a concomitant increase in cortical excitability.NEW & NOTEWORTHY Spinal disturbances confound the interpretation of excitability changes in motor areas and compromise the conclusions reached by previous studies using only a corticospinal marker for both vibrated and antagonist muscles. The time course recovery suggests that the H-reflex perturbations for the vibrated muscle do not only depend on changes in alpha motoneurons excitability. Local vibration induces neuromuscular changes in both vibrated and antagonist muscles at the spinal and cortical levels.

Neural Adjustments during Repeated Braking and Throttle Actions on a Motorcycle Setup

The aim of the study was to assess neuromuscular changes during an intermittent fatiguing task designed to replicate fundamental actions and ergonomics of road race motorcycling. Twenty-eight participants repeated a sequence of submaximal brake-pulling and gas throttle actions, interspaced by one maximal brake-pulling, until failure. During the submaximal brake-pulling actions performed at 30% MVC, force fluctuations, surface EMG, maximal M-wave (Mmax) and H-reflex were measured in the flexor digitorum superficialis. At the end of the task, the MVC force and associated EMG activity decreased (P<0.001) by 46% and 26%, respectively. During the task, force fluctuation and EMG activity increased gradually (106% and 61%, respectively) with respect to the pre-fatigue state (P≤0.029). The Mmax first phase did not change (P≥0.524), whereas the H-reflex amplitude, normalized to Mmax, increased (149%; P≤0.039). Noteworthy, the relative increase in H-reflex amplitude was correlated with the increase in EMG activity during the task (r=0.63; P<0.001). During the 10-min recovery, MVC force and EMG activity remained depressed (P≤0.05) whereas H-reflex amplitude and force fluctuation returned to pre-fatigue values. In conclusion, contrarily to other studies, our results bring forward that when mimicking motorcycling brake-pulling and gas throttle actions, supraspinal neural mechanisms primarily limit the duration of the performance.

Effects of facilitatory and inhibitory Kinesio taping on lateral gastrocnemius muscle activity, motor neuron excitability, and countermovement jump height in university athletes from multiple sports: A randomized controlled trial

Objectives

This study aims to investigate the temporal effects of two Kinesio Taping (KT) techniques on lateral gastrocnemius muscle activity, motor neuron excitability, and countermovement jump height in university athletes from hockey, football, basketball, and volleyball. Additionally, it investigates whether the athletes' playing positions-either attacker or defender-influence these outcomes following the KT application.

Methods

Forty-eight subjects were randomly assigned to one of three groups: Group A (n = 16), Group B (n = 16), and Group C (n = 16). All groups were further subdivided into attackers and defenders. Adhesive Kinesio tape was applied to the lateral gastrocnemius using the Y-shaped technique for three days. Facilitatory KT was applied from the origin to the insertion of the lateral gastrocnemius, while inhibitory KT was applied from the insertion to the origin. Motor neuron excitability, electromyographic activity, and countermovement jump height were tested at baseline, as well as after KT application, to evaluate if the dependent variables had changed. One-way ANOVA was used for baseline comparison, and mixed ANOVA was applied to assess post-interventional effects on the outcome measures.

Results

Significant group effects for lateral gastrocnemius activation were found, measured using percentage of maximum voluntary isometric contraction (% MVIC) average root mean square (RMS). In motor neuron excitability, maximal M-wave (Mmax) was significantly improved in group comparison. Further, there was also a significant increase in countermovement jump height. There was no significant difference in outcome measures based on playing position (attacker and defender).

Conclusion

Both KT techniques effectively influenced the lateral gastrocnemius muscle's activity, motor neuron excitability, and jump height when compared with the control group. Additionally, there is no effect of playing position, specifically attacker or defender, on the examined variables following KT application.

Novel Pulsed Ultrahigh-frequency Spinal Cord Stimulation Inhibits Mechanical Hypersensitivity and Brain Neuronal Activity in Rats after Nerve Injury

BACKGROUND

Spinal cord stimulation (SCS) is an important pain treatment modality. This study hypothesized that a novel pulsed ultrahigh-frequency spinal cord stimulation (pUHF-SCS) could safely and effectively inhibit spared nerve injury-induced neuropathic pain in rats.

METHODS

Epidural pUHF-SCS (± 3V, 2-Hz pulses comprising 500-kHz biphasic sinewaves) was implanted at the thoracic vertebrae (T9 to T11). Local field brain potentials after hind paw stimulation were recorded. Analgesia was evaluated by von Frey-evoked allodynia and acetone-induced cold allodynia.

RESULTS

The mechanical withdrawal threshold of the injured paw was 0.91 ± 0.28 g lower than that of the sham surgery (24.9 ± 1.2 g). Applying 5-, 10-, or 20-min pUHF-SCS five times every 2 days significantly increased the paw withdrawal threshold to 13.3 ± 6.5, 18.5 ± 3.6, and 21.0 ± 2.8 g at 5 h post-SCS, respectively (P = 0.0002, < 0.0001, and < 0.0001; n = 6 per group) and to 6.1 ± 2.5, 8.2 ± 2.7, and 14.3 ± 5.9 g on the second day, respectively (P = 0.123, 0.013, and < 0.0001). Acetone-induced paw response numbers decreased from pre-SCS (41 ± 12) to 24 ± 12 and 28 ± 10 (P = 0.006 and 0.027; n = 9) at 1 and 5 h after three rounds of 20-min pUHF-SCS, respectively. The areas under the curve from the C component of the evoked potentials at the left primary somatosensory and anterior cingulate cortices were significantly decreased from pre-SCS (101.3 ± 58.3 and 86.9 ± 25.5, respectively) to 39.7 ± 40.3 and 36.3 ± 20.7 (P = 0.021, and 0.003; n = 5) at 60 min post-SCS, respectively. The intensity thresholds for pUHF-SCS to induce brain and sciatic nerve activations were much higher than the therapeutic intensities and thresholds of conventional low-frequency SCS.

CONCLUSIONS

Pulsed ultrahigh-frequency spinal cord stimulation inhibited neuropathic pain-related behavior and paw stimulation evoked brain activation through mechanisms distinct from low-frequency SCS.

EDITOR’S PERSPECTIVE

Methods for automated delineation and assessment of EMG responses evoked by peripheral nerve stimulation in diagnostic and closed-loop therapeutic applications

Objective.Surface electromyography measurements of the Hoffmann (H-) reflex are essential in a wide range of neuroscientific and clinical applications. One promising emerging therapeutic application is H-reflex operant conditioning, whereby a person is trained to modulate the H-reflex, with generalized beneficial effects on sensorimotor function in chronic neuromuscular disorders. Both traditional diagnostic and novel realtime therapeutic applications rely on accurate definitions of the H-reflex and M-wave temporal bounds, which currently depend on expert case-by-case judgment. The current study automates such judgments.Approach.Our novel wavelet-based algorithm automatically determines temporal extent and amplitude of the human soleus H-reflex and M-wave. In each of 20 participants, the algorithm was trained on data from a preliminary 3 or 4 min recruitment-curve measurement. Output was evaluated on parametric fits to subsequent sessions' recruitment curves (92 curves across all participants) and on the conditioning protocol's subsequent baseline trials (∼1200 per participant) performed nearHmax. Results were compared against the original temporal bounds estimated at the time, and against retrospective estimates made by an expert 6 years later.Main results.Automatic bounds agreed well with manual estimates: 95% lay within ±2.5 ms. The resulting H-reflex magnitude estimates showed excellent agreement (97.5% average across participants) between automatic and retrospective bounds regarding which trials would be considered successful for operant conditioning. Recruitment-curve parameters also agreed well between automatic and manual methods: 95% of the automatic estimates of the current required to elicitHmaxfell within±1.4%of the retrospective estimate; for the 'threshold' current that produced an M-wave 10% of maximum, this value was±3.5%.Significance.Such dependable automation of M-wave and H-reflex definition should make both established and emerging H-reflex protocols considerably less vulnerable to inter-personnel variability and human error, increasing translational potential.

A Method for Quantification of Stretch Reflex Excitability During Ballistic Reaching

Stretch reflexes are crucial for performing accurate movements and providing rapid corrections for unpredictable perturbations. Stretch reflexes are modulated by supraspinal structures via corticofugal pathways. Neural activity in these structures is difficult to observe directly, but the characterization of reflex excitability during volitional movement can be used to study how these structures modulate reflexes and how neurological injuries impact this control, such as in spasticity after stroke. We have developed a novel protocol to quantify stretch reflex excitability during ballistic reaching. This novel method was implemented using a custom haptic device (NACT-3D) capable of applying high-velocity (270 °/s) joint perturbations in the plane of the arm while participants performed 3D reaching tasks in a large workspace. We assessed the protocol on four participants with chronic hemiparetic stroke and two control participants. Participants reached ballistically from a near to a far target, with elbow extension perturbations applied in random catch trials. Perturbations were applied before movement, during the early phase of movement, or near peak movement velocity. Preliminary results show that stretch reflexes were elicited in the stroke group in the biceps muscle during reaching, as measured by electromyographic (EMG) activity both before (pre-motion phase) and during (early motion phase) movement. Reflexive EMG was also seen in the anterior deltoid and pectoralis major in the pre-motion phase. In the control group, no reflexive EMG was seen, as expected. This newly developed methodology allows the study of stretch reflex modulation in new ways by combining multijoint movements with haptic environments and high-velocity perturbations.

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.

Differences in Trapezius Muscle H-Reflex between Asymptomatic Subjects and Symptomatic Shoulder Pain Subjects

In chronic shoulder pain, adaptations in the nervous system such as in motoneuron excitability, could contribute to impairments in scapular muscles, perpetuation and recurrence of pain and reduced improvements during rehabilitation. The present cross-sectional study aims to compare trapezius neural excitability between symptomatic and asymptomatic subjects. In 12 participants with chronic shoulder pain (symptomatic group) and 12 without shoulder pain (asymptomatic group), the H reflex was evoked in all trapezius muscle parts, through C3/4 nerve stimulation, and the M-wave through accessory nerve stimulation. The current intensity to evoke the maximum H reflex, the latency and the maximum peak-to-peak amplitude of both the H reflex and M-wave, as well as the ratio between these two variables, were calculated. The percentage of responses was considered. Overall, M-waves were elicited in most participants, while the H reflex was elicited only in 58-75% or in 42-58% of the asymptomatic and symptomatic participants, respectively. A comparison between groups revealed that the symptomatic group presented a smaller maximum H reflex as a percentage of M-wave from upper trapezius and longer maximal H reflex latency from the lower trapezius (p < 0.05). Subjects with chronic shoulder pain present changes in trapezius H reflex parameters, highlighting the need to consider trapezius neuromuscular control in these individuals' rehabilitation.

Electrical Stimulation and Muscle Strength Gains in Healthy Adults: A Systematic Review

ABSTRACT

Mukherjee, S, Fok, JR, and van Mechelen, W. Electrical stimulation and muscle strength gains in healthy adults: A systematic review. J Strength Cond Res 37(4): 938-950, 2023-Electrical muscle stimulation (EMS) is a popular method for strength gains among athletes and fitness enthusiasts. This review investigated the literature from 2008 to 2020 on EMS application protocols, strength adaptations, neural adaptations, and its use as an independent and combined training tool for strength gain in healthy adults. The investigation was modeled after the 2020 PRISMA guidelines. The eligibility criteria included studies that assessed the effect of EMS, either alone or in combination with voluntary resistance training (VRT) in healthy adult populations, involving a control group performing either usual or sham training, with at least 1 performance outcome measure assessed during experimental randomized controlled trials (RCTs), cluster RCT, randomized crossover trials, or nonrandomized studies. Ten studies met the eligibility criteria with a total of 174 subjects. Eight studies investigated the effect of EMS on lower limb muscles and 2 on elbow flexors. Five studies used concurrent VRT. Studies were heterogenous in methods, subject characteristics, intervention, and EMS protocols. All 10 studies reported significant strength gains as an outcome of EMS treatment, but there were no improvements in strength-related functional outcome measures. The optimal threshold for treatment duration, EMS intensity, pulse, and frequency could not be determined due to methodological differences and EMS application protocol inconsistency between studies. Protocol variations also existed between the studies that combined EMS with VRT. Standardized protocols are needed for electrode placement location, motor point identification, positioning of the body part being investigated, impulse type, intensity, and duration of stimulus.

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.

Modulation of spinal circuits following phase-dependent electrical stimulation of afferent pathways

Objective.Peripheral electrical stimulation (PES) of afferent pathways is a tool commonly used to induce neural adaptations in some neural disorders such as pathological tremor or stroke. However, the neuromodulatory effects of stimulation interventions synchronized with physiological activity (closed-loop strategies) have been scarcely researched in the upper-limb. Here, the short-term spinal effects of a 20-minute stimulation protocol where afferent pathways were stimulated with a closed-loop strategy named selective and adaptive timely stimulation (SATS) were explored in 11 healthy subjects.Approach. SATS was applied to the radial nerve in-phase (INP) or out-of-phase (OOP) with respect to the muscle activity of the extensor carpi radialis (ECR). The neural adaptations at the spinal cord level were assessed for the flexor carpi radialis (FCR) by measuring disynaptic Group I inhibition, Ia presynaptic inhibition, Ib facilitation from the H-reflex and estimation of the neural drive before, immediately after, and 30 minutes after the intervention.Main results.SATS strategy delivered electrical stimulation synchronized with the real-time muscle activity measured, with an average delay of 17 ± 8 ms. SATS-INP induced increased disynaptic Group I inhibition (77 ± 23% of baseline conditioned FCR H-reflex), while SATS-OOP elicited the opposite effect (125 ± 46% of baseline conditioned FCR H-reflex). Some of the subjects maintained the changes after 30 minutes. No other significant changes were found for the rest of measurements.Significance.These results suggest that the short-term modulatory effects of phase-dependent PES occur at specific targeted spinal pathways for the wrist muscles in healthy individuals. Importantly, timely recruitment of afferent pathways synchronized with specific muscle activity is a fundamental principle that shall be considered when tailoring PES protocols to modulate specific neural circuits. (NCT number 04501133).

Using a high-frequency carrier does not improve comfort of transcutaneous spinal cord stimulation

Objective.Spinal cord neuromodulation has gained much attention for demonstrating improved motor recovery in people with spinal cord injury, motivating the development of clinically applicable technologies. Among them, transcutaneous spinal cord stimulation (tSCS) is attractive because of its non-invasive profile. Many tSCS studies employ a high-frequency (10 kHz) carrier, which has been reported to reduce stimulation discomfort. However, these claims have come under scrutiny in recent years. The purpose of this study was to determine whether using a high-frequency carrier for tSCS is more comfortable at therapeutic amplitudes, which evoke posterior root-muscle (PRM) reflexes.Approach.In 16 neurologically intact participants, tSCS was delivered using a 1 ms long monophasic pulse with and without a high-frequency carrier. Stimulation amplitude and pulse duration were varied and PRM reflexes were recorded from the soleus, gastrocnemius, and tibialis anterior muscles. Participants rated their discomfort during stimulation from 0 to 10 at PRM reflex threshold.Main Results.At PRM reflex threshold, the addition of a high-frequency carrier (0.87 ± 0.2) was equally comfortable as conventional stimulation (1.03 ± 0.18) but required approximately double the charge to evoke the PRM reflex (conventional: 32.4 ± 9.2µC; high-frequency carrier: 62.5 ± 11.1µC). Strength-duration curves for tSCS with a high-frequency carrier had a rheobase that was 4.8× greater and a chronaxie that was 5.7× narrower than the conventional monophasic pulse, indicating that the addition of a high-frequency carrier makes stimulation less efficient in recruiting neural activity in spinal roots.Significance.Using a high-frequency carrier for tSCS is equally as comfortable and less efficient as conventional stimulation at amplitudes required to stimulate spinal dorsal roots.

The effect of loss of foot sole sensitivity on H-reflex of triceps surae muscles and functional gait

Objective: To investigate the effects of foot sole insensitivity on the outcomes of the triceps surae muscle H-reflex and functional gait. Material and Methods: People with peripheral neuropathy were recruited and divided into two groups: people with more (n = 13, 73.3 ± 4.3 years old) or less (n = 10, 73.5 ± 5.3) sensitive tactile sensation. Their monofilament testing scores were 9.0 ± 1.5 (range: 7-10) and 2.3 ± 2.4 (range: 0-6) out of 10, respectively. H-reflex of the triceps surae muscles during quiet standing and their relationship with functional gait, 6 min walking distance (6MWD), and timed-up-and-go duration (TUG), were compared between groups. Results: No significant difference was detected for H-reflex parameters between the groups. The less sensitive group showed reduced (p < .05) functional gait capacity compared to the other group, 38.4 ± 52.7 vs. 463.5 ± 47.6 m for 6MWD, and 9.0 ± 1.5 vs. 7.2 ± 1.1s for TUG, respectively. A significant correlation (p < .05), worse functional gait related to greater H/M ratio, was observed in the less sensitive group, not the other group. Conclusion: Although there was no significant H-reflex difference between the groups, more pronounced tactile sensation degeneration affected functional gaits and their relationship with H-reflex.

The Electrophysiological Findings in Spinocerebellar Ataxia Type 6: Evidence From 24 Patients

PURPOSE

Peripheral neuropathy has been reported commonly in several spinocerebellar ataxia (SCA) types. To date, there is a lack of robust evidence for neuropathy or neuronopathy in SCA type 6 (SCA6). Here, we aim to evaluate the presence of neuropathy or neuronopathy in a cohort of SCA6 patients.

METHODS

Twenty-four individuals with genetically confirmed SCA6 underwent detailed neurophysiological assessment. This included nerve conduction studies, and in some, cutaneous silent periods, blink reflexes, tilt table tests, quantitative sudomotor axon reflex tests, and somatosensory (median and tibial) evoked potentials.

RESULTS

Mean age was 56.1 years (range, 22-94 years) at the time of testing. Four patients were presymptomatic of SCA6 at recruitment. The mean disease duration of symptomatic patients was 11.9 years (range, 1-40 years). Most patients (79.2%, 19/24) had no neurophysiological evidence of a peripheral neuropathy. One with impaired glucose tolerance had mild, large, and small fiber sensorimotor polyneuropathy. One elderly patient had length-dependent axonal sensorimotor polyneuropathy. Two had minor sensory abnormalities (one had type II diabetes and previous chemotherapy). One other had minor small fiber abnormalities. Ten patients (41.7%) had median neuropathies at the wrist. All somatosensory evoked potential (15/15), and most autonomic function tests (13/14) were normal.

CONCLUSIONS

A large proportion of subjects (79.2%) in our cohort had no evidence of large or small fiber neuropathy. This study does not support the presence of neuropathy or neuronopathy as a common finding in SCA6 and confirms the importance of considering comorbidities as the cause of neurophysiological abnormalities.

Effects of voluntary contraction on the soleus H-reflex of different amplitudes in healthy young adults and in the elderly

A number of H-reflex studies used a moderate steady voluntary contraction in an attempt to keep the motoneuron pool excitability relatively constant. However, it is not clear whether the voluntary muscle activation itself represents a confounding factor for the elderly, as a few ongoing mechanisms of reflex modulation might be compromised. Further, it is well-known that the amount of either inhibition or facilitation from a given conditioning depends on the size of the test H-reflex. The present study aimed at evaluating the effects of voluntary contraction over a wide range of reflex amplitudes. A significant reflex facilitation during an isometric voluntary contraction of the soleus muscle (15% of the maximal voluntary isometric contraction-MVC) was found for both young adults and the elderly (p < 0.05), regardless of their test reflex amplitudes (considering the ascending limb of the H-reflex recruitment curve-RC). No significant difference was detected in the level of reflex facilitation between groups for all the amplitude parameters extracted from the RC. Simulations with a computational model of the motoneuron pool driven by stationary descending commands yielded qualitatively similar amount of reflex facilitation, as compared to human experiments. Both the experimental and modeling results suggest that possible age-related differences in spinal cord mechanisms do not significantly influence the reflex modulation during a moderate voluntary muscle activation. Therefore, a background voluntary contraction of the ankle extensors (e.g., similar to the one necessary to maintain upright stance) can be used in experiments designed to compare the RCs of both populations. Finally, in an attempt to elucidate the controversy around changes in the direct motor response (M-wave) during contraction, the maximum M-wave (Mmax) was compared between groups and conditions. It was found that the Mmax significantly increases (p < 0.05) during contraction and decreases (p < 0.05) with age arguably due to muscle fiber shortening and motoneuron loss, respectively.

Impaired rate-dependent depression of the H-reflex in type-2 diabetes, prediabetes, overweight and obesity: A cross-sectional study

Type-2 diabetes is a chronic metabolic disorder characterized by hyperglycemia, resulting from deficits in insulin secretion or insulin resistance. According to the International Diabetes Federation, approximately 463 million people suffered from this condition in 2019, with a rapidly increasing impact in low-and middle-income countries. Obesity is a well-known risk factor for diabetes, and current data project a continuous increase in diabetes prevalence worldwide in obese individuals. Among the common complications, diabetic peripheral neuropathy (DPN) causes sensory symptoms, including pain that contributes to foot ulceration, and if not controlled, limb amputation may occur. The diagnosis of DPN is a clinical problem. Rate-dependent depression (RDD) of the Hoffmann reflex in the lower limbs has been proposed as a test to determine the presence of neuropathic pain in subjects with type-1 and type-2 diabetes. Recently, impaired RDD has been described in obese and diabetic rodent models. In this study, we characterized the RDD by evaluating the H-reflex at 0.2, 1, 2, 5, and 10 Hz in 39 patients with type-2 Diabetes mellitus (T2DM) and 42 controls without diabetes, subsequently classified as overweight/obese and prediabetic. A significant decrease in the RDD of the H-reflex was found in T2DM subjects at 1, 2, 5, and 10 Hz (P < .001) stimulation frequencies compared to controls, but not at 0.2 Hz (P = .48). A major finding of this study is that impaired RDD was also found in 11/25 overweight and obese subjects in at least 2 stimulation frequencies, being 10 of those classified in prediabetic levels according to their HbA1C values. The RDD of the H-reflex could be used as a quantitative and sensitive tool to study T2DM subpopulations with peripheral neuropathy. RDD could be used as a screening tool in combination with clinical tests to diagnose DPN and evaluate the progression of this condition.

Ionic Plasticity: Common Mechanistic Underpinnings of Pathology in Spinal Cord Injury and the Brain

The neurotransmitter GABA is normally characterized as having an inhibitory effect on neural activity in the adult central nervous system (CNS), which quells over-excitation and limits neural plasticity. Spinal cord injury (SCI) can bring about a modification that weakens the inhibitory effect of GABA in the central gray caudal to injury. This change is linked to the downregulation of the potassium/chloride cotransporter (KCC2) and the consequent rise in intracellular Cl- in the postsynaptic neuron. As the intracellular concentration increases, the inward flow of Cl- through an ionotropic GABA-A receptor is reduced, which decreases its hyperpolarizing (inhibitory) effect, a modulatory effect known as ionic plasticity. The loss of GABA-dependent inhibition enables a state of over-excitation within the spinal cord that fosters aberrant motor activity (spasticity) and chronic pain. A downregulation of KCC2 also contributes to the development of a number of brain-dependent pathologies linked to states of neural over-excitation, including epilepsy, addiction, and developmental disorders, along with other diseases such as hypertension, asthma, and irritable bowel syndrome. Pharmacological treatments that target ionic plasticity have been shown to bring therapeutic benefits.

Conduction Velocity of Spinal Reflex in Patients with Acute Lateral Ankle Sprain

Recent literature has highlighted altered spinal-reflex excitability following acute lateral ankle sprain (ALAS), yet there is little information on the conduction velocity of spinal reflex pathways (CV-SRP) in these patients. Therefore, we aimed to investigate the effects of ALAS on the CV-SRP. We employed a cross-sectional study with two groups: ALAS (n = 30) and healthy controls (n = 30). The CV-SRP of the soleus, fibularis longus, and tibialis anterior was assessed using the H-index method. As secondary outcomes, H-reflex and M-wave latencies were assessed as well as acute symptoms including ankle swelling, pain, and self-reported ankle function. Separate group-by-limb ANOVA with repeated measures revealed a significant interaction for soleus CV-SRP (p < 0.001) and H-reflex latency (p < 0.001), showing significant slower CV-SRP and longer H-reflex latency in the involved limb of the ALAS group compared with both limbs in the control group. However, there was no significant interaction or main effect in any other ankle muscles (p > 0.05). A further correlation analysis showed a significant relationship between CV-SRP and acute symptoms, including ankle swelling (r = −0.37, p = 0.048) and self-reported ankle function (r = 0.44, p = 0.017) in ALAS patients. These results suggest a disrupted functionality of the afferent pathway and/or synaptic transmission following ALAS. Level of Evidence: 4.

Reduced wrist flexor H-reflex excitability is linked with increased wrist proprioceptive error in adults with cerebral palsy

Although most neurophysiological studies of persons with cerebral palsy (CP) have been focused on supraspinal networks, recent evidence points toward the spinal cord as a central contributor to their motor impairments. However, it is unclear if alterations in the spinal pathways are also linked to deficits in the sensory processing observed clinically. This investigation aimed to begin to address this knowledge gap by evaluating the flexor carpi radialis (FCR) H-reflex in adults with CP and neurotypical (NT) controls while at rest and during an isometric wrist flexion task. The maximal H-wave (Hmax) and M-wave (Mmax) at rest were calculated and utilized to compute Hmax/Mmax ratios (H:M ratios). Secondarily, the facilitation of the H-wave was measured while producing an isometric, voluntary wrist flexion contraction (i.e., active condition). Finally, a wrist position sense test was used to quantify the level of joint position sense. These results revealed that the adults with CP had a lower H:M ratio compared with the NT controls while at rest. The adults with CP were also unable to facilitate their H-reflexes with voluntary contraction and had greater position sense errors compared with the controls. Further, these results showed that the adults with CP that had greater wrist position sense errors tended to have a lower H:M ratio at rest. Overall, these findings highlight that aberration in the spinal cord pathways of adults with CP might play a role in the sensory processing deficiencies observed in adults with CP.

Influence of an Acute Exposure to a Moderate Real Altitude on Motoneuron Pool Excitability and Jumping Performance

The aim of the study was to test whether ascending to a moderate real altitude affects motoneuron pool excitability at rest, as expressed by a change in the H-reflex amplitude, and also to elucidate whether a possible alteration in the motoneuron pool excitability could be reflected in the execution of lower-body concentric explosive (squat jump; SJ) and fast eccentric-concentric (drop jump; DJ) muscle actions. Fifteen participants performed four experimental sessions that consisted of the combination of two real altitude conditions [low altitude (low altitude, 690 m), high altitude (higher altitude, 2,320 m)] and two testing procedures (H-reflex and vertical jumps). Participants were tested on each testing day at 8, 11, 14 and 17 h. The only significant difference (p < 0.05) detected for the H-reflex was the higher H-reflex response (25.6%) obtained 15 min after arrival at altitude compared to baseline measurement. In terms of motor behavior, DJ height was the only variable that showed a significant interaction between altitude conditions (LA and HA) and time of measurement (8, 11, 14 and 17 h) as DJ height increased more during successive measurements at HA compared to LA. The only significant difference between the LA and HA conditions was observed for DJ height at 17 h which was higher for the HA condition (p = 0.04, ES = 0.41). Although an increased H-reflex response was detected after a brief (15–20 min) exposure to real altitude, the effect on motorneuron pool excitability could not be confirmed since no significant changes in the H-reflex were detected when comparing LA and HA. On the other hand, the positive effect of altitude on DJ performance was accentuated after 6 h of exposure.

Neural Substrates of Transcutaneous Spinal Cord Stimulation: Neuromodulation across Multiple Segments of the Spinal Cord

Transcutaneous spinal cord stimulation (tSCS) has the potential to promote improved sensorimotor rehabilitation by modulating the circuitry of the spinal cord non-invasively. Little is currently known about how cervical or lumbar tSCS influences the excitability of spinal and corticospinal networks, or whether the synergistic effects of multi-segmental tSCS occur between remote segments of the spinal cord. The aim of this review is to describe the emergence and development of tSCS as a novel method to modulate the spinal cord, while highlighting the effectiveness of tSCS in improving sensorimotor recovery after spinal cord injury. This review underscores the ability of single-site tSCS to alter excitability across multiple segments of the spinal cord, while multiple sites of tSCS converge to facilitate spinal reflex and corticospinal networks. Finally, the potential and current limitations for engaging cervical and lumbar spinal cord networks through tSCS to enhance the effectiveness of rehabilitation interventions are discussed. Further mechanistic work is needed in order to optimize targeted rehabilitation strategies and improve clinical outcomes.

Temporal Profile of Descending Cortical Modulation of Spinal Excitability: Group and Individual-Specific Effects

Sensorimotor control is modulated through complex interactions between descending corticomotor pathways and ascending sensory inputs. Pairing sub-threshold transcranial magnetic stimulation (TMS) with peripheral nerve stimulation (PNS) modulates the Hoffmann’s reflex (H-reflex), providing a neurophysiologic probe into the influence of descending cortical drive on spinal segmental circuits. However, individual variability in the timing and magnitude of H-reflex modulation is poorly understood. Here, we varied the inter-stimulus interval (ISI) between TMS and PNS to systematically manipulate the relative timing of convergence of descending TMS-induced volleys with respect to ascending PNS-induced afferent volleys in the spinal cord to: (1) characterize effective connectivity between the primary motor cortex (M1) and spinal circuits, mediated by both direct, fastest-conducting, and indirect, slower-conducting descending pathways; and (2) compare the effect of individual-specific vs. standard ISIs. Unconditioned and TMS-conditioned H-reflexes (24 different ISIs ranging from −6 to 12 ms) were recorded from the soleus muscle in 10 able-bodied individuals. The magnitude of H-reflex modulation at individualized ISIs (earliest facilitation delay or EFD and individual-specific peak facilitation) was compared with standard ISIs. Our results revealed a significant effect of ISI on H-reflex modulation. ISIs eliciting earliest-onset facilitation (EFD 0 ms) ranged from −3 to −5 ms across individuals. No difference in the magnitude of facilitation was observed at EFD 0 ms vs. a standardized short-interval ISI of −1.5 ms. Peak facilitation occurred at longer ISIs, ranging from +3 to +11 ms. The magnitude of H-reflex facilitation derived using an individual-specific peak facilitation was significantly larger than facilitation observed at a standardized longer-interval ISI of +10 ms. Our results suggest that unique insights can be provided with individual-specific measures of top-down effective connectivity mediated by direct and/or fastest-conducting pathways (indicated by the magnitude of facilitation observed at EFD 0 ms) and other descending pathways that encompass relatively slower and/or indirect connections from M1 to spinal circuits (indicated by peak facilitation and facilitation at longer ISIs). By comprehensively characterizing the temporal profile and inter-individual variability of descending modulation of spinal reflexes, our findings provide methodological guidelines and normative reference values to inform future studies on neurophysiological correlates of the complex array of descending neural connections between M1 and spinal circuits.

Central nervous system physiology

This is the second chapter of the series on the use of clinical neurophysiology for the study of movement disorders. It focusses on methods that can be used to probe neural circuits in brain and spinal cord. These include use of spinal and supraspinal reflexes to probe the integrity of transmission in specific pathways; transcranial methods of brain stimulation such as transcranial magnetic stimulation and transcranial direct current stimulation, which activate or modulate (respectively) the activity of populations of central neurones; EEG methods, both in conjunction with brain stimulation or with behavioural measures that record the activity of populations of central neurones; and pure behavioural measures that allow us to build conceptual models of motor control. The methods are discussed mainly in relation to work on healthy individuals. Later chapters will focus specifically on changes caused by pathology.

Excitatory and inhibitory responses to cervical root magnetic stimulation in healthy subjects

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We studied excitatory and inhibitory responses to cervical root magnetic stimulation.

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CRMS elicited direct and reflex responses in hand muscles.

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CRMS is painless and well tolerated and, therefore, applicable to clinical studies.

Objectives

To characterize direct and reflex hand muscle responses to cervical root magnetic stimulation (CRMS) in healthy volunteers during sustained voluntary contraction.

Methods

In 18 healthy volunteers, we recorded from the first dorsal interosseous (FDI) muscle the responses to CRMS of progressively increasing intensity and level of muscle contraction. The compound muscle action potential (CMAP) and the silent period (SP) were compared to those obtained with plexus, midarm and wrist stimulation. Additionally, in a smaller number of subjects, we obtained the peristimulus time histogram (psth) of single motor unit firing in the FDI, examined the effects of vibration and recorded the modulation of sustained EMG activity in muscles of the lower limbs.

Results

Increasing CRMS intensity led to larger CMAP with no relevant changes in SP1 or SP2, except for lower amplitude of the burst interrupting the silent period (BISP). Increasing the level of muscle contraction led to reduced CMAP, shorter SP duration and increased BISP amplitude. The psth analysis showed the underlying changes in the motor unit firing frequency that corresponded to the changes seen in the CMAP and the SP with surface recordings. Progressively distal stimulation led to CMAPs of shorter latency and increased amplitude, SPs of longer latency and shorter duration, and a BISP of longer latency. Vibration led to reduction of the SP. CRMS induced SPs in muscles of the lower limb.

Conclusions

CRMS induces excitatory and inhibitory responses in hand muscles, fitting with the expected behavior of mixed nerve stimulation at very proximal sites.

Significance

Characterization of the effects of CRMS on hand muscles is of physiological and potentially clinical applicability, as it is a painless and reliable procedure.

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.