Recruitment Curves Research Articles

Endurance Trained Athletes Do Not per se Have Higher Hoffmann Reflexes Than Recreationally Active Controls

The impact of endurance training on spinal neural circuitries remains largely unknown. Some studies have reported higher H-reflexes in endurance trained athletes and therefore, adaptations within the Ia afferent pathways after long term endurance training have been suggested. In the present study we tested the hypothesis that cyclists (n = 12) demonstrate higher Hoffmann reflexes (H-reflexes) compared to recreationally active controls (n = 10). Notwithstanding, highly significant differences in endurance performance (VO2peak: 60.6 for cyclists vs. 46.3 ml/min/kg for controls (p < 0.001) there was no difference in the size of the SOL H-reflex between cyclists and controls (Hmax/Mmax ratio 61.3 vs. 60.0%, respectively (p = 0.840). Further analyses of the H and M recruitment curves for SOL revealed a significant steeper slope of the M recruitment curve in the group of cyclists (76.2 ± 3.8° vs. 72.0 ± 4.4°, p = 0.046) without a difference in the H-recruitment curve (84.6 ± 3.0° vs. 85.0 ± 2.8°, p = 0.784) compared to the control group. Cycling is classified as an endurance sport and thus the findings of the present study do not further support the assumption that long-term aerobic training leads to a general increase of the H-reflex. Amongst methodological differences in assessing the H-reflex, the training-specific sensorimotor control of the endurance sport itself might differently affect the responsiveness of spinal motoneurons on Ia-afferent inputs.

Can tDCS affect acquisition of a simple motor skill in neurotypical adults?

Abstract Excitatory tDCS (positive current into the brain) enhances skill acquisition and task dependent adaptation (TDA). Little is known about tDCS impact on spinal cord. Spinal reflexes participate in movements, change through life, and can be modified by experience or injury (Ann Rev Neurosci 24: 807-843). People can change spinal reflex size through operant conditioning (J Neurosci 29: 5784-5792). Acquiring this simple motor skill (larger or smaller reflex size) requires supraspinal drive (J Neurophysiol 87: 645-652). This study asks if tDCS affects the magnitude or consistency of H-reflex conditioning. The MUSC IRB approved the study. Neurotypical participants receive Active (excitatory, 2mA, 0.06mA/cm2, 30m) or Sham tDCS over contralateral leg region of primary motor cortex during soleus H-reflex down-conditioning. Tibial nerve stimulation elicits H-reflexes. Participants stand comfortably and maintain soleus background activity in a defined range. In 4 Baseline (BL) sessions, H-reflexes are simply recorded (control trials, three 75-trial blocks). In 12 subsequent Conditioning sessions (CS), participants practice reducing the H-reflex (conditioning trials, 3 blocks of 75). Visual feedback about reflex size follows each conditioning trial. Every session collects H-reflex recruitment curves (RCs) and control reflexes. TMS MEP RCs are collected periodically. A 1-month follow-up examines retention. To date, 8 participants (4 women, 5 Active tDCS, Age 22-36) have completed the study. Final conditioned H-reflex sizes (mean of CSs 10-12 as % BL) were 58, 67, 90, 99, 215% for Active and 71,115,125% for Sham participants. MEPmax change was -0.66, -0.28, -0.07, +0.04, +0.30mv and +0.09, +0.11, +1.11mV, Active and Sham respectively. The study is ongoing. The full results may illuminate tDCS impact on skill acquisition and guide its therapeutic applications as a neuromodulator. Keywords: tDCS, Hoffman reflex, operant conditioning, motor learning

Effects of Dry Needling on Spinal Reflexes: The Hoffman-reflex

<h3>Research Objectives</h3> Spasticity can lead to changes in spinal and supraspinal pathways which can lead to decreased function and quality of life in persons with stroke. Physical therapists worldwide have been administering trigger point deep dry needling (DDN) to treat post-stroke spasticity. Yet, currently, effects of DDN on spinal somatosensory processing are not well understood. To understand the neurophysiological mechanisms of DDN, we are currently investigating the effects of DDN on spinal somatosensory pathways in persons with and without stroke. <h3>Design</h3> Before, 0 minutes, 90 minutes, and 72 hours after DDN of the medial gastrocnemius (MG) the H-reflex and M-wave recruitment curves are measured in the soleus, MG and lateral gastrocnemius (LG). Passive ankle range of motion (ROM) before and after DDN. <h3>Setting</h3> University. <h3>Participants</h3> 11 adults 22-57 years old (median 37.5) without known neurological conditions. <h3>Interventions</h3> Trigger point dry needling to medial gastrocnemius. <h3>Main Outcome Measures</h3> H-Reflex and ROM. <h3>Results</h3> MG H-reflex latency was prolonged by 1-3 ms at 0- and 90-minutes post (in 9/11 individuals) and in 5/11 in the soleus and 3/11 in the LG. In the MG, H-reflex amplitude was significantly decreased at 72 hours post (p=0.004 by paired t-test) by 12%. Dorsiflexion ROM was increased by »4 deg at 0 minutes and 72 hours post. <h3>Conclusions</h3> DDN produces acute short-term effects (shown in the H-reflex latency shift at 0- and 90-minutes post) and significant long-term effects (shown in the reduced H-reflex at 72 hours post) in spinal pathways of the treated MG. To understand whether DDN affects the intact CNS and stroke-impaired CNS similarly or differently, studies of individuals after stroke are currently underway. <h3>Author(s) Disclosures</h3> The authors have no conflicts of interest to declare.

New Methodologies to Study DNA Repair Processes in Space and Time Within Living Cells.

DNA repair requires a coordinated effort from an array of factors that play different roles in the DNA damage response from recognizing and signaling the presence of a break, creating a repair competent environment, and physically repairing the lesion. Due to the rapid nature of many of these events, live-cell microscopy has become an invaluable method to study this process. In this review we outline commonly used tools to induce DNA damage under the microscope and discuss spatio-temporal analysis tools that can bring added information regarding protein dynamics at sites of damage. In particular, we show how to go beyond the classical analysis of protein recruitment curves to be able to assess the dynamic association of the repair factors with the DNA lesions as well as the target-search strategies used to efficiently find these lesions. Finally, we discuss how the use of mathematical models, combined with experimental evidence, can be used to better interpret the complex dynamics of repair proteins at DNA lesions.

Measurement error and reliability of TMS metrics collected from biceps and triceps in individuals with chronic incomplete tetraplegia.

Transcranial magnetic stimulation (TMS) is used to investigate corticomotor neurophysiology associated with functional recovery in individuals with spinal cord injury (SCI). There is insufficient evidence about test-retest measurement properties of TMS in SCI. Therefore, we investigated test-retest agreement and reliability of TMS metrics representing corticomotor excitability, output, gain, map (representation), and inhibition in individuals with cervical SCI. We collected TMS metrics from biceps and triceps muscles because of the relevance of this proximal muscle pair to the cervical SCI population. Twelve individuals with chronic C3-C6 SCI participated in two TMS sessions separated by ≥ 2weeks. Measurement agreement was evaluated using t tests, Bland-Altman limits of agreement and relative standard error of measurement (SEM%), while reliability was investigated using intra-class correlation coefficient (ICC) and concordance correlation coefficient (CCC). We calculated the smallest detectable change for all TMS metrics. All TMS metrics except antero-posterior map coordinates and corticomotor inhibition were in agreement upon repeated measurement though limits of agreement were generally large. Measures of corticomotor excitability, output and medio-lateral map coordinates had superior agreement (SEM% < 10). Metrics representing corticomotor excitability, output, and inhibition had good-to-excellent reliability (ICC/CCC > 0.75). The smallest detectable change for TMS metrics was generally high for a single individual, but this value reduced substantially with increase in sample size. We recommend use of corticomotor excitability and recruitment curve area owing to their superior measurement properties. A modest group size (20 or above) yields more stable measurements, which may favor use of TMS metrics in group level modulation after SCI.

Episodic recruitment in the saguaro cactus is driven by multidecadal periodicities.

Each year, an individual mature large saguaro cactus produces about one million seeds in attractive juicy fruits that lure seed predators and seed dispersers to a 3-month feast. From the million seeds produced, however, only a few will persist into mature saguaros. A century of research on saguaro population dynamics has led to the conclusion that saguaro recruitment is an episodic event that depends on the convergence of suitable conditions for survival during the critical early stages. Because most data have been collected in Arizona, particularly in the surroundings of Tucson, most research has relied on a limited amount of environmental variation. In this study, we upscaled this knowledge on saguaro recruitment to a regional scale with a new method that used the inverse-growth modeling of 1,487 saguaros belonging to 13 populations in a latitudinal gradient ranging from arid desert to tropical thornscrub forest in Sonora, Mexico. Using generalized linear and additive mixed models, we created two 110-yr-long saguaro recruitment curves: one driven only by previous size, and the second driven by size, drought, and soil structure. We found evidence that saguaro recruitment is indeed episodic, with periodicities of 20-30yr, possibly related to strong El Niño Southern Oscillation events. Our results suggest that saguaros rely on multidecadal periodic pulses of good beneficial years to incorporate new individuals into their populations. Inverse-growth modeling can be used in a wide variety of plant species to study their recruitment dynamics.

Transcutaneous Electrical Neuromodulation of the Cervical Spinal Cord Depends Both on the Stimulation Intensity and the Degree of Voluntary Activity for Training. A Pilot Study

Electrical enabling motor control (eEmc) through transcutaneous spinal cord stimulation offers promise in improving hand function. However, it is still unknown which stimulus intensity or which muscle force level could be better for this improvement. Nine healthy individuals received the following interventions: (i) eEmc intensities at 80%, 90% and 110% of abductor pollicis brevis motor threshold combined with hand training consisting in 100% handgrip strength; (ii) hand training consisting in 100% and 50% of maximal handgrip strength combined with 90% eEmc intensity. The evaluations included box and blocks test (BBT), maximal voluntary contraction (MVC), F wave persistency, F/M ratio, spinal and cortical motor evoked potentials (MEP), recruitment curves of spinal MEP and cortical MEP and short-interval intracortical inhibition. The results showed that: (i) 90% eEmc intensity increased BBT, MVC, F wave persistency, F/M ratio and cortical MEP recruitment curve; 110% eEmc intensity increased BBT, F wave persistency and cortical MEP and recruitment curve of cortical MEP; (ii) 100% handgrip strength training significantly modulated MVC, F wave persistency, F/M wave and cortical MEP recruitment curve in comparison to 50% handgrip strength. In conclusion, eEmc intensity and muscle strength during training both influence the results for neuromodulation at the cervical level.

WS2.2. Parameters of TMS

Transcranial magnetic stimulation (TMS) is a non-invasive painless technique to stimulate the human brain. This presentation will introduce basic physiological principles of how TMS excites human motor cortex. In addition, the following single-pulse and paired-pulse TMS measures of corticospinal and motor cortical excitability will be presented and discussed with respect to underpinning physiological mechanisms: motor threshold, motor evoked potential recruitment curve, cortical silent period, short-interval intracortical inhibition, intracortical facilitation, long- interval intracortical inhibition, short-interval intracortical facilitation, short-latency afferent inhibition. Pharmacological evidence will be summarized to explain the physiology of these measures. In addition, clinical utility will be exemplarily discussed by demonstrating abnormal TMS measures of corticospinal and motor cortical excitability in patients with epilepsy and Alzheimer's disease.

Transspinal stimulation and step training alter function of spinal networks in complete spinal cord injury.

Pilot study (case series). The objective of this study was to establish spinal neurophysiological changes following high-frequency transspinal stimulation during robot-assisted step training in individuals with chronic motor complete spinal cord injury (SCI). University research laboratory (Klab4Recovery). Four individuals with motor complete SCI received an average of 18 sessions of transspinal stimulation over the thoracolumbar region with a pulse train at 333 Hz during robotic-assisted step training. Each session lasted ~1 h, with an average of 240 stimulations delivered during each training session. Before and after the combined intervention, we evaluated the amplitude modulation of the long-latency tibialis anterior (TA) flexion reflex and transspinal evoked potentials (TEP) recorded from flexors and extensors during assisted stepping, and the TEP recruitment curves at rest. The long-latency TA flexion reflex was depressed in all phases of the step cycle and the phase-dependent amplitude modulation of TEPs was altered during assisted stepping, while spinal motor output based on TEP recruitment curves was increased after the combined intervention. This is the first study documenting noninvasive transspinal stimulation coupled with locomotor training depresses flexion reflex excitability and concomitantly increases motoneuron output over multiple spinal segments for both flexors and extensors in people with motor complete SCI. While both transspinal stimulation and locomotor training may act via similar activity-dependent neuroplasticity mechanisms, combined interventions for rehabilitation of neurological disorders has not been systematically assessed. Our current findings support locomotor training induced neuroplasticity may be augmented with transspinal stimulation.

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

Determine whether unilateral low-intensity blood-flow restricted (LIBFR) exercise is as effective as high-intensity (HI) resistance training for improving contralateral muscle strength. Thirty healthy adults (20-30 years) were randomly allocated to the following dynamic plantar-flexion training interventions: HI [75% of one-repetition maximum (1RM), 4 sets, 10 reps] and LIBFR [20% of 1RM, 4 sets, 30 + 15 + 15 + 15 reps]. Evoked V-wave and H-reflex recruitment curves, as well as maximal voluntary contraction (MVC) and panoramic ultrasound assessments of the trained and untrained soleus muscles were obtained pre-training, post-4 weeks of training and post-4 weeks of detraining. Both interventions failed to increase contralateral MVC and muscle cross-sectional area (CSA). Yet, contralateral rate of torque development (RTD) was enhanced by both regimens (12-26%) and this was accompanied by heightened soleus EMG within the first milliseconds of the rising torque-time curve (14-22%; p < 0.05). These improvements were dissipated after detraining. Contralateral adaptations were not accompanied by changes in V-wave or H-reflex excitability. Conversely, LIBFR and HI elicited a similar magnitude of ipsilateral increase in MVC, RTD and CSA post-training (10-18%). Improvements in V-wave amplitude and soleus EMG were limited to the trained leg assigned to LIBFR training (p < 0.05). While gains in strength and CSA remained preserved post-4 weeks of detraining, this did not occur with RTD. Since gains in RTD were similar between interventions, our findings indicate that both training regimens can be used interchangeably for improving contralateral rapid torque production. Ultimately, this may be beneficial in circumstances of limb immobilization after injury or surgery.

Influence of High Pass Filter Settings on Motor Evoked Potentials.

ObjectiveMotor evoked potentials (MEP), obtained by transcranial magnetic stimulation (TMS) are a common tool in clinical research and diagnostic. Nevertheless, reports regarding the influence of filter settings on MEP are sparse. Here, we compared MEP amplitudes and signal to noise ratio (SNR) using multiple high pass filter (HPF) and notch filter settings.Materials and MethodsTwenty healthy subjects were enrolled in the study. Recruitment curves were obtained with HPF settings varied at 10, 20, 50, and 100 Hz. The four HPF settings were tested both with and without 50 Hz active notch filter. Low pass filter was kept constant at 5 kHz.ResultsMEP amplitudes with HPF at 10 and 20 Hz were significantly higher than at 100 Hz, regardless of the notch filter. However, SNR did not differ among HPF settings. An active notch filter significantly improved SNR.ConclusionThe reduction in MEP amplitudes with HPF above 20 Hz may be due to noise reduction, since the different HPF conditions did not alter SNR. Thus, higher HPF above 50 Hz may be an option to reduce noise, the use of a notch filter may even improve SNR.SignificanceOur findings are relevant for the selection of filter settings and might be of importance to any researcher who utilizes TMS-MEP.

Assessing head acceleration to identify a motor threshold to galvanic vestibular stimulation.

Galvanic vestibular stimulation (GVS) is used to assess vestibular system function, but vestibulospinal responses can exhibit variability depending on protocols or intensities used. Here, we measured head acceleration in healthy subjects to identify an objective motor threshold on which to base GVS intensity when assessing standing postural responses. Thirteen healthy right-handed subjects stood on a force platform, eyes closed, and head facing forward. An accelerometer was placed on the vertex to detect head acceleration, and electromyography activity of the right soleus was recorded. GVS (200 ms; current steps 0.5, from 1 mA to 4 mA) was applied in a binaural and bipolar configuration. 1) GVS induced a biphasic accelerometer response at a latency of 15 ms. Based on response amplitude, we constructed a recruitment curve for all participants and determined the motor threshold. In parallel, the method of limits was used to devise a more rapid approach to determine motor threshold. 2) We observed significant differences between motor threshold based on a recruitment curve and all perceptual thresholds reported either by the subject (sensation of movement) or a standing experimenter observing the participant (perception of movement). No significant difference was observed between the motor threshold based on the method of limits and perceptual thresholds of movement. 3) Using orthogonal polynomial contrasts, we observed a linear progression between multiples of the objective motor threshold (0.5, 0.75, 1, 1.5× motor threshold) and the 95% confidence ellipse area, the first peak of center of pressure displacement velocity, and the short and medium latency responses in the soleus. Hence, an objective motor threshold for GVS based on head acceleration was identified in standing participants and a recruitment curve could be constructed for all participants. These novel approaches could enable better understanding of changes in the vestibular system in different conditions or over time.NEW & NOTEWORTHY Galvanic vestibular stimulation (GVS) has been used to assess the vestibular system, but the significant interindividual variability in the responses makes it difficult to quantitatively compare them between individuals or conditions. Using an accelerometer to quantify head movement induced by GVS, we were able to determine an objective motor threshold and construct a recruitment curve for all participants. These methods could help assess changes in the vestibular system under different conditions.

Infrared neurostimulation in ex-vivo rat sciatic nerve using 1470 nm wavelength

Objective. To design and implement a setup for ex-vivo optical stimulation for exploring the effect of several key parameters (optical power and pulse duration), activation features (threshold, spatial selectivity) and recovery characteristics (repeated stimuli) in peripheral nerves. Approach. A nerve chamber allowing ex-vivo electrical and optical stimulation was designed and built. A 1470 nm light source was chosen to stimulate the nerve. A photodiode module was implemented for synchronization of the electrical and optical channels. Main results. Compound neural action potentials (CNAPs) were successfully generated with infrared light pulses of 200–2000 µs duration and power in the range of 3–10 W. These parameters determine a radiant exposure for stimulation in the range 1.59–4.78 J cm−2. Recruitment curves were obtained by increasing durations at a constant power level. Neural activation threshold is reached at a mean radiant exposure of 3.16 ± 0.68 J cm−2 and mean pulse energy of 3.79 ± 0.72 mJ. Repetition rates of 2–10 Hz have been explored. In eight out of ten sciatic nerves (SNs), repeated light stimuli induced a sensitization effect in that the CNAP amplitude progressively grows, representing an increasing number of recruited fibres. In two out of ten SNs, CNAPs were composed of a succession of peaks corresponding to different conduction velocities. Significance. The reported sensitization effect could shed light on the mechanism underlying infrared neurostimulation. Our results suggest that, in sharp contrast with electrical stimuli, optical pulses could recruit slow fibres early on. This more physiological order of recruitment opens the perspective for specific neuromodulation of fibre population who remained poorly accessible until now. Short high-power light pulses at wavelengths below 1.5 µm offer interesting perspectives for neurostimulation.

Strengthened structure–function relationships of the corticospinal tract by free water correction after stroke

The corticospinal tract is the most intensively investigated tract of the human motor system in stroke rehabilitative research. Diffusion-tensor-imaging gives insights into its microstructure, and transcranial magnetic stimulation assesses its excitability. Previous data on the interrelationship between both measures are contradictory. Correlative or predictive models which associate them with motor outcome are incomplete. Free water correction has been developed to enhance diffusion-tensor-imaging by eliminating partial volume with extracellular water, which could improve capturing stroke-related microstructural alterations, thereby also improving structure-function relationships in clinical cohorts. In the present cross-sectional study, data of 18 chronic stroke patients and 17 healthy controls, taken from a previous study on cortico-cerebellar motor tracts, were re-analysed: The data included diffusion-tensor-imaging data quantifying corticospinal tract microstructure with and without free water correction, transcranial magnetic stimulation data assessing recruitment curve properties of motor evoked potentials and detailed clinical data. Linear regression modelling was used to interrelate corticospinal tract microstructure, recruitment curves properties and clinical scores. The main finding of the present study was that free water correction substantially strengthens structure-function associations in stroke patients: Specifically, our data evidenced a significant association between fractional anisotropy of the ipsilesional corticospinal tract and its excitability (P = 0.001, adj. R2 = 0.54), with free water correction explaining additional 20% in recruitment curve variability. For clinical scores, only free water correction leads to the reliable detection of significant correlations between ipsilesional corticospinal tract fractional anisotropy and residual grip (P = 0.001, adj. R2 = 0.70) and pinch force (P < 0.001, adj. R2 = 0.72). Finally, multimodal models can be improved by free water correction as well. This study evidences that corticospinal tract microstructure directly relates to its excitability in stroke patients. It also shows that unexplained variance in motor outcome is considerably reduced by free water correction arguing that it might serve as a powerful tool to improve existing models of structure-function associations and potentially also outcome prediction after stroke.

Cortical Representation and Excitability Increases for a Thenar Muscle Mediate Improvement in Short-Term Cellular Phone Text Messaging Ability.

Cortical representations expand during skilled motor learning. We studied a unique model of motor learning with cellular phone texting, where the thumbs are used exclusively to interact with the device and the prominence of use can be seen where 3200 text messages are exchanged a month in the 18–24 age demographic. The purpose of the present study was to examine the motor cortex representation and input–output (IO) recruitment curves of the abductor pollicis brevis (APB) muscle of the thumb and the ADM muscle with transcranial magnetic stimulation (TMS), relative to individuals’ texting abilities and short-term texting practice. Eighteen individuals performed a functional texting task (FTT) where we scored their texting speed and accuracy. TMS was then used to examine the cortical volumes and areas of activity in the two muscles and IO curves were constructed to measure excitability. Subjects also performed a 10-min practice texting task, after which we repeated the cortical measures. There were no associations between the cortical measures and the FTT scores before practice. However, after practice the APB cortical map expanded and excitability increased, whereas the ADM map constricted. The increase in the active cortical areas in APB correlated with the improvement in the FTT score. Based on the homogenous group of subjects that were already good at texting, we conclude that the cortical representations and excitability for the thumb muscle were already enlarged and more receptive to changes with short-term practice, as noted by the increase in FTT performance after 10-min of practice.

Modulation of the Corticomotor Excitability by Repetitive Peripheral Magnetic Stimulation on the Median Nerve in Healthy Subjects.

Objective: We aimed to examine the effects of repetitive peripheral nerve magnetic stimulation (rPNMS) on the excitability of the contralateral motor cortex and motor function of the upper limb in healthy subjects.Methods: Forty-six healthy subjects were randomly assigned to either a repetitive peripheral nerve magnetic stimulation group (n = 23) or a sham group (n = 23). The repetitive peripheral nerve magnetic stimulation group received stimulation using magnetic pulses at 20 Hz, which were applied on the median nerve of the non-dominant hand, whereas the sham group underwent the same protocol without the stimulation output. The primary outcome was contralateral transcranial magnetic stimulation (TMS)-induced corticomotor excitability for the abductor pollicis brevis of the stimulated hand in terms of resting motor threshold (rMT), the slope of recruitment curve, and peak amplitude of motor evoked potential (MEP), which were measured at baseline and immediately after each session. The secondary outcomes were motor hand function including dexterity and grip strength of the non-dominant hand assessed at baseline, immediately after stimulation, and 24 h post-stimulation.Results: Compared with the sham stimulation, repetitive peripheral nerve magnetic stimulation increased the peak motor evoked potential amplitude immediately after the intervention. The repetitive peripheral nerve magnetic stimulation also increased the slope of the recruitment curve immediately after intervention and enhanced hand dexterity after 24 h. However, the between-group difference for the changes was not significant. The significant changes in hand dexterity and peak amplitude of motor evoked potential after repetitive peripheral nerve magnetic stimulation were associated with their baseline value.Conclusions: Repetitive peripheral nerve magnetic stimulation may modulate the corticomotor excitability together with a possible lasting improvement in hand dexterity, indicating that it might be helpful for clinical rehabilitation.

M-wave and H-reflex recruitment curves in boys and men.

The aim of the present study was to check whether the M-wave and H-reflex recruitment curves differ between prepubertal boys and men. Eleven boys (9-11yr) and eleven men (18-35yr) were magnetically stimulated at the tibial nerve in a prone position. M-wave and H-reflex maximal amplitudes (Hmax; Mmax ; Hmax /Mmax ), thresholds, regression slopes (Hslp ; Mslp ; Hslp /Mslp ) were extracted from M-wave and H-reflex recruitment curves and compared between the two age groups. Overall, no significant difference in M-wave and H-reflex recruitment curve parameters was found between the two populations. Nevertheless, the size of the M-wave associated with maximal H-reflex amplitude was lower in boys as compared to men when expressed relative to maximal M-wave amplitude (MHmax /Mmax : 0.18±0.06 vs. 0.31±0.13; p<.05). This result suggests that the development of peripheral nerve was completed in 9 to 11-year-old boys and did not affect the M-wave and H-reflex recruitment curves parameters. In neuromuscular function studies, it implies that Hmax /Mmax and Hslp /Mslp could be used indifferently to compare spinal motoneuron excitability between 9-11-year-old boys and men. Conversely, evoking H-reflexes at a given percentage of Mmax may bias the comparison between boys and men.

Characterization of Motor-Evoked Responses Obtained with Transcutaneous Electrical Spinal Stimulation from the Lower-Limb Muscles after Stroke.

An increasing number of studies suggests that a novel neuromodulation technique targeting the spinal circuitry enhances gait rehabilitation, but research on its application to stroke survivors is limited. Therefore, we investigated the characteristics of spinal motor-evoked responses (sMERs) from lower-limb muscles obtained by transcutaneous spinal cord stimulation (tSCS) after stroke compared to age-matched and younger controls without stroke. Thirty participants (ten stroke survivors, ten age-matched controls, and ten younger controls) completed the study. By using tSCS applied between the L1 and L2 vertebral levels, we compared sMER characteristics (resting motor threshold (RMT), slope of the recruitment curve, and latency) of the tibialis anterior (TA) and medial gastrocnemius (MG) muscles among groups. A single pulse of stimulation was delivered in 5 mA increments, increasing from 5 mA to 250 mA or until the subjects reached their maximum tolerance. The stroke group had an increased RMT (27–51%) compared to both age-matched (TA: p = 0.032; MG: p = 0.005) and younger controls (TA: p < 0.001; MG: p < 0.001). For the TA muscle, the paretic side demonstrated a 13% increased latency compared to the non-paretic side in the stroke group (p = 0.010). Age-matched controls also exhibited an increased RMT compared to younger controls (TA: p = 0.002; MG: p = 0.007), suggesting that altered sMER characteristics present in stroke survivors may result from both stroke and normal aging. This observation may provide implications for altered spinal motor output after stroke and demonstrates the feasibility of using sMER characteristics as an assessment after stroke.

A MATLAB application for automated H-Reflex measurements and analyses

ObjectiveH-Reflex is a test that is carried out to measure the relative excitability of reflex pathways. Although reliable, conventional methods consist of performing many small steps, which requires a high level of attentiveness, and thus can carry an elevated risk of human error, despite proper training. Equipment that is available to perform those tests with different levels of automation are typically proprietary, inextensible by the user, and expensive. Here we present a novel MATLAB application that can accurately and reliably perform automated H-Reflex measurements, test the stimulating electrodes, and carry out typical subsequent analyses. MethodsThis application is a Graphical User Interface that works with inexpensive equipment and offers many important features such as measuring electrode impedance in-situ, automating lengthy measurements like recruitment curves and frequency response trials, standardizing electric stimulation properties, automatic exporting of digital data and metadata, and immediately analyzing acquired data with single-click events. ResultsOur new method was validated against conventional H-Reflex measurement methods with 2 anesthetized rats. The difference between acquired data using both methods was negligible (mean difference=0.0038; std=0.0121). Our app also detected electrode impedance with high accuracy (94%). ConclusionThe method presented here allows reliable and efficient automated H-reflex measurements and can accurately analyze the collected data. SignificanceThe features provided by our app can speed up data collection and reduce human error, and unlike conventional methods, allow the user to analyze data immediately after the record. This can result in higher research quality and give broader access to the technique.

Impact of β-range-induced oscillatory activity on human input–output relationship of the corticospinal pathway

ABSTRACT Objective: The aim of the study was to show that short-lasting (90 s) transcranial alternating current stimulation (tACS) at 20 Hz delivered over the left primary motor cortex (M1) is able to change the shape of recruitment curve of the corticospinal pathway. Methods: The corticospinal pathway was studied during tACS by means of the relationship between the intensity of transcranial magnetic stimulation (TMS) delivered over the left M1 and corresponding motor evoked potentials (MEPs) recorded from the right first dorsal interosseus muscle (FDI), in nine healthy subjects. In order to extract characteristics of the input–output relationship that have particular physiological relevance, data were fitted to the Boltzmann sigmoidal function by the Levenberg–Marquardt nonlinear, least mean squares algorithm. Results: The β-rhythm tACS influenced the shape and parameters of the input–output relation, so that the initial segment of the conditioned curve (from threshold to 30% of maximum muscle size) diverged, while the subsequent segment converged to overlap the unconditioned control curve. Discussion: β-rhythm tACS conditions only a definite subset of corticospinal elements influencing less than 30% of the entire motoneuronal pool. The fact that β-rhythm tACS mainly affects the most excitable motoneurons could explain the observed antikinetic effect of the tACS at β-rhythm applied in the motor regions.