Noninvasive stimulation of the human corticospinal tract

Reliability of Corticospinal and Motoneuronal Excitability Evaluation during Unfatiguing and Fatiguing Cycling Exercise

INTRODUCTION

Central nervous system excitability depends on the task performed, muscle group solicited, and contraction type. However, little is known on corticospinal and motoneuronal excitability measured during locomotor exercise. This study aimed at determining the reliability of motor-evoked potentials (MEP) and thoracic motor-evoked potentials (TMEP) in dynamic mode during unfatiguing and fatiguing cycling exercise.

METHODS

Twenty-two participants completed four visits. Visit 1 comprised familiarization and an incremental cycling test to determine maximal power output ( Wmax ). The remaining visits encompassed unfatiguing evaluations, which included a total of eight brief bouts of moderate- (50% Wmax ) and high-intensity cycling (80% Wmax ), four at each intensity. In each bout, a set of two TMEPs, five MEPs, and one M-max were obtained. Subsequently, a fatiguing exercise to exhaustion at 80% Wmax was performed, with four sets of measurements 3 min through the exercise and four additional sets at exhaustion, both measured at 50% Wmax .

RESULTS

Intraclass correlation coefficients (ICCs) for 5, 10, 15, and 20 MEP·Mmax -1 revealed excellent reliability at both intensities and during cycling to exhaustion (ICC ≥0.92). TMEP·Mmax -1 showed ICCs ≥0.82 for moderate and high intensity, and it was not affected by fatigability. Overall standard error of measurement was 0.090 (0.083, 0.097) for MEP·Mmax -1 and 0.114 (0.105, 0.125) for TMEP·Mmax -1 . A systematic bias associated with the number of stimulations, especially at high intensity, suggested that the evaluation itself may be influenced by fatigability. A mean reduction of 8% was detected in TMEP·Mmax -1 at exhaustion.

CONCLUSIONS

Motoneuronal and corticospinal excitability measured in dynamic mode presented good to excellent reliability in unfatiguing and fatiguing exercise. Further studies inducing greater fatigability must be conducted to assess the sensitivity of central nervous system excitability during cycling.

Differential modulation of corticomotor excitability in older compared to young adults following a single bout of strength -exercise

Evidence shows corticomotor plasticity diminishes with age. Nevertheless, whether strength-training, a proven intervention that induces corticomotor plasticity in younger adults, also takes effect in older adults, remains untested. This study examined the effect of a single-session of strength-exercise on corticomotor plasticity in older and younger adults. Thirteen older adults (72.3 ± 6.5 years) and eleven younger adults (29.9 ± 6.9 years), novice to strength-exercise, participated. Strength-exercise involved four sets of 6-8 repetitions of a dumbbell biceps curl at 70-75% of their one-repetition maximum (1-RM). Muscle strength, cortical, corticomotor and spinal excitability, before and up to 60-minutes after the strength-exercise session were assessed. We observed significant changes over time (p < 0.05) and an interaction between time and age group (p < 0.05) indicating a decrease in corticomotor excitability (18% p < 0.05) for older adults at 30- and 60-minutes post strength-exercise and an increase (26% and 40%, all p < 0.05) in younger adults at the same time points. Voluntary activation (VA) declined in older adults immediately post and 60-minutes post strength-exercise (36% and 25%, all p < 0.05). Exercise had no effect on the cortical silent period (cSP) in older adults however, in young adults cSP durations were shorter at both 30- and 60- minute time points (17% 30-minute post and 9% 60-minute post, p < 0.05). There were no differences in short-interval cortical inhibition (SICI) or intracortical facilitation (ICF) between groups. Although the corticomotor responses to strength-exercise were different within groups, overall, the neural responses seem to be independent of age.

Reliability of transcranial magnetic stimulation-evoked responses on knee extensor muscles during cycling

Transcranial magnetic stimulation (TMS) measures the excitability and inhibition of corticomotor networks. Despite its task-specificity, few studies have used TMS during dynamic movements and the reliability of TMS paired pulses has not been assessed during cycling. This study aimed to evaluate the reliability of motor evoked potentials (MEP) and short- and long-interval intracortical inhibition (SICI and LICI) on vastus lateralis and rectus femoris muscle activity during a fatiguing single-leg cycling task. Nine healthy adults (2 female) performed two identical sessions of counterweighted single-leg cycling at 60% peak power output until failure. Five single pulses and ten paired pulses were delivered to the motor cortex, and two maximal femoral nerve stimulations (Mmax) were administered during two baseline cycling bouts (unfatigued) and every 5 min throughout cycling (fatigued). When comparing both baseline bouts within the same session, MEP·Mmax-1 and LICI (both ICC: >0.9) were rated excellent while SICI was rated good (ICC: 0.7-0.9). At baseline, between sessions, in the vastus lateralis, Mmax (ICC: >0.9) and MEP·Mmax-1 (ICC: 0.7) demonstrated good reliability; LICI was moderate (ICC: 0.5), and SICI was poor (ICC: 0.3). Across the fatiguing task, Mmax demonstrated excellent reliability (ICC > 0.8), MEP·Mmax-1 ranged good to excellent (ICC: 0.7-0.9), LICI was moderate to excellent (ICC: 0.5-0.9), and SICI remained poorly reliable (ICC: 0.3-0.6). These results corroborate the cruciality of retaining mode-specific testing measurements and suggest that during cycling, Mmax, MEP·Mmax-1, and LICI measures are reliable whereas SICI, although less reliable across days, can be reliable within the same session.

Strength-trained adults demonstrate greater corticoreticular activation versus untrained controls

The rapid increase in strength following strength-training involves neural adaptations, however, their specific localisation remains elusive. Prior focus on corticospinal responses prompts this study to explore the understudied cortical/subcortical adaptations, particularly cortico-reticulospinal tract responses, comparing healthy strength-trained adults to untrained peers. Fifteen chronically strength-trained individuals (≥2 years of training, mean age: 24 ± 7 years) were compared with 11 age-matched untrained participants (mean age: 26 ± 8 years). Assessments included maximal voluntary force (MVF), corticospinal excitability using transcranial magnetic stimulation (TMS), spinal excitability (cervicomedullary stimulation), voluntary activation (VA) and reticulospinal tract (RST) excitability, utilizing StartReact responses and ipsilateral motor-evoked potentials (iMEPs) for the flexor carpi radialis muscle. Trained participants had higher normalized MVF (6.4 ± 1.1 N/kg) than the untrained participants (4.8 ± 1.3 N/kg) (p = .003). Intracortical facilitation was higher in the strength-trained group (156 ± 49%) (p = .02), along with greater VA (98 ± 3.2%) (p = .002). The strength-trained group displayed reduced short-interval-intracortical inhibition (88 ± 8.0%) compared with the untrained group (69 ± 17.5%) (p < .001). Strength-trained individuals exhibited a greater normalized rate of force development (38.8 ± 10.1 N·s-1/kg) (p < .009), greater reticulospinal gain (2.5 ± 1.4) (p = .02) and higher ipsilateral-to-contralateral MEP ratios compared with the untrained group (p = .03). Strength-trained individuals displayed greater excitability within the intrinsic connections of the primary motor cortex and the RST. These results suggest greater synaptic input from the descending cortico-reticulospinal tract to α-motoneurons in strength-trained individuals, thereby contributing to the observed increase in VA and MVF.

Linking corticospinal tract activation and upper-limb motor control in adults with cerebral palsy

AIM

To quantify the cervicomedullary motor evoked potentials (CMEPs) at the cervical spinal level in adults with cerebral palsy (CP) and determine if altered CMEPs are linked with upper-extremity motor function in this population.

METHOD

This cross-sectional study consisted of a cohort of adults with CP (n = 15; mean age = 33 years 5 months [SD = 11 years 8 months]); Manual Ability Classification System levels I-IV) and neurotypical controls (n = 18; mean age = 30 years 10 months [SD = 10 years 4 months]), who were recruited to participate at an academic medical center. Adults with CP and typical adults (controls) were stimulated at the cervicomedullary junction to assess CMEPs at the cervical spinal cord level. Upper-extremity motor function was quantified using the Box and Blocks and Purdue Pegboard tests, self-reported upper-extremity function (UEF), and assessments of selective motor control.

RESULTS

At higher stimulation levels, the contralateral CMEP responses of adults with CP were different from typical adults (p = 0.032). Reduced CMEP was correlated with reduced upper-limb function, including worse performance on the Box and Blocks (rho = 0.625, p = 0.025) and Purdue Pegboard tests (rho = 0.701, p = 0.010), lower self-reported UEF (rho = 0.761, p = 0.009), and overall selective motor control (rho = 0.731, p = 0.007).

INTERPRETATION

Changes in the activation of spinal motoneurons through corticospinal pathways may have an important role in the altered upper-extremity motor function of individuals with CP.

Transsynaptic activation of human lumbar spinal motoneurons by transvertebral magnetic stimulation

Noninvasive spinal stimulation has been increasingly used in research on motor control and neurorehabilitation. Despite advances in percutaneous electrical stimulation techniques, magnetic stimulation is not as commonly used as electrical stimulation. Therefore, it is still under discussion what neuronal elements are activated by magnetic stimulation of the human spinal cord. In this study, we demonstrated that transvertebral magnetic stimulation (TVMS) induced transsynaptic activation of spinal motoneuron pools in the lumbar cord. In healthy humans, paired-pulse TVMS was given over an intervertebral space between the L1-L2 vertebrae with an interpulse interval of 100 ms, and the stimulus-evoked electromyographic (EMG) responses were recorded in the lower limb muscles. The results show that the evoked EMG responses after the 2nd pulse were clearly suppressed compared with the widespread responses evoked after the 1st pulse in the muscles of the lower extremity, indicating that the transsynaptic activation of spinal motoneurons by the 2nd pulse was suppressed by the effects produced by the 1st pulse. The inconsistent modulation of response suppression to stimulus intensity across individuals suggests that the TVMS-evoked EMG responses are composed of the compound potentials mediated by the direct activation of motor axons and the transsynaptic activation of motoneuron pools through sensory afferents and that the recruitment order of those fibers by TVMS may be nonhomogeneous across individuals.

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.

Excitatory drive to spinal motoneurones is necessary for serotonin to modulate motoneurone excitability via 5-HT2 receptors in humans

Serotonin modulates corticospinal excitability, motoneurone firing rates and contractile strength via 5-HT2 receptors. However, the effects of these receptors on cortical and motoneurone excitability during voluntary contractions have not been explored in humans. Therefore, the purpose of this study was to investigate how 5-HT2 antagonism affects corticospinal and motoneuronal excitability with and without descending drive to motoneurones. Twelve individuals (aged 24 ± 4 years) participated in a double-blind, placebo-controlled, crossover study, whereby the 5-HT2 antagonist cyproheptadine was administered. Transcranial magnetic stimulation (TMS) was delivered to the motor cortex to produce motor evoked potentials (MEPs), and electrical stimulation at the cervicomedullary junction was used to generate cervicomedullary motor evoked potentials (CMEPs) in the biceps brachii at rest and during a range of submaximal elbow flexions. Evoked potentials were also obtained after a conditioning TMS pulse to produce conditioned MEPs and CMEPs (100 ms inter-stimulus interval). 5-HT2 antagonism reduced maximal torque (p < 0.001), and compared to placebo, reduced unconditioned MEP amplitude at rest (p = 0.003), conditioned MEP amplitude at rest (p = 0.033) and conditioned MEP amplitude during contractions (p = 0.020). 5-HT2 antagonism also increased unconditioned CMEP amplitude during voluntary contractions (p = 0.041) but not at rest. Although 5-HT2 antagonism increased long-interval intracortical inhibition, net corticospinal excitability was unaffected during voluntary contractions. Given that spinal motoneurone excitability was only affected when descending drive to motoneurones was present, the current study indicates that excitatory drive is necessary for 5-HT2 receptors to regulate motoneurone excitability but not intracortical circuits.

Serotonergic and noradrenergic contributions to motor cortical and spinal motoneuronal excitability in humans

Animal models indicate that motor behaviour is shaped by monoamine neuromodulators released diffusely throughout the brain and spinal cord. As an alternative to conducting a single study to explore the effects of neuromodulators on the human motor system, we have identified and collated human experiments investigating motor effects of well-characterised drugs that act on serotonergic and noradrenergic networks. In doing so, we present strong neuropharmacology evidence that human motor pathways are affected by neuromodulators across both healthy and clinical populations, insight that cannot be determined from a single reductionist experiment. We have focused our review on the effects that monoaminergic drugs have on muscle responses to non-invasive stimulation of the motor cortex and peripheral nerves, and other closely related tests of motoneuron excitability, and discuss how these measurement techniques elucidate the effects of neuromodulators at motor cortical and spinal motoneuronal levels. Although there is some heterogeneity in study methods, we find drugs acting to enhance extracellular concentrations of serotonin tend to reduce the excitability of the human motor cortex, and enhanced extracellular concentrations of noradrenaline increases motor cortical excitability by enhancing intracortical facilitation and reducing inhibition. Both monoamines tend to enhance the excitability of spinal motoneurons. Overall, this review details the importance of neuromodulators for the output of human motor pathways and suggests that commonly prescribed monoaminergic drugs target the motor system in addition to their typical psychiatric/neurological indications.

Increased paired stimuli enhance corticospinal-motoneuronal plasticity in humans with spinal cord injury

Paired corticospinal-motoneuronal stimulation (PCMS) has been used to enhance corticospinal excitability and functional outcomes in humans with spinal cord injury (SCI). Here, we examined the effect of increasing the number of paired pulses on PCMS-induced plasticity. During PCMS, corticospinal volleys evoked by transcranial magnetic stimulation (TMS) over the hand motor cortex were timed to arrive at corticospinal-motoneuronal synapses of the first dorsal interosseous (FDI) muscle 1-2 ms before the arrival of antidromic potentials elicited in motoneurons by electrical stimulation of the ulnar nerve. We tested motor-evoked potentials (MEPs) elicited by TMS over the hand motor cortex and electrical stimulation at the cervicomedullary junction (CMEPs) in the FDI muscle before and after 180 paired pulses (PCMS-180) followed up by another 180 paired pulses (PCMS-360) in humans with and without chronic incomplete cervical SCI. The nine-hole-peg-test (9HPT) was measured before and after PCMS paired pulses in individuals with SCI. We found that the size of MEPs and CMEPs increased after PCMS-180 in both groups compared with baseline and further increased after PCMS-360 in participants with SCI, suggesting a spinal origin for these effects. Notably, in people with SCI, the time to complete the 9HPT decreased after PCMS-180 and further decreased after PCMS-360 compared with baseline but not when the 9HPT was repeated overtime. Our findings demonstrate that increasing the number of PCMS paired pulses potentiates corticospinal excitability and voluntary motor output after SCI, likely through spinal plasticity. This proof-of-principle study suggests that increasing the PCMS dose represents a strategy to boost voluntary motor output after SCI.NEW & NOTEWORTHY Paired corticospinal-motoneuronal stimulation (PCMS) has been used to enhance corticospinal excitability and functional outcomes in humans with spinal cord injury (SCI). Here, we demonstrate that 360 paired pulses resulted in larger increases in motor-evoked potential size in a hand muscle and in a better ability to complete the nine-hold-peg-test compared with 180 paired pulses in people with SCI. This proof-of-principle study suggests that increasing the PCMS dose represents a strategy to boost motor output after SCI.

Multisite Hebbian Plasticity Restores Function in Humans with Spinal Cord Injury

OBJECTIVE

Spinal cord injury (SCI) damages synaptic connections between corticospinal axons and motoneurons of many muscles, resulting in devastating paralysis. We hypothesized that strengthening corticospinal-motoneuronal synapses at multiple spinal cord levels through Hebbian plasticity (i.e., "neurons that fire together, wire together") promotes recovery of leg and arm function.

METHODS

Twenty participants with chronic SCI were randomly assigned to receive 20 sessions of Hebbian or sham stimulation targeting corticospinal-motoneuronal synapses of multiple leg muscles followed by exercise. Based on the results from this study, in a follow-up prospective study, 11 more participants received 40 sessions of Hebbian stimulation targeting corticospinal-motoneuronal synapses of multiple arm and leg muscles followed by exercise. During Hebbian stimulation sessions, 180 paired pulses elicited corticospinal action potentials by magnetic (motor cortex) and/or electrical (thoracic spine) stimulation allowing volleys to arrive at the spinal cord 1-2 milliseconds before motoneurons were activated retrogradely via bilateral electrical stimulation (brachial plexus, ulnar, femoral, and common peroneal nerves) for biceps brachii, first dorsal interosseous, quadriceps femoris, and tibialis anterior muscles as needed.

RESULTS

We found in our randomized study that participants receiving Hebbian stimulation improved their walking speed and corticospinal function to a greater extent than individuals receiving sham stimulation. In agreement, prospective study participants improved their grasping and walking, corticospinal function, and quality of life metrics, exhibiting greater improvements with more sessions that persisted 9-month post-therapy.

INTERPRETATION

Our findings suggest that multisite Hebbian stimulation, informed by the physiology of the corticospinal system, represents an effective strategy to promote functional recovery following SCI. ANN NEUROL 2023;93:1198-1213.

Electrical stimulation for investigating and improving neuromuscular function in vivo: Historical perspective and major advances

This historical review summarizes the major advances - particularly from the last 50 years - in transcutaneous motor-level electrical stimulation, which can be used either as a tool to investigate neuromuscular function and its determinants (electrical stimulation for testing; EST) or as a therapeutic/training modality to improve neuromuscular and physical function (neuromuscular electrical stimulation; NMES). We focus on some of the most important applications of electrical stimulation in research and clinical settings, such as the investigation of acute changes, chronic adaptations and pathological alterations of neuromuscular function with EST, as well as the enhancement, preservation and restoration of muscle strength and mass with NMES treatment programs in various populations. For both EST and NMES, several major advances converge around understanding and optimizing motor unit recruitment during electrically-evoked contractions, also taking into account the influence of stimulation site (e.g., muscle belly vs nerve trunk) and type (e.g., pulse duration, frequency, and intensity). This information is equally important both in the context of mechanistic research of neuromuscular function as well as for clinicians who believe that improvements in neuromuscular function are required to provide health-related benefits to their patients.

Effects of trunk neuromuscular electrical stimulation on the motor circuits of able-bodied individuals

Upper- and lower-limb neuromuscular electrical stimulation (NMES) is known to modulate the excitability of the neural motor circuits. However, it remains unclear whether short-duration trunk muscle NMES could achieve similar neuromodulation effects. We assessed motor evoked potentials (MEPs) elicited through transcranial magnetic stimulation of the primary motor cortex representation of the trunk extensor muscles to evaluate corticospinal excitability. Moreover, cervicomedullary motor evoked potentials (CMEPs) were assessed through cervicomedullary junction magnetic stimulation to evaluate subcortical excitability. Twelve able-bodied individuals participated in the MEP study, and another twelve in the CMEP study. During the interventions, NMES was applied bilaterally to activate the erector spinae muscle and produce intermittent contractions (20 s ON/20 s OFF) for a total of 20 min while participants remained seated. Assessments were performed: (i) before; (ii) during (in brief periods when NMES was OFF); and (iii) immediately after the interventions to compare MEP or CMEP excitability. Our results showed that MEP responses were not affected by trunk NMES, while CMEP responses were facilitated for approximately 8 min during the intervention, and returned to baseline before the end of the 20 min stimulating period. Our findings therefore suggest that short-duration NMES of the trunk extensor muscles likely does not affect the corticospinal excitability, but it has a potential to facilitate subcortical neural circuits immediately after starting the intervention. These findings indicate that short-duration application of NEMS may be helpful in rehabilitation to enhance neuromodulation of the trunk subcortical neural motor circuits.

Corticospinal and spinal excitability during peripheral or central cooling in humans

Cold exposure can impair fine and gross motor control and threaten survival. Most motor task decrement is due to peripheral neuromuscular factors. Less is known about cooling on central neural factors. Corticospinal and spinal excitability were determined during cooling of the skin (T_sk) and core (T_co). Eight subjects (four female) were actively cooled in a liquid perfused suit for 90 min (2 °C inflow temperature), passively cooled for 7 min, and then rewarmed for 30 min (41 °C inflow temperature). Stimulation blocks included 10 transcranial magnetic stimulations [eliciting motor evoked potentials (MEPs) which indicate corticospinal excitability], 8 trans-mastoid electrical stimulations [eliciting cervicomedullary evoked potentials (CMEPs) which indicate spinal excitability] and 2 brachial plexus electrical stimulations [eliciting maximal compound motor action potentials (M_max)]. These stimulations were delivered every 30 min. Cooling for 90 min reduced T_sk to 18.2 °C while T_co did not change. At the end of rewarming T_sk returned to baseline while T_co decreased by 0.8 °C (afterdrop) (P < 0.001). Metabolic heat production was higher than baseline at the end of passive cooling (P = 0.01), and 7 min into rewarming (P = 0.04). MEP/M_max remained unchanged throughout. CMEP/M_max increased by 38% at end cooling (although increased variability at this time rendered the increase insignificant, P = 0.23) and 58% at end warming when T_co was 0.8 °C below baseline (P = 0.02). Cooling increased spinal excitability but not corticospinal excitability. Cooling may decrease cortical and/or supraspinal excitability which is compensated for by increased spinal excitability. This compensation is key to providing a motor task and survival advantage.

Cervical transcutaneous spinal stimulation for spinal motor mapping

Transcutaneous spinal stimulation (TSS) is a promising approach to restore upper-limb (UL) functions after spinal cord injury (SCI) in humans. We sought to demonstrate the selectivity of recruitment of individual UL motor pools during cervical TSS using different electrode placements. We demonstrated that TSS delivered over the rostrocaudal and mediolateral axes of the cervical spine resulted in a preferential activation of proximal, distal, and ipsilateral UL muscles. This was revealed by changes in motor threshold intensity, maximum amplitude, and the amount of post-activation depression of the evoked responses. We propose that an arrangement of electrodes targeting specific UL motor pools may result in superior efficacy, restoring more diverse motor activities after neurological injuries and disorders, including severe SCI.

How is neuromuscular fatigability affected by perceived fatigue and disability in people with multiple sclerosis?

Whereas fatigue is recognized to be the main complaint of patients with multiple sclerosis (PwMS), its etiology, and particularly the role of resistance to fatigability and its interplay with disability level, remains unclear. The purposes of this review were to (i) clarify the relationship between fatigue/disability and neuromuscular performance in PwMS and (ii) review the corticospinal and muscular mechanisms of voluntary muscle contraction that are altered by multiple sclerosis, and how they may be influenced by disability level or fatigue. Neuromuscular function at rest and during exercise are more susceptible to impairement, due to deficits in voluntary activation, when the disability is greater. Fatigue level is related to resistance to fatigability but not to neuromuscular function at rest. Neurophysiological parameters related to signal transmission such as central motor conduction time, motor evoked potentials amplitude and latency are affected by disability and fatigue levels but their relative role in the impaired production of torque remain unclear. Nonetheless, cortical reorganization represents the most likely explanation for the heightened fatigability during exercise for highly fatigued and/or disabled PwMS. Further research is needed to decipher how the fatigue and disability could influence fatigability for an ecological task, especially at the corticospinal level.

The recent history of afferent stimulation modulates corticospinal excitability

BACKGROUND

Transcranial magnetic stimulation (TMS) is widely used to probe corticospinal excitability and fast sensorimotor integration in the primary motor hand area (M1-HAND). A conditioning electrical stimulus, applied to the contralateral hand, can suppress the motor evoked potential (MEP) elicited by TMS of M1-HAND when the afferent stimulus arrives in M1-HAND at the time of TMS. The magnitude of this short-latency afferent inhibition (SAI) is expressed as the ratio between the conditioned and unconditioned MEP amplitude.

OBJECTIVE/HYPOTHESIS

We hypothesized that corticospinal excitability and SAI are influenced by the recent history of peripheral electrical stimulation.

METHODS

In twenty healthy participants, we recorded MEPs from the right first dorsal interosseus muscle. MEPs were evoked by single-pulse TMS of the left M1-HAND alone (unconditioned TMS) or by TMS preceded by electrical stimulation of the right index finger ("homotopic" conditioning) or little finger ("heterotopic" conditioning). The three conditions were either pseudo-randomly intermixed or delivered in blocks in which a single condition was repeated five or ten times. MEP amplitudes and SAI magnitudes were compared using linear mixed-effect models and one-way ANOVAs.

RESULTS

All stimulation protocols consistently produced SAI, which was stronger after homotopic stimulation. Randomly intermingling the three stimulation conditions reduced the relative magnitude of homotopic and heterotopic SAI as opposed to blocked stimulation. The apparent attenuation of SAI was caused by a suppression of the unconditioned but not the conditioned MEP amplitude during the randomly intermixed pattern.

CONCLUSION(S)

The recent history of afferent stimulation modulates corticospinal excitability. This "history effect" impacts on the relative magnitude of SAI depending on how conditioned and unconditioned responses are intermixed and needs to be taken into consideration when probing afferent inhibition and corticospinal excitability.

Non-local muscle fatigue is mediated at spinal and supraspinal levels

The objective was to measure the corticospinal excitability and motoneuron responsiveness of the right and left Biceps Brachii (BB), and left Abductor Digiti Minimi (ADM) muscles in response to submaximal isotonic fatiguing contractions performed by the right BB muscle. With the familiarization session, ten young moderately active male subjects came to the lab on seven occasions. Three sets of 3 min seated elbow curls at 25% of one-repetition maximum (1RM) separated by a 1-min rest performed by the right BB muscle were used as the fatiguing protocol. The motor evoked potential (MEP), cervicomedullary motor evoked potential (CMEP), and compound muscle action potential (Mmax) of the right BB muscle (baseline and after each set of the fatiguing task), the left BB and ADM muscles (baseline, post-fatigue, post-10, and post-20 min) were measured. MEP and CMEP were then normalized to Mmax for statistical analysis. The results showed that in the right BB muscle, there was a significant reduction in the MEP after performing the fatiguing task (p= 0.03), while no significant effect of time was seen in the CMEP (p= 0.07). In the left BB muscle, the MEP significantly decreased from pre-fatigue to post-fatigue (p= 0.01) and post-10 (p= 0.001), while there was a significant decline in the CMEP post-fatigue (p= 0.03). In the left ADM muscle, MEP significantly decreased post-fatigue (p= 0.03) and no changes were seen in the CMEP (p= 0.12). These results not only confirm the incidence of non-local muscle fatigue (NLMF) in response to performing submaximal isotonic fatiguing contractions but also as a new finding, imply that both spinal and supraspinal modulations account for the NLMF response.

5-HT2 receptor antagonism reduces human motoneuron output to antidromic activation but not to stimulation of corticospinal axons

The intrinsic electrical properties of motoneurons strongly affect motoneuron excitability to fast-acting excitatory ionotropic inputs. Serotonin (5-HT) is a neurochemical that alters the intrinsic properties of motoneurons, whereby animal models and in vitro experiments indicate that 5-HT increases motoneuron excitability by activating 5-HT₂ receptors on the somato-dendritic compartment. In the current study, we examined how antagonism of the 5-HT₂ receptor affects motoneuron excitability in humans. We hypothesised that motoneuron excitability would be reduced. The 5-HT₂ antagonist cyproheptadine was administered to ten healthy participants in a double-blinded, placebo-controlled, crossover trial. Electrical cervicomedullary stimulation was used to deliver a synchronised excitatory volley to motoneurons to elicit cervicomedullary motor evoked potentials (CMEPs) in the surface electromyography (EMG) signal of the resting biceps brachii. Likewise, electrical peripheral nerve stimulation was used to generate antidromic spikes in motoneurons and cause recurrent discharges, which were recorded with surface EMG as F-waves in a resting hand muscle. Compared to placebo, we found that 5-HT₂ antagonism reduced the amplitude and persistence of F-waves but did not affect CMEP amplitude. 5-HT₂ antagonism also reduced maximal contraction strength. The reduced recurrent discharge of motoneurons with 5-HT₂ antagonism suggests that 5-HT₂ receptors modulate the electrical properties of the initial segment or soma to promote excitability. Conversely, as cyproheptadine did not affect motoneuron excitability to brief synaptic input, but affected maximal contractions requiring sustained input, it seems likely that the 5-HT₂ mediated amplification of synaptic input at motoneuron dendrites is functionally significant only when excitatory input activates persistent inward currents.

The effects of pain induced by blood flow occlusion in one leg on exercise tolerance and corticospinal excitability and inhibition of the contralateral leg in males

Experiencing pain in one leg can alter exercise tolerance and neuromuscular fatigue (NMF) responses in the contralateral leg; however, the corticospinal modulations to non-local experimental pain induced by blood flow occlusion remain unknown. In three randomized visits, thirteen male participants performed 25% of isometric maximal voluntary contraction (25%IMVC) to task failure with one leg preceded by (i) 6-min rest (CON), (ii) cycling at 80% of peak power output until task failure with the contralateral leg (CYCL) or (iii) CYCL followed by blood flow occlusion (OCCL) during 25%IMVC. NMF assessments (IMVC, voluntary activation [VA] and potentiated twitch [Qtw]) were performed at baseline and task failure. During the 25%IMVC, transcranial magnetic stimulations were performed to obtain motor evoked potential (MEP), silent period (SP), and short intracortical inhibition (SICI). 25%IMVC was shortest in OCCL (105±50s) and shorter in CYCL (154±68s) than CON (219±105s) (P<0.05). IMVC declined less after OCCL (-24±19%) and CYCL (-27±18%) then CON (-35±11%) (P<0.05). Qtw declined less in OCCL (-40±25%) compared to CYCL (-50±22%) and CON (-50±21%) (P<0.05). VA was similar amongst conditions. MEP and SP increased and SICI decreased throughout the task while SP was longer for OCCL compared to CYC condition (P<0.05). The results suggest that pain in one leg diminishes contralateral limb exercise tolerance and NMF development and modulate corticospinal inhibition in males. Novelty: Pain in one leg diminished MVC and twitch force decline in the contralateral limb Experimental pain induced by blood flow occlusion may modulation corticospinal inhibition of the neural circuitries innervating the contralateral exercise limb.

Trans-Spinal Focused Ultrasound Stimulation Selectively Modulates Descending Motor Pathway

Compared to current non-invasive methods utilizing magnetic and electrical means, focused ultrasound provides greater spatial resolution and penetration depth. Despite the broad application of ultrasound stimulation, there is a lack of studies dedicated to the investigation of acoustic neuromodulation on the spinal cord. This study aims to apply focused ultrasound on the spinal cord to modulate the descending pathways in a non-invasive fashion. The application of trans-spinal focused ultrasound (tsFUS) was examined on the motor deficit mouse model. tsFUS was achieved using a single-element focused ultrasound transducer operating at 3 MHz. The sonication was performed on anesthetized 6 week-old mice targeting T12 and L3 vertebrae. The effect was analyzed by comparing electromyography responses from the hindlimb induced by electrical stimulation of the motor cortex. Further, the mouse model with the Harmaline-induced essential tremor was selected to investigate the potential clinical application of tsFUS. The safety was verified by histological assessment. Sonication at the T12 area inhibited motor response, while sonication over the L3 region provided signal enhancement. Sonication of T12 of the ET mouse also showed the ability of ultrasound to suppress tremors. Meanwhile, the histological examination did not show any abnormalities with the highest applied acoustic pressure. In this work, a non-invasive motor signal modulation was achieved using tsFUS. Moreover, the results showed the ability of focused ultrasound to manage tremors in a safe manner. This study provides a stepping stone for the trans-spinal application of focused ultrasound to motor-related disorders.

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

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

In vivo electrophysiological mechanisms underlying cervical epidural stimulation in adult rats

KEY POINTS

To electrophysiologically determine the predominant neural structures activated with cervical epidural stimulation (ES), well-established electrophysiological protocols (single-pulse, paired-pulse and multiple frequency stimulation) were delivered at rest, during motor activity and under anaesthesia in adult rats. Cervical ES resulted in spinal evoked motor responses with three different waveforms - early response (ER), middle response (MR) and late response (LR). ERs remained unmodulated by repeated stimulation protocols. In contrast, MRs and LRs were modulated by repeated stimulation protocols and volitional motor activity. ERs are consequential to the direct activation of motor efferents; MRs are secondary to type-I sensory afferent activation and LRs result from the engagement of wider spinal interneuronal circuitry with potential influence from supraspinal pathways. Evidence from this work is fundamental in enhancing our understanding of cervical ES, and critical in refining the design of neuromodulation-based rehabilitative strategies and in the construction of neuroprosthetics.

ABSTRACT

Epidural stimulation (ES) of the lumbar spinal cord has demonstrated significant improvements in various physiological functions after a traumatic spinal cord injury in humans. Electrophysiological evidence from rodent, human and computational studies collectively suggest that the functional recovery following lumbar ES is mediated via direct activation of sensory afferent fibres. However, the mechanisms underlying cervical ES have not been comprehensively studied, which greatly limits our understanding of its effectiveness in restoring upper limb function. In this work, we determined the predominant neural structures that are activated with cervical ES using in vivo cervical spinal evoked motor responses (SEMRs). Standard electrophysiological protocols (single-pulse, paired-pulse and multiple frequency stimulation) were implemented in 11 awake and anaesthetized rats in four experimental stages. Three distinct types of cervical SEMRs were identified based on latency of their appearance: early response (ER), middle response (MR) and late response (LR). ERs remained unmodulated by repeated stimulation protocols. MRs and LRs were modulated by repeated stimulation protocols and volitional motor activity. Except for LRs being completely abolished under urethane, ketamine or urethane anaesthesia did not affect the appearance of cervical SEMRs. Our data, backed by literature, suggest that ERs are secondary to the direct activation of motor efferents, MRs are elicited by activation of type-I sensory afferents and LRs result from the engagement of interneuronal circuitry with potential influence from supraspinal pathways. The gathered information paves the way to designing motor rehabilitation strategies that can utilize cervical ES to recover upper limb function following neurological deficits.

Remote muscle priming anodal transcranial direct current stimulation attenuates short interval intracortical inhibition and increases time to task failure of a constant workload cycling exercise

Anodal transcranial direct current stimulation (atDCS), a non-invasive neuromodulatory technique has been shown to increase the excitability of targeted brain area and influence endurance exercise performance. However, the effect of atDCS applied on an unexercised muscle motor cortex (M1) representation on GABA_A-mediated intracortical inhibition and endurance exercise performance remains unknown. In two separate sessions, twelve subjects performed fatigue cycling exercise (80% peak power output) sustained to task failure in a double-blinded design, following either ten minutes of bicephalic anodal tDCS (atDCS) or sham applied on a non-exercised hand muscle M1 representation. Short interval intracortical inhibition (SICI) was measured at baseline, post neuromodulation and post-exercise using paired-pulse transcranial magnetic stimulation (TMS) in a resting hand muscle. There was a greater decrease in SICI (P < 0.05) post fatigue cycling with atDCS priming compared to sham. Time to task failure (TTF) was significantly increased following atDCS compared to sham (P < 0.05). These findings suggest that atDCS applied over the non-exercised muscle M1 representation can augment cycling exercise performance; and although this outcome may be mediated via a multitude of mechanisms, a decrease in the global excitability of GABA_A inhibitory interneurons may be a possible contributing factor.

Reduced Cerebellar Brain Inhibition Measured Using Dual-Site TMS in Older Than in Younger Adults

Dual-site transcranial magnetic stimulation (TMS) can be used to measure the cerebellar inhibitory influence on the primary motor cortex, known as cerebellar brain inhibition (CBI), which is thought to be important for motor control. The aim of this study was to determine whether age-related differences in CBI (measured at rest) were associated with an age-related decline in bilateral motor control measured using the Purdue Pegboard task, the Four Square Step Test, and a 10-m walk. In addition, we examined test re-test reliability of CBI measured using dual-site TMS with a figure-of-eight coil in two sessions. There were three novel findings. First, CBI was less in older than in younger adults, which is likely underpinned by an age-related loss of Purkinje cells. Second, greater CBI was associated with faster 10-m walking performance in older adults, but slower 10-m walking performance in younger adults. Third, moderate intraclass correlation coefficients (ICCs: 0.53) were found for CBI in younger adults; poor ICCs were found for CBI (ICC: 0.40) in older adults. Together, these results have important implications for the use of dual-site TMS to increase our understanding of age- and disease-related changes in cortical motor networks, and the role of functional connectivity in motor control.

Muscle-specific movement-phase-dependent modulation of corticospinal excitability during upper-limb motor execution and motor imagery combined with virtual action observation

Corticospinal excitability in humans can be facilitated during imagination and/or observation of upper-limb motor tasks. However, it remains unclear to what extent facilitation levels may differ from those elicited during execution of the same tasks. Twelve able-bodied individuals were recruited in this study. Motor evoked potentials (MEPs) in extensor carpi radialis (ECR) and flexor carpi radialis (FCR) muscles were elicited through transcranial magnetic stimulation of the primary motor cortex during: (i) rest; (ii) wrist extension; and (iii) wrist flexion. Responses were compared between: (1) motor imagery combined with virtual action observation (MI + AO; first-person virtual wrist movements shown on a computer display, while participants remained at rest and imagined these movements); and (2) motor execution (ME; participants extended or flexed their wrist). During MI + AO, ECR MEPs were facilitated during the extension phase but not the flexion phase, while FCR MEPs were facilitated during the flexion phase but not extension phase, compared to rest. During the ME condition, same, but greater, modulations were shown as those during MI + AO, while background muscle activities were similar in the rest phase as during extension and flexion phase in the MI + AO condition. Our results demonstrated that kinesthetic MI that included imagination and observation of virtual hands can elicit phase-dependent muscles-specific corticospinal facilitation of wrist muscles, consistent to those during actual hand extension and flexion. Moreover, we showed that MI + AO can contribute considerably to the overall corticospinal facilitation (∼20 % of ME) even without muscle contractions. These findings support utility of computer graphics-based motor imagery, which may have implications for rehabilitation and development of brain-computer interfaces.

Assessing cerebellar-cortical connectivity using concurrent TMS-EEG: a feasibility study

Combined single-pulse transcranial magnetic stimulation (TMS) and electroencephalography (EEG) has been used to probe the features of local networks in the cerebral cortex. Here, we investigated whether we can use this approach to explore long-range connections between the cerebellum and cerebral cortex. Ten healthy adults received single-pulse suprathreshold TMS to the cerebellum and an occipital/parietal control site with double-cone and figure-of-eight coils while cerebral activity was recorded. A multisensory electrical control condition was used to simulate the sensation of the double-cone coil at the cerebellar site. Two cleaning pipelines were compared, and the spatiotemporal relationships of the EEG output between conditions were examined at sensor and source levels. Cerebellar stimulation with the double-cone coil resulted in large artifact in the EEG trace. The addition of SOUND filtering to the cleaning pipeline improved the signal such that further analyses could be undertaken. The cortical potentials evoked by the active TMS conditions showed strong relationships with the responses to the multisensory control condition after ∼50 ms. A distinct parietal component at ∼42 ms was found following cerebellar double-cone stimulation. Although evoked potentials differed across all conditions at early latencies, it is unclear as to whether these represented TMS-related network activation of the cerebellarthalamocortical tract, or whether components were dominated by sensory contamination and/or coil-driven artifact. This study highlights the need for caution when interpreting outcomes from cerebellar TMS-EEG studies.NEW & NOTEWORTHY This is the first study to systematically assess the feasibility of obtaining TMS-evoked potentials from cerebellar stimulation with concurrent EEG. An innovative control condition using electrical stimulation was modified to mimic the sensory aspects of cerebellar stimulation with a double-cone coil, and a state-of-the art cleaning pipeline was trialled. The extent of artifact contamination in signals from stimulation of a cerebellar and an occipital/parietal control site using two TMS coil types was highlighted.

Reductions in motoneuron excitability during sustained isometric contractions are dependent on stimulus and contraction intensity

Cervicomedullary stimulation provides a means of assessing motoneuron excitability. Previous studies demonstrated that during low-intensity sustained contractions, small cervicomedullary evoked potentials (CMEPs) conditioned using transcranial magnetic stimulation (TMS-CMEPs) are reduced, whereas large TMS-CMEPs are less affected. As small TMS-CMEPs recruit motoneurons most active during low-intensity contractions whereas large TMS-CMEPs recruit a high proportion of motoneurons inactive during the task, these results suggest that reductions in motoneuron excitability could be dependent on repetitive activation. To further test this hypothesis, this study assessed changes in small and large TMS-CMEPs across low- and high-intensity contractions. Twelve participants performed a sustained isometric contraction of the elbow flexor for 4.5 min at the electromyography (EMG) level associated with 20% maximal voluntary contraction force (MVC; low intensity) and 70% MVC (high intensity). Small and large TMS-CMEPs with amplitudes of ∼15% and ∼50% M_max at baseline, respectively, were delivered every minute throughout the tasks. Recovery measures were taken at 1-, 2.5- and 4-min postexercise. During the low-intensity trial, small TMS-CMEPs were reduced at 2-4 min (P ≤ 0.049) by up to -10% M_max, whereas large TMS-CMEPs remained unchanged (P ≥ 0.16). During the high-intensity trial, small and large TMS-CMEPs were reduced at all time points (P < 0.01) by up to -14% and -33% M_max, respectively, and remained below baseline during all recovery measures (P ≤ 0.02). TMS-CMEPs were unchanged relative to baseline during recovery following the low-intensity trial (P ≥ 0.24). These results provide novel insight into motoneuron excitability during and following sustained contractions at different intensities and suggest that contraction-induced reductions in motoneuron excitability depend on repetitive activation.NEW & NOTEWORTHY This study measured motoneuron excitability using cervicomedullary evoked potentials conditioned using transcranial magnetic stimulation (TMS-CMEPs) of both small and large amplitudes during sustained low- and high-intensity contractions of the elbow flexors. During the low-intensity task, only the small TMS-CMEP was reduced. During the high-intensity task, both small and large TMS-CMEPs were substantially reduced. These results indicate that repetitively active motoneurons are specifically reduced in excitability compared with less active motoneurons in the same pool.

Non-invasive brain stimulation to assess neurophysiologic underpinnings of lower limb motor impairment in multiple sclerosis

BACKGROUND

Multiple sclerosis (MS) is a neuroinflammatory disease resulting in axonal demyelination and an amalgamation of symptoms which commonly result in decreased quality of life due to mobility dysfunction and limited participation in meaningful activities.

NEW METHOD

The use of non-invasive brain stimulation (NIBS) techniques, specifically transcranial magnetic and transcranial direct current stimulation, have been essential in understanding the pathophysiological decrements related to disease progression, particularly with regard to motor impairments. Although the research in this area has primarily focused on the upper extremities, new interest has arisen in understanding the neurophysiological underpinnings of lower limb impairment. Therefore, the purpose of this review is to: first, provide an overview of common NIBS techniques used to explore sensorimotor neurophysiology; second, summarize lower limb neuromuscular and mobility impairments typically observed in PwMS; third, review the current knowledge regarding interactions between TMS-assessed neurophysiology and lower limb impairments in PwMS; and fourth, provide recommendations for future NIBS studies based on current gaps in the literature.

RESULTS

PwMS exhibit reduced excitability and increased inhibitory neurophysiologic function which has been related to disease severity and lower limb motor impairments. Comparison with existing methods: Moreover, promising results indicate that the use of repetitive stimulation and transcranial direct current stimulation may prime neural adaptability and prove useful as a therapeutic tool in ameliorating lower limb impairments.

CONCLUSIONS

While these studies are both informative and promising, additional studies are necessary to be conclusive. As such, studies assessing objective measures of lower limb impairments associated with neurophysiological adaptations need further evaluation.

The knowns and unknowns of neural adaptations to resistance training

The initial increases in force production with resistance training are thought to be primarily underpinned by neural adaptations. This notion is firmly supported by evidence displaying motor unit adaptations following resistance training; however, the precise locus of neural adaptation remains elusive. The purpose of this review is to clarify and critically discuss the literature concerning the site(s) of putative neural adaptations to short-term resistance training. The proliferation of studies employing non-invasive stimulation techniques to investigate evoked responses have yielded variable results, but generally support the notion that resistance training alters intracortical inhibition. Nevertheless, methodological inconsistencies and the limitations of techniques, e.g. limited relation to behavioural outcomes and the inability to measure volitional muscle activity, preclude firm conclusions. Much of the literature has focused on the corticospinal tract; however, preliminary research in non-human primates suggests reticulospinal tract is a potential substrate for neural adaptations to resistance training, though human data is lacking due to methodological constraints. Recent advances in technology have provided substantial evidence of adaptations within a large motor unit population following resistance training. However, their activity represents the transformation of afferent and efferent inputs, making it challenging to establish the source of adaptation. Whilst much has been learned about the nature of neural adaptations to resistance training, the puzzle remains to be solved. Additional analyses of motoneuron firing during different training regimes or coupling with other methodologies (e.g., electroencephalography) may facilitate the estimation of the site(s) of neural adaptations to resistance training in the future.

Cortical and Subcortical Neural Interactions Between Trunk and Upper-limb Muscles in Humans

Activities of daily living require simultaneous and coordinated activation of trunk and upper-limb segments, which involves complex interlimb interaction within the central nervous system. Although many studies have reported associations between activity of trunk and limb muscles during functional tasks, evidence on cortical and subcortical contributions to trunk-limb neural interactions is still not fully clear. Therefore, the aim of this study was to examine interactions between trunk and upper-limb muscles in the: (i) corticospinal circuits by using motor evoked potential (MEP) elicited through transcranial magnetic stimulation; and (ii) subcortical circuits by using cervicomedullary motor evoked potential (CMEP) elicited through cervicomedullary junction magnetic stimulation. Responses were evoked in the erector spinae (trunk) and flexor carpi radialis (upper-limb) muscles in twelve able-bodied individuals: (1) while participants were relaxed; (2) during trunk muscle contractions while arms were at rest; and (3) during upper-limb muscle contractions while the trunk was at rest. Our results showed that trunk muscle CMEP responses were not affected by upper-limb muscle contractions, while MEP responses were modulated. This indicates that at least the subcortical circuits may not attribute to facilitation of the trunk muscles during upper-limb contractions. On the other hand, in the upper-limb muscles, both CMEP and MEP responses were modulated during trunk contractions. These results indicate that cortical and subcortical mechanisms attributed to facilitation of upper-limb muscles during trunk contractions. In conclusion, our study demonstrated evidence that trunk-limb neural interactions may be attributed to cortical and/or subcortical mechanisms depending on the contracted muscle.

Motor cortical circuits contribute to crossed facilitation of trunk muscles induced by rhythmic arm movement

Training of one limb improves performance of the contralateral, untrained limb, a phenomenon known as cross transfer. It has been used for rehabilitation interventions, i.e. mirror therapy, in people with neurologic disorders. However, it remains unknown whether training of the upper limb can induce the cross-transfer effect to the trunk muscles. Using transcranial magnetic stimulation over the primary motor cortex (M1) we examined motor evoked potentials (MEPs) in the contralateral erector spinae (ES) muscle before and after 30 min of unilateral arm cycling in healthy volunteers. ES MEPs were increased after the arm cycling. To understand the origin of this facilitatory effect, we examined short-interval intracrotical inhibition (SICI) and cervicomedullary MEPs (CMEPs) in neural populations controlling in the ES muscle. Notably, SICI reduced after the arm cycling, while CMEPs remained the same. Using bilateral transcranial direct current stimulation (tDCS) in conjunction with 20 min of the arm cycling, the amplitude of ES MEPs increased to a similar extent as with 30 min of the arm cycling alone. These findings demonstrate that a single session of unilateral arm cycling induces short-term plasticity in corticospinal projections to the trunk muscle in healthy humans. The changes are likely driven by cortical mechanisms.

Acute intermittent hypoxia boosts spinal plasticity in humans with tetraplegia

Paired corticospinal-motoneuronal stimulation (PCMS) elicits spinal synaptic plasticity in humans with chronic incomplete cervical spinal cord injury (SCI). Here, we examined whether PCMS-induced plasticity could be potentiated by acute intermittent hypoxia (AIH), a treatment also known to induce spinal synaptic plasticity in humans with chronic incomplete cervical SCI. During PCMS, we used 180 pairs of stimuli where corticospinal volleys evoked by transcranial magnetic stimulation over the hand representation of the primary motor cortex were timed to arrive at corticospinal-motoneuronal synapses of the first dorsal interosseous (FDI) muscle ~1-2 ms before the arrival of antidromic potentials elicited in motoneurons by electrical stimulation of the ulnar nerve. During AIH, participants were exposed to brief alternating episodes of hypoxic inspired gas (1 min episodes of 9% O₂) and room air (1 min episodes of 20.9% O₂). We examined corticospinal function by measuring motor evoked potentials (MEPs) elicited by cortical and subcortical stimulation of corticospinal axons and voluntary motor output in the FDI muscle before and after 30 min of PCMS combined with AIH (PCMS+AIH) or sham AIH (PCMS+sham-AIH). The amplitude of MEPs evoked by magnetic and electrical stimulation increased after both protocols, but most after PCMS+AIH, consistent with the hypothesis that their combined effects arise from spinal plasticity. Both protocols increased electromyographic activity in the FDI muscle to a similar extent. Thus, PCMS effects on spinal synapses of hand motoneurons can be potentiated by AIH. The possibility of different thresholds for physiological vs behavioral gains needs to be considered during combinatorial treatments.

Long-term motor skill training with individually adjusted progressive difficulty enhances learning and promotes corticospinal plasticity

Motor skill acquisition depends on central nervous plasticity. However, behavioural determinants leading to long lasting corticospinal plasticity and motor expertise remain unexplored. Here we investigate behavioural and electrophysiological effects of individually tailored progressive practice during long-term motor skill training. Two groups of participants practiced a visuomotor task requiring precise control of the right digiti minimi for 6 weeks. One group trained with constant task difficulty, while the other group trained with progressively increasing task difficulty, i.e. continuously adjusted to their individual skill level. Compared to constant practice, progressive practice resulted in a two-fold greater performance at an advanced task level and associated increases in corticospinal excitability. Differences were maintained 8 days later, whereas both groups demonstrated equal retention 14 months later. We demonstrate that progressive practice enhances motor skill learning and promotes corticospinal plasticity. These findings underline the importance of continuously challenging patients and athletes to promote neural plasticity, skilled performance, and recovery.

Distinct Corticospinal and Reticulospinal Contributions to Voluntary Control of Elbow Flexor and Extensor Muscles in Humans with Tetraplegia

Humans with cervical spinal cord injury (SCI) often recover voluntary control of elbow flexors and, to a much lesser extent, elbow extensor muscles. The neural mechanisms underlying this asymmetrical recovery remain unknown. Anatomical and physiological evidence in animals and humans indicates that corticospinal and reticulospinal pathways differentially control elbow flexor and extensor motoneurons; therefore, it is possible that reorganization in these pathways contributes to the asymmetrical recovery of elbow muscles after SCI. To test this hypothesis, we examined motor-evoked potentials (MEPs) elicited by transcranial magnetic stimulation over the arm representation of the primary motor cortex, maximal voluntary contractions, the StartReact response (a shortening in reaction time evoked by a startling stimulus), and the effect of an acoustic startle cue on MEPs elicited by cervicomedullary stimulation (CMEPs) on biceps and triceps brachii in males and females with and without chronic cervical incomplete SCI. We found that SCI participants showed similar MEPs and maximal voluntary contractions in biceps but smaller responses in triceps compared with controls, suggesting reduced corticospinal inputs to elbow extensors. The StartReact and CMEP facilitation was larger in biceps but similar to controls in triceps, suggesting enhanced reticulospinal inputs to elbow flexors. These findings support the hypothesis that the recovery of biceps after cervical SCI results, at least in part, from increased reticulospinal inputs and that the lack of these extra inputs combined with the loss of corticospinal drive contribute to the pronounced weakness found in triceps.SIGNIFICANCE STATEMENT Although a number of individuals with cervical incomplete spinal cord injury show limited functional recovery of elbow extensors compared with elbow flexor muscles, to date, the neural mechanisms underlying this asymmetrical recovery remain unknown. Here, we provide for the first time evidence for increased reticulospinal inputs to biceps but not triceps brachii and loss of corticospinal drive to triceps brachii in humans with tetraplegia. We propose that this reorganization in descending control contributes to the asymmetrical recovery between elbow flexor and extensor muscles after cervical spinal cord injury.

Bipolar transcutaneous spinal stimulation evokes short-latency reflex responses in human lower limbs alike standard unipolar electrode configuration

Noninvasive electrical stimulation targeting the posterior lumbosacral roots has been applied recently in reflexes studies and as a neuromodulation intervention for modifying spinal cord circuitry after an injury. Here, we characterized short-latency responses evoked by four bipolar electrode configurations placed longitudinally over the spinal column at different vertebral levels from L1 to T9. They were compared with the responses evoked by the standard unipolar (aka monopolar) electrode configuration (cathode at T11/12, anode over the abdominal wall). Short-latency responses were recorded in the rectus femoris, medial hamstrings, tibialis anterior, and soleus muscles, bilaterally, in 11 neurologically intact participants. The response recruitment characteristics (maximal amplitude, motor threshold) and amplitude-matched onset latencies and paired-pulse suppression (35-ms interstimulus interval) were assessed with 1-ms current-controlled pulses at intensities up to 100 mA. The results showed that short-latency responses can be elicited with all bipolar electrode configurations. However, only with the cathode at T11/12 and the anode 10 cm cranially (∼T9), the maximum response amplitudes were statistical equivalent (P < 0.05) in the medial hamstrings, tibialis anterior, and soleus but not the rectus femoris, whereas motor thresholds were not significantly different across all muscles. The onset latency and paired-pulse suppression were also not significantly different across the tested electrode configurations, thereby confirming the reflex nature of the bipolar short-latency responses. We conclude that the bipolar configuration (cathode T11/12, anode ∼T9) produces reflex responses that are ostensibly similar to those evoked by the standard unipolar configuration. This provides an alternative approach for neuromodulation intervention.NEW & NOTEWORTHY Transcutaneous spinal stimulation with the identified bipolar electrode configuration may offer several advantages for neuromodulation interventions over commonly used unipolar configurations: there are no associated abdominal contractions, which improves the participant's comfort; additional dermatomes are not stimulated as when the anode is over the abdominal wall or iliac crest, which may have unwanted effects; and, due to a more localized electrical field, the bipolar configuration offers the possibility of targeting cord segments more selectively.

Spinal contribution to neuromuscular recovery differs between elbow-flexor and knee-extensor muscles after a maximal sustained fatiguing task

Data from studies of elbow-flexor (EF) or knee-extensor (KE) muscles suggest that a fatigue-related decrease in motoneuron excitability only occurs in EF. It is unknown how motoneuron excitability changes after sustained fatiguing maximal voluntary isometric contractions (MVICs) in EF and KE in the same participants. In two sessions, eight healthy men performed a 2-min MVIC of EF or KE to induce fatigue with brief MVICs before and six times after the 2-min MVIC. Electromyographic responses elicited by corticospinal tract stimulation at the transmastoid [cervicomedullary motor-evoked potential (CMEP)] or thoracic [thoracic motor-evoked potential (TMEP)] level were recorded from EF and KE, respectively. To account for muscle excitability, CMEPs and TMEPs were normalized to maximal M-wave (M_max) elicited by peripheral nerve stimulation during each brief MVIC. Immediately after the 2-min MVIC, biceps brachii and brachioradialis CMEP/M_max were 88% (SD 11%) (P = 0.026) and 87% (SD 12%) (P = 0.029) of pre-MVIC (PRE) values, respectively, and remained lower than PRE after 5 s of recovery [91% (SD 8%), P = 0.036 and 87% (SD 13%), P = 0.046, respectively]. No subsequent time points differed from PRE (all P ≥ 0.253). TMEP/M_max for rectus femoris and vastus lateralis were not different from PRE at any time during the recovery period (all P > 0.050). A different recovery pattern in motoneuron excitability occurred in EF as it recovered by 60 s whereas KE motoneurons were unaffected by the fatiguing task. The present findings may contribute to better understand muscle-specific neurophysiological differences in spinal excitability.NEW & NOTEWORTHY By comparing the changes in motoneuron excitability in elbow-flexor and knee-extensor muscles after sustained fatiguing maximal voluntary contractions, this study shows that motoneuron recovery behavior depends on the muscle performing the exercise. A different recovery pattern in motoneuron excitability occurs in elbow flexors as it recovered by 60 s whereas knee extensors were unaffected by fatigue. This finding can help to increase understanding of the effect of a fatigue and subsequent recovery on neural processes.

Cortical, Corticospinal, and Reticulospinal Contributions to Strength Training

Following a program of resistance training, there are neural and muscular contributions to the gain in strength. Here, we measured changes in important central motor pathways during strength training in 2 female macaque monkeys. Animals were trained to pull a handle with one arm; weights could be added to increase load. On each day, motor-evoked potentials in upper limb muscles were first measured after stimulation of the primary motor cortex (M1), corticospinal tract (CST), and reticulospinal tract (RST). Monkeys then completed 50 trials with weights progressively increased over 8-9 weeks (final weight ∼6 kg, close to the animal's body weight). Muscle responses to M1 and RST stimulation increased during strength training; there were no increases in CST responses. Changes persisted during a 2 week washout period without weights. After a further 3 months of strength training, an experiment under anesthesia mapped potential responses to CST and RST stimulation in the cervical enlargement of the spinal cord. We distinguished the early axonal volley and later spinal synaptic field potentials, and used the slope of the relationship between these at different stimulus intensities as a measure of spinal input-output gain. Spinal gain was increased on the trained compared with the untrained side of the cord within the intermediate zone and motor nuclei for RST, but not CST, stimulation. We conclude that neural adaptations to strength training involve adaptations in the RST, as well as intracortical circuits within M1. By contrast, there appears to be little contribution from the CST.

SIGNIFICANCE STATEMENT We provide the first report of a strength training intervention in nonhuman primates. Our results indicate that strength training is associated with neural adaptations in intracortical and reticulospinal circuits, whereas corticospinal and motoneuronal adaptations are not dominant factors.

Static stretch and dynamic muscle activity induce acute similar increase in corticospinal excitability

Even though the acute effects of pre-exercise static stretching and dynamic muscle activity on muscular and functional performance have been largely investigated, their effects on the corticospinal pathway are still unclear. For that reason, this study examined the acute effects of 5×20 s of static stretching, dynamic muscle activity and a control condition on spinal excitability, corticospinal excitability and plantar flexor neuromuscular properties. Fifteen volunteers were randomly tested on separate days. Transcranial magnetic stimulation was applied to investigate corticospinal excitability by recording the amplitude of the motor-evoked potential (MEP) and the duration of the cortical silent period (cSP). Peripheral nerve stimulation was applied to investigate (i) spinal excitability using the Hoffmann reflex (H_max), and (ii) neuromuscular properties using the amplitude of the maximal M-wave (M_max) and corresponding peak twitch torque. These measurements were performed with a background 30% of maximal voluntary isometric contraction. Finally, the maximal voluntary isometric contraction torque and the corresponding electromyography (EMG) from soleus, gastrocnemius medialis and gastrocnemius lateralis were recorded. These parameters were measured immediately before and 10 s after each conditioning activity of plantar flexors. Corticospinal excitability (MEP/M_max) was significantly enhanced after static stretching in soleus (P = 0.001; ES = 0.54) and gastrocnemius lateralis (P<0.001; ES = 0.64), and after dynamic muscle activity in gastrocnemius lateralis (P = 0.003; ES = 0.53) only. On the other hand, spinal excitability (H_max/M_max), cSP duration, muscle activation (EMG/M_max) as well as maximal voluntary and evoked torque remained unaltered after all pre-exercise interventions. These findings indicate the presence of facilitation of the corticospinal pathway without change in muscle function after both static stretching (particularly) and dynamic muscle activity.

Changes in motoneuron excitability during voluntary muscle activity in humans with spinal cord injury

The excitability of resting motoneurons increases following spinal cord injury (SCI). The extent to which motoneuron excitability changes during voluntary muscle activity in humans with SCI, however, remains poorly understood. To address this question, we measured F waves by using supramaximal electrical stimulation of the ulnar nerve at the wrist and cervicomedullary motor-evoked potentials (CMEPs) by using high-current electrical stimulation over the cervicomedullary junction in the first dorsal interosseous muscle at rest and during 5 and 30% of maximal voluntary contraction into index finger abduction in individuals with chronic cervical incomplete SCI and aged-matched control participants. We found higher persistence (number of F waves present in each set) and amplitude of F waves at rest in SCI compared with control participants. With increasing levels of voluntary contraction, the amplitude, but not the persistence, of F waves increased in both groups but to a lesser extent in SCI compared with control participants. Similarly, the CMEP amplitude increased in both groups but to a lesser extent in SCI compared with controls. These results were also found at matched absolutely levels of electromyographic activity, suggesting that these changes were not related to decreases in voluntary motor output after SCI. F-wave and CMEP amplitudes were positively correlated across conditions in both groups. These results support the hypothesis that the responsiveness of the motoneuron pool during voluntary activity decreases following SCI, which could alter the generation and strength of voluntary muscle contractions.NEW & NOTEWORTHY How the excitability of motoneurons changes during voluntary muscle activity in humans with spinal cord injury (SCI) remains poorly understood. We found that F-wave and cervicomedullary motor-evoked potential amplitude, outcomes reflecting motoneuronal excitability, increased during voluntary activity compared with rest in SCI participants but to a lesser extent that in controls. These results suggest that the responsiveness of motoneurons during voluntary activity decreases following SCI, which might affect functionally relevant plasticity after the injury.

Cortical and Subcortical Effects of Transcutaneous Spinal Cord Stimulation in Humans with Tetraplegia

An increasing number of studies supports the view that transcutaneous electrical stimulation of the spinal cord (TESS) promotes functional recovery in humans with spinal cord injury (SCI). However, the neural mechanisms contributing to these effects remain poorly understood. Here we examined motor-evoked potentials in arm muscles elicited by cortical and subcortical stimulation of corticospinal axons before and after 20 min of TESS (30 Hz pulses with a 5 kHz carrier frequency) and sham-TESS applied between C5 and C6 spinous processes in males and females with and without chronic incomplete cervical SCI. The amplitude of subcortical, but not cortical, motor-evoked potentials increased in proximal and distal arm muscles for 75 min after TESS, but not sham-TESS, in control subjects and SCI participants, suggesting a subcortical origin for these effects. Intracortical inhibition, elicited by paired stimuli, increased after TESS in both groups. When TESS was applied without the 5 kHz carrier frequency both subcortical and cortical motor-evoked potentials were facilitated without changing intracortical inhibition, suggesting that the 5 kHz carrier frequency contributed to the cortical inhibitory effects. Hand and arm function improved largely when TESS was used with, compared with without, the 5 kHz carrier frequency. These novel observations demonstrate that TESS influences cortical and spinal networks, having an excitatory effect at the spinal level and an inhibitory effect at the cortical level. We hypothesized that these parallel effects contribute to further the recovery of limb function following SCI.SIGNIFICANCE STATEMENT Accumulating evidence supports the view that transcutaneous electrical stimulation of the spinal cord (TESS) promotes recovery of function in humans with spinal cord injury (SCI). Here, we show that a single session of TESS over the cervical spinal cord in individuals with incomplete chronic cervical SCI influenced in parallel the excitability cortical and spinal networks, having an excitatory effect at the spinal level and an inhibitory effect at the cortical level. Importantly, these parallel physiological effects had an impact on the magnitude of improvements in voluntary motor output.

Effects of pre-induced fatigue vs. concurrent pain on exercise tolerance, neuromuscular performance and corticospinal responses of locomotor muscles

KEY POINTS

Fatigue and muscle pain induced in a remote muscle group has been shown to alter neuromuscular performance in exercising muscles. Inhibitory neural feedback associated with activation of mechano- and metabo-sensitive muscle afferents has been implicated in this phenomenon. The present study aimed to quantify and compare the effects of pre-induced fatigue and concurrent rising pain (evoked by muscle ischaemia) on the contralateral leg exercise capacity, neuromuscular performance, and corticomotor excitability and inhibition of knee extensor muscles. Pre-induced fatigue in one leg had a greater detrimental effect than the concurrent rising pain on the contralateral limb cycling capacity. Furthermore, pre-induced fatigue, but not concurrent rising pain, reduced corticospinal inhibition recorded from tested contralateral muscles. Regardless of the origin or mechanisms modulating sensory afferents during single-leg cycling exercise (i.e. pre-induced fatigue vs. concurrent rising pain), the limit of exercise tolerance remained the same and exercise was terminated upon achievement of a sensory tolerance limit.

ABSTRACT

Individuals often need to maintain voluntary contractions during high intensity exercise in the presence of fatigue and pain. This investigation examined the effects of pre-induced fatigue and concurrent rising pain (evoked by muscle ischaemia) in one leg on motor fatigability and corticospinal excitability/inhibition of the contralateral limb. Twelve healthy males undertook four experimental protocols including unilateral cycling to task failure at 80% of peak power output with: (i) the right-leg (RL); (ii) the left-leg (LL); (iii) RL immediately preceded by LL protocol (FAT-RL); and (iv) RL when blood flow was occluded in the contralateral (left) leg (PAIN-RL). Participants performed maximal and submaximal 5 s right-leg knee extensions during which transcranial magnetic and femoral nerve electrical stimuli were delivered to elicit motor-evoked and compound muscle action potentials, respectively. The pre-induced fatigue reduced the right leg cycling time-to-task failure (mean ± SD; 332 ± 137 s) to a greater extent than concurrent pain (460 ± 158 s), compared to RL (580 ± 226 s) (P < 0.001). The maximum voluntary contraction force declined less following FAT-RL (P < 0.019) and PAIN-RL (P < 0.032) compared to RL. Voluntary activation declined and the corticospinal excitability recorded from knee extensors increased similarly after the three conditions (P < 0.05). However, the pre-induced fatigue, but not concurrent pain, reduced corticospinal inhibition compared to RL (P < 0.05). These findings suggest that regardless of the origin and/or mechanisms modulating sensory afferent feedback during single-leg cycling (e.g. pre-induced fatigue vs. concurrent rising pain), the limit of exercise tolerance remains the same, suggesting that exercise will be terminated upon achievement of sensory tolerance limit.

Disparate kinetics of change in responses to electrical stimulation at the thoracic and lumbar level during fatiguing isometric knee extension

The present study compared the fatigue-induced change of matched-amplitude thoracic evoked potential (TMEP) and lumbar evoked potential (LEP) following electrical stimulation. Ten participants performed a 3 × 3 min isometric knee extension contraction separated by 4 min of recovery at the level of EMG required to produce 50% maximal voluntary contraction (MVC) force at baseline. The TMEP and LEP were evoked during the ongoing contraction at baseline and every minute into the fatiguing protocol and during recovery. Both responses were also assessed during a transcranial magnetic stimulation (TMS) evoked silent period to elicit a TMS-TMEP and TMS-LEP to assess responses without the confounding influence of descending drive. The results displayed disparate kinetics of the TMS-TMEP and TMS-LEP throughout the fatiguing protocol. The TMS-TMEP was reduced at all time points during exercise ( P < 0.001), whereas the TMS-LEP was reduced at 2 min into set 1 and 1 min into sets 2 and 3 ( P ≤ 0.04). TMS-LEPs were higher than the TMS-TMEPs at most time points ( P ≤ 0.04). No change was observed in the TMEP or LEP. When evoked during the silent period, the reduction in TMEP is greater than the LEP during fatiguing isometric exercise. The disparate kinetics of change suggest that differential mechanisms are responsible for evoked responses to thoracic and lumbar stimulation. More research is required to identify the mechanisms responsible for the TMEP and LEP before precise inferences can be made on what fatigue-induced changes in these variables reflect.

NEW & NOTEWORTHY Assessing spinal excitability using lumbar stimulation when measuring responses in lower limbs has been suggested as an alternative method that could circumvent the issues associated with thoracic stimulation. The present study compared responses to the two types of stimuli throughout a fatiguing protocol and demonstrated that lumbar evoked responses differ substantially from thoracic responses when measured in the absence of voluntary drive. These findings suggest that different mechanisms are responsible for evoked responses to thoracic and lumbar stimuli.

The use of electronic coil location control for focal magnetic stimulation at the cervical level

BACKGROUND

Accurate re-positioning of the coil is challenging in magnetic stimulation at the cervical spinal level. The applicability of coil location control for this type of stimulation is unexplored.

NEW METHOD

Utilizing a figure-of-eight coil and anatomy-specific models of the magnetic stimulation system, we developed a novel technique that enables probing corticospinal excitability at the cervical spinal level. Magnetic stimulation was performed in 9 healthy subjects at C2-C6 spinal levels using a figure-of-eight coil and a coil tracking system. MEPs were recorded from the abductor digiti minimi muscle. The functioning of the coil tracking system was tested with an estimated electric field maximum (eEFM) above the C1 cervical level (group 1) and below (group 2). Motor-evoked potential (MEP) reproducibility was assessed with intra-class correlation coefficient (ICC).

RESULTS

The use of coil location control in cervical level focal magnetic stimulation enabled the recording of highly reproducible MEPs. Within one co-registration, the ICC 95% confidence interval (CI) in group 1 was 0.89-0.99 and in group 2 was 0.24-0.85.

COMPARISON WITH EXISTING METHODS AND CONCLUSIONS

This method can be used for accurate maintenance and retrieval of the focal coil position at the cervical level with low spatial variability during stimulation. Existing methodologies employ determination of the coil location based on external landmarks, which makes the procedure cumbersome. This technique can optimize existing stimulation protocols and facilitate development of navigated spinal stimulation.

Corticospinal-Evoked Responses from the Biceps Brachii during Arm Cycling across Multiple Power Outputs

Background: We examined corticospinal and spinal excitability across multiple power outputs during arm cycling using a weak and strong stimulus intensity. Methods: We elicited motor evoked potentials (MEPs) and cervicomedullary motor evoked potentials (CMEPs) in the biceps brachii using magnetic stimulation over the motor cortex and electrical stimulation of corticospinal axons during arm cycling at six different power outputs (i.e., 25, 50, 100, 150, 200 and 250 W) and two stimulation intensities (i.e., weak vs. strong). Results: In general, biceps brachii MEP and CMEP amplitudes (normalized to maximal M-wave (M_max)) followed a similar pattern of modulation with increases in cycling intensity at both stimulation strengths. Specifically, MEP and CMEP amplitudes increased up until ~150 W and ~100 W when the weak and strong stimulations were used, respectively. Further increases in cycling intensity revealed no changes on MEP or CMEP amplitudes for either stimulation strength. Conclusions: In general, MEPs and CMEPs changed in a similar manner, suggesting that increases and subsequent plateaus in overall excitability are likely mediated by spinal factors. Interestingly, however, MEP amplitudes were disproportionately larger than CMEP amplitudes as power output increased, despite being initially matched in amplitude, particularly with strong stimulation. This suggests that supraspinal excitability is enhanced to a larger degree than spinal excitability as the power output of arm cycling increases.

Determining the early corticospinal-motoneuronal responses to strength training: a systematic review and meta-analysis

Several studies have used transcranial magnetic stimulation to probe the corticospinal-motoneuronal responses to a single session of strength training; however, the findings are inconsistent. This systematic review and meta-analysis examined whether a single bout of strength training affects the excitability and inhibition of intracortical circuits of the primary motor cortex (M1) and the corticospinal-motoneuronal pathway. A systematic review was completed, tracking studies between January 1990 and May 2018. The methodological quality of studies was determined using the Downs and Black quality index. Data were synthesised and interpreted from meta-analysis. Nine studies (n=107) investigating the acute corticospinal-motoneuronal responses to strength training met the inclusion criteria. Meta-analyses detected that after strength training compared to control, corticospinal excitability [standardised mean difference (SMD), 1.26; 95% confidence interval (CI), 0.88, 1.63; p<0.0001] and intracortical facilitation (ICF) (SMD, 1.60; 95% CI, 0.18, 3.02; p=0.003) were increased. The duration of the corticospinal silent period was reduced (SMD, −17.57; 95% CI, −21.12, −14.01; p=0.00001), but strength training had no effect on the excitability of the intracortical inhibitory circuits [short-interval intracortical inhibition (SICI) SMD, 1.01; 95% CI, −1.67, 3.69; p=0.46; long-interval intracortical inhibition (LICI) SMD, 0.50; 95% CI, −1.13, 2.13; p=0.55]. Strength training increased the excitability of corticospinal axons (SMD, 4.47; 95% CI, 3.45, 5.49; p<0.0001). This systematic review and meta-analyses revealed that the acute neural changes to strength training involve subtle changes along the entire neuroaxis from the M1 to the spinal cord. These findings suggest that strength training is a clinically useful tool to modulate intracortical circuits involved in motor control.