Changes in Corticospinal Excitability and Motor Control During Cerebellar Transcranial Direct Current Stimulation in Healthy Individuals.

Cerebellar transcranial direct current stimulation (ctDCS) modulates the primary motor cortex (M1) via cerebellar brain inhibition (CBI), which affects motor control in humans. However, the effects of ctDCS on motor control are inconsistent because of an incomplete understanding of the real-time changes in the M1 excitability that occur during ctDCS, which determines motor output under regulation by the cerebellum. This study investigated changes in corticospinal excitability and motor control during ctDCS in healthy individuals. In total, 37 healthy individuals participated in three separate experiments. ctDCS (2 mA) was applied to the cerebellar hemisphere during the rest condition or a pinch force-tracking task. Motor-evoked potential (MEP) amplitude and the F-wave were assessed before, during, and after ctDCS, and pinch force control was assessed before and during ctDCS. The MEP amplitudes were significantly decreased during anodal ctDCS from 13 min after the onset of stimulation, whereas the F-wave was not changed. No significant changes in MEP amplitudes were observed during cathodal and sham ctDCS conditions. The MEP amplitudes were decreased during anodal ctDCS when combined with the pinch force-tracking task, and pinch force control was impaired during anodal ctDCS relative to sham ctDCS. The MEP amplitudes were not significantly changed before and after all ctDCS conditions. Motor cortical excitability was suppressed during anodal ctDCS, and motor control was unskilled during anodal ctDCS when combined with a motor task in healthy individuals. Our findings provided a basic understanding of the clinical application of ctDCS to neurorehabilitation.

Shock-absorbing effect of flooring-adopted mechanical metamaterial technology and its influence on the gait and balance of older adults.

OBJECTIVE: To elucidate the performance of a shock-absorbing floor material with a mechanical metamaterial (MM-flooring) structure and its effect on the gait and balance of older adults. METHODS: The drop-weight impact was applied to evaluate the shock-absorbing performance. The falling weight was adjusted equivalent to the energy exerted on the femur of an older woman when she falls, which was evaluated on the MM-flooring and six other flooring materials.Nineteen healthy people over the age of 65 years participated in the gait and balance evaluations. The timed up and go and two-step tests were adopted as gait performance tests, and the sway-during-quiet-balance test with force plates and the functional reach test (FRT) were adopted as balance tests. All the participants underwent these tests on the MM-flooring, shock-absorbing mat and rigid flooring. RESULTS: The shock-absorbing performance test revealed that MM-flooring has sufficient shock-absorbing performance, and suggesting that it may reduce the probability of fractures in the older people when they fall. The results of the gait performance test showed that the participants demonstrated the same gait performance on the MM-flooring and the rigid floor. In the quiet standing test, MM-flooring did not affect the balance function of the participants to the same extent as the rigid floor, compared with the shock-absorbing mat. In the FRT, no significant differences were found for any of the flooring conditions. CONCLUSIONS: MM-flooring has the potential to prevent fractures attributed to falls and does not affect the gait or balance of older adults.

Electrical stimulation of the common peroneal nerve and its effects on the relationship between corticomuscular coherence and motor control in healthy adults.

BACKGROUND: Sensory input via neuromuscular electrical stimulation (NMES) may contribute to synchronization between motor cortex and spinal motor neurons and motor performance improvement in healthy adults and stroke patients. However, the optimal NMES parameters used to enhance physiological activity and motor performance remain unclear. In this study, we focused on sensory feedback induced by a beta-band frequency NMES (ß-NMES) based on corticomuscular coherence (CMC) and investigated the effects of ß-NMES on CMC and steady-state of isometric ankle dorsiflexion in healthy volunteers. Twenty-four participants received ß-NMES at the peak beta-band CMC or fixed NMES (f-NMES) at 100 Hz on different days. NMES was applied to the right part of the common peroneal nerve for 20 min. The stimulation intensity was 95% of the motor threshold with a pulse width of 1 ms. The beta-band CMC and the coefficient of variation of force (Force CV) were assessed during isometric ankle dorsiflexion for 2 min. In the complementary experiment, we applied ß-NMES to 14 participants and assessed beta-band CMC and motor evoked potentials (MEPs) with transcranial magnetic stimulation. RESULTS: No significant changes in the means of beta-band CMC, Force CV, and MEPs were observed before and after NMES conditions. Changes in beta-band CMC were correlated to (a) changes in Force CV immediately, at 10 min, and at 20 min after ß-NMES (all cases, p < 0.05) and (b) changes in MEPs immediately after ß-NMES (p = 0.01). No correlations were found after f-NMES. CONCLUSIONS: Our results suggest that the sensory input via NMES was inadequate to change the beta-band CMC, corticospinal excitability, and voluntary motor output. Whereas, the ß-NMES affects the relationship between changes in beta-band CMC, Force CV, and MEPs. These findings may provide the information to develop NMES parameters for neurorehabilitation in patients with motor dysfunction.

Single-Session Cerebellar Transcranial Direct Current Stimulation Affects Postural Control Learning and Cerebellar Brain Inhibition in Healthy Individuals.

Cerebellar transcranial direct current stimulation (ctDCS) modulates cerebellar activity and postural control. However, the effects of ctDCS on postural control learning and the mechanisms associated with these effects remain unclear. To examine the effects of single-session ctDCS on postural control learning and cerebellar brain inhibition (CBI) of the primary motor cortex in healthy individuals. In this triple-blind, sham-controlled study, 36 participants were allocated randomly to one of three groups: (1) anodal ctDCS group, (2) cathodal ctDCS group, and (3) sham ctDCS group. ctDCS (2 mA) was applied to the cerebellar brain for 20 min prior to six blocks of standing postural control training (each block consisted of five trials of a 30-s tracking task). CBI and corticospinal excitability of the tibialis anterior muscle were assessed at baseline, immediately after, 1 day after, and 7 days after training. Skill acquisition following training was significantly reduced in both the anodal and cathodal ctDCS groups compared with the sham ctDCS group. Changes in performance measured 1 day after and 7 days after training did not differ among the groups. In the anodal ctDCS group, CBI significantly increased after training, whereas corticospinal excitability decreased. Anodal ctDCS-induced CBI changes were correlated with the learning formation of postural control (r = 0.55, P = 0.04). Single-session anodal and cathodal ctDCS could suppress the skill acquisition of postural control in healthy individuals. The CBI changes induced by anodal ctDCS may affect the learning process of postural control.

Modulation of the rate of force development in hip abductor muscles by neuromuscular electrical stimulation during gait.

Purpose/aim of the study: An increase of hip abductor muscle strength contributes to the increase in gait speed. It is known that the rate of force development (RFD), an indicator of muscle strength, is increased by the combined use of low-intensity neuromuscular electrical stimulation (NMES) to the glutaeus medius (GM) and low-load resistance training (RT). However, it is unclear whether low-intensity neuromuscular electrical stimulation of the glutaeus medius during walking also increases the rate of force development. The aim of this study was to clarify whether NMES to the GM during gait modulates the RFD of the hip abductor muscles in healthy adults.Materials and methods: Twenty-two healthy adults randomly received both gait with sub-motor threshold NMES and gait with sham NMES conditions. The RFD was assessed at pre- and post-intervention. A two-way repeated measures analysis of variance was used to analyse the effects of time and intervention.Results: Gait with sub-motor threshold NMES condition significantly increased the RFD in shorter time interval (0-50 and 0-100 ms) compared to gait with sham NMES condition.Conclusions: These findings suggest that the adding low-intensity NMES of the GM to gait is effective in increasing the RFD of the hip abductor muscles.

Transcranial direct-current stimulation combined with attention increases cortical excitability and improves motor learning in healthy volunteers.

BACKGROUND: Transcranial direct current stimulation (tDCS) is a non-invasive brain stimulation technique that has the potential to induce motor cortical plasticity in humans. It is well known that motor cortical plasticity plays an essential role in motor learning and recovery in patients with stroke and neurodegenerative disorders. However, it remains unclear how cognitive function influences motor cortical plasticity induced by tDCS. The present study aimed to investigate whether anodal tDCS combined with attention to a target muscle could enhance motor cortical plasticity and improve motor learning in healthy individuals. METHODS: Thirty-three healthy volunteers were assigned to two experiments. In experiment 1, there were three interventional conditions: 1) anodal tDCS was applied while participants paid attention to the first dorsal interosseous (FDI) muscle, 2) anodal tDCS was applied while participants paid attention to the sound, and 3) anodal tDCS was applied without the participants paying attention to the FDI muscle or the sound. Anodal tDCS (2 mA, 10 min) was applied over the primary motor cortex (M1). Changes in motor evoked potentials (MEPs), short-interval intracortical inhibition (SICI), and intracortical facilitation (ICF) were assessed before and immediately after (0 min), and then 10 min, 30 min, and 60 min after each intervention. In experiment 2, we investigated whether the combination of anodal tDCS and attention to the abductor pollicis brevis (APB) muscle could facilitate the learning of a ballistic thumb movement. RESULTS: Anodal tDCS increased cortical excitability in all conditions immediately after the stimulation. Significant increases in MEPs and significant decreases in SICI were observed for at least 60 min after anodal tDCS, but only when participants paid attention to the FDI muscle. In contrast, no significant changes in ICF were observed in any condition. In experiment 2, the combination of tDCS and attention to the APB muscle significantly enhanced the acquisition of a ballistic thumb movement. The higher performance was still observed 7 days after the stimulation. CONCLUSIONS: This study shows that anodal tDCS over M1 in conjunction with attention to the target muscle enhances motor cortex plasticity and improves motor learning in healthy adults. These findings suggest that a combination of attention and tDCS may be an effective strategy to promote rehabilitation training in patients with stroke and neurodegenerative disorders. TRIAL REGISTRATION: Retrospectively registered (UMIN000036848).

Robot-assisted Gait Training Using Welwalk in Hemiparetic Stroke Patients: An Effectiveness Study with Matched Control.

OBJECTIVE: Although studies on the efficacy of the rehabilitation robot are increasing, there are few reports using the robot for gait training in the actual clinical setting. This study aimed to investigate the effectiveness of gait training using Welwalk in hemiparetic stroke patients in a real clinical setting. MATERIALS AND METHODS: This prospective study included 36 hemiparetic stroke patients who underwent gait training using Welwalk. We examined the walking ability improvement efficiency using Functional Independence Measure (FIM)-walk as the primary outcome, which was compared with that of 36 patients (matched control group) who underwent conventional rehabilitation. Other outcomes were the actual gait training period using Welwalk, raw FIM-walk score, lower extremity motor functions score in Stroke Impairment Assessment Set at discharge, and duration from stroke onset until discharge. RESULTS: The improvement efficiency of the FIM-walk was significantly higher in the Welwalk group than in the matched control group (control 0.48 ± 0.31, Welwalk 0.80 ± 0.38, p-value < 0.001). The mean gait training period using Welwalk was 5 weeks. No significant differences were found in other outcomes between the Welwalk group and the matched control group. CONCLUSION: This study demonstrated the effectiveness of gait training using Welwalk on the improvement efficiency of the FIM-walk in hemiparetic stroke patients in an actual clinical setting.

Time course of changes in corticospinal excitability induced by motor imagery during action observation combined with peripheral nerve electrical stimulation.

While previous studies assessed corticospinal excitability changes during and after motor imagery (MI) or action observation (AO) combined with peripheral nerve electrical stimulation (ES), we examined, for the first time, the time course of corticospinal excitability changes for MI during AO combined with ES (AO-MI + ES) using transcranial magnetic stimulation to measure motor evoked potentials (MEPs) in healthy individuals. Fourteen healthy volunteers participated in the following three sessions on different days: AO-MI alone, ES alone, and AO-MI + ES. In the AO-MI task, participants imagined squeezing and relaxing a ball, along with the respective actions shown in a movie, while passively holding the ball. We applied ES (intensity, 90% of the motor threshold) to the ulnar nerve at the wrist, which innervates the first dorsal interosseous (FDI) muscle. We assessed the FDI muscle MEPs at baseline and after every 5 min of the task for a total of 20 min. Additionally, participants completed the Vividness of Movement Imagery Questionnaire-2 (VMIQ-2) at the beginning of the experiment. Compared to baseline, AO-MI + ES significantly increased corticospinal excitability after 10 min, while AO-MI or ES alone had no effect on corticospinal excitability after 20 min. Moreover, the AO-MI + ES-induced cortical excitability changes were correlated with the VMIQ-2 scores for visual and kinaesthetic imagery. Collectively, our findings indicate that AO-MI + ES induces cortical plasticity earlier than does AO-MI or ES alone and that an individual's imagery ability plays an important role in inducing cortical excitability changes following AO-MI + ES.

Effect of electrical stimulation of antagonist muscles for voluntary motor drive.

Voluntary motor drive is an important central command that descends via the corticospinal tract to initiate muscle contraction. When electrical stimulation (ES) is applied to an antagonist or agonist muscle, it changes the agonist muscle's representative motor cortex and thus its voluntary motor drive. In this study, we used a reaction time task to compare the effects of weak and strong ES of the antagonist or agonist muscle during the premotor period of a wrist extension. We recorded motor evoked potentials (MEPs) induced by transcranial magnetic stimulation (TMS) that was applied to the extensor carpi radialis (ECR; agonist) and flexor carpi radialis (FCR; antagonist). When stronger ES intensities were applied to the antagonist, the MEP control ratio in the ECR significantly increased during the premotor time. Furthermore, the MEP control ratio with stronger antagonist ES intensity was significantly larger than that in the agonist for the same ES intensity. In the FCR, the MEP control ratio was also significantly greater at the strong ES intensity than at the weak ES intensity. Furthermore, the MEP control ratio in the antagonist with a strong ES intensity was significantly larger than that in the agonist with the same ES intensity. These results suggest that agonist corticomotor excitability might be enhanced by ES of the antagonist, which in turn strongly activates the descending motor system in the preparation of agonist contraction.

Combined effects of botulinum toxin type A and repetitive transcranial magnetic stimulation with intensive motor training immediately after injection in a patient with chronic stroke: A case report.

STUDY DESIGN: Single case report. INTRODUCTION: A previous study clarified that spasticity and motor function were improved by combined treatment with botulinum toxin type A (BTX) injection and 1-Hz repetitive transcranial magnetic stimulation (rTMS) with intensive motor training at 4 weeks after injection. However, it is not clear whether 1-Hz rTMS with intensive motor training immediately after BTX injection also improves spasticity and motor function in stroke patients. PURPOSE OF THE CASE REPORT: The purpose of this case report is to test the short- and long-term effects of BTX injection and rTMS with intensive motor training on the spasticity, motor function, and usefulness of the paretic hand in a stroke patient. METHODS: A 64-year-old male, who suffered from a right cerebral hemorrhage 53 months previously, participated in the present study. BTX was injected into the spastic muscles of the affected upper limb. He then received the new protocol for a total of 24 sessions. The Modified Ashworth Scale (MAS), Fugl-Meyer Assessment (FMA), and Motor Activity Log, consisting of the amount of use and quality of movement scales, were assessed before and immediately after BTX injection, at discharge, and monthly for up to 5 months after discharge. RESULTS: For the short-term effects of the therapy, the MAS scores of the elbow and wrist, FMA score, and quality of movement score improved. For the long-term effects of the therapy, the MAS score of the fingers, FMA score, and amount of use score improved for up to 5 months after discharge. CONCLUSIONS: The present case report showed the improvement of all assessments performed in the short and/or long term and suggest the possibility of shortening the intervention period of combined therapy of BTX and rTMS with intensive motor training.

Coordination of plantar flexor muscles during bipedal and unipedal stances in young and elderly adults.

To investigate the effects of aging on coordination of plantar flexor muscles during bipedal and unipedal stances, we examined a relationship between the center of pressure sway and electromyographic activity of these muscles, and also the common neural input, using a coherence analysis. Healthy young and elderly adults were asked to perform bipedal and unipedal standing. The electromyograms were recorded unilaterally from the medial and lateral gastrocnemius (MG and LG) and soleus (SL) muscles, and the common input was analyzed for MG-LG, MG-SL, and LG-SL pairs in two frequency bands: a delta band, that is associated with force variability, and a beta band, that could reflect the corticospinal drive. Main results indicated that the MG and SL muscles worked for lateral sway, while the LG muscle worked for medial sway during the unipedal stance. The delta-band coherence for the MG-SL pair and the beta-band coherences for all the pairs were larger during the unipedal than bipedal stance for both groups. The delta-band coherence for the MG-SL pair was larger for the elderly than young adults during the unipedal stance. In addition, the beta-band coherence for the MG-SL pair was larger than the other pairs during the unipedal stance for the elderly. These findings suggest that the oscillatory activity between the MG and SL muscles is strongly involved in the control of unipedal stance, and aging would increase the cortical drive to these muscles to deal with the postural sway that could be affected by forces generated cooperatively by them.

Influence of transcutaneous electrical nerve stimulation conditions on disynaptic reciprocal Ia inhibition and presynaptic inhibition in healthy adults.

This study investigated the influence of stimulus conditions of transcutaneous electrical nerve stimulation (TENS) on disynaptic reciprocal Ia inhibition (RI) and presynaptic inhibition (D1 inhibition) in healthy adults. Eight healthy participants received TENS (stimulus frequencies of 50, 100, and 200 Hz) over the deep peroneal nerve and tibialis anterior (TA) muscle in the resting condition for 30 min. At pre- and post-intervention, the RI from the TA to the soleus (SOL) and D1 inhibition of the SOL alpha motor neuron were assessed by evoked electromyography. The results showed that RI was not changed by TENS at any stimulus frequency condition. Conversely, D1 inhibition was significantly changed by TENS regardless of the stimulus frequency. The present results and previous studies pertaining to RI suggest that the resting condition might strongly influence the lack of pre- vs. post-intervention change in the RI. Regarding the D1 inhibition, the present results suggest that the effect of TENS might be caused by post-tetanic potentiation. The knowledge gained from the present study might contribute to a better understanding of fundamental studies of TENS in healthy adults and its clinical application for stroke survivors.

Auditory stimulus has a larger effect on anticipatory postural adjustments in older than young adults during choice step reaction.

PURPOSE: The study aim was to compare the influence of an auditory stimulus (AS) on anticipatory postural adjustments (APAs) between young and older adults during a choice step reaction. METHODS: Sixteen young and 19 older adults stepped forward in response to a visual imperative stimulus of an arrow. We used a choice reaction time (CRT) task and a Simon task which consisted of congruent and incongruent conditions. The direction of the presented arrow and its spatial location matched in the congruent condition while they did not in the incongruent condition. The AS was presented randomly and simultaneously with the visual stimulus. Incorrect weight shifts before lifting off the foot, termed APA errors, stepping errors, temporal parameters, and APA amplitudes were analyzed. RESULTS: The APA error rate was higher in trials with than without AS in all task conditions for the older group, while this increase occurred only in the incongruent condition for the young group. The stepping error rate was also increased in the presence of AS in the incongruent condition for the older group. Reaction times were faster with AS in both groups. The APA amplitude of erroneous APA trials became larger with AS in the incongruent condition for both groups, and this effect appeared greater for the older group. CONCLUSIONS: The effect of AS on APAs is larger in the elderly during a choice step reaction. In the presence of incongruent visual information, this effect becomes even greater, potentially inducing not only APA errors but also stepping errors.

Reliability and validity of a force-instrumented treadmill for evaluating balance: A preliminary study of feasibility in healthy young adults.

BACKGROUND: With the development of computer technology, computerized dynamic posturography provides objective assessments of balance and posture control under static and dynamic conditions. Although a force-instrumented treadmill-based balance assessment is feasible for balance evaluations, currently no data exists. OBJECTIVE: This study was undertaken to assess the reliability and validity of balance evaluations using a force-instrumented treadmill. METHODS: Ten healthy adults participated in evaluations using both the treadmill and the EquiTest. Four balance evaluations were conducted: Modified Clinical Test of Sensory Interaction on Balance, Unilateral Stance, Weight Bearing Squat, and Motor Control Test. RESULTS: All balance evaluations using the force-instrumented treadmill method shared good reliability (intraclass correlation coefficient ≥0.6). The Modified Clinical Test of Sensory Interaction on Balance, Unilateral Stance, and Weight Bearing Squat evaluations had a correlation of r < 0.5 with EquiTest, whereas the Motor Control Test balance evaluation had moderate correlations (r > 0.5) with the EquiTest. CONCLUSION: The results demonstrated that all balance evaluations using the force-instrumented treadmill were reliable, and that the Motor Control Test evaluation was moderately correlated with the EquiTest. Therefore, the use of a force-instrumented treadmill in balance evaluations might provide a certain level of value to clinical practice.

Estimation of 1RM for knee extension based on the maximal isometric muscle strength and body composition.

[Purpose] To create a regression formula in order to estimate 1RM for knee extensors, based on the maximal isometric muscle strength measured using a hand-held dynamometer and data regarding the body composition. [Subjects and Methods] Measurement was performed in 21 healthy males in their twenties to thirties. Single regression analysis was performed, with measurement values representing 1RM and the maximal isometric muscle strength as dependent and independent variables, respectively. Furthermore, multiple regression analysis was performed, with data regarding the body composition incorporated as another independent variable, in addition to the maximal isometric muscle strength. [Results] Through single regression analysis with the maximal isometric muscle strength as an independent variable, the following regression formula was created: 1RM (kg)=0.714 + 0.783 × maximal isometric muscle strength (kgf). On multiple regression analysis, only the total muscle mass was extracted. [Conclusion] A highly accurate regression formula to estimate 1RM was created based on both the maximal isometric muscle strength and body composition. Using a hand-held dynamometer and body composition analyzer, it was possible to measure these items in a short time, and obtain clinically useful results.

Validity of gait asymmetry estimation by using an accelerometer in individuals with hemiparetic stroke.

[Purpose] The purpose of this study was to evaluate the validity of estimating step time and length asymmetries, using an accelerometer against force plate measurements in individuals with hemiparetic stroke. [Subjects and Methods] Twenty-four individuals who previously had experienced a stroke were asked to walk without using a cane or manual assistance on a 16-m walkway. Step time and length were measured using force plates, which is the gold standard for assessing gait asymmetry. In addition to ground reaction forces, trunk acceleration was simultaneously measured using an accelerometer. To estimate step time asymmetry using accelerometer data, the time intervals between forward acceleration peaks for each leg were calculated. To estimate step length asymmetry using accelerometer data, the integration of the positive vertical accelerations following initial contact of each leg was calculated. Asymmetry was considered the affected side value divided by the unaffected side value. [Results] Significant correlations were found between the accelerometer and the force plates for step time and length asymmetries (rho=0.83 and rho=0.64, respectively). [Conclusion] An accelerometer might be useful for assessing step time and length asymmetries in individuals with hemiparetic stroke, although improvements are needed for estimating the accuracy of step length asymmetry.

Changes in postural strategy during exercise against perturbation using the balance exercise assist robot: a pilot study.

[Purpose] To clarify the changes in postural strategy by evaluating leg joint motion and muscle activity before and after continuous exercise against perturbation using the Balance Exercise Assist Robot (BEAR). [Subjects and Methods] Nine healthy subjects (male 7, female 2; mean age 23 ± 1 years) performed a postural perturbation coping exercise only. In the task, the robot leaned and moved automatically. Participants were instructed to maintain their default upright position and they performed the exercise five times in a row (1 minute/trial). Changes in total movement distance, range of motion of each joint (hip, knee, ankle), and mean activity of each muscle for the first and fifth trials were compared. [Results] The total movement distance of BEAR and range of motion in the hip decreased significantly from the first trial to the last trial. No change in muscle activity was observed in the rectus femoris, biceps femoris, tibialis anterior or gastrocnemius. [Conclusion] The results for exercise against perturbation using BEAR in this study suggest that BEAR may be a promising method to improve the ankle strategy for maintaining a standing posture.

Performance monitoring and response conflict resolution associated with choice stepping reaction tasks.

Choice reaction requires response conflict resolution, and the resolution processes that occur during a choice stepping reaction task undertaken in a standing position, which requires maintenance of balance, may be different to those processes occurring during a choice reaction task performed in a seated position. The study purpose was to investigate the resolution processes during a choice stepping reaction task at the cortical level using electroencephalography and compare the results with a control task involving ankle dorsiflexion responses. Twelve young adults either stepped forward or dorsiflexed the ankle in response to a visual imperative stimulus presented on a computer screen. We used the Simon task and examined the error-related negativity (ERN) that follows an incorrect response and the correct-response negativity (CRN) that follows a correct response. Error was defined as an incorrect initial weight transfer for the stepping task and as an incorrect initial tibialis anterior activation for the control task. Results revealed that ERN and CRN amplitudes were similar in size for the stepping task, whereas the amplitude of ERN was larger than that of CRN for the control task. The ERN amplitude was also larger in the stepping task than the control task. These observations suggest that a choice stepping reaction task involves a strategy emphasizing post-response conflict and general performance monitoring of actual and required responses and also requires greater cognitive load than a choice dorsiflexion reaction. The response conflict resolution processes appear to be different for stepping tasks and reaction tasks performed in a seated position.

Modulation of spinal inhibitory reflexes depends on the frequency of transcutaneous electrical nerve stimulation in spastic stroke survivors.

Neurophysiological studies in healthy subjects suggest that increased spinal inhibitory reflexes from the tibialis anterior (TA) muscle to the soleus (SOL) muscle might contribute to decreased spasticity. While 50 Hz is an effective frequency for transcutaneous electrical nerve stimulation (TENS) in healthy subjects, in stroke survivors, the effects of TENS on spinal reflex circuits and its appropriate frequency are not well known. We examined the effects of different frequencies of TENS on spinal inhibitory reflexes from the TA to SOL muscle in stroke survivors. Twenty chronic stroke survivors with ankle plantar flexor spasticity received 50-, 100-, or 200-Hz TENS over the deep peroneal nerve (DPN) of the affected lower limb for 30 min. Before and immediately after TENS, reciprocal Ia inhibition (RI) and presynaptic inhibition of the SOL alpha motor neuron (D1 inhibition) were assessed by adjusting the unconditioned H-reflex amplitude. Furthermore, during TENS, the time courses of spinal excitability and spinal inhibitory reflexes were assessed via the H-reflex, RI, and D1 inhibition. None of the TENS protocols affected mean RI, whereas D1 inhibition improved significantly following 200-Hz TENS. In a time-series comparison during TENS, repeated stimulation did not produce significant changes in the H-reflex, RI, or D1 inhibition regardless of frequency. These results suggest that the frequency-dependent effect of TENS on spinal reflexes only becomes apparent when RI and D1 inhibition are measured by adjusting the amplitude of the unconditioned H-reflex. However, 200-Hz TENS led to plasticity of synaptic transmission from the antagonist to spastic muscles in stroke survivors.

Effect of neuromuscular electrical stimulation on motor cortex excitability upon release of tonic muscle contraction.

The aim of the present study was to investigate the neurophysiological triggers underlying muscle relaxation from the contracted state, and to examine the mechanisms involved in this process and their subsequent modification by neuromuscular electrical stimulation (NMES). Single-pulse transcranial magnetic stimulation (TMS) was used to produce motor-evoked potentials (MEPs) and short-interval intracortical inhibition (SICI) in 23 healthy participants, wherein motor cortex excitability was examined at the onset of voluntary muscle relaxation following a period of voluntary tonic muscle contraction. In addition, the effects of afferent input on motor cortex excitability, as produced by NMES during muscle contraction, were examined. In particular, two NMES intensities were used for analysis: 1.2 times the sensory threshold and 1.2 times the motor threshold (MT). Participants were directed to execute constant wrist extensions and to release muscle contraction in response to an auditory "GO" signal. MEPs were recorded from the flexor carpi radialis (FCR) and extensor carpi radialis (ECR) muscles, and TMS was applied at three different time intervals (30, 60, and 90 ms) after the "GO" signal. Motor cortex excitability was greater during voluntary ECR and FCR relaxation using high-intensity NMES, and relaxation time was decreased. Each parameter differed significantly between 30 and 60 ms. Moreover, in both muscles, SICI was larger in the presence than in the absence of NMES. Therefore, the present findings suggest that terminating a muscle contraction triggers transient neurophysiological mechanisms that facilitate the NMES-induced modulation of cortical motor excitability in the period prior to muscle relaxation. High-intensity NMES might facilitate motor cortical excitability as a function of increased inhibitory intracortical activity, and therefore serve as a transient trigger for the relaxation of prime mover muscles in a therapeutic context.