Changes in segmental and motor cortical output with contralateral muscle contractions and altered sensory inputs in humans

Force irradiation effect of kinesiotaping on contralateral muscle activation

We aimed to determine the force irradiation effect of kinesiotaping (KT) on contralateral muscle activity during unilateral muscle contraction. Forty healthy (26 females, 14 males) subjects were divided into two groups: KT and control groups. KT was applied on the biceps brachii at the contralateral limb (non-dominant limb) in the KT group, whereas no taping was applied to the control group. All participants performed unilateral isometric, concentric, and eccentric contractions with their dominant upper limbs (exercised limb) by means of an isokinetic dynamometer, while the contralateral limb was in the resting condition, neutral position, and motionless during the testing procedure. During the exercise, contralateral biceps brachii muscle activity was recorded by surface electromyography (EMG). To quantify the muscle activation, EMG signals were expressed as a percentage of the maximal isometric voluntary contraction, which is referred to as %EMG_max. The KT group showed significantly higher %EMG_max in the biceps brachii compared to the control group at the contralateral limb during the isometric, concentric, and eccentric contractions (p = 0.035, p = 0.046, and p = 0.002, respectively) The median values of the contralateral muscle activity were 2.74 %EMG_max and 6.62 %EMG_max during the isometric contraction for the control and KT groups, respectively (p = 0.035). During the concentric contraction, the median values of the contralateral muscle activity were 1.61 %EMG_max and 9.39 %EMG_max for the control and KT groups, respectively (p = 0.046). The median values of the contralateral muscle activity were 4.49 %EMG_max and 22.89 %EMG_max for the eccentric contraction for the control and KT groups, respectively (p = 0.002). In conclusion, KT application on the contralateral limb increased the contralateral muscle activation in the biceps brachii during the unilateral isometric, concentric, and eccentric contractions.

Dominant and nondominant leg press training induce similar contralateral and ipsilateral limb training adaptations with children

Cross-education has been extensively investigated with adults. Adult studies report asymmetrical cross-education adaptations predominately after dominant limb training. The objective of the study was to examine unilateral leg press (LP) training of the dominant or nondominant leg on contralateral and ipsilateral strength and balance measures. Forty-two youth (10-13 years) were placed (random allocation) into a dominant (n = 15) or nondominant (n = 14) leg press training group or nontraining control (n = 13). Experimental groups trained 3 times per week for 8 weeks and were tested pre-/post-training for ipsilateral and contralateral 1-repetition maximum (RM) horizontal LP, maximum voluntary isometric contraction (MVIC) of knee extensors (KE) and flexors (KF), countermovement jump (CMJ), triple hop test (THT), MVIC strength of elbow flexors (EF) and handgrip, as well as the stork and Y balance tests. Both dominant and nondominant LP training significantly (p < 0.05) increased both ipsilateral and contralateral lower body strength (LP 1RM (dominant: 59.6%-81.8%; nondominant: 59.5%-96.3%), KE MVIC (dominant: 12.4%-18.3%; nondominant: 8.6%-18.6%), KF MVIC (dominant: 7.9%-22.3%; nondominant: nonsignificant-3.8%), and power (CMJ: dominant: 11.1%-18.1%; nondominant: 7.7%-16.6%)). The exception was that nondominant LP training demonstrated a nonsignificant change with the contralateral KF MVIC. Other significant improvements were with nondominant LP training on ipsilateral EF 1RM (6.2%) and THT (9.6%). There were no significant changes with EF and handgrip MVIC. The contralateral leg stork balance test was impaired following dominant LP training. KF MVIC exhibited the only significant relative post-training to pretraining (post-test/pre-test) ratio differences between dominant versus nondominant LP cross-education training effects. In conclusion, children exhibit symmetrical cross-education or global training adaptations with unilateral training of dominant or nondominant upper leg.

Effects of bilateral priming on motor cortex function in healthy adults

Bilateral priming is a rehabilitation adjuvant that can improve upper limb motor recovery poststroke. It uses a table-top device to couple the upper limbs together such that active flexion and extension of one wrist leads to passive movement of the opposite wrist in a mirror symmetric pattern. Bilateral priming increases corticomotor excitability (CME) in the primary motor cortex (M1) of the passively driven wrist; however, the neurophysiological mechanisms underlying this increase remain unclear. This study explored these mechanisms by using transcranial magnetic stimulation over the right M1 and recording motor-evoked potentials from the passively driven left extensor carpi radialis of healthy adults. Intracortical measures were recorded before and 5 and 35 min after a single 15-min session of priming. One-millisecond short-interval intracortical inhibition, long-interval intracortical inhibition, late cortical disinhibition (LCD), and intracortical facilitation were recorded with a posterior-anterior (PA) intracortical current, whereas CME and short-interval intracortical facilitation (SICF) were recorded with both PA and anterior-posterior (AP) currents. CME with PA stimulation was also recorded ~1 h postpriming. PA CME was elevated 35 min postpriming and remained elevated ~1 h postpriming. LCD decreased, and AP SICF increased at both 5 and 35 min postpriming. However, these changes in LCD and AP SICF are unlikely to be the cause of the increased PA CME because of the differing timelines of their effects and AP and PA currents activating separate interneuron circuits. These results suggest that bilateral priming does not increase CME through alterations of the intracortical circuits investigated here. NEW & NOTEWORTHY This is the first study to measure how bilateral priming modulates corticomotor excitability with posterior-anterior and anterior-posterior intracortical currents, 1-ms short-interval intracortical inhibition, late cortical disinhibition, intracortical facilitation, and short-interval intracortical facilitation. We found corticomotor excitability with a posterior-anterior current increased by 35 min until ~1 h postpriming. Short-interval intracortical facilitation with an anterior-posterior current was greater for at least 35 min postpriming. This provides further insight into the neurophysiological mechanisms underlying bilateral priming.

Time course of interlimb strength transfer after unilateral handgrip training

"Cross-education" is the increase in strength or functional performance of an untrained limb after unilateral training. A major limitation for clinical translation from unilateral injury includes knowledge on the minimum time for the emergence of crossed effects. Therefore, the primary purpose was to characterize the time course of bilateral strength changes during both "traditional" ( n = 11) and "daily" ( n = 8) unilateral handgrip training in neurologically intact participants. Traditional training included five sets of five maximal voluntary handgrip contractions 3 times/wk for 6 wk whereas daily training included the same number of sessions and contractions but over 18 consecutive days. Three pre- and one posttest session evaluated strength, muscle activation, and reflex excitability bilaterally. Time course information was assessed by recording handgrip force for every contraction in the trained limb and from a single contraction on every third training session in the untrained limb. Six weeks of traditional training increased handgrip strength in the trained limb after the 9th session whereas the untrained limb was stronger after the 12th session. This was accompanied by increased peak muscle activation and bilateral alterations in Hoffmann reflex excitability. Daily training revealed a similar number of sessions (15) were required to induce significant strength gains in the untrained limb (7.8% compared with 12.5%) in approximately half the duration of traditional training. Therefore, minimizing rest days may improve the efficiency of unilateral training when the trained limb is not the focus. Establishing a "dose" for the time course of adaptation to strength training is paramount for effective translation to rehabilitative interventions. NEW & NOTEWORTHY Unilateral handgrip training using a "traditional" protocol (3 times/wk; 6 wk) increased strength bilaterally after 9 (trained arm) and 12 (untrained arm) sessions. "Daily" training (18 consecutive days) increased strength in the untrained limb in a similar number of training sessions, which was accomplished in approximately half the time. Within clinical populations when the focus is on the untrained limb, reducing rest days may optimize the recovery of strength.

Electrical nerve stimulation modulates motor unit activity in contralateral biceps brachii during steady isometric contractions

The purpose of our study was to compare the influence of five types of electrical nerve stimulation delivered through electrodes placed over the right biceps brachii on motor unit activity in the left biceps brachii during an ongoing steady isometric contraction. The electrical stimulation protocols comprised different combinations of pulse duration (0.2 and 1.0 ms), stimulus frequency (50 and 90 Hz), and stimulus current (greater or less than motor threshold). The electrical nerve stimulation protocols were applied over the muscle of the right elbow flexors of 13 participants (26 ± 3 yr) while they performed voluntary contractions with the left elbow flexors to match a target force set at 10% of maximum. All five types of electrical nerve stimulation increased the absolute amplitude of the electromyographic (EMG) signal recorded from the left biceps brachii with high-density electrodes. Moreover, one stimulation condition (1 ms, 90 Hz) had a consistent influence on the centroid location of the EMG amplitude distribution and the average force exerted by the left elbow flexors. Another stimulation condition (0.2 ms, 90 Hz) reduced the coefficient of variation for force during the voluntary contraction, and both low-frequency conditions (50 Hz) increased the duration of the mean interspike interval of motor unit action potentials after the stimulation had ended. The findings indicate that the contralateral effects of electrical nerve stimulation on the motor neuron pool innervating the homologous muscle can be influenced by both stimulus pulse duration and stimulus frequency. NEW & NOTEWORTHY Different types of electrical nerve stimulation delivered through electrodes placed over the right biceps brachii modulated the ongoing motor unit activity in the left biceps brachii. Although the effects varied with stimulus pulse duration, frequency, and current, all five types of electrical nerve stimulation increased the amplitude of the electromyographic activity in the left biceps brachii. Moreover, most of the effects in the left arm occurred after the electrical nerve stimulation of the right arm had been terminated.

Determining the potential sites of neural adaptation to cross-education: implications for the cross-education of muscle strength

Cross-education describes the strength gain in the opposite, untrained limb following a unilateral strength training program. Since its discovery in 1894, several studies now confirm the existence of cross-education in contexts that involve voluntary dynamic contractions, eccentric contraction, electrical stimulation, whole-body vibration and, more recently, following mirror feedback training. Although many aspects of cross-education have been established, the mediating neural mechanisms remain unclear. Overall, the findings of this review show that the neural adaptations to cross-education of muscle strength most likely represent a continuum of change within the central nervous system that involves both structural and functional changes within cortical motor and non-motor regions. Such changes are likely to be the result of more subtle changes along the entire neuroaxis which include, increased corticospinal excitability, reduced cortical inhibition, reduced interhemispheric inhibition, changes in voluntary activation and new regions of cortical activation. However, there is a need to widen the breadth of research by employing several neurophysiological techniques (together) to better understand the potential mechanisms mediating cross-education. This fundamental step is required in order to better prescribe targeted and effective guidelines for the clinical practice of cross-education. There is a need to determine whether similar cortical responses also occur in clinical populations where, perhaps, the benefits of cross-education could be best observed.

Is the cross-over effect of a unilateral high-intensity leg extension influenced by the sex of the participants?

Background

While performing a unilateral muscle contraction, electrical muscle activity also arises in the contralateral homologous muscle, muscle group, or limb. When the muscle contraction induces muscle fatigue, females show not only a greater resistance than males but also a reduced contralateral muscle activation. The study aimed at investigating whether, during a high-intensity 30-s unilateral maximal effort isometric leg extension exercise, the contralateral non-exercising limb (NEL) knee extensor muscle activation would differ between females and males.

Methods

Twenty participants, 11 females (23.80 ± 2.15 years old) and 9 males (26.50 ± 2.45 years old), performed a unilateral 30-s exercise while surface electromyography (sEMG) was measured from the vastus lateralis (VL), vastus medialis (VM), and rectus femoris (RF) on both limbs. The maximal voluntary contraction (MVC) was measured for both the exercising limb (EL) and the NEL before (MVC PRE) and after (MVC POST) the 30-s exercise to assess muscle fatigue.

Results

While both females and males exhibited muscle fatigue in the EL (p = 0.015), females exhibited a lower MVC reduction than males (p = 0.042), suggesting that females were less fatigued than males. Although no muscle fatigue, i.e., no MVC force reduction was found in the NEL for either group before and after the 30-s exercise, the muscle activity of the VL was found to be of greater magnitude during the MVC POST only for females (p = 0.047) while it remained unchanged for males. During the 30-s exercise, the force output of the EL decreased only for males (p = 0.029) while females showed a preservation of the force output (p > 0.05). The sEMG activity of the NEL during the 30-s unilateral exercise increased for both groups in all measured muscles (all p-values < 0.03).

Conclusions

Likely, different underlying muscle fatigue mechanisms occurred in the EL between females and males. Yet, our findings suggest that the cross-over effect to the NEL during the 30-s exercise occurred in a similar fashion in both groups. The current study suggests that the contralateral muscle activation seen with a unilateral exercise is independent of the sex of individuals. Therefore, unilateral training or rehabilitation-based protocols would similarly impact females and males.

Unilateral movement preparation causes task-specific modulation of TMS responses in the passive, opposite limb

KEY POINTS

Activity in the primary motor cortices of both hemispheres increases during unilateral movement preparation, but the functional role of ipsilateral motor cortex activity is unknown. Ipsilateral motor cortical activity could represent subliminal 'motor planning' for the passive limb. Alternatively, it could represent the state of the active limb, to support coordination between the limbs should a bimanual movement be required. Here we assessed how preparation of forces toward different directions, with the left wrist, alters evoked responses to transcranial magnetic stimulation of left motor cortex. Preparation of a unilateral movement caused excitability increases in ipsilateral motor cortex that reflected forces produced with the active limb in an intrinsic (body-centred), rather than an extrinsic (world-centred), coordinate system. These results suggest that ipsilateral motor cortical activity prior to unilateral action reflects the state of the active limb, rather than subliminal motor planning for the passive limb.

ABSTRACT

Corticospinal excitability is modulated for muscles on both sides of the body during unilateral movement preparation. For the effector, there is a progressive increase in excitability, and a shift in direction of muscle twitches evoked by transcranial magnetic stimulation (TMS) toward the impending movement. By contrast, the directional characteristics of excitability changes in the opposite (passive) limb have not been fully characterized. Here we assessed how preparation of voluntary forces towards four spatially distinct visual targets with the left wrist alters muscle twitches and motor-evoked potentials (MEPs) elicited by TMS of left motor cortex. MEPs were facilitated significantly more in muscles homologous to agonist rather than antagonist muscles in the active limb, from 120 ms prior to voluntary EMG onset. Thus, unilateral motor preparation has a directionally specific influence on pathways projecting to the opposite limb that corresponds to the active muscles rather than the direction of movement in space. The directions of TMS-evoked twitches also deviated toward the impending force direction of the active limb, according to muscle-based coordinates, following the onset of voluntary EMG. The data indicate that preparation of a unilateral movement increases task-dependent excitability in ipsilateral motor cortex, or its downstream projections, that reflects the forces applied by the active limb in an intrinsic (body-centred), rather than an extrinsic (world-centred), coordinate system. The results suggest that ipsilateral motor cortical activity prior to unilateral action reflects the state of the active limb, rather than subliminal motor planning for the passive limb.

The effect of eccentric exercise and delayed onset muscle soreness on the homologous muscle of the contralateral limb

High intensity eccentric exercise induces muscle fiber damage and associated delayed-onset muscle soreness (DOMS) resulting in an impaired ability of the muscle to generate voluntary force. This study investigates the extent to which DOMS, induced by high intensity eccentric exercise, can affect the activation and performance of the non-exercised homologous muscle of the contralateral limb. Healthy volunteers performed maximal voluntary contractions of knee extension and sustained isometric knee extension at 50% of maximal force until task failure on both the ipsilateral exercised limb and the contralateral limb. Surface electromyography (EMG) was recorded from the ipsilateral and contralateral knee extensor muscles (vastus medialis, rectus femoris, and vastus lateralis). Maximal isometric knee extension force (13.7% reduction) and time to task failure (38.1% reduction) of the contralateral non-exercised leg decreased immediately after eccentric exercise, and persisted 24 h and 48 h later (p < 0.05). Moreover, the amplitude of muscle activity recorded from the contralateral knee extensor muscles was significantly lower during the post exercise maximal and submaximal contractions following high intensity eccentric exercise of the opposite limb (p < 0.05). Unilateral high intensity eccentric exercise of the quadriceps can contribute to reduced neuromuscular activity and physical work capacity of the non-exercised homologous muscle in the contralateral limb.

Neurophysiological adaptations in the untrained side in conjunction with cross-education of muscle strength: a systematic review and meta-analysis

We reviewed the evidence from randomized controlled trials (RCTs) focusing on the neurophysiological adaptations in the untrained side associated with cross-education of strength (CE) and pooled data into definite effect estimates for neurophysiological variables assessed in chronic CE studies. Furthermore, scoping directions for future research were provided to enhance the homogeneity and comparability of studies investigating the neural responses to CE. The magnitude of CE was 21.1 ± 18.2% (mean ± SD; P < 0.0001) in 22 RCTs ( n = 467 subjects) that measured at least 1 neurophysiological variable in the untrained side, including the following: electromyography (EMG; 14 studies); motor evoked potential (MEP; 8 studies); short-interval intracortical inhibition (SICI), recruitment curve, and M wave (6 studies); cortical silent period (cSP; 5 studies); interhemispheric inhibition, intracortical facilitation (ICF), and H reflex (2 studies); and V wave, short-interval ICF, short-latency afferent inhibition, and long-latency afferent inhibition (1 study). Only EMG, MEP, ICF, cSP, and SICI could be included in the meta-analysis (18 studies, n = 387). EMG ( P = 0.26, n = 235) and MEP amplitude ( P = 0.11, n = 145) did not change in the untrained limb after CE. cSP duration ( P = 0.02, n = 114) and SICI ( P = 0.001, n = 95) decreased in the untrained hemisphere according to body region and type and intensity of training. The magnitude of CE did not correlate with changes in these transcranial magnetic stimulation (TMS) measures. The design of this meta-analytical study and the lack of correlations prevented the ability to link mechanistically the observed neurophysiological changes to CE. Notwithstanding the limited amount of data available for pooling, the use of TMS to assess the ipsilateral neurophysiological responses to unilateral training still confirms the central neural origin hypothesis of chronic CE induced by strength training. However, how these neural adaptations contribute to CE remains unclear.

BDNF Val66Met polymorphism is associated with altered activity-dependent modulation of short-interval intracortical inhibition in bilateral M1

The BDNF Val66Met polymorphism is associated with impaired short-term plasticity in the motor cortex, short-term motor learning, and intermanual transfer of a procedural motor skill. Here, we investigated the impact of the Val66Met polymorphism on the modulation of cortical excitability and interhemispheric inhibition through sensorimotor practice of simple dynamic skills with the right and left first dorsal interosseous (FDI) muscles. To that end, we compared motor evoked potentials (MEP) amplitudes and short-interval intracortical inhibition (SICI) in the bilateral representations of the FDI muscle in the primary motor cortex (M1), and interhemispheric inhibition (IHI) from the left to right M1, before and after right and left FDI muscle training in an alternated sequence. Val66Met participants did not differ from their Val66Val counterparts on motor performance at baseline and following motor training, or on measures of MEP amplitude and IHI. However, while the Val66Val group displayed significant SICI reduction in the bilateral M1 in response to motor training, SICI remained unchanged in the Val66Met group. Further, Val66Val group's SICI decrease in the left M1, which was also observed following unimanual training with the right hand in the Control Right group, was correlated with motor improvement with the left hand. The potential interaction between left and right M1 activity during bimanual training and the implications of altered activity-dependent cortical excitability on short-term motor learning in Val66Met carriers are discussed.

Crossed corticospinal facilitation between arm and trunk muscles in humans

A voluntary contraction of muscles with one arm increases the excitability of corticospinal projections to the contralateral resting arm, a phenomenon known as crossed facilitation. Although many motor tasks engage simultaneous activation of the arm and trunk, interactions between corticospinal projections targeting these segments remain largely unknown. Using transcranial magnetic stimulation over the trunk representation of the primary motor cortex, we examined motor-evoked potentials (MEPs) in the resting erector spinae (ES) muscle when the contralateral arm remained at rest or performed 20% of isometric maximal voluntary contraction (MVC) into index finger abduction, thumb abduction, elbow flexion, and elbow extension. We found that MEP size in the ES increased during all voluntary contractions, with greater facilitation occurring during elbow flexion and index finger abduction. To further examine the origin of changes in MEP size, we measured short-interval intracortical inhibition (SICI) and cervicomedullary MEPs (CMEPs) in the ES muscle during elbow flexion and index finger abduction and when the arm remained at rest. Notably, SICI decreased and CMEPs remained unchanged in the ES during both voluntary contractions compared with rest, suggesting a cortical origin for the effects. Our findings reveal crossed facilitatory interactions between trunk extensor and proximal and distal arm muscles, particularly for elbow flexor and index finger muscles, likely involving cortical mechanisms. These interactions might reflect the different role of these muscles during functionally relevant arm and trunk movements. NEW & NOTEWORTHY Many of the tasks of daily life involve simultaneous activation of the arm and trunk. We found that responses in the erector spinae muscles evoked by motor cortical stimulation increased in size during elbow flexion and extension and during index finger abduction and thumb abduction. Crossed facilitation with the trunk was more pronounced during elbow flexion and index finger abduction. These results might reflect the different role of these muscles during arm and trunk movements.

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

INTRODUCTION

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

METHODS

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

RESULTS

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

DISCUSSION

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

The ipsilateral corticospinal responses to cross-education are dependent upon the motor-training intervention

This study aimed to identify the ipsilateral corticospinal responses of the contralateral limb following different types of unilateral motor-training. Three groups performing unilateral slow-paced strength training (SPST), non-paced strength training (NPST) or visuomotor skill training (VT) were compared to a control group. It was hypothesised that 4 weeks of unilateral SPST and VT, but not NPST, would increase ipsilateral corticospinal excitability (CSE) and reduce short-interval cortical inhibition (SICI), resulting in greater performance gains of the untrained limb. Tracking error of the untrained limb reduced by 29 and 41% following 2 and 4 weeks of VT. Strength of the untrained limb increased by 8 and 16% following 2 and 4 weeks of SPST and by 6 and 13% following NPST. There was no difference in cross-education of strength or tracking error. For the trained limb, SPST and NPST increased strength (28 and 26%), and VT improved by 47 and 58%. SPST and VT increased ipsilateral CSE by 89 and 71% at 2 weeks. Ipsilateral CSE increased 105 and 81% at 4 weeks following SPST and VT. The NPST group and control group showed no changes at 2 and 4 weeks. SPST and VT reduced ipsilateral SICI by 45 and 47% at 2 weeks; at 4 weeks, SPST and VT reduced SICI by 48 and 38%. The ipsilateral corticospinal responses are determined by the type of motor-training. There were no differences in motor performance between SPST, NPST and VT. The data suggests that the corticospinal responses to cross-education are different and determined by the type of motor-training.

High-definition transcranial direct-current stimulation of the right M1 further facilitates left M1 excitability during crossed facilitation

The crossed-facilitation (CF) effect refers to when motor-evoked potentials (MEPs) evoked in the relaxed muscles of one arm are facilitated by contraction of the opposite arm. The aim of this study was to determine whether high-definition transcranial direct-current stimulation (HD-tDCS) applied to the right primary motor cortex (M1) controlling the left contracting arm [50% maximum voluntary isometric contraction (MVIC)] would further facilitate CF toward the relaxed right arm. Seventeen healthy right-handed subjects participated in an anodal and cathodal or sham HD-tDCS session of the right M1 (2 mA for 20 min) separated by at least 48 h. Single-pulse transcranial magnetic stimulation (TMS) was used to elicit MEPs and cortical silent periods (CSPs) from the left M1 at baseline and 10 min into and after right M1 HD-tDCS. At baseline, compared with resting, CF (i.e., right arm resting, left arm 50% MVIC) increased left M1 MEP amplitudes (+97%) and decreased CSPs (-11%). The main novel finding was that right M1 HD-tDCS further increased left M1 excitability (+28.3%) and inhibition (+21%) from baseline levels during CF of the left M1, with no difference between anodal and cathodal HD-tDCS sessions. No modulation of CSP or MEP was observed during sham HD-tDCS sessions. Our findings suggest that CF of the left M1 combined with right M1 anodal or cathodal HD-tDCS further facilitated interhemispheric interactions during CF from the right M1 (contracting left arm) toward the left M1 (relaxed right arm), with effects on both excitatory and inhibitory processing. NEW & NOTEWORTHY This study shows modulation of the nonstimulated left M1 by right M1 HD-tDCS combined with crossed facilitation, which was probably achieved through modulation of interhemispheric interactions.

Does ipsilateral corticospinal excitability play a decisive role in the cross-education effect caused by unilateral resistance training? A systematic review

INTRODUCTION

Unilateral resistance training has been shown to improve muscle strength in both the trained and the untrained limb. One of the most widely accepted theories is that this improved performance is due to nervous system adaptations, specifically in the primary motor cortex. According to this hypothesis, increased corticospinal excitability (CSE), measured with transcranial magnetic stimulation, is one of the main adaptations observed following prolonged periods of training. The principal aim of this review is to determine the degree of adaptation of CSE and its possible functional association with increased strength in the untrained limb.

DEVELOPMENT

We performed a systematic literature review of studies published between January 1970 and December 2016, extracted from Medline (via PubMed), Ovid, Web of Science, and Science Direct online databases. The search terms were as follows: (transcranial magnetic stimulation OR excitability) AND (strength training OR resistance training OR force) AND (cross transfer OR contralateral limb OR cross education). A total of 10 articles were found.

CONCLUSION

Results regarding increased CSE were inconsistent. Although the possibility that the methodology had a role in this inconsistency cannot be ruled out, the results appear to suggest that there may not be a functional association between increases in muscle strength and in CSE.

Evidence for existence of trunk-limb neural interaction in the corticospinal pathway

In humans, trunk muscles have an essential role in postural control as well as walking. However, little is known about the mechanisms of interaction with different muscles, especially related to how trunk muscles interact with the limbs. Contraction of muscles can modulate the corticospinal excitability not only of the contracted muscle, but also of other muscles even in the remote segments of the body. However, "remote effect" mechanism has only been examined for inter-limb interactions. The aim of our current study was to test if there are trunk-limb interactions in the corticospinal pathways. We examined corticospinal excitability of: (a) trunk muscles at rest when hands, legs and jaw muscles were contracted and; (b) hand, leg, and jaw muscles at rest when trunk muscles were contracted. We measured motor evoked potentials elicited using transcranial magnetic stimulation in the rectus abdominis, flexor digitorum superficialis, masseter, tibialis anterior muscles under the following experimental conditions: (1) participants remained relaxed (Rest); (2) during trunk contraction (Trunk); (3) during bilateral hand clenching (Hands); (4) during jaw clenching (Jaw); and (5) during bilateral ankle dorsiflexion (Legs). Each condition was performed at three different stimulation intensities and conditions were randomized between participants. We found that voluntary contraction of trunk muscle facilitated the corticospinal excitability of upper-limb and lower-limb muscles during rest state. Furthermore, voluntary contraction of upper-limb muscle also facilitated the corticospinal excitability of trunk muscles during rest state. Overall, these results suggest the existence of trunk-limb interaction in the corticospinal pathway, which is likely depended on proximity of the trunk and limb representation in the motor cortex.

Corticospinal and Spinal Excitabilities Are Modulated during Motor Imagery Associated with Somatosensory Electrical Nerve Stimulation

Motor imagery (MI), the mental simulation of an action, influences the cortical, corticospinal, and spinal levels, despite the lack of somatosensory afferent feedbacks. The aim of this study was to analyze the effect of MI associated with somatosensory stimulation (SS) on the corticospinal and spinal excitabilities. We used transcranial magnetic stimulation and peripheral nerve stimulation to induce motor-evoked potentials (MEP) and H-reflexes, respectively, in soleus and medialis gastrocnemius (MG) muscles of the right leg. Twelve participants performed three tasks: (1) MI of submaximal plantar flexion, (2) SS at 65 Hz on the posterior tibial nerve with an intensity below the motor threshold, and (3) MI + SS. MEP and H-reflex amplitudes were recorded before, during, and after the tasks. Our results confirmed that MI increased corticospinal excitability in a time-specific manner. We found that MI + SS tended to potentiate MEP amplitude of the MG muscle compared to MI alone. We confirmed that SS decreased spinal excitability, and this decrease was partially compensated when combined with MI, especially for the MG muscle. The increase of CSE could be explained by a modulation of the spinal inhibitions induced by SS, depending on the amount of afferent feedbacks.

Somatosensory Electrical Stimulation Does Not Augment Motor Skill Acquisition and Intermanual Transfer in Healthy Young Adults-A Pilot Study

Sensory input can modify motor function and magnify interlimb transfer. We examined the effects of low-intensity somatosensory electrical stimulation (SES) on motor practice-induced skill acquisition and intermanual transfer. Participants practiced a visuomotor skill for 25 min and received SES to the practice or the transfer arm. Responses to single- and double-pulse transcranial magnetic stimulation were measured in both extensor carpi radialis. SES did not further increase skill acquisition (motor practice with right hand [RMP]: 30.8% and motor practice with right hand + somatosensory electrical stimulation to the right arm [RMP + RSES]: 27.8%) and intermanual transfer (RMP: 13.6% and RMP + RSES: 9.8%) when delivered to the left arm (motor practice with right hand + somatosensory electrical stimulation to the left arm [RMP + LSES]: 44.8% and 18.6%, respectively). Furthermore, transcranial magnetic stimulation measures revealed no changes in either hand. Future studies should systematically manipulate SES parameters to better understand the mechanisms of how SES affords motor learning benefits documented but not studied in patients.

Corticospinal excitability for hand muscles during motor imagery of foot changes with imagined force level

The object of this study was to clarify whether corticospinal excitability controlling hand muscles changes concurrently with increases in the imagined contraction level of foot dorsiflexion. Twelve participants performed actual and imagined dorsiflexion of their right foot at three different EMG levels (10, 40 or 80% of the maximum voluntary contraction). During isometric actual- or imagined- dorsiflexion, transcranial magnetic stimulation (TMS) was delivered to the right hand area of the left primary motor cortex. Motor evoked potentials (MEPs) were recorded from the right extensor carpi radialis (ECR) and flexor carpi radialis (FCR). During actual contraction, MEP amplitudes of ECR and FCR increased with an increased EMG level of dorsiflexion. Similarly, during imagery contraction, MEP amplitudes of ECR and FCR increased with the intensity of imagery contraction. Furthermore, a correlation between MEP amplitude during actual contraction and imagery contraction was observed for both ECR and FCR. Motor imagery of foot contraction induced an enhancement of corticospinal excitability for hand muscles that was dependent on the imagined contraction levels, just as what was observed when there was an actual contraction.

Transfer of Motor Learning Is More Pronounced in Proximal Compared to Distal Effectors in Upper Extremities

The current experiment investigated generalizability of motor learning in proximal versus distal effectors in upper extremities. Twenty-eight participants were divided into three groups: training proximal effectors, training distal effectors, and no training control group (CG). Performance was tested pre- and post-training for specific learning and three learning transfer conditions: (1) bilateral learning transfer between homologous effectors, (2) lateral learning transfer between non-homologous effectors, and (3) bilateral learning transfer between non-homologous effectors. With respect to specific learning, both training groups showed significant, similar improvement for the trained proximal and distal effectors, respectively. In addition, there was significant learning transfer to all three transfer conditions, except for bilateral learning transfer between non-homologous effectors for the distal training group. Interestingly, the proximal training group showed significantly larger learning transfer to other effectors compared to the distal training group. The CG did not show significant improvements from pre- to post-test. These results show that learning is partly effector independent and generalizable to different effectors, even though transfer is suboptimal compared to specific learning. Furthermore, there is a proximal-distal gradient in generalizability, in that learning transfer from trained proximal effectors is larger than from trained distal effectors, which is consistent with neuroanatomical differences in activation of proximal and distal muscles.

Ipsilateral corticomotor responses are confined to the homologous muscle following cross-education of muscular strength

Cross-education of strength occurs when strength-training 1 limb increases the strength of the untrained limb and is restricted to the untrained homologous muscle. Cortical circuits located ipsilateral to the trained limb might be involved. We used transcranial magnetic stimulation (TMS) to determine the corticomotor responses from the untrained homologous (biceps brachii) and nonhomologous (flexor carpi radialis) muscle following strength-training of the right elbow flexors. Motor evoked potentials were recorded from the untrained left biceps brachii and flexor carpi radialis during a submaximal contraction from 20 individuals (10 women, 10 men; aged 18-35 years; training group, n = 10; control group, n = 10) before and after 3 weeks of strength-training the right biceps brachii at 80% of 1-repetition maximum. Recruitment-curves for corticomotor excitability and inhibition of the untrained homologous and nonhomologous muscle were constructed and assessed by examining the area under the recruitment curve. Strength-training increased strength of the trained elbow flexors (29%), resulting in an 18% increase in contralateral strength of the untrained elbow flexors (P < 0.0001). The trained wrist flexors increased by 19%, resulting in a 12% increase in strength of the untrained wrist flexors (P = 0.005). TMS showed increased corticomotor excitability and decreased corticomotor inhibition for the untrained homologous muscle (P < 0.05); however, there were no changes in the untrained nonhomologous muscle (P > 0.05). These findings show that the cross-education of muscular strength is spatially distributed; however, the neural adaptations are confined to the motor pathway ipsilateral to the untrained homologous agonist.

Cross-Activation of the Motor Cortex during Unilateral Contractions of the Quadriceps

Transcranial magnetic stimulation (TMS) studies have demonstrated that unilateral muscle contractions in the upper limb produce motor cortical activity in both the contralateral and ipsilateral motor cortices. The increase in excitability of the corticomotor pathway activating the resting limb has been termed “cross-activation”, and is of importance due to its involvement in cross-education and rehabilitation. To date, very few studies have investigated cross-activation in the lower limb. Sixteen healthy participants (mean age 29 ± 9 years) took part in this study. To determine the effect of varying contraction intensities in the lower limb, we investigated corticomotor excitability and intracortical inhibition of the right rectus femoris (RF) while the left leg performed isometric extension at 0%, 25%, 50%, 75% and 100% of maximum force output. Contraction intensities of 50% maximal force output and greater produced significant cross-activation of the corticomotor pathway. A reduction in silent period duration was observed during 75% and 100% contractions, while the release of short-interval intracortical inhibition (SICI) was only observed during maximal (100%) contractions. We conclude that increasing isometric contraction intensities produce a monotonic increase in cross-activation, which was greatest during 100% force output. Unilateral training programs designed to induce cross-education of strength in the lower limb should therefore be prescribed at the maximal intensity tolerable.

Effects of 4 weeks of low-load unilateral resistance training, with and without blood flow restriction, on strength, thickness, V wave, and H reflex of the soleus muscle in men

PURPOSE

To test the effects of 4 weeks of unilateral low-load resistance training (LLRT), with and without blood flow restriction (BFR), on maximal voluntary contraction (MVC), muscle thickness, volitional wave (V wave), and Hoffmann reflex (H reflex) of the soleus muscle.

METHODS

Twenty-two males were randomly distributed into three groups: a control group (CTR; n = 8); a low-load blood flow restriction resistance training group (BFR-LLRT; n = 7), who were an inflatable cuff to occlude blood flow; and a low-load resistance training group without blood flow restriction (LLRT; n = 7). The training consisted of four sets of unilateral isometric LLRT (25% of MVC) three times a week over 4 weeks.

RESULTS

MVC increased 33% (P < 0.001) and 22% (P < 0.01) in the trained leg of both BFR-LLRT and LLRT groups, respectively. The soleus thickness increased 9.5% (P < 0.001) and 6.5% (P < 0.01) in the trained leg of both BFR-LLRT and LLRT groups, respectively. However, neither MVC nor thickness changed in either of the legs tested in the CTR group (MVC -1 and -5%, and muscle thickness 1.9 and 1.2%, for the control and trained leg, respectively). Moreover, V wave and H reflex did not change significantly in all the groups studied (V_wave/M _wave ratio -7.9 and -2.6%, and H _max/M _max ratio -3.8 and -4%, for the control and trained leg, respectively).

CONCLUSIONS

Collectively, the present data suggest that in spite of the changes occurring in soleus strength and thickness, 4 weeks of low-load resistance training, with or without BFR, does not cause any change in neural drive or motoneuronal excitability.

Preparation and execution of teeth clenching and foot muscle contraction influence on corticospinal hand-muscle excitability

Contraction of a muscle modulates not only the corticospinal excitability (CSE) of the contracting muscle but also that of different muscles. We investigated to what extent the CSE of a hand muscle is modulated during preparation and execution of teeth clenching and ipsilateral foot dorsiflexion either separately or in combination. Hand-muscle CSE was estimated based on motor evoked potentials (MEPs) elicited by transcranial magnetic stimulation (TMS) and recorded from the first dorsal interosseous (FDI) muscle. We found higher excitability during both preparation and execution of all the motor tasks than during mere observation of a fixation cross. As expected, the excitability was greater during the execution phase than the preparation one. Furthermore, both execution and preparation of combined motor tasks led to higher excitability than individual tasks. These results extend our current understanding of the neural interactions underlying simultaneous contraction of muscles in different body parts.

The effect of motor overflow on bimanual asymmetric force coordination

Motor overflow, typically described in the context of unimanual movements, refers to the natural tendency for a 'resting' limb to move during movement of the opposite limb and is thought to be influenced by inter-hemispheric interactions and intra-cortical networks within the 'resting' hemisphere. It is currently unknown, however, how motor overflow contributes to asymmetric force coordination task accuracy, referred to as bimanual interference, as there is need to generate unequal forces and corticospinal output for each limb. Here, we assessed motor overflow via motor evoked potentials (MEPs) and the regulation of motor overflow via inter-hemispheric inhibition (IHI) and short-intra-cortical inhibition (SICI) using transcranial magnetic stimulation in the presence of unimanual and bimanual isometric force production. All outcomes were measured in the left first dorsal interosseous (test hand) muscle, which maintained 30% maximal voluntary contraction (MVC), while the right hand (conditioning hand) was maintained at rest, 10, 30, or 70% of its MVC. We have found that as higher forces are generated with the conditioning hand, MEP amplitudes at the active test hand decreased and inter-hemispheric inhibition increased, suggesting reduced motor overflow in the presence of bimanual asymmetric forces. Furthermore, we found that subjects with less motor overflow (i.e., reduced MEP amplitudes in the test hemisphere) demonstrated poorer accuracy in maintaining 30% MVC across all conditions. These findings suggest that motor overflow may serve as an adaptive substrate to support bimanual asymmetric force coordination.

Slower but not faster unilateral fatiguing knee extensions alter contralateral limb performance without impairment of maximal torque output

PURPOSE

The purpose of the present study was to examine the effects of unilateral fatigue of the knee extensors at different movement velocities on neuromuscular performance in the fatigued and non-fatigued leg.

METHODS

Unilateral fatigue of the knee extensors was induced in 11 healthy young men (23.7 ± 3.8 years) at slower (60°/s; FAT60) and faster movement velocities (240°/s; FAT240) using an isokinetic dynamometer. A resting control (CON) condition was included. The fatigue protocols consisted of five sets of 15 maximal concentric knee extensions using the dominant leg. Before and after fatigue, peak isokinetic torque (PIT) and time to PIT (TTP) of the knee extensors as well as electromyographic (EMG) activity of vastus medialis, vastus lateralis, and biceps femoris muscles were assessed at 60 and 240°/s movement velocities in the fatigued and non-fatigued leg.

RESULTS

In the fatigued leg, significantly greater PIT decrements were observed following FAT60 and FAT240 (11-19%) compared to CON (3-4%, p = .002, d = 2.3). Further, EMG activity increased in vastus lateralis and biceps femoris muscle following FAT240 only (8-28%, 0.018 ≤ p ≤ .024, d = 1.8). In the non-fatigued leg, shorter TTP values were found after the FAT60 protocol (11-15%, p = .023, d = 2.4). No significant changes were found for EMG data in the non-fatigued leg.

CONCLUSION

The present study revealed that both slower and faster velocity fatiguing contractions failed to show any evidence of cross-over fatigue on PIT. However, unilateral knee extensor fatigue protocols conducted at slower movement velocities (i.e., 60°/s) appear to modulate torque production on the non-fatigued side (evident in shorter TTP values).

Unilateral Eccentric Contraction of the Plantarflexors Leads to Bilateral Alterations in Leg Dexterity

Eccentric contractions can affect musculotendon mechanical properties and disrupt muscle proprioception, but their behavioral consequences are poorly understood. We tested whether repeated eccentric contractions of plantarflexor muscles of one leg affected the dexterity of either leg. Twenty healthy male subjects (27.3 ± 4.0 yrs) compressed a compliant and slender spring prone to buckling with each isolated leg. The maximal instability they could control (i.e., the maximal average sustained compression force, or lower extremity dexterity force, LED_force) quantified the dexterity of each leg. We found that eccentric contractions did not affect LED_force, but reduced force variability (LED_SD). Surprisingly, LED_force increased in the non-exposed, contralateral leg. These effects were specific to exposure to eccentric contractions because an effort-matched exposure to walking did not affect leg dexterity. In the exposed leg, eccentric contractions (i) reduced voluntary error corrections during spring compressions (i.e., reduced 0.5–4 Hz power of LED_force); (ii) did not change spinal excitability (i.e., unaffected H-reflexes); and (iii) changed the structure of the neural drive to the α-motoneuron pool (i.e., reduced EMG power within the 4–8 Hz physiological tremor band). These results suggest that repeated eccentric contractions alter the feedback control for dexterity in the exposed leg by reducing muscle spindle sensitivity. Moreover, the unexpected improvement in LED_force in the non-exposed contralateral leg was likely a consequence of crossed-effects on its spinal and supraspinal feedback control. We discuss the implications of these bilateral effects of unilateral eccentric contractions, their effect on spinal and supraspinal control of dynamic foot-ground interactions, and their potential to facilitate rehabilitation from musculoskeletal and neuromotor impairments.

Unilateral Fatigue Affects the Unipedal Postural Balance in Individuals With Intellectual Disability

This study aimed to explore the effect of local muscle fatigue on the unipedal stance in men with intellectual disability (ID). The Centre of pressure (CoP) excursions and the isometric maximal voluntary contraction (MVC) were measured before and after a fatiguing exercise. Higher baseline values of CoP excursions and lower MVC values were recorded in the ID group. After the fatiguing exercise, this group showed higher MVC decrease and higher percentage of increase of the mean CoP velocity. In conclusion, men with ID are more vulnerable to the disturbing effects of fatigue during the unipedal stance compared to men without ID.

Knee extensors neuromuscular fatigue changes the corticospinal pathway excitability in biceps brachii muscle

Equivocal evidence indicates that high-intensity muscle contractions can affect the corticospinal responses in muscles not directly involved in the task. In the present study, the responsiveness of corticomotor pathway innervating non-dominant biceps brachii was measured in eleven healthy participants before and after: (i) two 100-s isometric unilateral knee extension maximal voluntary contractions (MVCs) on dominant leg (FATIGUE) and (ii) rest (CONTROL). Transcranial magnetic stimulation, transmastoid electrical and brachial plexus electrical stimulation were used to evoke motor evoked potential (MEP), cervicomedullary motor evoked potential (CMEP) and compound muscle action potential (Mmax) in biceps brachii muscle. The three stimuli were elicited at 2, 3.5 and 5s while participants were performing 6-s elbow flexion contractions at 100, 50, and 5% of MVC interspersed with 10-s rest. The results demonstrated opposing behaviors of MEP responses at 100% (23% higher, p=0.08) and 5% MVC (34% lower, p=0.019) following FATIGUE compared to CONTROL. Similarly, MEP·CMEP^-1 ratio changes indicated that the supraspinal motor response was significantly higher during 100% (42%, p=0.027) but lower during 5% MVC (28%, p=0.009) following FATIGUE. Yet, the elbow flexor MVC force did not exhibit any difference between FATIGUE and CONTROL conditions. These results suggest that the upper limb muscles' corticomotor pathway responsiveness recorded during voluntary contractions were modulated by lower limbs fatiguing contractions and this modulation depends on the force produced during testing, i.e. level of central motor drive. However, these changes have little effect on upper limb muscle maximal performance.

Maximal intermittent contractions of the first dorsal interosseous inhibits voluntary activation of the contralateral homologous muscle

The aim of this study was to investigate how maximal intermittent contractions for a hand muscle influence cortical and reflex activity, as well as the ability to voluntarily activate, the homologous muscle in the opposite limb. Twelve healthy subjects (age: 24 ± 3 years, all right hand dominant) performed maximal contractions of the dominant limb first dorsal interosseous (FDI), and activity of the contralateral FDI was examined in a series of experiments. Index finger abduction force, FDI EMG, motor evoked potentials and heteronomous reflexes were obtained from the contralateral limb during brief non-fatiguing contractions. The same measures, as well as the ability to voluntarily activate the contralateral FDI, were then assessed in an extended intermittent contraction protocol that elicited fatigue. Brief contractions under non-fatigued conditions increased index finger abduction force, FDI EMG, and motor evoked potential amplitude of the contralateral limb. However, when intermittent maximal contractions were continued until fatigue, there was an inability to produce maximal force with the contralateral limb (~30%) which was coupled to a decrease in the level of voluntary activation (~20%). These declines were present without changes in reflex activity, and regardless of whether cortical or motor point stimulation was used to assess voluntary activation. It is concluded that performing maximal intermittent contractions with a single limb causes an inability of the CNS to maximally drive the homologous muscle of the contralateral limb. This was, in part, mediated by mechanisms that involve the motor cortex ipsilateral to the contracting limb.

Muscle Relaxation of the Foot Reduces Corticospinal Excitability of Hand Muscles and Enhances Intracortical Inhibition

The object of this study was to clarify the effects of foot muscle relaxation on activity in the primary motor cortex (M1) of the hand area. Subjects were asked to volitionally relax the right foot from sustained contraction of either the dorsiflexor (tibialis anterior; TA relaxation) or plantarflexor (soleus; SOL relaxation) in response to an auditory stimulus. Single- and paired-pulse transcranial magnetic stimulation (TMS) was delivered to the hand area of the left M1 at different time intervals before and after the onset of TA or SOL relaxation. Motor evoked potentials (MEPs) were recorded from the right extensor carpi radialis (ECR) and flexor carpi radialis (FCR). MEP amplitudes of ECR and FCR caused by single-pulse TMS temporarily decreased after TA and SOL relaxation onset, respectively, as compared with those of the resting control. Furthermore, short-interval intracortical inhibition (SICI) of ECR evaluated with paired-pulse TMS temporarily increased after TA relaxation onset. Our findings indicate that muscle relaxation of the dorsiflexor reduced corticospinal excitability of the ipsilateral hand muscles. This is most likely caused by an increase in intracortical inhibition.

Anodal Transcranial Direct Current Stimulation Prolongs the Cross-education of Strength and Corticomotor Plasticity

PURPOSE

This study aimed to assess the efficacy of applying anodal transcranial direct-current stimulation (a-tDCS) to the ipsilateral motor cortex (iM1) during unilateral strength training to enhance the neurophysiological and functional effects of cross-education.

METHODS

Twenty-four healthy volunteers were randomly allocated to perform either of the following: strength training during a-tDCS (ST + a-tDCS), strength training during sham tDCS (ST + sham), or a-tDCS during rest (a-tDCS) across 2 wk. Strength training of the right biceps brachii involved four sets of six repetitions at 80% of one-repetition maximum three times per week. Anodal tDCS was applied to the iM1 at 1.5 mA for 15 min during each strength training session. Outcome measures included one-repetition maximum strength of the untrained biceps brachii, corticomotoneuronal excitability, cross-activation, and short-interval intracortical inhibition (SICI) of the iM1 determined by transcranial magnetic stimulation.

RESULTS

Immediately after the final training session, there was an increase in strength for both the ST + a-tDCS (12.5%, P < 0.001) and the ST + sham group (9.4%, P = 0.007), which was accompanied by significant increases in corticomotoneuronal excitability and decreases in SICI for both groups. After a 48-h retention period, strength increase was maintained in the ST + a-tDCS (13.0%, P = 0.001) group, which was significantly greater than the ST + sham group (7.6%, P = 0.039). Similarly, increases in corticomotoneuronal excitability and decreases in SICI were maintained in the ST + a-tDCS group but not in the ST + sham group. No main effects were reported for the a-tDCS group (all P > 0.05).

CONCLUSIONS

The addition of a-tDCS to the iM1 during unilateral strength training prolongs the benefits of cross-education, which may have significant implications to enhancement of rehabilitation outcomes after a single-limb injury or impairment.

Neural Adaptations Associated with Interlimb Transfer in a Ballistic Wrist Flexion Task

Cross education is the process whereby training of one limb gives rise to increases in the subsequent performance of its opposite counterpart. The execution of many unilateral tasks is associated with increased excitability of corticospinal projections from primary motor cortex (M1) to the opposite limb. It has been proposed that these effects are causally related. Our aim was to establish whether changes in corticospinal excitability (CSE) arising from prior training of the opposite limb determine levels of interlimb transfer. We used three vision conditions shown previously to modulate the excitability of corticospinal projections to the inactive (right) limb during wrist flexion movements performed by the training (left) limb. These were: (1) mirrored visual feedback of the training limb; (2) no visual feedback of either limb; and (3) visual feedback of the inactive limb. Training comprised 300 discrete, ballistic wrist flexion movements executed as rapidly as possible. Performance of the right limb on the same task was assessed prior to, at the mid point of, and following left limb training. There was no evidence that variations in the excitability of corticospinal projections (assessed by transcranial magnetic stimulation (TMS)) to the inactive limb were associated with, or predictive of, the extent of interlimb transfer that was expressed. There were however associations between alterations in muscle activation dynamics observed for the untrained limb, and the degree of positive transfer that arose from training of the opposite limb. The results suggest that the acute adaptations that mediate the bilateral performance gains realized through unilateral practice of this ballistic wrist flexion task are mediated by neural elements other than those within M1 that are recruited at rest by single-pulse TMS.

Excitability of hand motor areas during articulation of syllables

It is known that articulating different syllables is linked to different grasp actions, e.g. [ti] is linked to precision grip, and [kɑ] to power grip. The aim of the present study was to test whether articulating or hearing these syllables would result in an increased activity in the representation of hand muscles involved in these two actions in a muscle-specific manner. To this end, we used transcranial magnetic stimulation (TMS) to investigate changes in the excitability of the left primary motor cortex (M1) innervating hand muscles while participants articulated or listened to meaningless syllables, listened to a metronome, or observed a fixation cross. The motor-evoked potentials of two hand muscles associated with either a precision or power grip exhibited significantly greater amplitudes during articulation than in passive listening, metronome, and fixation cross conditions. Moreover, these muscles exhibited similar patterns of excitability during articulation regardless of which syllable was articulated. The increased excitability of the left M1 hand area during articulation, but not during perception of the syllables, might be due to the cortico-cortical interaction between the motor representations of oral organs with the hand area.

The influence of unilateral contraction of hand muscles on the contralateral corticomuscular coherence during bimanual motor tasks

The mechanisms behind how muscle contractions in one hand influence corticomuscular coherence in the opposite hand are still undetermined. Twenty-two subjects were recruited to finish bimanual and unimanual motor tasks. In the unimanual tasks, subjects performed precision grip using their right hand with visual feedback of exerted forces. The bimanual tasks involved simultaneous finger abduction of their left hand with visual feedback and precision grip of their right hand. They were divided into four conditions according to the two contraction levels of the left-hand muscles and whether visual feedback existed for the right hand. Measures of coherence and power spectrum were calculated from EEG and EMG data and statistically analyzed to identify changes in corticomuscular coupling and oscillatory activity. Results showed that compared with the unimanual task, a significant increase in the mean corticomuscular coherence of the right hand was found when left-hand muscles contracted at 5% of the maximal isometric voluntary contraction (MVC). No significant changes were found when the contraction level was 50% of the MVC. Furthermore, both the increase of muscle contraction levels and the elimination of visual feedback for right hand can significantly decrease the corticomuscular coupling in right hand during bimanual tasks. In summary, the involvement of moderate left-hand muscle contractions resulted in an increase tendency of corticomuscular coherence in right hand while strong left-hand muscle contractions eliminated it. We speculated that the perturbation of activities in one corticospinal tract resulted from the movement of the opposite hand can enhance the corticomuscular coupling when attention distraction is limited.

Unilateral elbow flexion fatigue modulates corticospinal responsiveness in non-fatigued contralateral biceps brachii

Exercise-induced fatigue can change motor performance in non-exercised muscles. The objective was to investigate unilateral elbow flexion (EF) fatigue effects on the maximal voluntary force (MVC) and corticospinal excitability of contralateral non-exercised biceps brachii (BB). Transcranial magnetic, transmastoid electrical, and brachial plexus electrical stimulation were used to elicit motor evoked potentials (MEP), cervicomedullary motor evoked potentials (CMEP), and compound muscle action potentials in the contralateral non-exercised BB of 12 participants before and after (i) two bouts of 100-s unilateral EF (fatigue) or (ii) control. Three stimuli were evoked every 1.5 s during a series of 6-s isometric EF at 100%, 50%, and 5% of MVC. The non-exercised EF MVC force, electromyographic activity, and voluntary activation were not significantly different between fatigue and control. Non-exercised BB MEP and CMEP amplitudes during 100% MVCs demonstrated significantly higher (P = 0.03) and lower values (P = 0.01), respectively, after fatigue compared with control. There was no difference between the two conditions for MEP and CMEP amplitudes during 50% and 5% MVCs. Unilateral exercise-induced EF fatigue did not lead to cross-over central fatigue to the contralateral homologous muscle but enhanced the supraspinal responsiveness (MEP/CMEP) of the neural circuitries supplying central commands to non-exercised muscles at higher contraction intensity.