Effect of antagonistic voluntary contraction on motor responses in the forearm.

OBJECTIVE: We investigated the effects of voluntary contraction of agonist and antagonist muscles on motor evoked potentials (MEP) and on myoelectric activities in the target (agonist) muscle following transcranial magnetic stimulation (TMS). METHODS: The left extensor carpi radialis (ECR) and flexor carpi radialis (FCR) muscles were studied in 16 healthy subjects. H reflexes, MEP induced by TMS, and background electromyographic (EMG) activity were recorded using surface electrodes at rest and during voluntary contraction of either agonist or antagonist muscles. RESULTS: Voluntary contraction of antagonist muscles (at 10% of maximum contraction) enhanced the amplitudes of MEP for both muscles. The H reflex of the FCR muscle was inhibited by contraction (10% of maximum) of the ECR muscle. Background EMG activity did not differ between H-reflex trials and TMS trials. Enhancement of MEP amplitudes and background EMG activity during voluntary antagonist contraction was comparable in the two muscles. Appearance rate of MEP recorded by needle electrodes in response to subthreshold TMS was increased by antagonistic voluntary contraction. CONCLUSION: Facilitation occurs during voluntary contraction of antagonist muscles. Differences between the effects of voluntary contraction of the ECR muscle for the MEP and the H reflex of the FCR suggest that cortical facilitatory spread occurs between agonist and antagonist muscles.     

Inhibition in the human motor cortex is reduced just before a voluntary contraction.

OBJECTIVE: To examine inhibition in the human motor cortex before and during voluntary movements. METHODS: The balance between the excitation and inhibition of corticospinal neurons in the human motor cortex was tested by conditioning the motor evoked potentials (MEP) evoked in forearm muscles by transcranial magnetic stimulation with a preceding subthreshold stimulus delivered through the same coil. RESULTS: When normal individuals (n = 9) made a tonic wrist extension, inhibition of the forearm extensor MEP decreased, whereas that of the forearm flexors was unchanged. When these individuals made a tonic wrist flexion, inhibition of the forearm flexor MEP diminished, whereas that of the forearm extensors was unchanged. When normal individuals (n = 10) made a phasic wrist extension in response to an auditory signal, inhibition of the extensor MEP began to decline about 95 msec before the onset of the agonist EMG activity. CONCLUSIONS: The changes in balance of excitation and inhibition of corticospinal neurons associated with a voluntary movement precede the movement and are directed at the corticospinal neurons projecting to the agonists. These changes may help to select the population of cortical neurons responsible for the movement.     

Different patterns of excitation and inhibition of the small hand and forearm muscles from magnetic brain stimulation in humans.

OBJECTIVES: The objective of the present study is to quantify the effects of voluntary muscle contraction of the small hand (abductor pollicis brevis, first dorsal interosseus (FDI)) and forearm muscles (extensor carpi radialis (ECR), extensor carpi ulnaris (ECU), flexor carpi radialis (FCR) and flexor carpi ulnaris (FCU)) on motor evoked potentials (MEPs). METHODS: MEPs were recorded in 12 healthy subjects by a circular coil placed over the vertex at 1.2 times the resting motor threshold at different levels of the muscle contraction (0-100% of maximum voluntary contraction (MVC)). The effects of transcranial magnetic stimulation (TMS) on the onset latency, MEP area and silent period (SP) as a function of the %MVC were evaluated using a piecewise linear regression analysis. RESULTS: The MEP areas for the small hand muscles were almost completely saturated at 20% of MVC. In contrast, the MEP areas for radial muscles (ECR, FCR) had a dual increase at 40% of MVC while the ulnar muscles (ECU, FCU) had a dual increase at 20% of MVC. A uniform latency shift (1.5-3 ms reduction) was observed in all muscles with a changing point at 10% of MVC. The SPs were the longest for FDI and were not significantly influenced by MVC for any muscles. CONCLUSIONS: The excitatory and inhibitory effects of TMS on the MEPs differed for the small hand and forearm muscles and also between the ulnar and radial muscles. These results probably resulted from the different degrees of direct corticomotoneuronal inputs to each muscle and the inherent properties of the spinal motoneurons.     

Reciprocal effect of single corticomotoneuronal cells on wrist extensor and flexor muscle activity in the primate

Action potentials of single motor cortex cells were recorded extracellularly in awake monkeys trained to make alternating wrist movements between flexion and extension position zones. Spike-triggered averaging of rectified EMG activity was used to test each cell's output effect on both the coactivated (agonist) forearm muscles and their antagonists. Five of 18 adequately tested motor cortex cells, whose activity covaried strongly with alternating wrist movement, not only produced postspike facilitation of agonist muscles but also postspike suppression of antagonist muscles. Five additional cells only facilitated the agonist muscles and had no effect on the recorded antagonists; 8 cells had no effect on either agonist or antagonist muscles. We conclude that the synaptic terminations of some motor cortex cells with flexor and extensor spinal motoneurons are reciprocally organized.     

Pathophysiological mechanisms in cerebral palsy.

To investigate some of the pathophysiological mechanisms in cerebral palsy, surface electromyograms (EMG) were recorded from pairs of flexor/extensor muscles during both voluntary and passive flexion/extension of upper and lower limbs of 20 patients. Elbow, knee, or ankle joint angles were measured simultaneously, as well as the force required to flex/extend the limbs passively at frequencies of 0.1--1.0 Hz. In addition, single motor units were recorded from the first dorsal interosseous muscles of six of the patients. Almost all patients showed resistance to passive movements (hypertonia). This hypertonia did not necessarily impair voluntary flexion/extension movements if alternating EMG activity was maintained in at least one of the pairs of flexor/extensor muscles involved in the movement. In six severly involved patients, there was a complete breakdown in the reciprocal relationship between reciprocally acting pairs of flexor/extensor motoneurones, which resulted in synchronous activation (co-contractions) of flexor/extensor muscles during both voluntary and passive movements. In these patients the hyperactive segmental reflex added to the disabling effects of co-contractions during voluntary movements. Single motor units recorded from patients with dystonic movements were recruited with variable delays (2--10 s) and usually discharged intermittently at high frequencies (60--120/s). This abnormla motor unit discharge pattern may relate to pathology of the basal ganglia.     

Temporal changes of pyramidal tract activities after decision of movement: a study using transcranial magnetic stimulation of the motor cortex in humans

To elucidate the effects of the decision to move on the pyramidal tract in humans, we examined the changes in the motor evoked potentials (MEP) of the forearm muscles following transcranial magnetic cortical stimulation (TMS) of the hand area during a go/no-go hand-movement task in 10 normal subjects. The subjects performed an extension of the right wrist according to the go, no-go and control signals, one of which was randomly presented on a TV. A single TMS was applied to the primary hand motor area in the left hemisphere 0–300 ms after each signal. The MEPs recorded from the wrist extensor and flexor muscles changed in amplitude after both go and no-go signals. In comparison with the control, the MEPs were significantly facilitated in the agonistic muscles (wrist extensor muscles) and attenuated in the antagonistic muscles (wrist flexor muscles), at the latencies of 100–200 ms after the go signal (P<0.02). In contrast, the MEPs of both the extensor and flexor muscles were significantly attenuated during the period of 100–200 ms after the no-go signal (P<0.001). We speculate that there is strong inhibition on the pyramidal tract after the no-go signal and that the inhibitory effect is non-specific to the target muscles. This inhibition differs from the reciprocal inhibition of the MEP observed in antagonistic muscles after the go signal, and it is probably related to the movement decision originating in the prefrontal cortex.     

Subthreshold corticospinal control of anticipatory actions in humans

Previous findings suggest that, by influencing the subthreshold state of motoneurons, the corticospinal pathways can set and reset the threshold position at which wrist muscle recruitment begins. Here we assumed that the corticospinal system can change the threshold position in a similar way before anticipated perturbation to pre-determine an appropriate emerging response to it. We first analyzed motor-evoked potentials (MEPs) elicited by transcranial magnetic stimulation (TMS) applied to the wrist area of motor cortex before unloading of preloaded wrist flexors, i.e. before the subsequent involuntary wrist motion to another position (natural unloading). Subjects then learned to diminish the post-unloading movement extent without activating antagonist (extensor) muscles before unloading or making intentional movement corrections after unloading (adjusted unloading). Although activity levels of wrist muscles before unloading were similar, MEPs of extensor but not pre-loaded flexor muscles were higher before adjusted unloading. We also applied TMS in combination with a torque pulse that shortened extensor muscles such that the MEP occurred when the motoneuronal excitability was minimized. Although diminished following muscle shortening, MEPs before adjusted unloading were still higher than before natural unloading. Results suggest that the corticospinal system, possibly together with other descending systems participated in the tonic subthreshold facilitation of antagonist motoneurons before adjusted unloading, which appears sufficient in modifying motor commands and motion leading to adjusted unloading. This study reinforces previous findings that descending systems, in particular, the corticospinal system can employ threshold position control during and after learning a novel action.     

Facilitation and reciprocal inhibition by imagining thumb abduction.

It is well known that motor evoked potentials (MEPs) elicited by transcranial magnetic stimulation (TMS) of the motor cortex are facilitated by voluntary muscle contraction. We evaluated the effects of imagination of movements on MEP latencies of agonist and antagonist muscles in the hand using TMS. Twenty-two healthy volunteers were studied. TMS delivered at rest and while imagining tonic abduction of the right thumb. MEPs were recorded in response to magnetic stimulation over the scalp and cervical spine (C7-T1), and central motor conduction times (CMCT) were calculated. MEPs were recorded from right abductor pollicis brevis muscle (APB) and adductor pollicis muscle (AP) simultaneously. Imagination of abduction resulted in a shortened latency of MEPs in the APB muscle, and a prolonged latency in the AP muscle. But the imagination caused no significant change in the latency of MEPs elicited by stimulation over the cervical spine. The changes of the CMCT may account for these latency changes with imagination of movement. These findings indicate that imagination of thumb abduction facilitates motoneurons of agonist muscle and has an inhibitory effect on those of antagonist muscle (reciprocal inhibition).     

Strength deficits in primary focal hand dystonia.

Cortical activation is reduced when patients with focal dystonia perform movements that do not induce dystonic posturing. This finding suggests that the cortical drive to muscles may in some circumstances actually be reduced not increased, as suggested by basal ganglia models of dystonia as a hyperkinetic disorder. The purpose of this study was to examine flexor and extensor strength at the wrist (a clinically affected joint) and elbow (a nonclinically affected joint) in 18 patients with primary focal hand dystonia compared to matched control subjects. We measured peak torque from maximum voluntary contractions, and agonist and antagonist muscle activation by means of surface electromyograms. Patients were significantly weaker than controls at both the elbow and wrist joints and in both flexors and extensors compared to controls. Peak elbow flexion torque was, on average, 14.4% lower in the dystonic compared to the control group, elbow extensor peak torque was 28.6% lower, wrist flexor peak torque was 17.4% lower, and wrist extensor peak torque was 20.7% lower. Strength did not differ as a function of clinical severity. Reductions in peak torque were accompanied by reduced agonist activation, although this finding only reached statistical significance at the elbow. The amount of co-contraction of antagonistic muscles was not significantly different between the two groups. These results are discussed in the context of dystonia as a disorder resulting from dysfunction of basal ganglia output.     

Roles of primate spinal interneurons in preparation and execution of voluntary hand movement

To study the contribution of primate cervical interneurons (INs) to preparation and execution of normal voluntary hand movement we investigated their activity and correlational linkages to muscles in monkeys performing tracking tasks. During ramp-and-hold flexion–extension torques about the wrist most task-related spinal INs exhibited some activity during both flexion and extension, in unexpected contrast to the strictly unidirectional activity of corticomotoneuronal (CM) cells and motoneurons. Most INs increased their activity more in one of these two directions; response patterns in their preferred direction were typically tonic or phasic-tonic. Spike-triggered averages of EMG detected significant features in muscle activity for many task-related INs. Premotor INs (PreM-INs) were identified by post-spike facilitation or suppression with appropriate onset latencies after the trigger spike. Muscle fields of PreM-INs were smaller than those of supraspinal PreM cells in cortex and red nucleus, and rarely involved reciprocal effects on antagonist muscles. To investigate the relation of spinal INs to a repertoire of different muscle synergies, activity of INs was recorded from a macaque performing a multidirectional wrist task. The monkey generated isometric torques in flexion/extension, radial/ulnar deviation, pronation/supination, and executed a power grip that co-contracted wrist flexor and extensor muscles. Many INs showing task-modulated activity had preferred directions in this multidirectional space, typically with broadly tuned activation. The role of spinal INs in preparation for voluntary movement was revealed in monkeys performing instructed delay tasks. During the delay between a transient visual cue and a go signal a third of the tested INs showed significant delay modulation (SDM) of firing rate relative to the pre-cue rate. The SDM responses often differed from the INs’ responses during the subsequent active torque period. In a monkey instructed by either visual or proprioceptive cues the delay period activity for many INs was similar in visual and perturbation trials, although other INs exhibited different SDM for visually and proprioceptively cued trials. These results suggest that spinal INs are involved, with cortex, in the earliest stages of movement preparation. The sensory input to INs could be identified in transient responses to the torque pulse, which showed two predominant patterns, consistent with inputs from cutaneous or proprioceptive receptors. We also investigated the task-dependent modulation of neural responses to peripheral input in a monkey performing wrist flexion/extension movements in a visually cued instructed delay task. Monosynaptic responses evoked by electrical stimulation of the superficial radial nerve through a cuff electrode were suppressed or abolished during the dynamic movement phase. Since task-related activity of the INs increased at the same time, the suppression was mediated by presynaptic rather than postsynaptic inhibition. These observations indicate that under normal behavioral conditions many spinal INs have response properties comparable to those previously documented for cortical neurons in behaving animals.     

Vibration prolongs the cortical silent period in an antagonistic muscle

We tested whether the silent period, an indicator of inhibitory neuronal activity, is modulated by muscle vibration. Vibration was applied to the right extensor carpi radialis (ECR) muscle in 17 healthy subjects and, as a control experiment, to the dorsal terminal phalanges in 5 subjects. Data before vibration were compared with those during vibration. The cortical silent period (CSP) was evoked by transcranial magnetic stimuli (TMS) during voluntary wrist flexion or during voluntary wrist extension. TMS‐evoked motor potentials (MEPs) of the flexor carpi radialis (FCR) muscle were recorded during muscle relaxation. The mixed nerve silent period (MNSP) was obtained by electrical stimulation of the median nerve during wrist flexion. ECR vibration induced a significant prolongation of the CSP in FCR. CSP increases induced by vibration of the dorsal terminal phalanges were significantly less pronounced. In ECR, the CSP tended to be shortened. MEPs and MNSP remained unchanged. We conclude that vibration enhances inhibitory neuronal properties in a non‐vibrated antagonistic muscle, presumably at a supraspinal level. These results may be relevant for the treatment of spasticity of the upper extremity. Muscle Nerve, 2009     

Impaired modulation of motor cortex excitability by homonymous and heteronymous muscle afferents in focal hand dystonia.

A decrease of heteronymous median nerve-evoked inhibition of corticospinal projections to forearm extensor muscles was reported in a group of 10 dystonic patients by Bertolasi and colleagues in 2003. Here we tested the excitability of corticomotoneuronal connections to both wrist extensor (ECR) and flexor (FCR) muscles after conditioning stimulation of median and also radial nerve at rest in a group of 25 patients with focal hand dystonia compared to 20 healthy subjects. We also investigated the effect of the wrist dystonic posture, either in flexion or in extension, on the afferent modulation of ECR and FCR motor evolved potentials (MEPs). The heteronymous (median-induced) but also homonymous (radial-induced) inhibitions (interstimuli intervals 13-21 ms) of ECR MEP size observed in healthy subjects were decreased in patients. In addition, homonymous (median-induced) facilitation of FCR MEP size was also decreased in patients while heteronymous inhibition (radial-induced) was not. Neither the involvement of the target muscle in the dystonic posture nor the origin of the afferent volley (from a dystonic muscle) influenced the degree of impairment of afferent modulation of the MEP. These findings support the view that a global abnormal somatosensory coupling in focal hand dystonia may contribute to an inadequate motor command to wrist muscles.     

Step-tracking movements of the wrist in humans. II. EMG analysis

We asked human subjects to make accurate step-tracking movements of the wrist to targets that required 5 degrees-30 degrees of radial or ulnar deviation. Speed instructions were given prior to each trial. Muscle activity was recorded from extensor carpi radialis longus (ECRL) and extensor carpi ulnaris (ECU) using surface electrodes. The agonist muscle initiated each movement with a brief burst of activity which began approximately 45 msec before movement onset. Then, the antagonist muscle displayed a brief burst of activity which began approximately 10 msec after movement onset. The magnitude, but not the timing, of these bursts was modulated by changes in the task requirements. The area of the initial agonist burst varied with changes in both displacement and intended speed. This burst was most highly correlated with the initial peaks of acceleration and jerk. In contrast, the area of the initial antagonist burst varied with changes in intended speed and was less well modulated by changes in displacement. This burst was highly correlated with the reciprocal of movement duration. Some small, fast movements had the same agonist bursts as some large, slow movements. However, the antagonist bursts for these movements differed greatly. This observation provides clear evidence that the magnitudes of the agonist and antagonist bursts are independently controlled. In a prior paper (Hoffman and Strick, 1986b), we proposed that step-tracking movements of different amplitudes and intended speeds are centrally generated by adjusting 2 kinematic variables: (1) the peak value and (2) the duration of a derivative of displacement. The present results suggest that these 2 kinematic parameters are separately generated by independently modulating the magnitudes of the agonist and antagonist bursts. Thus, the peak displacement of a step-tracking movement must be determined by the appropriate adjustment of both bursts of muscle activity.     

Voluntary movement at the elbow in spastic hemiparesis

The relative importance of hyperreflexia and paresis in disturbances of voluntary arm movement was studied in a group of patients (n = 25) with spasticity arising from a unilateral ischemic cerebral lesion. Patient performance was evaluated against data obtained from normal subjects (n = 15). Spastic patients achieved lower maximum movement velocities during flexion or extension than did normal subjects. The more marked the paresis of the elbow flexor and extensor muscles of the patients, relative to the strength of the normal subjects, the greater was this reduction in maximum velocity. For a given velocity, however, the time taken to complete a movement and the time to reach the peak velocity were normal. No relationship was found between the degree of impairment of voluntary movement and the level of passive muscle hypertonia in the antagonist. Although overactivity of the antagonist muscle may play some role in disturbance of movements made at low velocities without an opposing load, antagonist activity during movements made against a load (i.e., under more natural conditions) was at or below normal levels, even in those patients with the most marked passive muscle hypertonia. It is concluded that agonist muscle paresis, rather than antagonist muscle hypertonia, plays the dominant role in the disturbance of voluntary elbow movement following stroke.     

The basis and functional role of the late EMG activity in human forearm muscles following wrist displacement.

The present paper examines the hypothesis that the long latency EMG activity produced by muscle stretch is the result of long loop reflex pathways involved in the control of limb stiffness. We recorded the cerebral responses and late EMG activity in agonist and antagonist muscles following sudden stretch of the wrist extensor muscles under 4 experimental conditions in 11 subjects. In each experiment subjects held their right wrist extended isometrically against a constant force of 2.3 N and a trial was begun with a step increase in the force from 2.3 N to 18.4 N, to stretch the extensor muscle. In the first and second experiments the force change occurred unpredictably and subjects had to either oppose the perturbation (Unpredictable Oppose) or relax the forearm muscles once the increase in force was detected (Unpredictable Let-Go). In the third and fourth experiments the force change occurred predictably when subjects pressed a thumb switch with the left hand to cause it. As before, subjects were instructed to either oppose the perturbation (Predictable Oppose) or relax the forearm muscles (Predictable Let-Go). Responses were recorded from the flexor and extensor carpi radialis muscles and from the scalp. When the perturbing force occurred unpredictably, early latency EMG activity (the MI response) was seen in the stretched extensor muscle, and longer latency EMG activity was seen simultaneously in both extensor and flexor muscles. When the force change occurred predictably the late EMG activity was considerably attenuated, especially in the Predictable Let-Go condition. Cerebral responses similarly depended upon the predictability of the perturbation.(ABSTRACT TRUNCATED AT 250 WORDS)     

Task-dependence of muscle afferent monosynaptic inputs to human extensor carpi radialis motoneurones

The task-dependence of homonymous muscle afferent inputs was investigated in motor units of the extensor carpi radialis muscles during voluntary isometric contraction involving either the activation of agonist extensor muscles (wrist extension) or the co-activation of antagonist extensor and flexor muscles (hand clenching). The effectiveness of the muscle afferent monosynaptic inputs was tested by delivering either tendon taps or electrical stimulation to the radial nerve. In both cases, the motor unit responses, which took the form of narrow peaks in the peri-stimulus time histograms, were found to be significantly greater during hand clenching. The parallel enhancement of the responses to both mechanical and electrical stimulations observed during hand clenching could not be explained in terms of changes in the muscle spindle responsiveness. The enhancement of the motor units' responsiveness was apparent during the first 0.5 ms of the peaks in the peri-stimulus time histograms, taken to be uncontaminated by any polysynaptic components. It may therefore have reflected an increase in the amplitude of the excitatory monosynaptic potentials generated by the muscle spindle primary afferents. This is interpreted in terms of changes in the presynaptic inhibition, which might be depressed as the result of the large-scale activation of palm and finger cutaneous afferents liable to occur during hand clenching.     

一种新型腕手矫形器的结构特点及其使用

目的：介绍一种新型腕手矫形器。 方法：该腕手矫形器由腕背护板和腕托架构成的主架，主架的前侧固定安装一U形前支架；食指套圈、中指套圈、无名指套圈和小指套圈分别通过弹力带连接在主架的腕背护扳上；拇指套圈通过钢丝安装在腕背护板的食指套圈一侧；一外展弹力套通过钢丝设置在小指套圈外侧的腕背护板上。使用时可根据矫形需要，将手腕以手心朝上或朝下之方式伸入腕背护板和腕托架构成的主架内，手位于其前方，指套置于滚动套管上的前方，手指伸至滚动套管下的前方，指套圈分别套在食指、中指、无名指和小指上，拇指套圈套在拇指上。然后根据矫形需要活动手腕，实现其各种矫形功能。 结果：该新型腕手矫形器可用于伸指肌腱损伤术后保持手指呈伸直状态，以限制手指的活动范围，促进断端的愈合；用于屈指肌腱损伤损伤术后保持腕关节和指关节处于屈曲状态；改善腕关节屈、伸，增加腕关节屈曲的活动范围；增加腕关节桡侧偏移的活动范围；增加桡侧腕屈肌和尺侧腕屈肌肌力、桡侧腕伸肌和尺侧腕伸肌肌力、尺侧伸腕肌和尺侧屈腕肌肌力、指总伸肌肌力、指浅屈肌及指深屈肌肌力。 结论：该种型腕手矫形器适用于腕及手的神经、肌腱、肌肉、血管损伤术后保护，腕及手骨折所致的腕关节、指关节运动功能障碍恢复，可改善腕关节活动范围，增加肌力。     

Oscillatory electromyographic responses to limb displacement in Parkinsonism.

Forty Parkinsonian patients and 26 normal subjects were instructed not to resist movements of a handle which they maintained in a specified position (1) during tonic activation of muscles against the force produced by a torque motor and (2) while no force was produced by the motor. Electromyographic responses to handle displacements were recorded in biceps muscle (pronating or supinating displacements) or in wrist extensor and flexor muscles (displacements which extended or flexed the wrist). Displacements involving changes of muscle length elicited (1) excitation and inhibition occurring at monosynaptic latency in muscles which were stretched and shortened, respectively; (2) a "silent period" following the initial excitation in the stretched muscle and excitation following the initial inhibition in the shortened muscle (shortening reaction); and (3) (in Parkinsonian patients) sustained oscillations at about 4 to 5 Hz (at rest) or about 6 to 8 Hz (during maintained posture). It was also observed that the initial muscle responses in both the stretched and shortened muscle could be reciprocal and biphasic, with the two peaks of excitation in the agonist occurring during reduced activity of antagonist muscles, and vice versa.     

Muscle imaging: mapping responses to transcranial magnetic stimulation with high-density surface electromyography

Representations of different body parts or muscles in the human primary motor cortex overlap extensively. At the effector level, most muscles are surrounded by and overlap with several neighbours as well. This hampers the assessment of excitability in individual muscles with transcranial magnetic stimulation (TMS), even if so-called "focal" stimulating coils are used. Here we used a novel mapping paradigm based on high-density surface electromyography (HD-sEMG) to investigate the spatial selectivity of TMS in the forearm musculature. In addition, we tested the hypothesis that selective stimulation can be improved by a voluntary background contraction of the target muscle. We mapped and compared the topographies of motor evoked potential (MEP) amplitudes during rest and during background contractions of two forearm muscles (extensor carpi radialis and extensor digitorum communis). The MEP topographies were also compared to the amplitude topography of voluntary EMG. The results indicate that under many conditions a large proportion of the MEP activity recorded at the surface originated from the target muscle's neighbours. There was a systematic relationship between TMS intensity and the topographic distribution of MEP responses during voluntary contraction. With increasing stimulus intensity, the MEP topography deviated increasingly more from the topography of voluntary EMG. We conclude that when standard EMG montages are used, the recorded MEPs are not necessarily evoked in the target muscle alone. Stimulation during a voluntary background contraction of the target muscle may enhance the selectivity of TMS. It however remains essential to use stimulus intensities as low as possible, to minimize the contribution of surrounding non-target muscles to the MEP.     

Evidence for facilitation of motor evoked potentials (MEPs) induced by motor imagery

This study examined the extent to which motor imagery can facilitate to specific pools of motoneurons. Motor commands induced by motor imagery were subthreshold for muscle activity and were presumably not associated with any change in background afferent activity. To estimate excitability changes of flexor carpi radialis (FCR) muscle motoneuron in spinal and cortical level, electric stimuli for recording H-reflex and transcranial magnetic stimulation (TMS) for recording motor evoked potentials (MEPs) were used. During motor imagery of wrist flexion, remarkable increases in the amplitude of the MEP of FCR were observed with no change in the H-reflex. Furthermore, facilitation of antagonist (extensor carpi radialis; ECR) was also observed. Therefore, it is concluded that internal motor command can activate precisely cortical excitability with no change in spinal level without recourse to afferent feedback.