Stepping before standing: hip muscle function in stepping and standing balance after stroke

OBJECTIVE: To compare the pattern of pelvic girdle muscle activation in normal subjects and hemiparetic patients while stepping and maintaining standing balance. DESIGN: Group comparison. METHOD: Seventeen patients who had regained the ability to walk after a single hemiparetic stroke were studied together with 16 normal controls. Median interval between stroke and testing was 17 months. Amplitude and onset latency of surface EMG activity in hip abductors and adductors were recorded in response to sideways pushes in either direction while standing. Similar recordings were made in the same subjects during gait initiation and a single stride. RESULTS: In the standing balance task, normal subjects resisted a sideways push to the left with the left gluteus medius (74 ms) and with the right adductor (111 ms), and vice versa. In hemiparetic patients, the amplitude of activity was reduced in the hemiparetic muscles, the onset latencies of which were delayed (gluteus medius 96 ms, adductor 144 ms). Contralateral, non-paretic, adductor activity was increased after a push towards the hemiparetic side of patients with stroke and the latency was normal (110 ms). During self initiated sideways weight shifts at gait initiation, hemiplegic muscle activation was impaired. By contrast, the pattern and peak amplitude of hip muscle activation in stepping was normal in both hemiparetic and non-hemiparetic muscles of the subjects with stroke. CONCLUSIONS: In ambulant patients with stroke, a normal pattern of activation of hemiparetic muscles is seen in stepping whereas the response of these muscles to a perturbation while standing remains grossly impaired and is compensated by increased activity of the contralateral muscles. This suggests that hemiparetic patients should be able to step before regaining standing balance.     

Magnetic stimulation of the human motor cortex: facilitation and its relationship to a visual motor task

Transcranial magnetic stimulation of the motor cortex can evoke motor responses in small hand muscles. This response can be facilitated by a background muscle contraction of the target muscle, resulting in an enhanced compound muscle action potential (CMAP) with a shorter onset latency. A number of intracortical mechanisms may account for this facilitatory effect, including, in part, direct input from visual to motor cortex. We studied the facilitation produced by a visual-motor task and compared those results with the facilitation produced by the same task without the visual cues. No differences in facilitation of amplitude or latency were observed. This suggests that there is no direct influence exerted by the visual cortex upon those elements of the motor cortex activated by a tangential magnetic stimulus, i.e., corticocortical and corticospinal neurons and their processes. Also, the large majority of facilitation (90%) was produced by a very small background voluntary contraction (less than 5% of maximum), indicating that any mild-to-moderate contraction of the target muscles will produce a consistent response for clinical measurements.     

Effect of discharge desynchronization on the size of motor evoked potentials: an analysis.

OBJECTIVE: Motor evoked potentials (MEPs) after transcranial magnetic brain stimulation (TMS) are smaller than CMAPs after peripheral nerve stimulation, because desynchronization of the TMS-induced motor neurone discharges occurs (i.e. MEP desynchronization). This desynchronization effect can be eliminated by use of the triple stimulation technique (TST; Brain 121 (1998) 437). The objective of this paper is to study the effect of discharge desynchronization on MEPs by comparing the size of MEP and TST responses. METHODS: MEP and TST responses were obtained in 10 healthy subjects during isometric contractions of the abductor digiti minimi, during voluntary background contractions between 0% and 20% of maximal force, and using 3 different stimulus intensities. Additional data from other normals and from multiple sclerosis (MS) patients were obtained from previous studies. RESULTS: MEPs were smaller than TST responses in all subjects and under all stimulating conditions, confirming the marked influence of desynchronization on MEPs. There was a linear relation between the amplitudes of MEPs vs. TST responses, independent of the degree of voluntary contraction and stimulus intensity. The slope of the regression equation was 0.66 on average, indicating that desynchronization reduced the MEP amplitude on average by one third, with marked inter-individual variations. A similar average proportion was found in MS patients. CONCLUSIONS: The MEP size reduction induced by desynchronization is not influenced by the intensity of TMS and by the level of facilitatory voluntary background contractions. It is similar in healthy subjects and in MS patients, in whom increased desynchronization of central conduction was previously suggested to occur. Thus, the MEP size reduction observed may not parallel the actual amount of desynchronization.     

Facilitatory effect of thinking about movement on magnetic motor-evoked potentials

To investigate the facilitatory effect of thinking about movement on motor evoked potential (MEP) amplitude, we recorded MEPs in two test muscles during rest, with the subject thinking about contracting the test muscle but without subsequent contraction, and during 10% maximum voluntary contraction. Stimuli were delivered at 10% above resting motor threshold and at 90–100% stimulator output. H-reflexes, recorded in flexor carpi radialis, were obtained during rest and think conditions. MEP threshold was lower during the think condition (P=0.004). At both stimulus intensities, median MEP amplitudes and areas were significantly (P<0.001) larger during the think paradigm compared with rest. This effect was greater at the lower stimulus intensity. There was no significant difference in latency (P=0.15). In 4/8 subjects, H-reflex amplitudes were mildly facilitated (P<0.05) during the think condition. We conclude that thinking about movement without detectable EMG activity has a facilitatory effect on magnetic MEPs. The absence of a MEP latency shift between rest and think conditions and absence of a consistent increase in H-reflex amplitude suggests this effect occurs largely at the cortical level. In some subjects, however, an increase in spinal motoneuron excitability may also contribute.     

Reproducibility of tendon jerk reflexes during a voluntary contraction.

OBJECTIVES: The present study explored whether testing tendon jerks during voluntary contraction of the test muscle would improve reproducibility by effectively 'clamping' the excitability of the motoneurone pool at firing threshold. METHODS: Tendon jerk reflexes of soleus, tibialis anterior and vastus lateralis and the soleus H reflex were recorded in 12 healthy subjects at rest and during voluntary contractions of 10-20% of maximum. Recordings were repeated 8-10 times in 5 subjects, in whom reflex symmetry was also determined. RESULTS: Not all tendon jerk reflexes could be recorded at rest, and the variability of latency and amplitude was high. All reflexes could be recorded in each subject during contractions. The latency of tendon jerk reflexes decreased by approximately 2 ms during contractions, but H-reflex latency decreased by only 0.2-0.3 ms. For the tendon jerks, an asymmetry of >3.0 ms at rest and >2.5 ms contracting would be outside 3 SD of the normal mean. In repeat studies, the coefficient of variation of reflex latency was <4% for the tendon jerk. CONCLUSIONS: A voluntary contraction could potentiate the tendon jerk by a number of mechanisms, but the most important is probably enhancement of the excitability of the motoneurone pool. The present techniques should increase the value of tendon reflex testing when assessing possible peripheral nerve, plexus and root disturbances.     

Inhibition of dynamic thigh muscle contraction by electrical stimulation of the posterior cruciate ligament in humans.

We investigated the influence of electrical stimulation of the posterior cruciate ligament (PCL) on the motoneuron pool of the thigh muscle during voluntary static and dynamic muscle contraction. The study group comprised nine young men with no history of injury to the knee joints. Multistranded Teflon-insulated stainless-steel wires were inserted into the PCL guided by ultrasound. In three subjects wires were also inserted into the fat pad of the knee. The PCL was electrically stimulated during static, concentric, or eccentric muscle contraction with a constant load of 20% of the maximal voluntary contraction of either the quadriceps or the hamstrings. Electromyographic signals were recorded with bipolar surface electrodes placed over the vastus medialis, rectus femoris, vastus lateralis, biceps femoris caput longum, and semitendinosus muscles. The stimuli consisted of four pulses delivered at 200 HZ; the stimulus amplitude was two to three times the sensory threshold. The electrical stimulation of the PCL inhibited the ongoing muscle activity in both the quadriceps and hamstrings with latencies of 114-150 ms and 99-130 ms, respectively. Stimulation of the fat pad of the knee did not influence the muscle activity. The study suggests that the mechanoreceptors in the PCL are involved in controlling muscle activity during both static and active muscle contractions. The relative long latency of the reflex makes it unlikely that it can serve as a directly protective reflex for the cruciate ligaments.     

Effects of fatigue on the stretch reflex in a human muscle.

The effects of fatigue on the electromyographic (EMG) reflex activities were compared during sustained voluntary contractions and contractions evoked by electrical stimulation (30 Hz) in the human first dorsal interosseus (FDI). Short latency (SL), medium latency (ML) and long latency (LL) reflex responses to a ramp-and-hold stretch of the muscle were recorded and analysed in 27 healthy subjects of both sexes. The amplitude of the reflex components was normalized as function of the amplitude of the surface action potential (SAP) recorded in response to the supramaximal stimulation of the motor nerve. The results indicate that for a similar reduction of force, SL and ML are significantly reduced after fatigue induced by voluntary contractions but they are not when the fatigue test is performed by electrical stimulation at the motor point. In voluntary fatigue experiments, the LL component showed no significant decrease below control values, but an enhancement was observed during electrically evoked contraction. This enhancement remained above control values for at least 15 min during the recovery period, whereas SL and ML decreases returned to control within 5 min after the fatigue tests. The electrical stimulation applied to the skin overlying the FDI at an intensity lower than the motor threshold did not affect SL and ML, but enhanced LL for about 15 min. On the contrary, the anaesthesia of the skin overlying the FDI induced a decrease in LL without significant change of SL and ML. It is concluded that muscle reflex fatigue is present during sustained voluntary contractions and decreases SL and ML responses to quick stretches.(ABSTRACT TRUNCATED AT 250 WORDS)     

Facilitation of motor evoked potentials: Timing of jendrassik maneuver effects

Remote voluntary contraction, such as the classical Jendrassik maneuver (JM), is a procedure routinely used to increase the amplitude of tendon reflexes in the lower limb. In 8 healthy subjects we studied the effects of JM on the motor evoked potentials (MEP) recorded from tibialis anterior muscle, produced by transcranial magnetic stimulation (stimulus output of 5–10% over motor threshold). In this study, JM consisted here of a bilateral violent handgrip, preceding magnetic stimulation from 100 to 50 ms (steps of 100 ms). Compared to the control test, latencies remained unchanged. MEP amplitudes were greatly enhanced with a JM test interval from 200 to 400 ms (170% of control amplitude at 300 ms). We also studied 6 patients with severe alterations of MEPs from tibialis anterior muscle. In each case, JM preceding magnetic stimulation (stimulus output 100%) from 300 ms induced reappearance of response or marked enhancement of amplitude, allowing calculation of central conduction time. Such a technique, which is easy to perform, may be useful in clinical practice to calculate central motor conduction time, where it would otherwise be difficult or impossible. © 1995 John Wiley & Sons, Inc.     

Corticospinal conduction studied with magnetic double stimulation in the intact human.

A group of 8 healthy normal subjects (24-36 years old, mean age 29 years) were investigated. Transcranial magnetic double stimulation of the motor cortex was carried out at different interstimulus intervals. With both stimuli suprathreshold, an attenuation of the test response was found at interstimulus intervals of less than 200 msec (target relaxed or contracted). The manifestation of this attenuation correlated with central signs in 31 patients with multiple sclerosis. This phenomenon is (at least at longer intervals) probably not a result of the refractory spinal motoneuron pool, but of a supraspinal inhibitory mechanism or lack of corticospinal drive caused otherwise. At interstimulus intervals between 10 and 30 msec, the test response increases significantly (magnetic double stimulation 10% suprathreshold, target relaxed). This result is also seen with voluntary muscle contraction and with vibration applied to a relaxed target muscle. The facilitatory effect is probably caused by slowly conducted corticospinal volleys enabling summation, with descending impulses generated by the test stimulus. With the conditioning stimulus subthreshold and target muscle relaxed an intracortical inhibition of the test response could be confirmed at short interstimulus intervals.     

Examination of the descending pathway to the external anal sphincter and pelvic floor muscles by transcranial cortical stimulation.

In 26 neurologically normal patients and 9 healthy volunteers EMG responses after transcranial cortical stimulation (TCCS) were recorded from the external anal sphincter (EAS), the anterior tibial muscle (TA), the bulbocavernosus muscle (BC) and the rectus abdominis muscle (RA). Electrical TCCS was used in 29 subjects and magnetic TCCS in 6 subjects. Response patterns in the different muscles in relation to the strength of the stimulus were analyzed. It was found that the response patterns related to the strength of stimulation differed totally between the TA and the EAS. When the stimulus strength was increased stepwise, a response with a latency of 31.9 +/- 2.5 msec was first recorded in the TA, followed at higher strength by a secondary response with a latency of approximately 100 msec. In contrast, a response with a latency of 105.5 +/- 23.9 msec was first recorded in the EAS. The latency of this response gradually shortened with increasing stimulus strength until a response with a constant latency of 36.1 +/- 6.1 was obtained. In some subjects the response pattern in the BC was similar to that in the TA, and in others it was similar to that in the EAS. Responses in the TA, RA and EAS were all facilitated during voluntary contraction of the EAS. Both responses in the TA and in the EAS were facilitated by voluntary contraction of the TA. During voluntary contraction of the TA an inhibitory period was always recorded, while in the EAS no inhibitory periods were observed during either contraction or relaxation. The hypothesis that the fastest cortico-motoneuronal pathway to the EAS is polysynaptic is proposed.     

The soleus late response elicited by transcranial magnetic stimulation reflects agonist-antagonist postural adjustment in the lower limbs.

OBJECTIVES: We studied the origin and underlying mechanism of the soleus late response (SLR) at a mean latency of 90 ms following transcranial magnetic stimulation. METHODS: The soleus primary response (SPR) and SLR were recorded from the soleus (SOL) muscle in 27 normal subjects under various conditions using a double-cone coil. We also tested 28 patients demonstrating neurological disorders with postural disturbance. RESULTS: The amplitude of the SPR gradually increased and its latency gradually decreased against the voluntary contraction (0-80%) of the tibialis anterior (TA) muscle. In contrast, the SLR amplitude was the greatest at a 20% TA contraction while the SLR latency was the shortest at a 40% TA contraction. The preactivation of SOL enhanced the SPR response but did not evoke the SLR. The SPR amplitude was significantly augmented while standing, however, the SLR amplitude tended to decrease. The SLR was never obtained following the stimulation of the brainstem, lumbar roots and peroneal nerve. The SLR was abnormal in patients with cerebellar ataxia and Parkinson's disease while the SPR was normal. CONCLUSIONS: A lack of any correlation between the SPR and SLR suggests that the SLR does not originate in the corticospinal tract. The SLR may thus be a polysynaptic response related to the postural control of the agonist and antagonist organization between the TA and SOL.     

Interaction of nociceptive and non-nociceptive cutaneous afferents from foot sole in common reflex pathways to tibialis anterior motoneurones in humans

In six healthy subjects, the reflex responses of the tibialis anterior muscle (TA) to stimulation of the cutaneous afferents arising from plantar foot, were studied at rest and during different levels of steady voluntary contraction of the TA. At rest, the threshold of the response and the threshold of subjective pain sensation coincided. The mean latency of this TA nociceptive response was 84.7 ms. Steady voluntary contractions of the TA, which was increased progressively from 3% to 15% of the maximum voluntary contraction, produced a significant and parallel reduction in the threshold and latency of the response: at 15%, the mean latency was about 26 ms shorter than at rest and its threshold was about half (i.e. below the pain threshold). The conduction velocity of the afferents responsible for TA response at rest was within the range of A-δ pain afferents (mean 27.4 m/s), whereas during voluntary contraction it was within the A-β fibre range (mean 45.1 m/s). This suggests that descending command makes the discharge of low-threshold, fast-conducting fibres sufficient for reflex activation of TA motoneurones (MNs). Central delay (about 4 ms) and MN recruitment order (according to the size principle) were found to be the same for both nociceptive and non-nociceptive TA reflex responses. Finally, experiments of spatial summation revealed an interaction between nociceptive and non-nociceptive inputs at a premotoneuronal level. It is therefore proposed that nociceptive and non-nociceptive cutaneous afferents arising from the foot sole use the same short-latency spinal pathway to contact TA MNs and that their relative contribution to its segmental activation is contingent upon descending command.     

Effects of voluntary activity on the excitability of motor axons in the peroneal nerve

To investigate whether there are inter-nerve differences in the extent and pattern of axonal excitability changes produced by voluntary contractions of tibialis anterior (TA) and abductor pollicis brevis (APB), threshold tracking was used to measure axonal excitability parameters [threshold, supernormality and strength-duration time constant (tauSD)] of peroneal and median motor axons in 11 healthy subjects. Maximal contractions for 1 min resulted in an increase in threshold, an increase in supernormality, a decrease in tauSD and an increase in latency, all of which indicate axonal hyperpolarization. The increase in threshold was less in peroneal axons (18 +/- 4%) than median axons (37 +/- 6%, mean +/- SEM, P < 0.001), and was accompanied by smaller absolute changes in latency, supernormality, and tauSD. Peroneal axons had less supernormality at rest but a greater change in supernormality for the change in threshold. There were major contraction-induced changes in the compound muscle action potential of TA but not that of APB. Voluntary contractions depress axonal excitability, but the changes are quantitatively different for motor axons innervating different muscles. There are three clinical implications. First, weakness and fatigue due to activity-dependent conduction block may vary for different muscles, independent of disease severity, and therapeutic strategies to overcome activity-dependent conduction block may not be equally effective for different muscles. Second, in motor control studies using the H reflex to document motoneuron excitability, a constant stimulus will not produce a constant neural volley if the stimulated axons have been activated by, for example, a voluntary contraction. Third, TA is probably not optimal for testing for activity-dependent conduction block.     

Motor cortical excitability studied with repetitive transcranial magnetic stimulation in patients with Huntington's disease.

OBJECTIVE: TMS techniques have provided controversial information on motor cortical function in Huntington's disease (HD). We investigated the excitability of motor cortex in patients with HD using repetitive transcranial magnetic stimulation (rTMS). METHODS: Eleven patients with HD, and 11 age-matched healthy subjects participated in the study. The clinical features of patients with HD were evaluated with the United Huntington's Disease Rating Scale (UHDRS). rTMS was delivered with a Magstim Repetitive Magnetic Stimulator through a figure-of-8 coil placed over the motor area of the first dorsal interosseus (FDI) muscle. Trains of 10 stimuli were delivered at 5 Hz frequency and suprathreshold intensity (120% resting motor threshold) with the subjects at rest and during voluntary contraction of the target muscle. RESULTS: In healthy subjects at rest, rTMS produced motor evoked potentials (MEPs) that increased in amplitude over the course of the trains. Conversely in patients, rTMS left the MEP size almost unchanged. In both groups, during voluntary contraction rTMS increased the silent period (SP) duration. CONCLUSIONS: Because rTMS modulates motor cortical excitability by activating cortical excitatory and inhibitory interneurons these findings suggest that in patients with HD the excitability of facilitatory intracortical interneurones is decreased. SIGNIFICANCE: We suggest that depressed excitability of the motor cortex in patients with HD reflects a disease-related weakening of cortical facilitatory mechanisms.     

Differences in recruitment properties of the corticospinal pathway between lengthening and shortening contractions in human soleus muscle

The purpose of this study was to investigate how the recruitment properties of the corticospinal pathway are modulated in the soleus muscle of the lower limb during lengthening (LEN) and shortening (SHO) contractions by comparing the shape of the input-output (I/O) relation of the corticospinal pathway. To this end, we investigated the relationship between various stimulus intensities applied via transcranial magnetic stimulation and the size of motor-evoked potentials in 14 healthy subjects during voluntary plantarflexion and dorsiflexion (active lengthening) with a similar background activity (BGA) level. The shape of this relationship was sigmoidal and was characterized by a plateau value, maximum slope and threshold. The plateau value was clearly lower during LEN contractions than during SHO contractions. Likewise, the maximum slope was lower during LEN contractions. However, the threshold did not differ significantly between the two tasks. Since the plateau value and the maximum slope clearly differed between LEN and SHO contractions despite the similarity of their BGA levels, the central nervous system appears to have a different activation strategy for each of these tasks. Namely, the relative balance between excitatory and inhibitory components of the corticospinal volleys, as well as the subliminal fringe of the corticospinal pathway, were reduced during LEN contractions compared with SHO contractions. These strategies may help to avoid reflexive contractions brought about by higher discharge of muscle spindles and enable fine motor actions in voluntary lengthening contractions.     

Voluntary eyelid contraction modifies the blink reflex recovery cycle

In order to determine the extent to which the recovery cycle of the blink reflex is modified by voluntary contraction of the eyelids we investigated the electrically elicited blink reflex with paired stimuli of identical intensity in 9 healthy subjects. We pseudorandomly administered two interstimulus intervals (150 ms and 250 ms) in three different conditions of voluntary contraction of the orbicularis oculi muscle (relaxed, mild lid closure and strong lid closure). Our results show that inhibition of the R2 response following the second stimulus is significantly reduced with voluntary contraction. In addition, we found significantly larger amplitudes and shorter latencies of R1 and R2 with voluntary contraction. We conclude that in healthy subjects, voluntary eyelid contraction causes facilitation or disinhibition not only in the nuclei of the seventh cranial nerve, but also in the polysynaptic pathway of the R2. The possible implications on the interpretation of clinical data are discussed.     

Repetitive magnetic stimulation of cortical motor areas in Parkinson's disease: implications for the pathophysiology of cortical function.

We investigated the neurophysiological and clinical effects of repetitive magnetic stimulation (rTMS) delivered to the cortical motor areas in healthy subjects and patients with Parkinson's disease. rTMS was delivered with a high speed magnetic stimulator (Cadwell, Kennewick, WA) through a figure-eight coil centred on the primary motor area at a stimulus intensity of 120% motor threshold. Trains of 10 stimuli were delivered at frequencies of 5 Hz while subjects were at rest and during a voluntary contraction of the contralateral first dorsal interosseous muscle. In normal subjects at rest, the muscle evoked responses (MEPs) to each stimulus in a train of magnetic stimuli progressively increased in size during the train. rTMS left the MEPs unchanged in patients off therapy and had a small facilitatory effect in those on therapy. In normal subjects and patients, 5-Hz rTMS trains delivered during a voluntary contraction of the target muscle left the MEP unchanged in size. MEPs were followed by a silent period that increased in duration during the course of the train. The silent period duration increased to a similar extent in patients and controls. The reduced rTMS-induced facilitation of MEPs in patients with Parkinson's disease reflects a decreased facilitation of the excitatory cells in the cortical motor areas.     

The soleus late response elicited by transcranial magnetic stimulation reflects agonist–antagonist postural adjustment in the lower limbs

Objectives: We studied the origin and underlying mechanism of the soleus late response (SLR) at a mean latency of 90 ms following transcranial magnetic stimulation.Methods: The soleus primary response (SPR) and SLR were recorded from the soleus (SOL) muscle in 27 normal subjects under various conditions using a double-cone coil. We also tested 28 patients demonstrating neurological disorders with postural disturbance.Results: The amplitude of the SPR gradually increased and its latency gradually decreased against the voluntary contraction (0–80%) of the tibialis anterior (TA) muscle. In contrast, the SLR amplitude was the greatest at a 20% TA contraction while the SLR latency was the shortest at a 40% TA contraction. The preactivation of SOL enhanced the SPR response but did not evoke the SLR. The SPR amplitude was significantly augmented while standing, however, the SLR amplitude tended to decrease. The SLR was never obtained following the stimulation of the brainstem, lumbar roots and peroneal nerve. The SLR was abnormal in patients with cerebellar ataxia and Parkinson's disease while the SPR was normal.Conclusions: A lack of any correlation between the SPR and SLR suggests that the SLR does not originate in the corticospinal tract. The SLR may thus be a polysynaptic response related to the postural control of the agonist and antagonist organization between the TA and SOL.     

Modulation of single motor unit discharges using magnetic stimulation of the motor cortex in incomplete spinal cord injury

OBJECTIVES: Motor evoked potentials (MEPs) and inhibition of voluntary contraction to transcranial magnetic stimulation (TMS) of the motor cortex have longer latencies than normal in patients with incomplete spinal cord injury (iSCI) when assessed using surface EMG. This study now examines the modulation of single motor unit discharges to TMS with the aim of improving resolution of the excitatory and inhibitory responses seen previously in surface EMG recordings. METHODS: A group of five patients with iSCI (motor level C4-C7) was compared with a group of five healthy control subjects. Single motor unit discharges were recorded with concentric needle electrodes from the first dorsal interosseus muscle during weak voluntary contraction (2%-5% maximum). TMS was applied with a 9 cm circular stimulating coil centred over the vertex. Modulation of single motor unit discharges was assessed using peristimulus time histograms (PSTHs). RESULTS: Mean (SEM) threshold (expressed as percentage of maximum stimulator output (%MSO)) for the excitatory peak (excitation) or inhibitory trough (inhibition) in the PSTHs was higher (p<0.05) in the patients (excitation = 47.1 (5.9) %MSO; inhibition = 44.3 (3.2) %MSO) than in controls (excitation=31.6 (1.2) %MSO; inhibition = 27.4 (1.0) %MSO). Mean latencies of excitation and inhibition were longer (p<0.05) in the patients (excitation=35 (1.8) ms; inhibition = 47.1 (1.8) ms) than in the controls (excitation = 21.1 (1.6) ms; inhibition = 27 (0.4) ms). Furthermore, the latency difference (inhibition-excitation) was longer (p<0.05) in the patients (10.4 (2.1) ms) than in the controls (6.2 (0.6) ms). CONCLUSION: Increased thresholds and latencies of excitation and inhibition may reflect degraded corticospinal transmission in the spinal cord. However, the relatively greater increase in the latency of inhibition compared with excitation in the patients with iSCI may reflect a weak or absent early component of cortical inhibition. Such a change in cortical inhibition may relate to the restoration of useful motor function after iSCI.     

Change of facilitation during voluntary bilateral hand activation after stroke

BACKGROUND AND PURPOSE: The relearning of daily activities after stroke also involves performance of bimanual tasks. This raises the possibility that concurrent activation of the healthy hemisphere interferes with reorganization processes in the affected hemisphere due to inhibitory pathways between homologous motor cortex representations. This study investigated the effect of voluntary, simultaneous activation of both hands upon the non-dominant (healthy subjects) or affected (stroke patients) hemisphere. METHODS: Eleven healthy subjects and 16 stroke patients were investigated using transcranial stimulation (TMS). TMS was applied over the non-dominant/affected hemisphere during performance of an isometric pinch grip at different force levels (10% or 50% of maximal voluntary contraction) with the contralateral hand. The ipsilateral hand had to perform the pinch grip at various force levels (10%, 50%, or 100% of maximal voluntary contraction) simultaneously. Peak-to-peak amplitudes of motor evoked potentials (MEPs) were compared to assess differences in motor cortex excitability. RESULTS: Unilateral activity of either hand alone exerted a facilitatory effect upon the non-dominant or affected hemisphere. In healthy subjects, the activation of the ipsilateral hand simultaneously with the contralateral hand did not produce any significant change of the MEP amplitudes compared to unilateral activation of the contralateral hand. In patients, however, the additional activation of the ipsilateral hand caused an additional increase of the peak-to-peak amplitudes. CONCLUSION: In healthy subjects voluntary activation of the ipsilateral hand does not change the excitability of the motor cortex of the non-dominant hemisphere, when the contralateral hand is simultaneously activated. The facilitation of the contralateral hand seems to gate further facilitation by the ipsilateral hand. However, in stroke patients simultaneous activation of both hands causes an additional facilitation compared to activation of the affected hand alone.