Central fatigue and motor cortical excitability during repeated shortening and lengthening actions

A decline in voluntary muscle activation and adaptations in motor cortical excitability contribute to the progressive decline in voluntary force during sustained isometric contractions. However, the neuronal control of muscle activation differs between isometric and dynamic contractions. This study was designed to investigate voluntary activation, motor cortex excitability, and intracortical inhibition during fatiguing concentric and eccentric actions. Eight subjects performed 143 torque motor-controlled, repeated shortening and lengthening actions of the elbow flexor muscles. Transcranial magnetic stimulation (TMS) was applied three times every 20 cycles. Magnetic evoked motor potentials (MEP), duration of the silent period (SP), and the torque increase due to TMS were analyzed. TMS resulted in a small torque increase in unfatigued actions. With repeated actions, voluntary torque dropped rapidly and the amplitude of the TMS-induced twitches increased, especially during repeated lengthening actions. MEP area of biceps brachii and brachioradialis muscles increased during repeated actions to a similar extent during lengthening and shortening fatigue. The duration of biceps and brachioradialis SP did not change with fatigue. Thus, voluntary activation became suboptimal during fatiguing dynamic actions and motor cortex excitability increased without any changes in intracortical inhibition. The apparent dissociation of voluntary activation and motor cortex excitability suggests that the central fatigue observed, especially during lengthening actions, did not result from changes in motor cortex excitability.     

Chronic trauma-induced neck pain impairs the neural control of the deep semispinalis cervicis muscle

ObjectiveThe deep cervical extensors show structural changes in patients with neck pain however their activation has never been investigated in patients. This study is the first to present neurophysiological data from the deep semispinalis cervicis muscle in patients.MethodsTen women with chronic neck pain and 10 healthy controls participated. Activity of the semispinalis cervicis was measured as subjects performed isometric contractions at 15 and 30 N force with continuous change in force direction in the range 0–360°. Tuning curves of the EMG amplitude (average rectified value, ARV) were computed and the mean point of the ARV curves defined a directional vector, which determined the directional specificity of the muscle activity.ResultsPatients displayed reduced directional specificity of the semispinalis cervicis (P < 0.05). Furthermore, the EMG amplitude during the circular contraction was lower for the patients (86.3 ± 38.0 and 104.4 ± 47.0 μV for 15 and 30 N, respectively) compared to controls (226.4 ± 128.5 and 315.8 ± 205.5 μV; P < 0.05).ConclusionsThe activity of the semispinalis cervicis muscle is reduced and less defined in patients with neck pain confirming a disturbance in the neural control of this muscle.SignificanceThis finding suggests that exercises that target the deep semispinalis cervicis muscle may be relevant to include in the management of patients with neck pain.     

Automated-parameterization of the motor evoked potential and cortical silent period induced by transcranial magnetic stimulation

ObjectiveTo standardize the characterization of motor evoked potential (MEP) and cortical silent period (CSP) recordings elicited with transcranial magnetic stimulation (TMS).MethodsA computer-based, automated-parameterization program (APP) was developed and tested which provides a comprehensive set of electromyography (EMG) magnitude and temporal measures. The APP was tested using MEP, CSP, and isolated CSP (iCSP) TMS stimulus–response data from a healthy adult population (N = 13).ResultsThe APP had the highest internal reliability (Cronbach’s alpha = .98) for CSP offset time compared with two prominent automated methods. The immediate post-CSP EMG recovery level was 49% higher than the pre-TMS EMG level. MEP size (peak amplitude, mean amplitude, peak-to-peak amplitude, and area) correlated higher with effective E-field (E_eff) than other intensity measures (r ≈ 0.5 vs. r ≈ 0.3) suggesting that E_eff is better suited for standardizing MEP stimulus–response relationships.ConclusionsThe APP successfully characterized individual and mean epochs containing MEP, CSP, and iCSP responses. The APP provided common signal and temporal measures consistent with previous studies and novel additional parameters.SignificanceWith the use of the APP modeling method and the E_eff, a standard approach for the analysis and reporting of MEP–CSP complex and iCSP measurements is achievable.     

Lateralization of forelimb motor evoked potentials by transcranial magnetic stimulation in rats

ObjectivesTo approximate methods for human transcranial magnetic stimulation (TMS) in rats, we tested whether lateralized cortical stimulation resulting in selective activation of one forelimb contralateral to the site of stimulation could be achieved by TMS in the rat.MethodsMotor evoked potentials (MEP) were recorded from the brachioradialis muscle bilaterally in adult male anesthetized rats (n = 13). A figure-of-eight TMS coil was positioned lateral to midline. TMS intensity was increased stepwise from subthreshold intensities to maximal machine output in order to generate input–output curves and to determine the motor threshold (MT) for brachioradialis activation.ResultsIn 100% of the animals, selective activation of the contralateral brachioradialis, in the absence of ipsilateral brachioradialis activation was achieved, and the ipsilateral brachioradialis was activated only at TMS intensities exceeding contralateral forelimb MT. With increasing TMS intensity, the amplitudes of both the ipsilateral and contralateral signals increased in proportion to TMS strength. However, the input–output curves for the contralateral and ipsilateral brachioradialis were significantly different (p < 0.001) such that amplitude of the ipsilateral MEP was reliably lower than the contralateral signal.ConclusionsWe demonstrate that lateralized TMS leading to asymmetric brachioradialis activation is feasible with conventional TMS equipment in anesthetized rats.SignificanceThese data show that TMS can be used to assess the unilateral excitability of the forelimb descending motor pathway in the rat, and suggest that rat TMS protocols analogous to human TMS may be applied in future translational research.     

Fatigue modulates synchronous but not asynchronous soleus activation during stimulation of paralyzed muscle

ObjectiveElectrical stimulation over a motor nerve yields muscle force via a combination of direct and reflex-mediated activation. We determined the influence of fatigue on reflex-mediated responses induced during supra-maximal electrical stimulation in humans with complete paralysis.MethodsWe analyzed soleus electromyographic (EMG) activity during repetitive stimulation (15 Hz, 125 contractions) in 22 individuals with complete paralysis. The bout of stimulation caused significant soleus muscle fatigue (53.1% torque decline).ResultsBefore fatigue, EMG at all latencies after the M-wave was less than 1% of the maximal M-wave amplitude (% MaxM). After fatigue there was a fourfold (p < 0.05) increase in EMG at the H-reflex latency; however, the overall magnitude remained low (<2% change in % MaxM). There was no increase in “asynchronous” EMG ～ 1 s after the stimulus train.ConclusionsFatigue enhanced the activation to the paralyzed soleus muscle, but primarily at the H-reflex latency. The overall influence of this reflex modulation was small. Soleus EMG was not elevated during fatigue at latencies consistent with asynchronous activation.SignificanceThese findings support synchronous reflex responses increase while random asynchronous reflex activation does not change during repetitive supra-maximal stimulation, offering a clinical strategy to consistently dose stress to paralyzed tissues.     

Measurement of voluntary activation of the back muscles using transcranial magnetic stimulation

ObjectiveTwitch interpolation using transcranial magnetic stimulation (TMS) has recently been used to measure the level of drive from the motor cortex to contracting muscles of the upper and lower limbs, termed voluntary activation. It has yet to be used to assess voluntary activation in trunk muscles. The aim of this study was to assess the feasibility of using TMS to measure voluntary activation in back muscles.MethodsSixteen healthy subjects performed a series of brief maximal and submaximal isometric contractions of the back extensors during which TMS was delivered to the motor cortex. The evoked (superimposed) twitch was measured using dynamometry and simultaneous surface electromyographic (EMG) recordings were taken from the left and right erector spinae at vertebral level T12. Voluntary activation was derived using the expression: (1-superimposed twitch amplitude/resting twitch amplitude) × 100. The resting twitch amplitude was estimated by extrapolation of the linear correlation between voluntary torque and superimposed twitch amplitude to zero torque.ResultsThe relationship between superimposed twitch size and voluntary contraction strength for contraction strengths of 50–100% MVC was linear but regression revealed variability between subjects. When data were included from those subjects with a good linear regression fit a strong linear relationship was found for the group means between voluntary contraction strength and voluntary activation (r² = 1) and superimposed twitch size (r² = 0.99) for contraction strengths of 50–100% MVC. Voluntary activation was found to be less than maximal (67.71 ± 5.22%) during maximal efforts. Time-to-peak amplitude decreased linearly with increasing voluntary torque. The amplitudes of the motor evoked potentials (MEPs) increased with increasing voluntary torque.ConclusionsTwitch interpolation using TMS can be used to quantify voluntary activation in back extensors. The results of this study reveal that neural drive to the back extensors during strong contractions is submaximal.SignificanceThe assessment of voluntary activation of the back muscles may aid our understanding of the mechanisms of alteration in control of these muscles implicated in chronic low back pain.     

Coactivation of ankle muscles during stance phase of gait in patients with lower limb hypertonia after acquired brain injury

ObjectiveExamine (1) coactivation between tibialis anterior (TA) and medial gastrocnemius (MG) muscles during stance phase of gait in patients with moderate-to-severe resting hypertonia after stroke or traumatic brain injury (TBI) and (2) the relationship between coactivation and stretch velocity-dependent increase in MG activity.MethodsGait and surface EMG were recorded from patients with stroke or TBI (11 each) and corresponding healthy controls (n = 11) to determine the magnitude and duration of TA–MG coactivation. The frequency and gain of positive (>0) and significant positive (p < 0.05) EMG–lengthening velocity (EMG–LV) slope in MG were related to coactivation parameters.ResultsThe magnitude of coactivation was increased on the more-affected (MA) side, whereas the duration was prolonged on the less-affected (LA) side of both stroke and TBI patients. The difference reached significance during the initial and late double support. The magnitude of coactivation positively correlated with the gain of significant positive EMG–LV slope in TBI patients.ConclusionsIncreased coactivation between TA and MG during initial and late double support is a unique feature of gait in stroke and TBI patients with muscle hypertonia.SignificanceIncreased coactivation may represent an adaptation to compensate for impaired stability during step transition after stroke and TBI.     

Effect of pain on the modulation in discharge rate of sternocleidomastoid motor units with force direction

ObjectiveTo compare the behavior of sternocleidomastoid motor units of patients with chronic neck pain and healthy controls.MethodsNine women (age, 40.4 ± 3.5 yr) with chronic neck pain and nine age- and gender-matched healthy controls participated. Surface and intramuscular EMG were recorded from the sternocleidomastoid muscle bilaterally as subjects performed isometric contractions of 10-s duration in the horizontal plane at a force of 15 N in eight directions (0–360°; 45° intervals) and isometric contractions at 15 and 30 N force with continuous change in force direction in the range 0–360°. Motor unit behavior was monitored during the 10-s contractions and the subsequent resting periods.ResultsThe mean motor unit discharge rate depended on the direction of force in the control subjects (P < 0.05) but not in the patients. Moreover, in three of the nine patients, but in none of the controls, single motor unit activity continued for 8.1 ± 6.1 s upon completion of the contraction. The surface EMG amplitude during the circular contraction at 15 N was greater for the patients (43.5 ± 54.2 μV) compared to controls (16.9 ± 14.9 μV; P < 0.05).ConclusionsThe modulation in discharge rate of individual motor units with force direction is reduced in the sternocleidomastoid muscle in patients with neck pain, with some patients showing prolonged motor unit activity when they were instructed to rest.SignificanceThese observations suggest that chronic neck pain affects the change in neural drive to muscles with force direction.     

Power spectral analysis of surface electromyography (EMG) at matched contraction levels of the first dorsal interosseous muscle in stroke survivors

ObjectiveThe objective of this study was to help assess complex neural and muscular changes induced by stroke using power spectral analysis of surface electromyogram (EMG) signals.MethodsFourteen stroke subjects participated in the study. They were instructed to perform isometric voluntary contractions by abducting the index finger. Surface EMG signals were collected from the paretic and contralateral first dorsal interosseous (FDI) muscles with forces ranging from 30% to 70% maximum voluntary contraction (MVC) of the paretic muscle. Power spectral analysis was performed to characterize features of the surface EMG in paretic and contralateral muscles at matched forces. A Linear Mixed Model was applied to identify the spectral changes in the hemiparetic muscle and to examine the relation between spectral parameters and contraction levels. Regression analysis was performed to examine the correlations between spectral characteristics and clinical features.ResultsDifferences in power spectrum distribution patterns were observed in paretic muscles when compared with their contralateral pairs. Nine subjects showed increased mean power frequency (MPF) in the contralateral side (>15 Hz). No evident spectrum difference was observed in 3 subjects. Only 2 subjects had higher MPF in the paretic muscle than the contralateral muscle. Pooling all subjects’ data, there was a significant reduction of MPF in the paretic muscle compared with the contralateral muscle (paretic: 168.7 ± 7.6 Hz, contralateral: 186.1 ± 8.7 Hz, mean ± standard error, F = 36.56, p < 0.001). Examination of force factor on the surface EMG power spectrum did not confirm a significant correlation between the MPF and contraction force in either hand (F = 0.7, p > 0.5). There was no correlation between spectrum difference and Fugl–Meyer or Chedoke scores, or ratio of paretic and contralateral MVC (p > 0.2).ConclusionsThere appears to be complex muscular and neural processes at work post stroke that may impact the surface EMG power spectrum. The majority of the tested stroke subjects had lower MPF in the paretic muscle than in the contralateral muscle at matched isometric contraction force. The reduced MPF of paretic muscles can be attributed to different factors such as increased motor unit synchronization, impairments in motor unit control properties, loss of large motor units, and atrophy of muscle fibers.SignificanceSurface EMG power spectral analysis can serve as a useful tool to indicate complex neural and muscular changes after stroke.     

Zonisamide decreases cortical excitability in patients with idiopathic generalized epilepsy.

ObjectiveTo evaluate changes in cortical excitability after long-term zonisamide (ZNS) administration.MethodsFifteen drug-naïve idiopathic generalized epilepsy (IGE) patients (8 male, mean age 24.9 years) were enrolled. The transcranial magnetic stimulation (TMS) parameters obtained using two Magstim 200 stimulators were resting motor threshold (RMT), motor evoked potential (MEP) amplitudes, cortical silent period (CSP), intracortical inhibition (ICI), and intracortical facilitation (ICF). TMS parameters were compared before and after ZNS administration.ResultsAll patients were administered ZNS monotherapy (200 mg/day) for 8 weeks. No patient reported seizures during the study period. After ZNS treatment MEP amplitudes were significantly reduced in right (−34.2%) and left hemispheres (−37.0%) (Wilcoxon’s signed rank test after Bonferroni’s correction for multiple comparisons, P < 0.05). Mean RMT, CSP, and ICI/ICF were not changed by ZNS (P > 0.05).ConclusionsThese findings suggest that ZNS decreases cortical excitability in patients with IGE and a MEP amplitude is a useful TMS parameter for evaluating changes in cortical excitability induced by ZNS.SignificanceThe findings in this study are helpful to understand how ZNS affects the excitability of the motor cortex in patients with IGE.     

Differential distribution of coherence between beta-band subthalamic oscillations and forearm muscles in Parkinson’s disease during isometric contraction

ObjectiveUnder rest condition, beta-band (13–30 Hz) activity in patients with Parkinson’s disease (PD) is prominent in the subthalamic nucleus (STN). However, the beta-band coupling between STN and muscle activity, its distribution and relation to motor symptoms remains unclear.MethodsUsing up to five electrodes, we recorded local field potentials (LFPs) above (zona incerta, ZI) and within the STN at different recording heights in 20 PD patients during isometric contraction. Simultaneously, we registered activity of the contralateral flexor and extensor muscle. We analysed LFP–EMG coherence to estimate coupling in the frequency domain.ResultsCoherence analysis showed beta–associated coupling in the ZI and STN with more significant LFP–EMG coherences in the STN. Coherence varied depending on the localisation of the LFP and muscles. We found significant difference between coherence of the extensor and the flexor muscle to the same LFP (p = 0.045).ConclusionsWe demonstrated that coherence between beta-band oscillations and forearm muscles are differentially distributed in the subthalamic region and between the forearm muscles in Parkinson’s disease during isometric contraction. However, the significant LFP–EMG coupling did not associate with motor deficits in PD patients.SignificanceThe differential distribution of beta-band activity in the STN highlights the importance of a topographically distinct therapeutic modulation.     

The relation between spasticity and muscle behavior during the swing phase of gait in children with cerebral palsy

There is much debate about how spasticity contributes to the movement abnormalities seen in children with spastic cerebral palsy (CP). This study explored the relation between stretch reflex characteristics in passive muscles and markers of spasticity during gait. Twenty-four children with CP underwent 3D gait analysis at three walking velocity conditions (self-selected, faster and fastest). The gastrocnemius (GAS) and medial hamstrings (MEHs) were assessed at rest using an instrumented spasticity assessment that determined the stretch-reflex threshold, expressed in terms of muscle lengthening velocity. Muscle activation was quantified with root mean square electromyography (RMS-EMG) during passive muscle stretch and during the muscle lengthening periods in the swing phase of gait. Parameters from passive stretch were compared to those from gait analysis.In about half the children, GAS peak muscle lengthening velocity during the swing phase of gait did not exceed its stretch reflex threshold. In contrast, in the MEHs the threshold was always exceeded. In the GAS, stretch reflex thresholds were positively correlated to peak muscle lengthening velocity during the swing phase of gait at the faster (r = 0.46) and fastest (r = 0.54) walking conditions. In the MEHs, a similar relation was found, but only at the faster walking condition (r = 0.43). RMS-EMG during passive stretch showed moderate correlations to RMS-EMG during the swing phase of gait in the GAS (r = 0.46–0.56) and good correlations in the MEHs (r = 0.69–0.77) at all walking conditions. RMS-EMG during passive stretch showed no correlations to peak muscle lengthening velocity during gait.We conclude that a reduced stretch reflex threshold in the GAS and MEHs constrains peak muscle lengthening velocity during gait in children with CP. With increasing walking velocity, this constraint is more marked in the GAS, but not in the MEHs. Hyper-activation of stretch reflexes during passive stretch is related to muscle activation during the swing phase of gait, but has a limited contribution to reduced muscle lengthening velocity during swing. Larger studies are required to confirm these results, and to investigate the contribution of other impairments such as passive stiffness and weakness to reduced muscle lengthening velocity during the swing phase of gait.     

Cortical and segmental excitability during fatiguing contractions of the soleus muscle in humans

ObjectiveThe aim of this study was to examine the cortical and segmental excitability changes during fatigue of the soleus muscle.MethodsTen healthy young subjects performed 45 plantar flexion maximal voluntary contractions (MVCs) (7-s on/3-s off) in 9 epochs of five contractions. Motor evoked potentials (MEPs) using transcranial magnetic stimulation and H-reflexes were assessed during the task.ResultsThe torque and the soleus EMG activity both showed the greatest decline during the 1st epoch, followed by a gradual, but significant decrease by the end of the task (～70% pre-fatigue). The H-reflex sampled at rest after each epoch decreased to 66.6 ± 18.3% pre-fatigue after the first epoch, and then showed no further change. The MEP on 10% pre-fatigue MVC after each epoch increased progressively (252.9 ± 124.2% pre-fatigue). There was no change in the MEPs on the 3rd MVC in each epoch. The silent period on the MVC increased (109.0 ± 9.2% pre-fatigue) early with no further changes during the task.ConclusionsThese findings support that the motor cortex increases excitability during fatigue, but with a concomitant inhibition.SignificanceThese findings are in contrast to upper extremity muscles and may reflect a distinct response specific to postural, fatigue-resistant muscle.     

The pain-induced change in relative activation of upper trapezius muscle regions is independent of the site of noxious stimulation

ObjectiveTo assess the effect of local excitation of nociceptors at different locations of the upper trapezius muscle on the spatial distribution of upper trapezius electromyographic (EMG) amplitude during sustained contraction.MethodsSurface (EMG) signals were recorded from the upper trapezius muscle with a grid of 10 × 5 electrodes from nine healthy men during 90° shoulder abduction sustained for 60 s. In one experimental session, the subjects received separate injections of 0.4 ml of hypertonic saline (experimental muscle pain) into the cranial and caudal region of the upper trapezius. In a separate experimental session the same subjects received two injections of 0.2 ml each of hypertonic saline simultaneously in the cranial and caudal region. The EMG root mean square (RMS) values were computed for each electrode location to provide a topographical map of EMG amplitude.ResultsThe RMS value averaged across all electrode locations decreased following injection of hypertonic saline (P < 0.05) by a similar amount for the two experimental sessions. The pain-induced decrease was larger in the cranial than in the caudal region for both experimental sessions, as evidenced by a shift of the EMG amplitude distribution towards the caudal region of the muscle (P < 0.0001).ConclusionMuscle pain induces a consistent change in the spatial activation of the upper trapezius muscle which is independent of the site of noxious stimulation.SignificancePain-induced changes in the spatial distribution of muscle activity may induce overload of specific muscle regions in the long term.     

Blink reflexes in chronic tension-type headache patients and healthy controls

ObjectiveTo use the R2 component of nociceptive-specific blink reflex (nBR) to probe the trigeminal nociceptive system and compare chronic tension-type headache (CTTH) patients and healthy controls.MethodsThirty patients with CTTH and 30 age- and sex-matched healthy control subjects were included. nBR were evoked by painful electrical pulses (0.5 ms duration), delivered by a concentric electrode placed on the left lower forehead close to the supraorbital foramen. The EMG activity in the orbicularis oculi muscles was recorded bilaterally. The electrical stimulus intensities to evoke individual sensory threshold (Is) and pin-prick pain sensation (Ip) were assessed. A fixed stimulation intensity of 1.5× Ip was used to evoke the nBR. The perceived pain intensity of the electrical stimulus (I_BR) was assessed by the subjects on a 0–10 cm visual analogue scale (VAS).ResultsIs did not differ between CTTH patients and controls (P = 0.687) but were lower in females than in males (P = 0.020). CTTH patients had higher scores on I_BR than controls (P = 0.026). ANOVA showed significantly higher pre-stimulus EMG values in CTTH patients on the left (stimulated) side (P < 0.001), whereas there were no differences between males and females (P > 0.168). There were no significant differences in the absolute values of the nBR, however CTTH patients had significantly lower values of the normalized root mean square (RMS) (P = 0.035) and area under the curve (AUC) (P = 0.042) of the nBR on the left side compared with control subjects with no sex-related differences (P > 0.070). The onset latencies and duration were not significantly different between CTTH and control subjects or between female and male subjects (P > 0.270). There was no significant correlation between any of the BR parameters (RMS, AUC) and clinical characteristics of CTTH (headache intensity and duration) (P > 0.163).ConclusionsThe results of the present study did not detect a significantly different blink reflex response in CTTH patients, but suggested that painful electrical stimulation was associated with consistent increases in eye muscle activity on the same side.SignificanceThese findings add further information to central nociceptive pathways in CTTH patients.     

Differential effect of baclofen on cortical and spinal inhibitory circuits

ObjectiveThe cutaneous silent period (SP) is a spinal inhibitory reflex, which suppresses activity in spinal motor nuclei. Transcranial magnetic stimulation (TMS) elicits a cortical SP, which represents GABA_B receptor-mediated inhibition of cortical excitability. Baclofen as a strong GABA_B agonist effectively reduces muscle hypertonia, however, it is not known whether intrathecal baclofen (ITB) may modulate spinal inhibitory circuits.MethodsWe evaluated clinical and neurophysiological effects of ITB in ten patients with severe spasticity due to spinal cord injury (n = 9) and chronic progressive multiple sclerosis (n = 1). Neurophysiological assessment included H reflex and cutaneous and cortical SPs, before and 15, 30, 60, 90, 120, and 180 min after ITB bolus administration.ResultsITB suppressed soleus H reflex as early as 15 min after lumbar bolus injection; MAS scores declined after 1 h. Cortical SP end latency and duration increased progressively with a significant maximum 3 h following ITB bolus, whereas cutaneous SP latency and duration did not change significantly.ConclusionThe present findings suggest that baclofen does not affect the cutaneous SP, but prolongs the cortical SP.SignificanceThe spinal inhibitory circuitry of the cutaneous SP is not modulated by GABA_B receptor-mediated activity, in contrast to the cortical inhibitory circuitry of the cortical SP, which is subject to powerful GABA_B control.     

Stretch reflex responses in Complex Regional Pain Syndrome-related dystonia are not characterized by hyperreflexia

ObjectiveTo evaluate if hyperreflexia (exaggerated reflexes) due to disinhibition is associated with dystonia in Complex Regional Pain Syndrome (CRPS).MethodsStretch reflexes at the wrist were assessed in healthy controls (n = 10) and CRPS-patients with dystonia (n = 10). Subjects exerted a wrist flexion torque of 5% of maximum voluntary contraction torque (T_MVC) to a manipulandum which applied ramp-and-hold stretches to the wrist flexors. Since reflex responses scale with background contraction, controls additionally performed the task at 1% and 3% T_MVC to attain similar torques as patients who have reduced T_MVC.The M1 onset and the magnitudes of the short latency M1 and long latency M2 were assessed using the electromyographic signals (EMG) of the flexor carpi radialis. EMG of the extensor carpi radialis was recorded to monitor cocontraction.ResultsCompared to controls, patients had a substantially reduced T_MVC. Ramp velocity had a significant effect on M1 onset time and magnitude.ConclusionsSince M1 magnitude decreased with flexion torque, no significant difference was found between patients and controls at 5% T_MVC, while comparison at similar absolute torques (controls at 1% T_MVC) resulted in significantly smaller M1 magnitudes for patients with dystonia.SignificanceThis study suggests that CRPS-patients with dystonia are not hyperreflexive.     

Estimating motor unit discharge patterns from high-density surface electromyogram

ObjectiveWe systematically tested the capability of the Convolution Kernel Compensation (CKC) method to identify motor unit (MU) discharge patterns from the simulated and experimental surface electromyogram (sEMG) during low-force contractions.MethodssEMG was detected with a grid of 13 × 5 electrodes. In simulated signals with 20 dB signal-to-noise ratio, 11 ± 3 out of 63 concurrently active MUs were identified with sensitivity >95% in the estimation of their discharge times. In experimental signals recorded at 0–10% of the maximal force, the discharge patterns of (range) 11–19 MUs (abductor pollicis; n = 8 subjects), 9–17 MUs (biceps brachii; n = 2), 7–11 MUs (upper trapezius; n = 2), and 6–10 MUs (vastus lateralis; n = 2) were identified. In the abductor digiti minimi muscle of one subject, the decomposition results from concurrently recorded sEMG and intramuscular EMG (iEMG) were compared; the two approaches agreed on 98 ± 1% of MU discharges.ConclusionIt is possible to identify the discharge patterns of several MUs during low-force contractions from high-density sEMG.SignificancesEMG can be used for the analysis of individual MUs when the application of needles is not desirable or in combination with iEMG to increase the number of sampled MUs.     

Comparison between the degree of motor unit short-term synchronization and recurrence quantification analysis of the surface EMG in two human muscles

ObjectiveTo verify if non-linear recurrence analysis of the surface EMG is a suitable tool for assessing motor unit short-term synchronization.MethodsSurface and intramuscular EMG signals were recorded from the abductor digiti minimi and vastus medialis muscles of 12 and 10 healthy men, respectively, during isometric contractions. In the abductor digiti minimi, EMG signals were additionally recorded after a contraction sustained for 1 min at 50% of the maximal force. In both muscles, percent of determinism (%DET) was estimated from the surface EMG and common input strength (CIS) index was computed from motor unit recordings.ResultsFor both muscles, CIS did not correlate with %DET (abductor digiti minimi: R² = 0.11, P = 0.12; vastus medialis: R² = 0.04, P = 0.56). Although the values of CIS for the vastus medialis were lower than those of the abductor digiti minimi (P < 0.001), the %DET values did not differ between the two muscles (71.6 ± 5.5% vs 66.9 ± 8.7%; P = 0.12).ConclusionThe variable %DET extracted from the surface EMG is a poor indicator of the degree of motor unit short-term synchronization.SignificanceThe study provides a systematic evaluation of a technique previously proposed for the estimation of a clinically relevant characteristic of motor unit behavior.