scholarly journals Changes in motoneuron afterhyperpolarization duration in stroke survivors

2014 ◽  
Vol 112 (6) ◽  
pp. 1447-1456 ◽  
Author(s):  
Aneesha K. Suresh ◽  
Xiaogang Hu ◽  
Randall K. Powers ◽  
C. J. Heckman ◽  
Nina L. Suresh ◽  
...  
Keyword(s):  
Firing Rate ◽  
Motor Unit ◽  
Force Production ◽  
Motor Units ◽  
Surface Emg ◽  
Stroke Survivors ◽  
Single Motor Unit ◽  
Control Subjects ◽  
First Dorsal Interosseous Muscle ◽  
Hemiparetic Stroke

Hemispheric brain injury resulting from a stroke is often accompanied by muscle weakness in limbs contralateral to the lesion. In the present study, we investigated whether weakness in contralesional hand muscle in stroke survivors is partially attributable to alterations in motor unit activation, including alterations in firing rate modulation range. The afterhyperpolarization (AHP) potential of a motoneuron is a primary determinant of motoneuron firing rate. We examined differences in AHP duration in motoneurons innervating paretic and less impaired (contralateral) limb muscles of hemiparetic stroke survivors as well as in control subjects. A novel surface EMG (sEMG) electrode was used to record motor units from the first dorsal interosseous muscle. The sEMG data were subsequently decomposed to derive single-motor unit events, which were then utilized to produce interval (ISI) histograms of the motoneuron discharges. A modified version of interval death rate (IDR) analysis was used to estimate AHP duration. Results from data analyses performed on both arms of 11 stroke subjects and in 7 age-matched control subjects suggest that AHP duration is significantly longer for motor units innervating paretic muscle compared with units in contralateral muscles and in units of intact subjects. These results were supported by a coefficient of variation (CV) analysis showing that paretic motor unit discharges have a lower CV than either contralateral or control units. This study suggests that after stroke biophysical changes occur at the motoneuron level, potentially contributing to lower firing rates and potentially leading to less efficient force production in paretic muscles.

Rectification is required to extract oscillatory envelope modulation from surface electromyographic signals

2014 ◽  
Vol 112 (7) ◽  
pp. 1685-1691 ◽  
Author(s):  
Christopher J. Dakin ◽  
Brian H. Dalton ◽  
Billy L. Luu ◽  
Jean-Sébastien Blouin
Keyword(s):  
Motor Unit ◽  
Stimulus Frequency ◽  
Motor Units ◽  
Surface Emg ◽  
Medial Gastrocnemius ◽  
Single Motor Unit ◽  
Single Motor ◽  
Single Motor Units ◽  
Emg Signal ◽  
Envelope Modulation

Rectification of surface electromyographic (EMG) recordings prior to their correlation with other signals is a widely used form of preprocessing. Recently this practice has come into question, elevating the subject of EMG rectification to a topic of much debate. Proponents for rectifying suggest it accentuates the EMG spike timing information, whereas opponents indicate it is unnecessary and its nonlinear distortion of data is potentially destructive. Here we examine the necessity of rectification on the extraction of muscle responses, but for the first time using a known oscillatory input to the muscle in the form of electrical vestibular stimulation. Participants were exposed to sinusoidal vestibular stimuli while surface and intramuscular EMG were recorded from the left medial gastrocnemius. We compared the unrectified and rectified surface EMG to single motor units to determine which method best identified stimulus-EMG coherence and phase at the single-motor unit level. Surface EMG modulation at the stimulus frequency was obvious in the unrectified surface EMG. However, this modulation was not identified by the fast Fourier transform, and therefore stimulus coherence with the unrectified EMG signal failed to capture this covariance. Both the rectified surface EMG and single motor units displayed significant coherence over the entire stimulus bandwidth (1–20 Hz). Furthermore, the stimulus-phase relationship for the rectified EMG and motor units shared a moderate correlation ( r = 0.56). These data indicate that rectification of surface EMG is a necessary step to extract EMG envelope modulation due to motor unit entrainment to a known stimulus.

Doublet Discharges in Motoneurons of Young and Older Adults

2006 ◽  
Vol 95 (5) ◽  
pp. 2787-2795 ◽  
Author(s):  
Anita Christie ◽  
Gary Kamen
Keyword(s):  
Firing Rate ◽  
Motor Unit ◽  
Force Production ◽  
Motor Units ◽  
Voluntary Contraction ◽  
Older Individuals ◽  
Motor Unit Firing ◽  
Older Subjects ◽  
Young Subjects ◽  
Ramp Contractions

The purpose of this study was to investigate the occurrence of motor unit doublet discharges in young and older individuals at different rates of increasing force. Participants included eight young (21.9 ± 3.56 yr) and eight older (74.1 ± 8.79 yr) individuals, with equal numbers of males and females in each group. Motor unit activity was recorded from the tibialis anterior during isometric dorsiflexion using a four-wire needle electrode. Subjects performed three ramp contractions from zero to 50% maximal voluntary contraction (MVC) force at each of three rates: 10, 30, and 50% MVC/s. Overall, the occurrence of doublets was significantly higher in the young than in the older individuals. However, neither group showed differences in the occurrence of doublets across the three rates of force production. Doublet firings were observed in 45.6 (young) and 35.1% (old) of motor units at 10% MVC/s; 48.6 (young) and 22.5% (old) of motor units at 30% MVC/s; and 48.4 (young) and 31.4% (old) at 50% MVC/s. The maximal firing rate was significantly higher and the force at which the motor units were recruited was significantly lower for those units that fired doublets than those that did not. The force at which doublets occurred ranged from 3.42 to 50% MVC in the young subjects and from 0 (force onset) to 50% MVC in the older subjects. The results of this study suggest that the occurrence of doublets is dependent on both motor unit firing rate and force level. The lower incidence of doublets in older individuals may be attributable to changes in the intrinsic properties of the motoneurons with aging, which appear to play a role in doublet discharges.

Factors Governing the Form of the Relation Between Muscle Force and the EMG: A Simulation Study

2004 ◽  
Vol 92 (5) ◽  
pp. 2878-2886 ◽  
Author(s):  
Ping Zhou ◽  
William Zev Rymer
Keyword(s):  
Firing Rate ◽  
Motor Unit ◽  
Muscle Force ◽  
Dominant Role ◽  
Motor Units ◽  
Surface Emg ◽  
Emg Amplitude ◽  
Force Relation ◽  
Motor Unit Firing ◽  
Simulation Results

The dependence of the form of the EMG-force relation on key motoneuron and muscle properties was explored using a simulation approach. Surface EMG signals and isometric forces were simulated using existing motoneuron pool, muscle force, and surface EMG models, based primarily on reported properties of the first dorsal interosseous (FDI) muscle in humans. Our simulation results indicate that the relation between electrical and mechanical properties of the individual motor unit level plays the dominant role in determining the overall EMG amplitude-force relation of the muscle, while the underlying motor unit firing rate strategy appears to be a less important factor. However, different motor unit firing rate strategies result in substantially different relations between counts of the numbers of motoneuron discharges and the isometric force. Our simulation results also show that EMG amplitude (estimated as the average rectified value) increases as a result of synchronous discharges of different motor units within the pool, but the magnitude of this increase is determined primarily by the action potential duration of the synchronized motor units. Furthermore, when the EMG effects are normalized to their maximum levels, motor unit synchrony does not exert significant effects on the form of the EMG-force relation, provided that the synchrony level is held similar at different excitation levels.

Low-threshold motor unit membrane properties vary with contraction intensity during sustained activation with surface EMG visual feedback

2004 ◽  
Vol 96 (4) ◽  
pp. 1505-1515 ◽  
Author(s):  
Dario Farina ◽  
Marco Gazzoni ◽  
Federico Camelia
Keyword(s):  
Firing Rate ◽  
Motor Unit ◽  
Visual Feedback ◽  
Low Frequency ◽  
Practical Method ◽  
Surface Emg ◽  
Membrane Properties ◽  
Single Motor Unit ◽  
Low Threshold ◽  
Over Time

Single-motor unit (MU) activities were detected from the abductor pollicis and abductor digiti minimi muscles providing the subjects with visual feedback of multichannel surface electromyogram (EMG) signals. The subjects could modulate the force to observe on the surface recordings a single dominant MU and modulate its firing rate for contractions of 300 s with a noninvasive EMG feedback. The firing rate was maintained at ∼8 pulses per second [low-frequency (LF) contraction] and at ∼12 pulses per second [high-frequency (HF) contraction]. Single-MU conduction velocity (CV) decreased slightly but significantly over time, and it was possible to identify a significantly larger rate of decrease of CV during the HF with respect to the LF contractions. CV initial value significantly increased with the average firing rate, and CV values were significantly correlated to the instantaneous firing rate ( R ranging from 0.21 to 0.39). Both additional MU recruitment and substitution were observed during the contractions. The study provides evidence that 1) it is possible to follow the same MU in a hand muscle at two different intensities (HF and LF) for 300-s durations by using visual feedback of surface EMG, 2) low-threshold single-MU CV changes over time since the beginning of the contraction, and 3) it is possible to distinguish between CV changes of the same MU at slightly different firing rates. The technique provides a practical method for the noninvasive assessment of both control and membrane properties of single MUs.

Motor Unit Substitution in Long-Duration Contractions of the Human Trapezius Muscle

1999 ◽  
Vol 82 (1) ◽  
pp. 501-504 ◽  
Author(s):  
R. H. Westgaard ◽  
C. J. de Luca
Keyword(s):  
Motor Unit ◽  
Trapezius Muscle ◽  
Motor Units ◽  
Surface Emg ◽  
Single Motor Unit ◽  
Recruitment Threshold ◽  
Low Level ◽  
Emg Signal ◽  
Short Period ◽  
Low Threshold

We examined the activity pattern of low-threshold motor units in the human trapezius muscle during contractions of 10 min duration. Three procedures were applied in sequence: 1) static contraction controlled by maintaining a constant low level of the surface electromyogram (EMG)-detected root-mean-square signal, 2) a manipulation task with mental concentration, and 3) copying a text on a word processor. A quadrifilar fine-wire electrode was used to record single motor unit activity. Simultaneously, surface electrodes recorded the surface EMG signal. During these contractions, low-threshold motor units showed periods of inactivity and were substituted by motor units of higher recruitment threshold. This phenomenon was not observed during the first few minutes of the contraction. In several cases the substitution process coincided with a short period of inactivity in the surface EMG pattern. Substitution was observed in five of eight experiments. These observations may be explained by a time-variant recruitment threshold of motor units, sensitive to their activation history and temporal variation in the activity patterns. We speculate that the substitution phenomenon protects motor units in postural muscles from excessive fatigue when there is a demand for sustained low-level muscle activity.

Nonlinear behaviour of human motoneurons

1995 ◽  
Vol 73 (1) ◽  
pp. 113-123 ◽  
Author(s):  
Louise Smith ◽  
Tao Zhong ◽  
Parveen Bawa
Keyword(s):  
Steady State ◽  
Firing Rate ◽  
Motor Unit ◽  
Human Subjects ◽  
Motor Units ◽  
Rate Of Change ◽  
Emg Activity ◽  
Nonlinear Behaviour ◽  
Single Motor Unit ◽  
Single Motor

When ramp-and-hold currents are injected into a motoneuron of an anesthetized cat, the motoneuron responds with a high initial firing rate (dynamic phase), which then adapts to a lower steady-state firing rate. The firing rates during the dynamic and the steady-state phases are linearly related to the rate of change and the magnitude of the injected current, respectively. In human subjects, where inputs to the motoneurons are not accessible, force parameters are used to describe motoneuron behaviour. Population responses of human motoneurons, measured in terms of gross electromyographic (EMG) activity, increase linearly with the magnitude and the rate of change of force. No study has attempted to examine the question of linearity of single motor units during the dynamic as well as the steady-state phases. The following study recorded single motor unit and EMG activities simultaneously from the flexor carpi radialis muscle in human subjects completing ramp-and-hold force trajectories. Although the results confirmed the linear relationship between EMG activities and the rate and magnitude of the force, a nonlinear activity pattern was observed between the single motor unit firing and the force parameters, suggesting that recruitment must be responsible for the linear behaviour of EMG activity. Comparisons of different background activity levels on the firing patterns of a given motor unit, as well as comparisons of two simultaneously recorded units, further supported nonlinear response patterns of single motor units.Key words: human, motoneurons, motor units, nonlinearity, force trajectory, repetitive firing.

scholarly journals Beta-band motor unit coherence and nonlinear surface EMG features of the first dorsal interosseous muscle vary with force

2019 ◽  
Vol 122 (3) ◽  
pp. 1147-1162 ◽  
Author(s):  
Lara McManus ◽  
Matthew W. Flood ◽  
Madeleine M. Lowery
Keyword(s):  
Motor Unit ◽  
Motor Units ◽  
Surface Emg ◽  
Sample Entropy ◽  
Beta Band ◽  
Motoneuron Pool ◽  
Motor Unit Firing ◽  
First Dorsal Interosseous Muscle ◽  
Nonlinear Surface ◽  
Dorsal Interosseous Muscle

Motor unit firing times are weakly coupled across a range of frequencies during voluntary contractions. Coherent activity within the beta-band (15–35 Hz) has been linked to oscillatory cortical processes, providing evidence of functional connectivity between the motoneuron pool and motor cortex. The aim of this study was to investigate whether beta-band motor unit coherence is altered with increasing abduction force in the first dorsal interosseous muscle. Coherence between motor unit firing times, extracted from decomposed surface electromyography (EMG) signals, was investigated in 17 subjects at 10, 20, 30, and 40% of maximum voluntary contraction. Corresponding changes in nonlinear surface EMG features (specifically sample entropy and determinism, which are sensitive to motor unit synchronization) were also examined. A reduction in beta-band and alpha-band coherence was observed as force increased [ F(3, 151) = 32, P < 0.001 and F(3, 151) = 27, P < 0.001, respectively], accompanied by corresponding changes in nonlinear surface EMG features. A significant relationship between the nonlinear features and motor unit coherence was also detected ( r = −0.43 ± 0.1 and r = 0.45 ± 0.1 for sample entropy and determinism, respectively; both P < 0.001). The reduction in beta-band coherence suggests a change in the relative contribution of correlated and uncorrelated presynaptic inputs to the motoneuron pool, and/or a decrease in the responsiveness of the motoneuron pool to synchronous inputs at higher forces. The study highlights the importance of considering muscle activation when investigating changes in motor unit coherence or nonlinear EMG features and examines other factors that can influence coherence estimation. NEW & NOTEWORTHY Intramuscular alpha- and beta-band coherence decreased as muscle contraction force increased. Beta-band coherence was higher in groups of high-threshold motor units than in simultaneously active lower threshold units. Alterations in motor unit coherence with increases or decreases in force and with the onset of fatigue were accompanied by corresponding changes in surface electromyography sample entropy and determinism. Mixed-model analysis indicated mean firing rate and number of motor units also influenced the coherence estimate.

scholarly journals Origins of Spontaneous Firing of Motor Units in the Spastic–Paretic Biceps Brachii Muscle of Stroke Survivors

2010 ◽  
Vol 104 (6) ◽  
pp. 3168-3179 ◽  
Author(s):  
C. J. Mottram ◽  
C. L. Wallace ◽  
C. N. Chikando ◽  
W. Z. Rymer
Keyword(s):  
Motor Unit ◽  
Biceps Brachii ◽  
Motor Units ◽  
Afferent Feedback ◽  
Spontaneous Discharge ◽  
Stroke Survivors ◽  
Spontaneous Firing ◽  
Descending Pathways ◽  
Hemiparetic Stroke ◽  
Ramp Contractions

One potential expression of altered motoneuron excitability following a hemispheric stroke is the spontaneous unit firing (SUF) of motor units at rest. The elements contributing to this altered excitability could be spinal descending pathways, spinal interneuronal networks, afferent feedback, or intrinsic motoneuron properties. Our purpose was to examine the characteristics of spontaneous discharge in spastic–paretic and contralateral muscles of hemiparetic stroke survivors, to determine which of these mechanisms might contribute. To achieve this objective, we examined the statistics of spontaneous discharge of individual motor units and we conducted a coherence analyses on spontaneously firing motor unit pairs. The presence of significant coherence between units might indicate a common driving source of excitation to multiple motoneurons from descending pathways or regional interneurons, whereas a consistent lack of coherence might favor an intrinsic cellular mechanism of hyperexcitability. Spontaneous firing of motor units (i.e., ongoing discharge in the absence of an ongoing stimulus) was observed to a greater degree in spastic–paretic muscles (following 83.2 ± 16.7% of ramp contractions) than that in contralateral muscles (following just 14.1 ± 10.5% of ramp contractions; P < 0.001) and was not observed at all in healthy control muscle. The average firing rates of the spontaneously firing units were 8.4 ± 1.8 pulses/s (pps) in spastic–paretic muscle and 9.6 ± 2.2 pps in contralateral muscle ( P < 0.001). In 37 instances ( n = 63 pairs), we observed spontaneous discharge of two or more motor units simultaneously in spastic–paretic muscle. Seventy percent of the dually firing motor unit pairs exhibited significant coherence ( P < 0.001) in the 0- to 4-Hz bandwidth (average peak coherence: 0.14 ± 0.13; range: 0.01–0.75) and 22% of pairs exhibited significant coherence ( P < 0.001) in the 15- to 30-Hz bandwidth (average peak coherence: 0.07 ± 0.06; range: 0.01–0.31). We suggest that the spontaneous firing was likely not attributable solely to enhanced intrinsic motoneuron activation, but attributable, at least in part, to a low-level excitatory synaptic input to the resting spastic–paretic motoneuron pool, possibly from regional or supraspinal centers.

Amplitude cancellation reduces the size of motor unit potentials averaged from the surface EMG

2006 ◽  
Vol 100 (6) ◽  
pp. 1928-1937 ◽  
Author(s):  
Kevin G. Keenan ◽  
Dario Farina ◽  
Roberto Merletti ◽  
Roger M. Enoka
Keyword(s):  
Motor Unit ◽  
Conduction Velocity ◽  
Motor Units ◽  
Surface Emg ◽  
Excitation Level ◽  
Bipolar Electrode ◽  
Emg Signal ◽  
Motor Unit Synchronization ◽  
Rate Coding ◽  
Simulated Surface

The purpose of the study was to evaluate the influence of selected physiological parameters on amplitude cancellation in the simulated surface electromyogram (EMG) and the consequences for spike-triggered averages of motor unit potentials derived from the interference and rectified EMG signals. The surface EMG was simulated from prescribed recruitment and rate coding characteristics of a motor unit population. The potentials of the motor units were detected on the skin over a hand muscle with a bipolar electrode configuration. Averages derived from the EMG signal were generated using the discharge times for each of the 24 motor units with lowest recruitment thresholds from a population of 120 across three conditions: 1) excitation level; 2) motor unit conduction velocity; and 3) motor unit synchronization. The area of the surface-detected potential was compared with potentials averaged from the interference, rectified, and no-cancellation EMGs. The no-cancellation EMG comprised motor unit potentials that were rectified before they were summed, thereby preventing cancellation between the opposite phases of the potentials. The percent decrease in area of potentials extracted from the rectified EMG was linearly related to the amount of amplitude cancellation in the interference EMG signal, with the amount of cancellation influenced by variation in excitation level and motor unit conduction velocity. Motor unit synchronization increased potentials derived from both the rectified and interference EMG signals, although cancellation limited the increase in area for both potentials. These findings document the influence of amplitude cancellation on motor unit potentials averaged from the surface EMG and the consequences for using the procedure to characterize motor unit properties.

Sign in / Sign up

Export Citation Format

Share Document