A computational approach for generating continuous estimates of motor unit discharge rates and visualizing population discharge characteristics

2021 ◽  
Author(s):  
James A Beauchamp ◽  
Obaid U Khurram ◽  
Julius Dewald ◽  
C J Heckman ◽  
Gregory Pearcey
Keyword(s):  
Motor Unit ◽  
Neural Control ◽  
Polynomial Regression ◽  
Discharge Rate ◽  
Medial Gastrocnemius ◽  
Support Vector ◽  
Discharge Characteristics ◽  
Discharge Rates ◽  
Entire Duration ◽  
Discharge Profile

Abstract Objective: Successive improvements in high density surface electromyography and decomposition techniques have facilitated an increasing yield in decomposed motor unit (MU) spike times. Though these advancements enhance the generalizability of findings and promote the application of MU discharge characteristics to inform the neural control of motor output, limitations remain. Specifically, 1) common approaches for generating smooth estimates of MU discharge rates introduce artifacts in quantification, which may bias findings, and 2) discharge characteristics of large MU populations are often difficult to visualize. Approach: In the present study, we propose support vector regression (SVR) as an improved approach for generating smooth continuous estimates of discharge rate and compare the fit characteristics of SVR to traditionally used methods, including Hanning window filtering and polynomial regression. Furthermore, we introduce ensembles as a method to visualize the discharge characteristics of large MU populations. We define ensembles as the average discharge profile of a subpopulation of MUs, composed of a time normalized ensemble average of all units within this subpopulation. Analysis was conducted with MUs decomposed from the tibialis anterior (N = 2128), medial gastrocnemius (N = 2673), and soleus (N = 1190) during isometric plantarflexion and dorsiflexion contractions. Main Result: Compared to traditional approaches, we found SVR to alleviate commonly observed inaccuracies and produce significantly less absolute fit error in the initial phase of MU discharge and throughout the entire duration of discharge. Additionally, we found the visualization of MU populations as ensembles to intuitively represent population discharge characteristics with appropriate accuracy for visualization. Significance: The results and methods outlined here provide an improved method for generating estimates of MU discharge rate with SVR and present a unique approach to visualizing MU populations with ensembles. In combination, the use of SVR and generation of ensembles represent an efficient method for rendering population discharge characteristics.

scholarly journals A computational approach for generating smooth estimates of motor unit discharge rates and visualizing population discharge characteristics

2021 ◽  
Author(s):  
James A Beauchamp ◽  
Obaid U Khurram ◽  
Julius PA Dewald ◽  
CJ Heckman ◽  
Gregory EP Pearcey
Keyword(s):  
Motor Unit ◽  
Neural Control ◽  
Polynomial Regression ◽  
Discharge Rate ◽  
Medial Gastrocnemius ◽  
Support Vector ◽  
Discharge Characteristics ◽  
Discharge Rates ◽  
Entire Duration ◽  
Discharge Profile

Objective: Successive improvements in high density surface electromyography and decomposition techniques have facilitated an increasing yield in decomposed motor unit (MU) spike times. Though these advancements enhance the generalizability of findings and promote the application of MU discharge characteristics to inform the neural control of motor output, limitations remain. Specifically, 1) common approaches for generating smooth estimates of MU discharge rates introduce artifacts in quantification, which may bias findings, and 2) discharge characteristics of large MU populations are often difficult to visualize. Approach: In the present study, we propose support vector regression (SVR) as an improved approach for generating continuous estimates of discharge rate and compare the fit characteristics of SVR to traditionally used methods, including Hanning window filtering and polynomial regression. Furthermore, we introduce ensembles as a method to visualize the discharge characteristics of large MU populations. We define ensembles as the average discharge profile of a subpopulation of MUs, composed of a time normalized ensemble average of all units within this subpopulation. Analysis was conducted with MUs decomposed from the tibialis anterior (N = 2128), medial gastrocnemius (N = 2673), and soleus (N = 1190) during isometric plantarflexion and dorsiflexion contractions. Main Result: Compared to traditional approaches, we found SVR to alleviate commonly observed inaccuracies and produce significantly less absolute fit error in the initial phase of MU discharge and throughout the entire duration of discharge. Additionally, we found the visualization of MU populations as ensembles to intuitively represent population discharge characteristics with appropriate accuracy for visualization. Significance: The results and methods outlined here provide an improved method for generating smooth estimates of MU discharge rate with SVR and present a unique approach to visualizing MU populations with ensembles. In combination, the use of SVR and generation of ensembles represent an efficient method for rendering population discharge characteristics.

Discharge Properties of Motor Units of the Abductor Hallucis Muscle During Cramp Contractions

2009 ◽  
Vol 102 (3) ◽  
pp. 1890-1901 ◽  
Author(s):  
Marco A. Minetto ◽  
Aleš Holobar ◽  
Alberto Botter ◽  
Dario Farina
Keyword(s):  
Motor Unit ◽  
Discharge Rate ◽  
Motor Units ◽  
Abductor Hallucis ◽  
Significant Peak ◽  
Discharge Rates ◽  
Abductor Hallucis Muscle ◽  
Active Motor ◽  
Number Of Discharges ◽  
Voluntary Contractions

We analyzed individual motor units during electrically elicited cramp contractions with the aim of characterizing the variability and degree of common oscillations in their discharges. Intramuscular and surface electromyographic (EMG) signals were detected from the abductor hallucis muscle of 11 healthy subjects (age 27.0 ± 3.7 yr) during electrically elicited cramps. In all, 48 motor units were identified from the intramuscular EMG. These motor units were active for 23.6 ± 16.2 s, during which their average discharge rate was 14.5 ± 5.1 pulses/s (pps) and their minimum and maximum rates were, respectively, 6.0 ± 0.8 and 25.0 ± 8.0 pps ( P < 0.001). The coefficient of variation for the interspike interval (ISI) was 44.6 ± 9.7% and doublet discharges constituted 4.1 ± 4.7% of the total number of discharges. In 38 motor units, the SD of the ISI was positively correlated to the mean ISI ( R2 = 0.37, P < 0.05). The coherence spectrum between smoothed discharge rates of pairs of motor units showed one significant peak at 1.4 ± 0.4 Hz for 29 of the 96 motor unit pairs and two significant peaks at 1.3 ± 0.5 and 1.5 ± 0.5 Hz for 8 motor unit pairs. The cross-correlation function between pairs of discharge rates showed a significant peak (0.52 ± 0.11) in 26 motor unit pairs. In conclusion, motor units active during cramps showed a range of discharge rates similar to that observed during voluntary contractions but larger ISI variability, probably due to large synaptic noise. Moreover, the discharge rates of the active motor units showed common oscillations.

An integrative model of motor unit activity during sustained submaximal contractions

2010 ◽  
Vol 108 (6) ◽  
pp. 1550-1562 ◽  
Author(s):  
Jakob L. Dideriksen ◽  
Dario Farina ◽  
Martin Baekgaard ◽  
Roger M. Enoka
Keyword(s):  
Motor Unit ◽  
Unit Activity ◽  
Discharge Rate ◽  
Motor Units ◽  
Compartment Model ◽  
Integrative Model ◽  
Motor Unit Activity ◽  
Discharge Rates ◽  
Motor Unit Discharge ◽  
Experimental Findings

The purpose of the study was to expand a model of motor unit recruitment and rate coding ( 30 ) to simulate the adjustments that occur during a fatiguing contraction. The major new components of the model were the introduction of time-varying parameters for motor unit twitch force, recruitment, discharge rate, and discharge variability, and a control algorithm that estimates the net excitation needed by the motoneuron pool to maintain a prescribed target force. The fatigue-induced changes in motor unit activity in the expanded model are a function of changes in the metabolite concentrations that were computed with a compartment model of the intra- and extracellular spaces. The model was validated by comparing the simulation results with data available from the literature and experimentally recorded in the present study during isometric contractions of the first dorsal interosseus muscle. The output of the model was able to replicate a number of experimental findings, including the time to task failure for a range of target forces, the changes in motor unit discharge rates, the skewness and kurtosis of the interspike interval distributions, discharge variability, and the discharge characteristics of newly recruited motor units. The model output provides an integrative perspective of the adjustments during fatiguing contractions that are difficult to measure experimentally.

Vastus lateralis surface and single motor unit EMG following submaximal shortening and lengthening contractions

2008 ◽  
Vol 33 (6) ◽  
pp. 1086-1095 ◽  
Author(s):  
Teatske M. Altenburg ◽  
Cornelis J. de Ruiter ◽  
Peter W.L. Verdijk ◽  
Willem van Mechelen ◽  
Arnold de Haan
Keyword(s):  
Motor Unit ◽  
Muscle Activation ◽  
Discharge Rate ◽  
Vastus Lateralis ◽  
Motor Units ◽  
Voluntary Contraction ◽  
Knee Extensors ◽  
Single Motor Unit ◽  
Discharge Rates ◽  
Active Motor

A single shortening contraction reduces the force capacity of muscle fibers, whereas force capacity is enhanced following lengthening. However, how motor unit recruitment and discharge rate (muscle activation) are adapted to such changes in force capacity during submaximal contractions remains unknown. Additionally, there is limited evidence for force enhancement in larger muscles. We therefore investigated lengthening- and shortening-induced changes in activation of the knee extensors. We hypothesized that when the same submaximal torque had to be generated following shortening, muscle activation had to be increased, whereas a lower activation would suffice to produce the same torque following lengthening. Muscle activation following shortening and lengthening (20° at 10°/s) was determined using rectified surface electromyography (rsEMG) in a 1st session (at 10% and 50% maximal voluntary contraction (MVC)) and additionally with EMG of 42 vastus lateralis motor units recorded in a 2nd session (at 4%–47%MVC). rsEMG and motor unit discharge rates following shortening and lengthening were normalized to isometric reference contractions. As expected, normalized rsEMG (1.15 ± 0.19) and discharge rate (1.11 ± 0.09) were higher following shortening (p < 0.05). Following lengthening, normalized rsEMG (0.91 ± 0.10) was, as expected, lower than 1.0 (p < 0.05), but normalized discharge rate (0.99 ± 0.08) was not (p > 0.05). Thus, muscle activation was increased to compensate for a reduced force capacity following shortening by increasing the discharge rate of the active motor units (rate coding). In contrast, following lengthening, rsEMG decreased while the discharge rates of active motor units remained similar, suggesting that derecruitment of units might have occurred.

Behavior of motor units in human biceps brachii during a submaximal fatiguing contraction

1994 ◽  
Vol 76 (6) ◽  
pp. 2411-2419 ◽  
Author(s):  
S. J. Garland ◽  
R. M. Enoka ◽  
L. P. Serrano ◽  
G. A. Robinson
Keyword(s):  
Motor Unit ◽  
Biceps Brachii ◽  
Discharge Rate ◽  
Human Subjects ◽  
Absolute Threshold ◽  
Motor Units ◽  
Single Motor ◽  
Threshold Force ◽  
Single Motor Units ◽  
Discharge Rates

The activity of 50 single motor units was recorded in the biceps brachii muscle of human subjects while they performed submaximal isometric elbow flexion contractions that were sustained to induce fatigue. The purposes of this study were to examine the influence of fatigue on motor unit threshold force and to determine the relationship between the threshold force of recruitment and the initial interimpulse interval on the discharge rates of single motor units during a fatiguing contraction. The discharge rate of most motor units that were active from the beginning of the contraction declined during the fatiguing contraction, whereas the discharge rates of most newly recruited units were either constant or increased slightly. The absolute threshold forces of recruitment and derecruitment decreased, and the variability of interimpulse intervals increased after the fatigue task. The change in motor unit discharge rate during the fatigue task was related to the initial rate, but the direction of the change in discharge rate could not be predicted from the threshold force of recruitment or the variability in the interimpulse intervals. The discharge rate of most motor units declined despite an increase in the excitatory drive to the motoneuron pool during the fatigue task.

Aging, Resistance Training, and Motor Unit Discharge Behavior

2005 ◽  
Vol 30 (3) ◽  
pp. 341-351 ◽  
Author(s):  
Gary Kamen
Keyword(s):  
Motor Unit ◽  
Muscular Strength ◽  
Discharge Rate ◽  
Recruitment Rate ◽  
Unit Discharge ◽  
Discharge Rates ◽  
Motor Unit Synchronization ◽  
Motor Unit Discharge ◽  
Rate Coding ◽  
Neural Factors

Researchers have alluded to the existence of "neural factors" in the expression and development of muscular strength. Candidate neural factors including motor unit recruitment, rate coding, doublet firing, and motor unit synchronization are discussed in this review. Aging is generally accompanied by lower motor unit discharge rates. However, both young and older adults exhibit rapid changes in muscular strength with repeated strength testing. These strength changes occur with concomitant albeit transient increases in motor unit discharge rate. These and other neural factors may contribute to the initial increases in muscular strength observed during the early phases of resistance exercise training. Key words: firing rate, muscle, exercise

Experimental muscle pain reduces initial motor unit discharge rates during sustained submaximal contractions

2005 ◽  
Vol 98 (3) ◽  
pp. 999-1005 ◽  
Author(s):  
Dario Farina ◽  
Lars Arendt-Nielsen ◽  
Thomas Graven-Nielsen
Keyword(s):  
Motor Unit ◽  
Conduction Velocity ◽  
Muscle Pain ◽  
Tibialis Anterior ◽  
Discharge Rate ◽  
Human Study ◽  
Unit Discharge ◽  
Discharge Rates ◽  
Motor Unit Discharge ◽  
The Right

The aim of this human study was to investigate the effect of experimentally induced muscle pain on the modifications of motor unit discharge rate during sustained, constant-force contractions. Intramuscular and multichannel surface electromyographic (EMG) signals were collected from the right and left tibialis anterior muscle of 11 volunteers. The subjects performed two 4-min-long isometric contractions at 25% of the maximal dorsiflexion torque, separated by a 20-min rest. Before the beginning of the second contraction, hypertonic (painful; right leg) or isotonic (nonpainful; left leg) saline was injected into the tibialis anterior. Pain intensity scores did not change significantly in the first 150 s of the painful contraction. Exerted torque and its coefficient of variation were the same for the painful and nonpainful contractions. Motor unit discharge rate was higher in the beginning of the nonpainful contraction than the painful contraction on the right side [means ± SE, 11.3 ± 0.2 vs. 10.6 ± 0.2 pulses/s (pps); P < 0.01] whereas it was the same for the two contractions on the left side (11.6 ± 0.2 vs. 11.5 ± 0.2 pps). The decrease in discharge rate in 4 min was smaller for the painful (0.4 ± 0.1 pps) than for the control contractions (1.3 ± 0.1 pps). Initial value and decrease in motor unit conduction velocity were not different in the four contractions (right leg, 4.0 ± 0.1 m/s with decrease of 0.6 ± 0.1 m/s in 4 min; left leg, 4.1 ± 0.1 m/s with 0.7 ± 0.1 m/s decrease). In conclusion, stimulation of nociceptive afferents by injection of hypertonic saline did not alter motor unit conduction velocity but reduced the initial motor unit discharge rates and the difference between initial and final discharge rates during sustained contraction.

scholarly journals Differences in Human Motoneuron Excitability Between Functionally Diverse Muscles

CommonHealth ◽  
2020 ◽  
Vol 1 (1) ◽  
pp. 12-23
Author(s):  
Christopher Taylor ◽  
Tyler Kmiec ◽  
Christopher Thompson
Keyword(s):  
Motor Unit ◽  
Discharge Rate ◽  
Statistical Significance ◽  
Spike Trains ◽  
The Body ◽  
Intrinsic Excitability ◽  
Functional Movement ◽  
Discharge Rates ◽  
Density Surface ◽  
Functionally Diverse

Introduction: Spinal motoneurons (MN) transmit neural commands from the brain to the muscles they innervate and, as a result, produce functional movement. However, MNs are not simply passive conduits of these command and, instead, actively shape motor output through alterations in intrinsic excitability. We hypothesize that the excitability of MNs is not fixed across the body; instead, MNs are functionally tuned to the tasks they control. Here, we investigate this mapping of MN excitability across motor pools. Methods: High-density surface electromyography of the tibialis anterior (TA) and first dorsal interosseous (FDI) was recorded from four neurologically intact participants while they performed low-level, isometric contractions. The data were decomposed into underlying motor unit action potentials and paired motor unit analyses were subsequently performed on these spike trains to quantify MN excitability (ΔF). Results: 1,638 motor unit spike trains were extracted across all contractions. Mann-Whitney U test revealed that all subjects (4/4) had significantly higher maximal discharge rates in FDI, 19.13 [17.62 – 20.59] pps, when compared to the TA, 13.08 [11.51 – 15.46] pps. All subjects (4/4) had a higher ΔF in the TA (4.22 [2.89 – 5.61] pps) than the FDI (3.62 [1.23 – 5.94] pps), with 3/4 reaching statistical significance. Conclusions: Our findings suggest that the discharge rate and intrinsic excitability of human MNs differs across TA and FDI motor pools during similar isometric tasks. These results support the notion that motor pools are functionally tuned to their environmental demands.

scholarly journals Analysis of motor unit spike trains estimated from high-density surface electromyography is highly reliable across operators

2021 ◽  
Author(s):  
François Hug ◽  
Simon Avrillon ◽  
Alessandro Del Vecchio ◽  
Andrea Casolo ◽  
Jaime Ibanez ◽  
...  
Keyword(s):  
Motor Unit ◽  
Surface Electromyography ◽  
Neural Control ◽  
Discharge Rate ◽  
Intraclass Correlation ◽  
Motor Units ◽  
Spike Trains ◽  
High Density ◽  
Density Surface ◽  
Unit Spike

AbstractThere is a growing interest in decomposing high-density surface electromyography (HDsEMG) into motor unit spike trains to improve knowledge on the neural control of muscle contraction. However, the reliability of decomposition approaches is sometimes questioned, especially because they require manual editing of the outputs. We aimed to assess the inter-operator reliability of the identification of motor unit spike trains. Eight operators with varying experience in HDsEMG decomposition were provided with the same data extracted using the convolutive kernel compensation method. They were asked to manually edit them following established procedures. Data included signals from three lower leg muscles and different contraction intensities. After manual analysis, 126 ± 5 motor units were retained (range across operators: 119-134). A total of 3380 rate of agreement values were calculated (28 pairwise comparisons × 11 contractions/muscles × 4-28 motor units). The median rate of agreement value was 99.6%. Inter-operator reliability was excellent for both mean discharge rate and time at recruitment (intraclass correlation coefficient > 0.99). These results show that when provided with the same decomposed data and the same basic instructions, operators converge toward almost identical results. Our data have been made available so that they can be used for training new operators.

Discharge Rate of Sternohyoid Motor Units Activated With Surface EMG Feedback

2009 ◽  
Vol 101 (2) ◽  
pp. 624-632 ◽  
Author(s):  
Dario Farina ◽  
Deborah Falla
Keyword(s):  
Motor Unit ◽  
Motor Neurons ◽  
Discharge Rate ◽  
Maximum Level ◽  
Motor Units ◽  
Surface Emg ◽  
Discharge Rates ◽  
Average Maximum ◽  
Motor Unit Discharge ◽  
Voluntary Contractions

We analyzed individual motor units of the sternohyoid muscle with the aim of characterizing their minimum and maximum discharge rates and their variability in discharge during voluntary contractions. Surface EMG signals were recorded with an array of eight electrodes from the sternohyoid muscle of seven healthy men (age: 30.2 ± 3.5 yr). The multichannel surface EMG signals were displayed as feedback for the subjects who identified and modulated the activity of one target motor unit in 30-s contractions during which the discharge rate was increased from minimum to maximum (ramp contraction), sustained at maximum level (sustained), or increased in brief bursts (burst). During the ramp contractions, the minimum average discharge rate over epochs of 1 s was 11.6 ± 1.5 pulses per second (pps) and the maximum 57.0 ± 5.7 pps ( P < 0.001). During the sustained contractions, the motor unit discharge rate decreased from 65.5 ± 8.4 pps at the beginning to 52.9 ± 7.6 pps at the end of the contraction ( P < 0.05). The coefficient of variation for the interspike interval during the sustained contractions was 40.2 ± 9.8% and a large percentage of discharges had instantaneous rates >50 pps (52.2 ± 12.5%) and >100 pps (8.0 ± 1.2%), with peak values >150 pps. During the burst contractions, the instantaneous discharge rate reached average maximum values of 97.6 ± 36.8 pps. The observed discharge rates and their variability are higher than those reported for limb muscles, which may be due to large synaptic input and noise received by these motor neurons.

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