scholarly journals Motor unit contributions to activation reduction and torque steadiness following active lengthening: a study of residual torque enhancement

2020 ◽  
Vol 123 (6) ◽  
pp. 2209-2216 ◽  
Author(s):  
Jennifer M. Jakobi ◽  
Samantha L. Kuzyk ◽  
Chris J. McNeil ◽  
Brian H. Dalton ◽  
Geoffrey A. Power
Keyword(s):  
Motor Unit ◽  
Discharge Rate ◽  
Motor Units ◽  
Electromyographic Activity ◽  
Torque Capacity ◽  
Active Motor ◽  
Motor Neuron Pool ◽  
Acute Condition ◽  
The Impact ◽  
Neuron Pool

Our findings indicate that lower electromyographic activity during the torque-enhanced condition following active lengthening compared with a purely isometric contraction arises from fewer active motor units and a lower discharge rate of those that are active. We used an acute condition of increased torque capacity to induce a decrease in net output of the motor neuron pool during a submaximal task to demonstrate, in humans, the impact of motor unit activity on torque steadiness.

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.

scholarly journals Adjustments Differ Among Low-Threshold Motor Units During Intermittent, Isometric Contractions

2009 ◽  
Vol 101 (1) ◽  
pp. 350-359 ◽  
Author(s):  
Dario Farina ◽  
Aleš Holobar ◽  
Marco Gazzoni ◽  
Damjan Zazula ◽  
Roberto Merletti ◽  
...  
Keyword(s):  
Motor Unit ◽  
Conduction Velocity ◽  
Muscle Fiber ◽  
Discharge Rate ◽  
Motor Units ◽  
Abductor Pollicis Brevis ◽  
Muscle Fiber Conduction Velocity ◽  
Neuron Activation ◽  
Active Motor ◽  
Low Threshold

We investigated the changes in muscle fiber conduction velocity, recruitment and derecruitment thresholds, and discharge rate of low-threshold motor units during a series of ramp contractions. The aim was to compare the adjustments in motor unit activity relative to the duration that each motor unit was active during the task. Multichannel surface electromyographic (EMG) signals were recorded from the abductor pollicis brevis muscle of eight healthy men during 12-s contractions ( n = 25) in which the force increased and decreased linearly from 0 to 10% of the maximum. The maximal force exhibited a modest decline (8.5 ± 9.3%; P < 0.05) at the end of the task. The discharge times of 73 motor units that were active for 16–98% of the time during the first five contractions were identified throughout the task by decomposition of the EMG signals. Action potential conduction velocity decreased during the task by a greater amount for motor units that were initially active for >70% of the time compared with that of less active motor units. Moreover, recruitment and derecruitment thresholds increased for these most active motor units, whereas the thresholds decreased for the less active motor units. Another 18 motor units were recruited at an average of 171 ± 32 s after the beginning of the task. The recruitment and derecruitment thresholds of these units decreased during the task, but muscle fiber conduction velocity did not change. These results indicate that low-threshold motor units exhibit individual adjustments in muscle fiber conduction velocity and motor neuron activation that depended on the relative duration of activity during intermittent contractions.

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.

The motor unit

2016 ◽  
Author(s):  
David Burke ◽  
James Howells
Keyword(s):  
Motor Unit ◽  
Neuromuscular Junctions ◽  
Discharge Rate ◽  
Motor Units ◽  
Motor Unit Action Potential ◽  
Muscle Fibres ◽  
Reflex Gain ◽  
Active Motor ◽  
Motor Unit Action ◽  
Fast Twitch

The motor unit represent the final output of the motor system. Each consists of a motoneuron, its axon, neuromuscular junctions, and muscle fibres innervated by that axon. The discharge of a motor unit can be followed by recording its electromyographic signature, the motor unit action potential. Motoneurons are not passive responders to the excitatory and inhibitory influences on them from descending and segmental sources. Their properties can change, e.g. due to descending monoaminergic pathways, which can alter their responses to other inputs (changing ‘reflex gain’). Contraction strength depends on the number of active motor units, their discharge rate, and whether the innervated muscle fibres are slow-twitch producing low force, but resistant to fatigue, fast-twitch producing more force, but susceptible to fatigue, or intermediate fast-twitch fatigue-resistant. These properties are imposed by the parent motoneurons, and the innervated muscle fibres have different histochemical profiles (oxidative, glycolytic, or oxidative-glycolytic, respectively).

scholarly journals Recruitment of motor units in the medial gastrocnemius muscle during human quiet standing: is recruitment intermittent? What triggers recruitment?

2012 ◽  
Vol 107 (2) ◽  
pp. 666-676 ◽  
Author(s):  
Taian M. M. Vieira ◽  
Ian D. Loram ◽  
Silvia Muceli ◽  
Roberto Merletti ◽  
Dario Farina
Keyword(s):  
Motor Unit ◽  
Planning Process ◽  
Discharge Rate ◽  
Motor Units ◽  
Median Number ◽  
The Body ◽  
Medial Gastrocnemius ◽  
Quiet Standing ◽  
Rate Coding ◽  
Active Motor

The recruitment and the rate of discharge of motor units are determinants of muscle force. Within a motoneuron pool, recruitment and rate coding of individual motor units might be controlled independently, depending on the circumstances. In this study, we tested whether, during human quiet standing, the force of the medial gastrocnemius (MG) muscle is predominantly controlled by recruitment or rate coding. If MG control during standing was mainly due to recruitment, then we further asked what the trigger mechanism is. Is it determined internally, or is it related to body kinematics? While seven healthy subjects stood quietly, intramuscular electromyograms were recorded from the MG muscle with three pairs of wire electrodes. The number of active motor units and their mean discharge rate were compared for different sway velocities and positions. Motor unit discharges occurred more frequently when the body swayed faster and forward (Pearson R = 0.63; P < 0.0001). This higher likelihood of observing motor unit potentials was explained chiefly by the recruitment of additional units. During forward body shifts, the median number of units detected increased from 3 to 11 ( P < 0.0001), whereas the discharge rate changed from 8 ± 1.1 (mean ± SD) to 10 ± 0.9 pulses/s ( P = 0.001). Strikingly, motor units did not discharge continuously throughout standing. They were recruited within individual, forward sways and intermittently, with a modal rate of two recruitments per second. This modal rate is consistent with previous circumstantial evidence relating the control of standing to an intrinsic, higher level planning process.

Neurophysiology in amyotrophic lateral sclerosis and other motor degenerations

2016 ◽  
Author(s):  
Mamede de Carvalho ◽  
Michael Swash
Keyword(s):  
Amyotrophic Lateral Sclerosis ◽  
Motor Neuron ◽  
Motor Unit ◽  
Chronic Conditions ◽  
Motor Units ◽  
Disease Course ◽  
Motor Neuron Diseases ◽  
Motor Neuron Pool ◽  
Neuron Pool ◽  
Lateral Sclerosis

Electromyography is critical for the diagnosis of motor neuron disease, as its findings exclude mimicking disorders, and confirm signs of widespread motor unit loss and reinnervation. In chronic conditions the slow disease course allows giant, stable motor unit potentials to appear. In contrast, in amyotrophic lateral sclerosis, the rapid degenerative process is characterized by signs of denervation and unstable motor unit potentials, where motor units become dysfunctional before having time to sustain very large reinnervated motor unit potentials. Fasciculation potentials are observed in both conditions. In amyotrophic lateral sclerosis fasciculation potentials are important supporting electrodiagnostic evidence, permitting earlier diagnosis. Many methods have been developed to quantify and monitor the lower motor neuron pool, but few have been used in clinical trials. Their role as tools to follow interventions or to interpret pathogenesis remains incompletely explored. Electromyography is a sensitive and reliable test in the diagnosis and assessment of motor neuron diseases.

Change in Muscle Fascicle Length Influences the Recruitment and Discharge Rate of Motor Units During Isometric Contractions

2005 ◽  
Vol 94 (5) ◽  
pp. 3126-3133 ◽  
Author(s):  
Benjamin Pasquet ◽  
Alain Carpentier ◽  
Jacques Duchateau
Keyword(s):  
Motor Unit ◽  
Ankle Joint ◽  
Discharge Rate ◽  
Motor Units ◽  
Motor Unit Recruitment ◽  
Isometric Contractions ◽  
Neutral Position ◽  
Proprioceptive Information ◽  
Fascicle Length ◽  
Muscle Fascicle

This study examines the effect of fascicle length change on motor-unit recruitment and discharge rate in the human tibialis anterior (TA) during isometric contractions of various intensities. The torque produced during dorsiflexion and the surface and intramuscular electromyograms (EMGs) from the TA were recorded in eight subjects. The behavior of the same motor unit ( n = 59) was compared at two ankle joint angles (+10 and −10° around the ankle neutral position). Muscle fascicle length of the TA was measured noninvasively using ultrasonography recordings. When the ankle angle was moved from 10° plantarflexion to 10° dorsiflexion, the torque produced during maximal voluntary contraction (MVC) was significantly reduced [35.2 ± 3.3 vs. 44.3 ± 4.2 (SD) Nm; P < 0.001] and the average surface EMG increased (0.47 ± 0.08 vs. 0.43 ± 0.06 mV; P < 0.05). At reduced ankle joint angle, muscle fascicle length declined by 12.7% ( P < 0.01) at rest and by 18.9% ( P < 0.001) during MVC. Motor units were activated at a lower recruitment threshold for short compared with long muscle fascicle length, either when expressed in absolute values (2.1 ± 2.5 vs. 3.6 ± 3.7 Nm; P < 0.001) or relative to their respective MVC (5.2 ± 6.1 vs. 8.8 ± 9.0%). Higher discharge rate and additional motor-unit recruitment were observed at a given absolute or relative torque when muscle fascicles were shortened. However, the data indicate that increased rate coding was mainly present at low torque level (<10% MVC), when the muscle-tendon complex was compliant, whereas recruitment of additional motor units played a dominant role at higher torque level and decreased compliance (10–35% MVC). Taken together, the results suggest that the central command is modulated by the afferent proprioceptive information during submaximal contractions performed at different muscle fascicle lengths.

Spike-triggered average torque and muscle fiber conduction velocity of low-threshold motor units following submaximal endurance contractions

2005 ◽  
Vol 98 (4) ◽  
pp. 1495-1502 ◽  
Author(s):  
Dario Farina ◽  
Lars Arendt-Nielsen ◽  
Thomas Graven-Nielsen
Keyword(s):  
Action Potential ◽  
Motor Unit ◽  
Conduction Velocity ◽  
Surface Potential ◽  
Muscle Fiber ◽  
Discharge Rate ◽  
Motor Units ◽  
Muscle Fiber Conduction Velocity ◽  
Sustained Contraction ◽  
Low Threshold

The motor unit twitch torque is modified by sustained contraction, but the association to changes in muscle fiber electrophysiological properties is not fully known. Thus twitch torque, muscle fiber conduction velocity, and action potential properties of single motor units were assessed in 11 subjects following an isometric submaximal contraction of the tibialis anterior muscle until endurance. The volunteers activated a target motor unit at the minimum discharge rate in eight 3-min-long contractions, three before and five after an isometric contraction at 40% of the maximal torque, sustained until endurance. Multichannel surface electromyogram signals and joint torque were averaged with the target motor unit potential as trigger. Discharge rate (mean ± SE, 6.6 ± 0.2 pulses/s) and interpulse interval variability (33.3 ± 7.0%) were not different in the eight contractions. Peak twitch torque and recruitment threshold increased significantly (93 ± 29 and 12 ± 5%, P < 0.05) in the contraction immediately after the endurance task with respect to the preendurance values (0.94 ± 0.26 mN·m and 3.7 ± 0.5% of the maximal torque), whereas time to peak of the twitch torque did not change (74.4 ± 10.1 ms). Muscle fiber conduction velocity decreased and action potential duration increased in the contraction after the endurance (6.3 ± 1.8 and 9.8 ± 1.8%, respectively, P < 0.05; preendurance values, 3.9 ± 0.2 m/s and 11.1 ± 0.8 ms), whereas the surface potential peak-to-peak amplitude did not change (27.1 ± 3.1 μV). There was no significant correlation between the relative changes in muscle fiber conduction velocity or surface potential duration and in peak twitch torque ( R2 = 0.04 and 0.10, respectively). In conclusion, modifications in peak twitch torque of low-threshold motor units with sustained contraction are mainly determined by mechanisms not related to changes in action potential shape and in its propagation velocity.

Cutaneous stimulation fails to alter motor unit recruitment in the decerebrate cat

1993 ◽  
Vol 70 (4) ◽  
pp. 1433-1439 ◽  
Author(s):  
B. D. Clark ◽  
S. M. Dacko ◽  
T. C. Cope
Keyword(s):  
Motor Unit ◽  
Conduction Velocity ◽  
Motor Units ◽  
Firing Frequency ◽  
Medial Gastrocnemius ◽  
Electromyographic Activity ◽  
Size Principle ◽  
Decerebrate Cat ◽  
Slow Twitch ◽  
Recruitment Order

1. An attempt was made to repeat the observation that cutaneous input to the cat medial gastrocnemius (MG) muscle sometimes had the differential effect of inhibiting motoneurons with slow axonal conduction velocity while simultaneously exciting others with fast conduction velocity. Dual microelectrode recording from intact ventral root filaments was used to study the effects of cutaneous inputs on recruitment order and on firing frequency of physiologically characterized MG motor units in decerebrate cats. Motor responses to pinch of the skin over the lateral surface of the ankle as well as electrical stimulation of the caudal cutaneous sural (CCS) nerve were contrasted with the responses to static muscle stretch as well as muscle vibration. 2. In contrast to the prediction, recruitment order in pairwise tests was the same for skin pinch or CCS stimulation as it was for MG stretch or vibration in all 32 tested pairs of motor units. This sample included seven pairs comprising one slow-twitch (S) and one fast-twitch motor unit, where the predicted reversal of recruitment should have been most apparent. Regardless of the source of excitation, recruitment of motor units of the MG was consistent with Henneman's size principle in approximately 90% of trials. 3. Skin pinch increased the firing rate of 30 of 32 individual motor units previously activated by stretch or vibration, including 7 slow-twitch units. In the remaining two units, skin pinch transiently (100-400 ms) slowed the firing of an S unit in 11 of 13 vibration + pinch trials. The other unit (type unknown) showed one or two retarded spikes in each of four vibration + pinch trials. In three S units, including the lone inhibitable unit and two others that were only excited by skin pinch, there was a significant positive rank correlation between change in unit firing frequency and change in soleus integrated electromyographic activity.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

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