Distribution of motor unit properties across human muscles

2021 ◽  
Author(s):  
Jacques Duchateau ◽  
Roger M. Enoka
Keyword(s):  
Motor Unit ◽  
Neurological Disorders ◽  
Motor Units ◽  
Current Data ◽  
Influential Factor ◽  
Discharge Rates ◽  
Human Muscles ◽  
Hand Muscle ◽  
Voluntary Contractions ◽  
General Organization

The purpose of our review was to compare the distribution of motor unit properties across human muscles of different sizes and recruitment ranges. Although motor units can be distinguished based on several different attributes, we focused on four key parameters that have a significant influence on the force produced by muscle during voluntary contractions: the number of motor units, average innervation number, and the distributions of contractile characteristics and discharge rates within motor unit pools. Despite relatively few publications on this topic, current data indicate that the most influential factor in the distribution of these motor unit properties between muscles is innervation number. Nonetheless, despite a five-fold difference in innervation number between a hand muscle (first dorsal interosseus) and a lower leg muscle (tibialis anterior), the general organization of their motor unit pools and the range of discharge rates appears to be relatively similar. These observations provide foundational knowledge for studies on the control of movement and the changes that occur with ageing and neurological disorders.

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.

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.

Motor-unit discharge rates in maximal voluntary contractions of three human muscles

1983 ◽  
Vol 50 (6) ◽  
pp. 1380-1392 ◽  
Author(s):  
F. Bellemare ◽  
J. J. Woods ◽  
R. Johansson ◽  
B. Bigland-Ritchie
Keyword(s):  
Motor Unit ◽  
Biceps Brachii ◽  
Motor Units ◽  
Adductor Pollicis ◽  
Maximum Force ◽  
Mean Values ◽  
Firing Rates ◽  
Discharge Rates ◽  
The Mean ◽  
Voluntary Contractions

Single motor<unit firing rates have been recorded during maximal voluntary contractions using tungsten microelectrodes. Over 300 units from four subjects were sampled from each of three muscles. These were the biceps brachii, adductor pollicis, and soleus, chosen because of known differences in their fiber<type composition and contractile properties. In all cases the contraction maximality was assured by delivering single supramaximal shocks during the voluntary contractions. All motor units were deemed to have already been fully activated if no additional force resulted. Thus for each muscle, the firing rates elicited by a maximal voluntary effort are sufficient to generate a fully fused tetanus in each motor unit. For the biceps brachii and adductor pollicis muscles, the mean firing rates (+/- SD) were 31.1 +/- 10.1 and 29.9 +/- 8.6 Hz, respectively, while for soleus they were only 10.7 +/- 2.9 HZ. For each muscle the firing rates distribution covered approximately a four-fold range about the mean value. The mean firing rates for each muscle varied roughly in proportion to their respective twitch contraction and half relaxation times. These contractile time measurements for both biceps brachii and adductor pollicis agreed well with the mean values reported for human fast-twitch motor units, while those for soleus fell in the range observed for human slow-twitch units. An argument is presented that suggests that, in response to voluntary effort, the range of discharge rates of each motor-unit pool is limited to those only just sufficient to produce maximum force in each motor unit. This suggestion is based on the relationship between the range of motor-unit firing frequencies observed during maximum voluntary contractions, their range of contraction times, and the stimulation frequencies required for maximum force generation. The implications of this hypothesis for motor control are discussed.

scholarly journals Stroke increases ischemia-related decreases in motor unit discharge rates

2018 ◽  
Vol 120 (6) ◽  
pp. 3246-3256 ◽  
Author(s):  
Spencer A. Murphy ◽  
Francesco Negro ◽  
Dario Farina ◽  
Tanya Onushko ◽  
Matthew Durand ◽  
...  
Keyword(s):  
Motor Unit ◽  
Time Constant ◽  
Motor Units ◽  
Unit Discharge ◽  
Firing Rates ◽  
Sensory Pathways ◽  
Discharge Rates ◽  
Motor Unit Firing ◽  
Motor Unit Discharge ◽  
Density Surface

Following stroke, hyperexcitable sensory pathways, such as the group III/IV afferents that are sensitive to ischemia, may inhibit paretic motor neurons during exercise. We quantified the effects of whole leg ischemia on paretic vastus lateralis motor unit firing rates during submaximal isometric contractions. Ten chronic stroke survivors (>1 yr poststroke) and 10 controls participated. During conditions of whole leg occlusion, the discharge timings of motor units were identified from decomposition of high-density surface electromyography signals during repeated submaximal knee extensor contractions. Quadriceps resting twitch responses and near-infrared spectroscopy measurements of oxygen saturation as an indirect measure of blood flow were made. There was a greater decrease in paretic motor unit discharge rates during the occlusion compared with the controls (average decrease for stroke and controls, 12.3 ± 10.0% and 0.1 ± 12.4%, respectively; P < 0.001). The motor unit recruitment thresholds did not change with the occlusion (stroke: without occlusion, 11.68 ± 5.83%MVC vs. with occlusion, 11.11 ± 5.26%MVC; control: 11.87 ± 5.63 vs. 11.28 ± 5.29%MVC). Resting twitch amplitudes declined similarly for both groups in response to whole leg occlusion (stroke: 29.16 ± 6.88 vs. 25.75 ± 6.78 Nm; control: 38.80 ± 13.23 vs 30.14 ± 9.64 Nm). Controls had a greater exponential decline (lower time constant) in oxygen saturation compared with the stroke group (stroke time constant, 22.90 ± 10.26 min vs. control time constant, 5.46 ± 4.09 min; P < 0.001). Ischemia of the muscle resulted in greater neural inhibition of paretic motor units compared with controls and may contribute to deficient muscle activation poststroke. NEW & NOTEWORTHY Hyperexcitable inhibitory sensory pathways sensitive to ischemia may play a role in deficient motor unit activation post stroke. Using high-density surface electromyography recordings to detect motor unit firing instances, we show that ischemia of the exercising muscle results in greater inhibition of paretic motor unit firing rates compared with controls. These findings are impactful to neurophysiologists and clinicians because they implicate a novel mechanism of force-generating impairment poststroke that likely exacerbates baseline weakness.

scholarly journals Blood Flow Restriction Alters Motor Unit Behavior During Resistance Exercise

2019 ◽  
Vol 40 (09) ◽  
pp. 555-562 ◽  
Author(s):  
Pedro Fatela ◽  
Goncalo V. Mendonca ◽  
António Prieto Veloso ◽  
Janne Avela ◽  
Pedro Mil-Homens
Keyword(s):  
Blood Flow ◽  
Firing Rate ◽  
Motor Unit ◽  
Motor Units ◽  
Blood Flow Restriction ◽  
Motor Unit Recruitment ◽  
Recruitment Threshold ◽  
Firing Rates ◽  
Flow Restriction ◽  
Voluntary Contractions

AbstractWe aimed to determine whether blood flow restriction (BFR) alters the characteristics of individual motor units during low-intensity (LI) exercise. Eight men (26.0±3.8 yrs) performed 5 sets of 15 knee extensions at 20% of one-repetition maximum (with and without BFR). Maximal isometric voluntary contractions (MVC) were performed before and after exercise to quantify force decrement. Submaximal isometric voluntary contractions were additionally performed for 18 s, matching trapezoidal target-force trajectories at 40% pre-MVC. EMG activity was recorded from the vastus lateralis muscle. Then, signals were decomposed to extract motor unit recruitment threshold, firing rates and action potential amplitudes (MUAP). Force decrement was only seen after LI BFR exercise (–20.5%; p<0.05). LI BFR exercise also induced greater decrements in the linear slope coefficient of the regression lines between motor unit recruitment threshold and firing rate (BFR: –165.1±120.4 vs. non-BFR: –44.4±33.1%, p<0.05). Finally, there was a notable shift towards higher values of firing rate and MUAP amplitude post-LI BFR exercise. Taken together, our data indicate that LI BFR exercise increases the activity of motor units with higher MUAP amplitude. They also indicate that motor units with similar MUAP amplitudes become activated at higher firing rates post-LI BFR exercise.

Motor-Unit Synchronization Alters Spike-Triggered Average Force in Simulated Contractions

2002 ◽  
Vol 88 (1) ◽  
pp. 265-276 ◽  
Author(s):  
Anna M. Taylor ◽  
Julie W. Steege ◽  
Roger M. Enoka
Keyword(s):  
Motor Unit ◽  
Motor Units ◽  
Contraction Time ◽  
Average Force ◽  
Twitch Force ◽  
Discharge Rates ◽  
Spike Triggered Averaging ◽  
Motor Unit Synchronization ◽  
Contraction Times ◽  
Twitch Characteristics

The purpose of the study was to quantify the effect of motor-unit synchronization on the spike-triggered average forces of a population of motor units. Muscle force was simulated by defining mechanical and activation characteristics of the motor units, specifying motor neuron discharge times, and imposing various levels of motor-unit synchronization. The model comprised 120 motor units. Simulations were performed for motor units 5–120 to compare the spike-triggered average responses in the presence and absence of motor-unit synchronization with the motor-unit twitch characteristics defined in the model. To synchronize motor-unit activity, selected motor-unit discharge times were adjusted; this kept the number of action potentials constant across the three levels of synchrony for each motor unit. Because there was some overlap of motor-unit twitches even at minimal discharge rates, the simulations indicated that spike-triggered averaging underestimates the twitch force of all motor units and the contraction time of motor units with contraction times longer than 49 ms. Although motor-unit synchronization increased the estimated twitch force and decreased the estimated contraction time of all motor units, spike-triggered average force changed systematically with the level of synchrony in motor units 59–120 (upper 90% of the range of twitch forces). However, the reduction in contraction time was similar for moderate and high synchrony. In conclusion, spike-triggered averaging appears to provide a biased estimate of the distribution of twitch properties for a population of motor units because twitch fusion causes an underestimation of twitch force for slow units and motor-unit synchronization causes an overestimation of force for fast motor units.

scholarly journals Neural and muscular determinants of maximal rate of force development

2020 ◽  
Vol 123 (1) ◽  
pp. 149-157 ◽  
Author(s):  
Jakob L. Dideriksen ◽  
Alessandro Del Vecchio ◽  
Dario Farina
Keyword(s):  
Motor Unit ◽  
Maximum Rate ◽  
Rapid Development ◽  
Motor Units ◽  
Rate Of Force Development ◽  
Time Range ◽  
Limiting Factor ◽  
Force Development ◽  
Twitch Contraction ◽  
Discharge Rates

The ability to produce rapid forces requires quick motor unit recruitment, high motor unit discharge rates, and fast motor unit force twitches. The relative importance of these parameters for maximum rate of force development (RFD), however, is poorly understood. In this study, we systematically investigated these relationships using a computational model of motor unit pool activity and force. Across simulations, neural and muscular properties were systematically varied in experimentally observed ranges. Motor units were recruited over an interval starting from contraction onset (range: 22–233 ms). Upon recruitment, discharge rates declined from an initial rate (range: 89–212 pulses per second), with varying likelihood of doublet (interspike interval of 3 ms; range: 0–50%). Finally, muscular adaptations were modeled by changing average twitch contraction time (range: 42–78 ms). Spectral analysis showed that the effective neural drive to the simulated muscle had smaller bandwidths than the average motor unit twitch, indicating that the bandwidth of the motor output, and thus the capacity for explosive force, was limited mainly by neural properties. The simulated RFD increased by 1,050 ± 281% maximal voluntary contraction force per second from the longest to the shortest recruitment interval. This effect was more than fourfold higher than the effect of increasing the initial discharge rate, more than fivefold higher than the effect of increasing the chance of doublets, and more than sixfold higher than the effect of decreasing twitch contraction times. The simulated results suggest that the physiological variation of the rate by which motor units are recruited during ballistic contractions is the main determinant for the variability in RFD across individuals. NEW & NOTEWORTHY An important limitation of human performance is the ability to generate explosive movements by means of rapid development of muscle force. The physiological determinants of this ability, however, are poorly understood. In this study, we show using extensive simulations that the rate by which motor units are recruited is the main limiting factor for maximum rate of force development.

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.

Short interspike intervals and double discharges of anconeus motor unit action potentials for the production of dynamic elbow extensions

2014 ◽  
Vol 111 (10) ◽  
pp. 2039-2046 ◽  
Author(s):  
B. Harwood ◽  
C. L. Rice
Keyword(s):  
Motor Unit ◽  
Motor Units ◽  
Maximal Velocity ◽  
Short Isis ◽  
Interspike Intervals ◽  
Young Males ◽  
Motor Unit Action ◽  
Human Muscles ◽  
Low Incidence ◽  
Extensor Torque

Incidence of double discharges (DDs; >100 Hz) and short interspike intervals (ISIs; >50 to <100 Hz) is reported to vary widely among different muscles and tasks, with a higher incidence in motor unit (MU) trains of fast muscles and for the production of fast contractions in humans. However, it is unclear whether human muscles with a large composition of slower motor units exhibit DDs or short ISIs when activated with maximal synaptic drive, such as those required for maximal velocity dynamic contractions. Thus the purpose of this study was to determine the effect of increasing peak contraction velocity on the incidence of DDs and short ISIs in the anconeus muscle. Seventeen anconeus MUs in 10 young males were recorded across dynamic elbow extensions ranging from low submaximal velocities (16% of maximal velocity) up to maximal velocities. A low incidence of DDs (4%) and short ISIs (29%) was observed among the 583 MU trains recorded. Despite the low incidence in individual MU trains, a majority (71% and 94%, respectively) of MUs exhibited at least one DD or short ISI. The number of short ISIs shared no variance with MU recruitment threshold ( R2 = 0.02), but their distribution was skewed toward higher peak velocities ( G = −1.26) and a main effect of peak elbow extension velocity was observed ( P < 0.05). Although a greater number of short ISIs was observed with increasing velocity, the low incidence of DDs and short ISIs in the anconeus muscle is likely related to the function of the anconeus as a stabilizer rather than voluntary elbow extensor torque and velocity production.

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