Pain-induced changes in motor unit discharge depend on recruitment threshold and contraction speed

2021 ◽  
Author(s):  
Eduardo Martinez-Valdes ◽  
Francesco Negro ◽  
Michail Arvanitidis ◽  
Dario Farina ◽  
Deborah Falla
Keyword(s):  
Discharge Rate ◽  
Tibialis Anterior Muscle ◽  
Maximum Voluntary Contraction ◽  
Motor Units ◽  
Inhibitory Effect ◽  
Voluntary Contraction ◽  
Recruitment Threshold ◽  
Contraction Speed ◽  
Discharge Rates ◽  
Hypertonic Saline Injection

At high forces, the discharge rates of lower and higher threshold motor units (MU) are influenced in a different way by muscle pain. These differential effects may be particularly important for performing contractions at different speeds since the proportion of lower and higher threshold MUs recruited varies with contraction velocity. We investigated whether MU discharge and recruitment strategies are differentially affected by pain depending on their recruitment threshold (RT), across a range of contraction speeds. Participants performed ankle dorsiflexion sinusoidal-isometric contractions at two frequencies (0.25Hz and 1Hz) and two modulation amplitudes [5% and 10% of the maximum voluntary contraction (MVC)] with a mean target torque of 20%MVC. High-density surface electromyography recordings from the tibialis anterior muscle were decomposed and the same MUs were tracked across painful (hypertonic saline injection) and non-painful conditions. Torque variability, mean discharge rate (MDR), DR variability (DRvar), RT and the delay between the cumulative spike train and the resultant torque output (neuromechanical delay, NMD) were assessed. The average RT was greater at faster contraction velocities (p=0.01) but was not affected by pain. At the fastest contraction speed, torque variability and DRvar were reduced (p<0.05) and MDR was maintained. Conversely, MDR decreased and DRvar and NMD increased significantly during pain at slow contraction speeds (p<0.05). These results show that reductions in contraction amplitude and increased recruitment of higher threshold MUs at fast contraction speeds appears to compensate for the inhibitory effect of nociceptive inputs on lower threshold MUs, allowing the exertion of fast submaximal contractions during pain.

Discharge properties of abductor hallucis before, during, and after an isometric fatigue task

2013 ◽  
Vol 110 (4) ◽  
pp. 891-898 ◽  
Author(s):  
Luke A. Kelly ◽  
Sebastien Racinais ◽  
Andrew G. Cresswell
Keyword(s):  
Discharge Rate ◽  
Weight Bearing ◽  
Maximum Voluntary Contraction ◽  
Motor Units ◽  
Abductor Hallucis ◽  
Recruitment Threshold ◽  
Longitudinal Arch ◽  
Discharge Rates ◽  
Significant Difference ◽  
Ramp Contractions

Abductor hallucis is the largest muscle in the arch of the human foot and comprises few motor units relative to its physiological cross-sectional area. It has been described as a postural muscle, aiding in the stabilization of the longitudinal arch during stance and gait. The purpose of this study was to describe the discharge properties of abductor hallucis motor units during ramp and hold isometric contractions, as well as its discharge characteristics during fatigue. Intramuscular electromyographic recordings from abductor hallucis were made in 5 subjects; from those recordings, 42 single motor units were decomposed. Data were recorded during isometric ramp contractions at 60% maximum voluntary contraction (MVC), performed before and after a submaximal isometric contraction to failure (mean force 41.3 ± 15.3% MVC, mean duration 233 ± 116 s). Motor unit recruitment thresholds ranged from 10.3 to 54.2% MVC. No significant difference was observed between recruitment and derecruitment thresholds or their respective discharge rates for both the initial and postfatigue ramp contractions (all P > 0.25). Recruitment threshold was positively correlated with recruitment discharge rate ( r = 0.47, P < 0.03). All motor units attained similar peak discharge rates (14.0 ± 0.25 pulses/s) and were not correlated with recruitment threshold. Thirteen motor units could be followed during the isometric fatigue task, with a decline in discharge rate and increase in discharge rate variability occurring in the final 25% of the task (both P < 0.05). We have shown that abductor hallucis motor units discharge relatively slowly and are considerably resistant to fatigue. These characteristics may be effective for generating and sustaining the substantial level of force that is required to stabilize the longitudinal arch during weight bearing.

Changes in muscle fiber conduction velocity indicate recruitment of distinct motor unit populations

2003 ◽  
Vol 95 (3) ◽  
pp. 1045-1054 ◽  
Author(s):  
C. J. Houtman ◽  
D. F. Stegeman ◽  
J. P. Van Dijk ◽  
M. J. Zwarts
Keyword(s):  
Conduction Velocity ◽  
Muscle Fiber ◽  
Tibialis Anterior Muscle ◽  
Maximum Voluntary Contraction ◽  
Isometric Exercise ◽  
Motor Units ◽  
Voluntary Contraction ◽  
Type I ◽  
Muscle Fiber Conduction Velocity ◽  
Depth Study

To obtain more insight into the changes in mean muscle fiber conduction velocity (MFCV) during sustained isometric exercise at relatively low contraction levels, we performed an in-depth study of the human tibialis anterior muscle by using multichannel surface electromyogram. The results show an increase in MFCV after an initial decrease of MFCV at 30 or 40% maximum voluntary contraction in all of the five subjects studied. With a peak velocity analysis, we calculated the distribution of conduction velocities of action potentials in the bipolar electromyogram signal. It shows two populations of peak velocities occurring simultaneously halfway through the exercise. The MFCV pattern implies the recruitment of two different populations of motor units. Because of the lowering of MFCV of the first activated population of motor units, the newly recruited second population of motor units becomes visible. It is most likely that the MFCV pattern can be ascribed to the fatiguing of already recruited predominantly type I motor units, followed by the recruitment of fresh, predominantly type II, motor units.

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.

scholarly journals Recruitment of motor units in two fascicles of the semispinalis cervicis muscle

2012 ◽  
Vol 107 (11) ◽  
pp. 3078-3085 ◽  
Author(s):  
Jochen Schomacher ◽  
Jakob Lund Dideriksen ◽  
Dario Farina ◽  
Deborah Falla
Keyword(s):  
Motor Unit ◽  
Discharge Rate ◽  
Maximum Voluntary Contraction ◽  
Motor Units ◽  
Voluntary Contraction ◽  
Common Input ◽  
Motor Unit Synchronization ◽  
Motor Unit Action ◽  
Input Strength ◽  
Individual Motor

This study investigated the behavior of motor units in the semispinalis cervicis muscle. Intramuscular EMG recordings were obtained unilaterally at levels C2 and C5 in 15 healthy volunteers (8 men, 7 women) who performed isometric neck extensions at 5%, 10%, and 20% of the maximal force [maximum voluntary contraction (MVC)] for 2 min each and linearly increasing force contractions from 0 to 30% MVC over 3 s. Individual motor unit action potentials were identified. The discharge rate and interspike interval variability of the motor units in the two locations did not differ. However, the recruitment threshold of motor units detected at C2 ( n = 16, mean ± SD: 10.3 ± 6.0% MVC) was greater than that of motor units detected at C5 ( n = 92, 6.9 ± 4.3% MVC) ( P < 0.01). A significant level of short-term synchronization was identified in 246 of 307 motor unit pairs when computed within one spinal level but only in 28 of 110 pairs of motor units between the two levels. The common input strength, which quantifies motor unit synchronization, was greater for pairs within one level (0.47 ± 0.32) compared with pairs between levels (0.09 ± 0.07) ( P < 0.05). In a second experiment on eight healthy subjects, interference EMG was recorded from the same locations during a linearly increasing force contraction from 0 to 40% MVC and showed significantly greater EMG amplitude at C5 than at C2. In conclusion, synaptic input is distributed partly independently and nonuniformly to different fascicles of the semispinalis cervicis muscle.

scholarly journals Motor control differs for increasing and releasing force

2016 ◽  
Vol 115 (6) ◽  
pp. 2924-2930 ◽  
Author(s):  
Seoung Hoon Park ◽  
MinHyuk Kwon ◽  
Danielle Solis ◽  
Neha Lodha ◽  
Evangelos A. Christou
Keyword(s):  
Older Adults ◽  
Force Control ◽  
Discharge Rate ◽  
Maximum Voluntary Contraction ◽  
Motor Units ◽  
Voluntary Contraction ◽  
Force Variability ◽  
Force Output ◽  
Force Increase ◽  
Change In Mean

Control of the motor output depends on our ability to precisely increase and release force. However, the influence of aging on force increase and release remains unknown. The purpose of this study, therefore, was to determine whether force control differs while increasing and releasing force in young and older adults. Sixteen young adults (22.5 ± 4 yr, 8 females) and 16 older adults (75.7 ± 6.4 yr, 8 females) increased and released force at a constant rate (10% maximum voluntary contraction force/s) during an ankle dorsiflexion isometric task. We recorded the force output and multiple motor unit activity from the tibialis anterior (TA) muscle and quantified the following outcomes: 1) variability of force using the SD of force; 2) mean discharge rate and variability of discharge rate of multiple motor units; and 3) power spectrum of the multiple motor units from 0–4, 4–10, 10–35, and 35–60 Hz. Participants exhibited greater force variability while releasing force, independent of age ( P < 0.001). Increased force variability during force release was associated with decreased modulation of multiple motor units from 35 to 60 Hz ( R2 = 0.38). Modulation of multiple motor units from 35 to 60 Hz was further correlated to the change in mean discharge rate of multiple motor units ( r = 0.66) and modulation from 0 to 4 Hz ( r = −0.64). In conclusion, these findings suggest that force control is altered while releasing due to an altered modulation of the motor units.

scholarly journals Sex differences in motor unit discharge rates at maximal and submaximal levels of force output

2020 ◽  
Vol 45 (11) ◽  
pp. 1197-1207
Author(s):  
J. Greig Inglis ◽  
David A. Gabriel
Keyword(s):  
Sex Differences ◽  
Motor Unit ◽  
Discharge Rate ◽  
Maximum Voluntary Contraction ◽  
Voluntary Contraction ◽  
Force Output ◽  
Neural Drive ◽  
Discharge Rates ◽  
Intramuscular Electrodes ◽  
Motor Unit Discharge

This study evaluated potential sex differences in motor unit (MU) behaviour at maximal and submaximal force outputs. Forty-eight participants, 24 females and 24 males, performed isometric dorsiflexion contractions at 20%, 40%, 60%, 80%, and 100% of a maximum voluntary contraction (MVC). Tibialis anterior electromyography was recorded both by surface and intramuscular electrodes. Compared with males, females had a greater MU discharge rate (MUDR) averaged across all submaximal intensities (Δ 0.45 pps, 2.56%). Males exhibited greater increases in MUDR above 40% MVC, surpassing females at 100% MVC (p’s < 0.01). Averaged across all force outputs, females had a greater incidence of doublet and rapid discharges and a greater percentage of MU trains with doublet and rapid (5–10 ms) discharges (Δ 75.55% and 61.48%, respectively; p’s < 0.01). A subset of males (n = 8) and females (n = 8), matched for maximum force output, revealed that females had even greater MUDR (Δ 1.38 pps, 7.47%) and percentage of MU trains with doublet and rapid discharges (Δ 51.62%, 56.68%, respectively; p’s < 0.01) compared with males at each force output, including 100% MVC. Analysis of the subset of strength-matched males and females suggest that sex differences in MU behaviour may be a result of females needing to generate greater neural drive to achieve fused tetanus. Novelty Females had higher MUDRs and greater percentage of MU trains with doublets across submaximal force outputs (20%–80% MVC). Differences were even greater for a strength matched subset. Differences in motor unit behaviour may arise from musculoskeletal differences, requiring greater neural drive in females.

Multiple Features of Motor-Unit Activity Influence Force Fluctuations During Isometric Contractions

2003 ◽  
Vol 90 (2) ◽  
pp. 1350-1361 ◽  
Author(s):  
Anna M. Taylor ◽  
Evangelos A. Christou ◽  
Roger M. Enoka
Keyword(s):  
Motor Unit ◽  
Discharge Rate ◽  
Maximum Voluntary Contraction ◽  
Power Spectra ◽  
Motor Units ◽  
Voluntary Contraction ◽  
Total Power ◽  
Force Fluctuations ◽  
Synchronization Model ◽  
Rate Coding

To identify the mechanisms responsible for the fluctuations in force that occur during voluntary contractions, experimental measurements were compared with simulated forces in the time and frequency domains at contraction intensities that ranged from 2 to 98% of the maximum voluntary contraction (MVC). The abduction force exerted by the index finger due to an isometric contraction of the first dorsal interosseus muscle was measured in 10 young adults. Force was simulated with computer models of motor-unit recruitment and rate coding for a population of 120 motor units. The models varied recruitment and rate-coding properties of the motor units and the activation pattern of the motor-unit population. The main finding was that the experimental observations of a minimum in the coefficient of variation (CV) for force (1.7%) at approximately 30% MVC and a plateau at higher forces could not be replicated by any of the models. The model that increased the level of short-term synchrony with excitatory drive provided the closest fit to the experimentally observed relation between the CV for force and the mean force. In addition, the results for the synchronization model extended previous modeling efforts to show that the effect of synchronization is independent from that of discharge-rate variability. Most of the power in the force power spectra for the models was contained in the frequency bins below 5 Hz. Only a model that included a low-frequency oscillation in excitation, however, could approximate the experimental finding of peak power at a frequency below 2 Hz: 38% of total power at 0.99 Hz and 43% at 1.37 Hz, respectively. In contrast to the experimental power spectra, all model spectra included a second peak at a higher frequency. The secondary peak was less prominent in the synchronization model because of greater variability in discharge rate. These results indicate that the variation in force fluctuations across the entire operating range of the muscle cannot be explained by a single mechanism that influences the output of the motor-unit population.

Fine-wire recordings of flexor hallucis brevis motor units up to maximal voluntary contraction reveal a flexible, nonrigid mechanism for force control

2020 ◽  
Vol 123 (5) ◽  
pp. 1766-1774
Author(s):  
J. Aeles ◽  
L. A. Kelly ◽  
Y. Yoshitake ◽  
A. G. Cresswell
Keyword(s):  
Full Range ◽  
Maximum Voluntary Contraction ◽  
Motor Units ◽  
Voluntary Contraction ◽  
Motor Unit Action Potential ◽  
Single Motor Unit ◽  
Discharge Rates ◽  
Motor Unit Action ◽  
First Time ◽  
Unit Action

We recorded for the first time single motor unit action potential trains in the flexor hallucis brevis, a short toe muscle, over the full range of maximum voluntary contraction. Its motor units are recruited up to very high (98%) recruitment thresholds with a substantial range of discharge rates. We further show high variability with crossover of discharge rates as a function of recruitment threshold both between participants and between motor units within participants.

scholarly journals Discharge properties of motor units during steady isometric contractions performed with the dorsiflexor muscles

2012 ◽  
Vol 112 (11) ◽  
pp. 1897-1905 ◽  
Author(s):  
Mark Jesunathadas ◽  
Malgorzata Klass ◽  
Jacques Duchateau ◽  
Roger M. Enoka
Keyword(s):  
Tibialis Anterior ◽  
Interspike Interval ◽  
Motor Units ◽  
Voluntary Contraction ◽  
Isometric Contractions ◽  
Discharge Characteristics ◽  
Recruitment Threshold ◽  
Discharge Rates ◽  
Contraction Times ◽  
Experimental Values

The purpose of this study was to record the discharge characteristics of tibialis anterior motor units over a range of target forces and to import these data, along with previously reported observations, into a computational model to compare experimental and simulated measures of torque variability during isometric contractions with the dorsiflexor muscles. The discharge characteristics of 44 motor units were quantified during brief isometric contractions at torques that ranged from recruitment threshold to an average of 22 ± 14.4% maximal voluntary contraction (MVC) torque above recruitment threshold. The minimal [range: 5.8–19.8 pulses per second (pps)] and peak (range: 8.6–37.5 pps) discharge rates of motor units were positively related to the recruitment threshold torque ( R2 ≥ 0.266; P < 0.001). The coefficient of variation for interspike interval at recruitment was positively associated with recruitment threshold torque ( R2 = 0.443; P < 0.001) and either decreased exponentially or remained constant as target torque increased above recruitment threshold torque. The variability in the simulated torque did not differ from the experimental values once the recruitment range was set to ∼85% MVC torque, and the association between motor twitch contraction times and peak twitch torque was defined as a weak linear association ( R2 = 0.096; P < 0.001). These results indicate that the steadiness of isometric contractions performed with the dorsiflexor muscle depended more on the distributions of mechanical properties than discharge properties across the population of motor units in the tibialis anterior.

scholarly journals Rate modulation of human anconeus motor units during high-intensity dynamic elbow extensions

2016 ◽  
Vol 121 (2) ◽  
pp. 475-482 ◽  
Author(s):  
Brianna L. Cowling ◽  
Brad Harwood ◽  
David B. Copithorne ◽  
Charles L. Rice
Keyword(s):  
High Intensity ◽  
Peak Power ◽  
Maximum Voluntary Contraction ◽  
Motor Units ◽  
Voluntary Contraction ◽  
Elbow Extension ◽  
Concentric Contractions ◽  
Motor Unit Firing ◽  
Maximal Effort ◽  
Isometric Maximum Voluntary Contraction

Investigations of high-intensity isometric fatiguing protocols report decreases in motor unit firing rates (MUFRs), but little is known regarding changes in MUFRs following fatigue induced by high-intensity dynamic contractions. Our purpose was to evaluate MUFRs of the anconeus (an accessory elbow extensor) and elbow extension power production as a function of time to task failure (TTF) during high-velocity fatiguing concentric contractions against a moderately heavy resistance. Fine-wire intramuscular electrode pairs were inserted into the anconeus to record MUs in 12 male participants (25 ± 3 yr), over repeated sessions on separate days. MUs were tracked throughout a three-stage, varying load dynamic elbow extension protocol designed to extend the task duration for >1 min thereby inducing substantial fatigue. Mean MUFRs and peak power were calculated for three relative time ranges: 0–15% TTF (beginning), 45–60% TTF (middle) and 85–100% TTF (end). Mean duration of the overall fatigue protocol was ∼80 s. Following the protocol, isometric maximum voluntary contraction (MVC), highest velocity at 35% MVC load, and peak power decreased 37, 60, and 64% compared with baseline, respectively. Data from 20 anconeus MUs tracked successfully throughout the protocol indicated a reduction in MUFRs in relation to power loss from 36 Hz/160 W (0–15% TTF) to 28 Hz/97 W (45–60% TTF) to 23 Hz/43 W (85–100% TTF). During these high-intensity maximal effort concentric contractions, anconeus MUFRs decreased substantially (>35%). Although the absolute MUFRs were higher in the present study than those reported previously for other muscles during sustained high-intensity isometric tasks, the relative decrease in MUFRs was similar between the two tasks.

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