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.

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.

Effect of temperature on spike-triggered average torque and electrophysiological properties of low-threshold motor units

2005 ◽  
Vol 99 (1) ◽  
pp. 197-203 ◽  
Author(s):  
Dario Farina ◽  
Lars Arendt-Nielsen ◽  
Thomas Graven-Nielsen
Keyword(s):  
Action Potential ◽  
Skin Temperature ◽  
Motor Unit ◽  
Conduction Velocity ◽  
Discharge Rate ◽  
Peak Amplitude ◽  
Single Motor Unit ◽  
Time To Peak ◽  
Low Threshold ◽  
Electromyographic Signals

The aim of the study was to jointly analyze temperature-induced changes in low-threshold single motor unit twitch torque and action potential properties. Joint torque, multichannel surface, and intramuscular electromyographic signals were recorded from the tibialis anterior muscle of 12 subjects who were instructed to identify the activity of a target motor unit using intramuscular electromyographic signals as feedback. The target motor unit was activated at the minimum stable discharge rate in seven 3-min-long contractions. The first three contractions (C1–C3) were performed at 33°C skin temperature. After 5 min, the subject performed three contractions at 33°C (T1), 39°C (T2), and 45°C (T3), followed by a contraction at 33°C (C4) skin temperature. Twitch torque and multichannel surface action potential of the target motor unit were obtained by spike-triggered averaging. Discharge rate (mean ± SE, 7.1 ± 0.5 pulses/s), interpulse interval variability (35.8 ± 9.2%), and recruitment threshold (4.5 ± 0.4% of the maximal voluntary torque) were not different among the seven contractions. None of the investigated variables were different among C1–C3, T1, and C4. Conduction velocity and peak twitch torque increased with temperature ( P < 0.05; T1: 3.53 ± 0.21 m/s and 0.82 ± 0.23 mN·m, T2: 3.93 ± 0.24 m/s and 1.17 ± 0.36 mN·m, T3: 4.35 ± 0.25 m/s and 1.46 ± 0.40 mN·m, respectively). Twitch time to peak and surface action potential peak-to-peak amplitude were smaller in T3 (61.8 ± 2.0 ms and 27.4 ± 5.1 μV, respectively) than in T1 (71.9 ± 4.1 ms and 35.0 ± 6.5 μV, respectively) ( P < 0.05). The relative increase in conduction velocity between T1 and T3 was positively correlated ( P < 0.05) with the increase in twitch peak amplitude ( r2 = 0.48), with the decrease in twitch time to peak ( r2 = 0.43), and with the decrease in action potential amplitude ( r2 = 0.50). In conclusion, temperature-induced modifications in fiber membrane conduction properties may have a direct effect on contractile motor unit properties.

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.

scholarly journals Reduced Endplate Currents Underlie Motor Unit Dysfunction in Canine Motor Neuron Disease

2002 ◽  
Vol 88 (6) ◽  
pp. 3293-3304 ◽  
Author(s):  
Mark M. Rich ◽  
Robert. F. Waldeck ◽  
Linda C. Cork ◽  
Rita J. Balice-Gordon ◽  
Robert E. W. Fyffe ◽  
...  
Keyword(s):  
Motor Neuron ◽  
Motor Neuron Disease ◽  
Motor Unit ◽  
Conduction Velocity ◽  
Motor Neurons ◽  
Muscle Fiber ◽  
Muscular Atrophy ◽  
Motor Units ◽  
Postnatal Maturation ◽  
Axonal Conduction

Hereditary canine spinal muscular atrophy (HCSMA) is an autosomal dominant degenerative disorder of motor neurons. In homozygous animals, motor units produce decreased force output and fail during repetitive activity. Previous studies suggest that decreased efficacy of neuromuscular transmission underlies these abnormalities. To examine this, we recorded muscle fiber endplate currents (EPCs) and found reduced amplitudes and increased failures during nerve stimulation in homozygotes compared with wild-type controls. Comparison of EPC amplitudes with muscle fiber current thresholds indicate that many EPCs from homozygotes fall below threshold for activating muscle fibers but can be raised above threshold following potentiation. To determine whether axonal abnormalities might play a role in causing motor unit dysfunction, we examined the postnatal maturation of axonal conduction velocity in relation to the appearance of tetanic failure. We also examined intracellularly labeled motor neurons for evidence of axonal neurofilament accumulations, which are found in many instances of motor neuron disease including HCSMA. Despite the appearance of tetanic failure between 90 and 120 days, average motor axon conduction velocity increased with age in homozygotes and achieved adult levels. Normal correlations between motor neuron properties (including conduction velocity) and motor unit properties were also observed. Labeled proximal motor axons of several motor neurons that supplied failing motor units exhibited little or no evidence of axonal swellings. We conclude that decreased release of transmitter from motor terminals underlies motor unit dysfunction in HCSMA and that the mechanisms determining the maturation of axonal conduction velocity and the pattern of correlation between motor neuron and motor unit properties do not contribute to the appearance or evolution of motor unit dysfunction.

Conduction Velocity of Quiescent Muscle Fibers Decreases During Sustained Contraction

2005 ◽  
Vol 94 (1) ◽  
pp. 387-394 ◽  
Author(s):  
Marco Gazzoni ◽  
Federico Camelia ◽  
Dario Farina
Keyword(s):  
Motor Unit ◽  
Conduction Velocity ◽  
Muscle Fibers ◽  
Motor Units ◽  
Membrane Properties ◽  
Control Group ◽  
Experimental Session ◽  
Sustained Contraction ◽  
Voluntary Contractions ◽  
Percent Decrease

We tested the hypothesis that conduction velocity of quiescent muscle fibers decreases during sustained contraction due to the activity of the active motor units in the muscle. Ten subjects trained for the identification of a target motor unit in the abductor pollicis brevis with feedback on surface EMG signals detected with a two-dimensional array of 61 electrodes. The subjects activated the target motor unit in two 10-s long contractions, before (contraction C1) and after (C3) a 3-min contraction (C2), all in ischemic condition. The target motor unit was not activated during C2. Eight of the 10 subjects (control group) performed a second experimental session identical to the first but with a resting period of 3 min instead of the contraction C2. Exerted force and target motor unit discharge rate were not different between the two subject groups and between C1 and C3 (mean ± SD, over C1 and C3; C2 group: 15.8 ± 10.4% maximal voluntary contractions and 13.1 ± 1.9 pps; control group: 15.6 ± 22.1% maximal voluntary contractions and 14.5 ± 1.9 pps, respectively). Muscle fiber conduction velocity of the target motor unit decreased in C3 with respect to C1 in the C2 group (3.59 ± 0.57 and 3.34 ± 0.47 m/s for C1 and C3, respectively; P < 0.05) but not in the control group (3.47 ± 0.68 and 3.46 ± 0.73 m/s). In the C2 group, the percent decrease in conduction velocity of the target motor unit between C1 and C3 (6.4 ± 7.1%) was not significantly different from the percent decrease in the average conduction velocity of the motor units active during C2 (9.6 ± 5.4%). In conclusion, the contraction-induced modifications in electrophysiological membrane properties of muscle fibers are partly independent on fiber activation.

Effect of power, pedal rate, and force on average muscle fiber conduction velocity during cycling

2004 ◽  
Vol 97 (6) ◽  
pp. 2035-2041 ◽  
Author(s):  
Dario Farina ◽  
Andrea Macaluso ◽  
Richard A. Ferguson ◽  
Giuseppe De Vito
Keyword(s):  
Conduction Velocity ◽  
Muscle Fiber ◽  
External Force ◽  
Power Output ◽  
Vastus Lateralis ◽  
Force Production ◽  
Motor Units ◽  
Angular Speed ◽  
Muscle Fiber Conduction Velocity ◽  
Pedal Rate

Muscle fiber conduction velocity (MFCV) provides indications on motor unit recruitment strategies due to the relation between conduction velocity and fiber diameter. The aim of this study was to investigate MFCV of thigh muscles during cycling at varying power outputs, pedal rates, and external forces. Twelve healthy male participants aged between 19 and 30 yr cycled on an electronically braked ergometer at 45, 60, 90, and 120 rpm. For each pedal rate, subjects performed two exercise intensities, one at an external power output corresponding to the previously determined lactate threshold (100% LT) and the other at half of this power output (50% LT). Surface electromyogram signals were detected during cycling from vastus lateralis and medialis muscles with linear adhesive arrays of eight electrodes. In both muscles, MFCV was higher at 100% LT compared with 50% LT for all average pedal rates except 120 rpm (mean ± SE, 4.98 ± 0.19 vs. 4.49 ± 0.18 m/s; P < 0.001). In all conditions, MFVC increased with increasing instantaneous knee angular speed (from 4.14 ± 0.16 to 5.08 ± 0.13 m/s in the range of instantaneous angular speeds investigated; P < 0.001). When MFCV was compared at the same external force production (i.e., 90 rpm/100% LT vs. 45 rpm/50% LT, and 120 rpm/100% LT vs. 60 rpm/50% LT), MFCV was higher at the faster pedal rate (5.02 ± 0.17 vs. 4.64 ± 0.12 m/s, and 4.92 ± 0.19 vs. 4.49 ± 0.11 m/s, respectively; P < 0.05) due to the increase in inertial power required to accelerate the limbs. It was concluded that, during repetitive dynamic movements, MFCV increases with the external force developed, instantaneous knee angular speed, and average pedal rate, indicating progressive recruitment of large, high conduction velocity motor units with increasing muscle force.

scholarly journals Higher muscle fiber conduction velocity and early rate of torque development in chronically strength-trained individuals

2018 ◽  
Vol 125 (4) ◽  
pp. 1218-1226 ◽  
Author(s):  
A. Del Vecchio ◽  
F. Negro ◽  
D. Falla ◽  
I. Bazzucchi ◽  
D. Farina ◽  
...  
Keyword(s):  
Conduction Velocity ◽  
Muscle Fiber ◽  
Motor Units ◽  
Control Group ◽  
Time Intervals ◽  
Neural Drive ◽  
Muscle Fiber Conduction Velocity ◽  
Rate Of Torque Development ◽  
First Time ◽  
Torque Development

Strength-trained individuals (ST) develop greater levels of force compared with untrained subjects. These differences are partly of neural origin and can be explained by training-induced changes in the neural drive to the muscles. In the present study we hypothesize a greater rate of torque development (RTD) and faster recruitment of motor units with greater muscle fiber conduction velocity (MFCV) in ST compared with a control cohort. MFCV was assessed during maximal voluntary isometric explosive contractions of the elbow flexors in eight ST and eight control individuals. MFCV was estimated from high-density surface electromyogram recordings (128 electrodes) in intervals of 50 ms starting from the onset of the electromyogram. RTD and MFCV were computed and normalized to their maximal voluntary torque (MVT) values. The explosive torque of the ST was greater than in the control group in all time intervals analyzed ( P < 0.001). The absolute MFCV values were also greater for the ST than for controls at all time intervals ( P < 0.001). ST also achieved greater normalized RTD in the first 50 ms of contraction [887.6 (152) vs. 568.5 (148.66)%MVT/s, mean (SD), P < 0.001] and normalized MFCV before the rise in force compared with controls. We have shown for the first time that ST can recruit motor units with greater MFCV in a shorter amount of time compared with untrained subjects during maximal voluntary isometric explosive contractions. NEW & NOTEWORTHY Strength-trained individuals show neuromuscular adaptations. These adaptations have been partly related to changes in the neural drive to the muscles. Here, we show for the first time that during the initial phase of a maximal isometric explosive contraction, strength-trained individuals achieve higher levels of force and recruit motor units with greater conduction velocities.

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.

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