Motor unit firing rates during isometric voluntary contractions performed at different muscle lengths

2004 ◽  
Vol 82 (8-9) ◽  
pp. 769-776 ◽  
Author(s):  
Alejandro Del Valle ◽  
Christine K Thomas
Keyword(s):  
Motor Unit ◽  
Muscle Activation ◽  
Muscle Length ◽  
Surface Emg ◽  
Voluntary Contraction ◽  
Force Frequency ◽  
Triceps Brachii ◽  
Firing Rates ◽  
Motor Unit Firing ◽  
Voluntary Contractions

Firing rates of motor units and surface EMG were measured from the triceps brachii muscles of able-bodied subjects during brief submaximal and maximal isometric voluntary contractions made at 5 elbow joint angles that covered the entire physiological range of muscle lengths. Muscle activation at the longest, midlength, and shortest muscle lengths, measured by twitch occlusion, averaged 98%, 97%, and 93% respectively, with each subject able to achieve complete activation during some contractions. As expected, the strongest contractions were recorded at 90° of elbow flexion. Mean motor unit firing rates and surface EMG increased with contraction intensity at each muscle length. For any given absolute contraction intensity, motor unit firing rates varied when muscle length was changed. However, mean motor unit firing rates were independent of muscle length when contractions were compared with the intensity of the maximal voluntary contraction (MVC) achieved at each joint angle.Key words: muscle activation, length–tension relationships, force–frequency relationships.

Differences in the motor unit firing rates and amplitudes in relation to recruitment thresholds during submaximal contractions of the first dorsal interosseous between chronically resistance-trained and physically active men

2018 ◽  
Vol 43 (8) ◽  
pp. 759-768 ◽  
Author(s):  
Adam J. Sterczala ◽  
Jonathan D. Miller ◽  
Michael A. Trevino ◽  
Hannah L. Dimmick ◽  
Trent J. Herda
Keyword(s):  
Motor Unit ◽  
Voluntary Contraction ◽  
Force Frequency ◽  
First Dorsal Interosseous ◽  
Physically Active ◽  
Firing Rates ◽  
Motor Unit Firing ◽  
Frequency Relationship ◽  
Muscle Actions ◽  
Active Controls

Previous investigations report no changes in motor unit (MU) firing rates during submaximal contractions following resistance training. These investigations did not account for MU recruitment or examine firing rates as a function of recruitment threshold (REC). Therefore, MU recruitment and firing rates in chronically resistance-trained (RT) and physically active controls (CON) were examined. Surface electromyography signals were collected from the first dorsal interosseous during isometric muscle actions at 40% and 70% maximal voluntary contraction (MVC). For each MU, force at REC, mean firing rate (MFR) during the steady force, and MU action potential amplitude (MUAPAMP) were analyzed. For each individual and contraction, the MFRs were linearly regressed against REC, whereas, exponential models were applied to the MFR versus MUAPAMP and MUAPAMP versus REC relationships with the y-intercepts and slopes (linear) and A and B terms (exponential) calculated. For the 40% MVC, the RT had less negative slopes (p = 0.001) and lower y-intercepts (p = 0.006) of the MFR versus REC relationships and lower B terms (p = 0.011) of the MUAPAMP versus REC relationships. There were no differences in either relationship between groups for the 70% MVC. During the 40% MVC, the RT had a smaller range of MFRs and MUAPAMPS in comparison with the CON, likely because of reduced MU recruitment. The RT had lower MFRs and recruitment during the 40% MVC, which may indicate a leftward shift in the force–frequency relationship, and thus require less excitation to the motoneuron pool to match the same relative force.

Initial phase of maximal voluntary and electrically stimulated knee extension torque development at different knee angles

2004 ◽  
Vol 97 (5) ◽  
pp. 1693-1701 ◽  
Author(s):  
C. J. de Ruiter ◽  
R. D. Kooistra ◽  
M. I. Paalman ◽  
A. de Haan
Keyword(s):  
Muscle Activation ◽  
Knee Extensor ◽  
Surface Emg ◽  
Knee Extension ◽  
Voluntary Contraction ◽  
Voluntary Activation ◽  
Knee Angle ◽  
Time Integral ◽  
Voluntary Contractions ◽  
Torque Development

We investigated the capacity for torque development and muscle activation at the onset of fast voluntary isometric knee extensions at 30, 60, and 90° knee angle. Experiments were performed in subjects ( n = 7) who had high levels (>90%) of activation at the plateau of maximal voluntary contractions. During maximal electrical nerve stimulation (8 pulses at 300 Hz), the maximal rate of torque development (MRTD) and torque time integral over the first 40 ms (TTI40) changed in proportion with torque at the different knee angles (highest values at 60°). At each knee angle, voluntary MRTD and stimulated MRTD were similar ( P < 0.05), but time to voluntary MRTD was significantly longer. Voluntary TTI40 was independent ( P > 0.05) of knee angle and on average (all subjects and angles) only 40% of stimulated TTI40. However, among subjects, the averaged (across knee angles) values ranged from 10.3 ± 3.1 to 83.3 ± 3.2% and were positively related ( r2 = 0.75, P < 0.05) to the knee-extensor surface EMG at the start of torque development. It was concluded that, although all subjects had high levels of voluntary activation at the plateau of maximal voluntary contraction, among subjects and independent of knee angle, the capacity for fast muscle activation varied substantially. Moreover, in all subjects, torque developed considerably faster during maximal electrical stimulation than during maximal voluntary effort. At different knee angles, stimulated MRTD and TTI40 changed in proportion with stimulated torque, but voluntary MRTD and TTI40 changed less than maximal voluntary torque.

scholarly journals The Effects of Resistance Training on Motor Unit Firing Rates and Muscle Activation

2018 ◽  
Vol 50 (5S) ◽  
pp. 567
Author(s):  
Adam J. Sterczala ◽  
Jonathan D. Miller ◽  
Mandy E. Wray ◽  
Hannah L. Dimmick ◽  
Michael A. Trevino ◽  
...  
Keyword(s):  
Resistance Training ◽  
Motor Unit ◽  
Muscle Activation ◽  
Firing Rates ◽  
Motor Unit Firing

Probabilities Associated With Counting Average Motor Unit Firing Rates in Active Human Muscle

1998 ◽  
Vol 23 (1) ◽  
pp. 87-94 ◽  
Author(s):  
Christopher Rich ◽  
George L. O′Brien ◽  
Enzo Cafarelli
Keyword(s):  
Motor Unit ◽  
Vastus Lateralis ◽  
Small Diameter ◽  
Motor Units ◽  
Voluntary Contraction ◽  
Human Muscle ◽  
Cell Diameter ◽  
Unit Recording ◽  
Firing Rates ◽  
Motor Unit Firing

Motor unit firing rates in human muscle can be determined from recordings made with small-diameter microelectrodes inserted directly into the muscle during voluntary contraction. Frequently, these counts are pooled to give an average motor unit firing rate under a given set of conditions. Since the fibers of one motor unit are dispersed among the cells of several others, it is conceivable that discharge rates can be measured in more than one cell from the same unit. If this occurred frequently, the distribution of firing rates could be influenced by those from units counted more than once. Based on literature values, we made the following assumptions: vastus lateralis contains approximately 300 motor units, with an average innervation ratio of 1500. Muscle cell diameter is about 50 to 100 μm and cells are randomly distributed over a motor unit territory of 10 μm diameter. The recording range of a microelectrode is about 600 μm. Given the distribution of cells normally found in whole human muscle, the probability of recording from two or more cells of the same motor unit at 50% MVC follows a Poisson distribution with a mean of 0.44. This model suggests that although there is a low probability of some duplication in this technique, the extent to which it influences the distribution of average motor unit firing rates is minimal over the entire range of forces produced by vastus lateralis. Key words: probability, motor unit, single unit recording, human muscle, rate coding

An examination of a potential organized motor unit firing rate and recruitment scheme of an antagonist muscle during isometric contractions

2021 ◽  
Author(s):  
Tanner Micah Reece ◽  
Trent J Herda
Keyword(s):  
Action Potential ◽  
Firing Rate ◽  
Motor Unit ◽  
Motor Unit Action Potential ◽  
Triceps Brachii ◽  
Antagonist Muscle ◽  
Recruitment Threshold ◽  
Firing Rates ◽  
Antagonist Muscles ◽  
Motor Unit Firing

The primary purpose of the present study is to determine if an organized control scheme exists for the antagonist muscle during steady isometric torque. A secondary focus is to better understand how firing rates of the antagonist muscle changes from a moderate- to higher-contraction intensity. Fourteen subjects performed two submaximal isometric trapezoid muscle actions of the forearm flexors that included a linearly increasing, steady force at both 40% and 70% maximum voluntary contraction, and linearly decreasing segments. Surface electromyographic signals of the biceps and triceps brachii were collected and decomposed into constituent motor unit action potential trains. Motor unit firing rate vs. recruitment threshold, motor unit action potential amplitude vs. recruitment threshold, and motor unit firing rate vs. action potential amplitude relationships of the biceps brachii (agonist) and triceps brachii (antagonist) muscles were analyzed. Moderate- to-strong relationships (|r| ³ 0.69) were present for the agonist and antagonist muscles for each relationship with no differences between muscles (p = 0.716, 0.428, 0.182). The y-intercepts of the motor unit firing rate vs. recruitment threshold relationship of the antagonist did not increase from 40% to 70% maximal voluntary contractions (p = 0.96), unlike for the agonist (p = 0.009). The antagonist muscle exhibits a similar motor unit control scheme to the agonist. Unlike the agonist, however, the firing rates of the antagonist did not increase with increasing intensity. Future research should investigate how antagonist firing rates adapt to resistance training and changes in antagonist firing rates in the absence of peripheral feedback.

Relationships between surface EMG variables and motor unit firing rates

2009 ◽  
Vol 107 (2) ◽  
pp. 177-185 ◽  
Author(s):  
Anita Christie ◽  
J. Greig Inglis ◽  
Gary Kamen ◽  
David A. Gabriel
Keyword(s):  
Motor Unit ◽  
Surface Emg ◽  
Firing Rates ◽  
Motor Unit Firing

Caffeine increases time to fatigue by maintaining force and not by altering firing rates during submaximal isometric contractions

2005 ◽  
Vol 99 (3) ◽  
pp. 1056-1063 ◽  
Author(s):  
B.M. Meyers ◽  
E. Cafarelli
Keyword(s):  
Motor Unit ◽  
Repeated Measures ◽  
Muscle Activation ◽  
Isometric Contractions ◽  
Double Blind ◽  
Single Motor Unit ◽  
Instantaneous Rate ◽  
Firing Rates ◽  
Motor Unit Firing ◽  
Placebo Capsule

Caffeine increases time to fatigue [limit of endurance (Tlim)] during submaximal isometric contractions without altering whole muscle activation or neuromuscular junction transmission. We used 10 male volunteers in a randomized, double-blind, repeated-measures experiment to examine single motor unit firing rates during intermittent submaximal contractions and to determine whether administering caffeine increased Tlim by maintaining higher firing rates. On 2 separate days, subjects performed intermittent 50% maximal voluntary contractions of the quadriceps to Tlim, 1 h after ingesting a caffeine (6 mg/kg) or placebo capsule. Average motor unit firing rates recorded with tungsten microelectrodes were constant for the duration of contractions. Caffeine increased average Tlim by 20.5 ± 8.1% ( P < 0.05) compared with placebo conditions. This increase was due to seven subjects, termed responders, who increased Tlim significantly. Two other subjects showed no response, and a third had a shorter Tlim. Neither the increased Tlim nor the responders' performance could be explained by alterations in firing rates or other neuromuscular variables. However, the amplitude of the evoked twitch and its maximal instantaneous rate of relaxation did not decline to the same degree in the caffeine trial of the responders; this resulted in values 20 and 30% higher at the time point matching the end of the placebo trial ( P < 0.05). The amplitude of the evoked twitch and the maximal instantaneous rate of relaxation were linearly correlated (caffeine r = 0.72, placebo r = 0.80, both P < 0.001), suggesting that the increase in Tlim may be partially explained by caffeine's effects on calcium reuptake and twitch force.

Motor Unit Firing During and After Voluntary Contractions of Human Thenar Muscles Weakened by Spinal Cord Injury

2003 ◽  
Vol 89 (4) ◽  
pp. 2065-2071 ◽  
Author(s):  
Inge Zijdewind ◽  
Christine K. Thomas
Keyword(s):  
Spinal Cord ◽  
Spinal Cord Injury ◽  
Motor Unit ◽  
Cervical Spinal Cord ◽  
Motor Units ◽  
Firing Rates ◽  
Rate Modulation ◽  
Motor Unit Firing ◽  
Cord Injury ◽  
Voluntary Contractions

Spinal cord injury may change both the distribution and the strength of the synaptic input within a motoneuron pool and therefore alter force gradation. Here, we have studied the relative contributions of motor unit recruitment and rate modulation to force gradation during voluntary contractions of thenar muscles performed by five individuals with chronic (>1 yr) cervical spinal cord injury. Mean ± SD thenar unit firing rates were low during both steady-level 25% (8.3 ± 2.2 Hz, n = 27 units) and 100% maximal voluntary contractions (MVCs, 9.2 ± 3.1 Hz, n = 23 units). Thus modest rate modulation, or a lack of it in some units, was seen despite an average fourfold increase in integrated surface electromyographic activity and force. During ramp contractions, units were recruited at 5.7 ± 2.5 Hz, but still only reached maximal firing rates of 12.8 ± 4.9 Hz. Motor units were recruited up to 85% of the maximal force achieved (14.6 ± 5.6 N). In contrast, unit recruitment in control hand muscles is largely complete by 30% MVC. Thus, during voluntary contractions of thenar muscles weakened by cervical spinal cord injury, motor unit rate modulation was limited and recruitment occurred over a wider than usual force range. Those motor units that were stopped voluntarily had significantly lower derecruitment versus recruitment thresholds. However, 8 units (24%) continued to fire long after the signal to end the voluntary contraction at a mean frequency of 5.9 ± 0.8 Hz. The forces generated by this prolonged unit activity ranged from 0.3 to 7.2% maximum. Subjects were unable to stop this involuntary unit activity even with the help of feedback. The mechanisms that underlie this prolonged motor unit firing need to be explored further.

scholarly journals Effects Of Resistance Training On Maximal Motor Unit Firing Rates In Young And Older Males

2018 ◽  
Vol 50 (5S) ◽  
pp. 429-430
Author(s):  
Phuong L. Ha ◽  
Garrett M. Hester ◽  
Ryan J. Colquhoun ◽  
Mitchel A. Magrini ◽  
Zachary K. Pope ◽  
...  
Keyword(s):  
Resistance Training ◽  
Motor Unit ◽  
Firing Rates ◽  
Motor Unit Firing ◽  
Older Males
Sign in / Sign up

Export Citation Format

Share Document