Is the motor unit mean firing rate versus recruitment threshold relationship linear?

2019 ◽  
Vol 40 (9) ◽  
pp. 095002 ◽  
Author(s):  
Kylie K Harmon ◽  
Ryan M Girts ◽  
Rob J MacLennan ◽  
Matt S Stock
Keyword(s):  
Firing Rate ◽  
Motor Unit ◽  
Recruitment Threshold ◽  
Rate Versus

VASTUS LATERALIS MOTOR UNIT RECRUITMENT THRESHOLDS ARE COMPRESSED TOWARDS LOWER FORCES IN OLDER MEN

2020 ◽  
pp. 1-6
Author(s):  
R.M. Girts ◽  
J.A. Mota ◽  
K.K. Harmon ◽  
R.J. MacLennan ◽  
M.S. Stock
Keyword(s):  
Firing Rate ◽  
Motor Unit ◽  
Vastus Lateralis ◽  
Motor Units ◽  
Older Men ◽  
Force Level ◽  
Recruitment Threshold ◽  
Rate Versus ◽  
Younger Men ◽  
The Mean

Background: Aging results in adaptations which may affect the control of motor units. Objective: We sought to determine if younger and older men recruit motor units at similar force levels. Design: Cross-sectional, between-subjects design. Setting: Controlled laboratory setting. Participants: Twelve younger (age = 25 ± 3 years) and twelve older (age = 75 ± 8 years) men. Measurements: Participants performed isometric contractions of the dominant knee extensors at a force level corresponding to 50% maximal voluntary contraction (MVC). Bipolar surface electromyographic (EMG) signals were detected from the vastus lateralis. A surface EMG signal decomposition algorithm was used to quantify the recruitment threshold of each motor unit, which was defined as the force level corresponding to the first firing. Recruitment thresholds were expressed in both relative (% MVC) and absolute (N) terms. To further understand age-related differences in motor unit control, we examined the mean firing rate versus recruitment threshold relationship at steady force. Results: MVC force was greater in younger men (p = 0.010, d = 1.15). Older men had lower median recruitment thresholds in both absolute (p = 0.005, d = 1.29) and relative (p = 0.001, d = 1.53) terms. The absolute recruitment threshold range was larger for younger men (p = 0.020; d = 1.02), though a smaller difference was noted in relative terms (p = 0.235, d = 0.50). These findings were complimented by a generally flatter slope (p = 0.070; d = 0.78) and lower y-intercept (p = 0.009; d = 1.17) of the mean firing rate versus recruitment threshold relationship in older men. Conclusion: Older men tend to recruit more motor units at lower force levels. We speculate that recruitment threshold compression may be a neural adaptation serving to compensate for lower motor unit firing rates and/or denervation and subsequent re-innervation in aged muscle.

Effects of fatigue on motor unit firing rate versus recruitment threshold relationships

2011 ◽  
Vol 45 (1) ◽  
pp. 100-109 ◽  
Author(s):  
Matt S. Stock ◽  
Travis W. Beck ◽  
Jason M. Defreitas
Keyword(s):  
Firing Rate ◽  
Motor Unit ◽  
Recruitment Threshold ◽  
Rate Versus ◽  
Motor Unit Firing

scholarly journals The Motor Unit Mean Firing Rate versus Recruitment Threshold Relationship is Unaffected by Short-Term Disuse

2019 ◽  
Vol 51 (Supplement) ◽  
pp. 342-343
Author(s):  
Rob J. MacLennan ◽  
David Ogilvie ◽  
John McDorman ◽  
Ernest Vargas ◽  
Nathan Becker ◽  
...  
Keyword(s):  
Firing Rate ◽  
Motor Unit ◽  
Short Term ◽  
Recruitment Threshold ◽  
Rate Versus

scholarly journals Is the Mean Firing Rate versus Recruitment Threshold Relationship Linear?

2019 ◽  
Vol 51 (Supplement) ◽  
pp. 344-345
Author(s):  
Kylie K. Harmon ◽  
Ryan M. Girts ◽  
Robert J. MacLennan ◽  
Matt S. Stock
Keyword(s):  
Firing Rate ◽  
Recruitment Threshold ◽  
Rate Versus ◽  
The Mean

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.

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.

scholarly journals Hierarchical control of motor units in voluntary contractions

2012 ◽  
Vol 107 (1) ◽  
pp. 178-195 ◽  
Author(s):  
Carlo J. De Luca ◽  
Paola Contessa
Keyword(s):  
Firing Rate ◽  
Motor Unit ◽  
Hierarchical Control ◽  
Motor Units ◽  
Exponential Functions ◽  
Size Principle ◽  
Recruitment Threshold ◽  
Firing Rates ◽  
Control Scheme ◽  
Voluntary Contractions

For the past five decades there has been wide acceptance of a relationship between the firing rate of motor units and the afterhyperpolarization of motoneurons. It has been promulgated that the higher-threshold, larger-soma, motoneurons fire faster than the lower-threshold, smaller-soma, motor units. This relationship was based on studies on anesthetized cats with electrically stimulated motoneurons. We questioned its applicability to motor unit control during voluntary contractions in humans. We found that during linearly force-increasing contractions, firing rates increased as exponential functions. At any time and force level, including at recruitment, the firing rate values were inversely related to the recruitment threshold of the motor unit. The time constants of the exponential functions were directly related to the recruitment threshold. From the Henneman size principle it follows that the characteristics of the firing rates are also related to the size of the soma. The “firing rate spectrum” presents a beautifully simple control scheme in which, at any given time or force, the firing rate value of earlier-recruited motor units is greater than that of later-recruited motor units. This hierarchical control scheme describes a mechanism that provides an effective economy of force generation for the earlier-recruited lower force-twitch motor units, and reduces the fatigue of later-recruited higher force-twitch motor units—both characteristics being well suited for generating and sustaining force during the fight-or-flight response.

scholarly journals Characteristics of the Electrophysiological Properties of Neuromuscular Motor Units and Its Adaptive Strategy Response in Lower Extremity Muscles for Seniors with Pre-Sarcopenia: A Preliminary Study

2021 ◽  
Vol 18 (6) ◽  
pp. 3063
Author(s):  
Chia-Han Hu ◽  
Chia-Chi Yang ◽  
Shihfan Jack Tu ◽  
Ing-Jer Huang ◽  
Danaa Ganbat ◽  
...  
Keyword(s):  
Firing Rate ◽  
Motor Unit ◽  
Muscle Fiber ◽  
Young Group ◽  
Muscle Performance ◽  
Related Phenomenon ◽  
Unit Force ◽  
Recruitment Threshold ◽  
Healthy Elderly ◽  
Function Loss

Older adults with sarcopenia, which is an aging-related phenomenon of muscle mass loss, usually suffer from decreases in both strength and functional performance. However, the causality between function loss and physiological changes is unclear. This study aimed to explore the motor unit characteristics of the neurological factors between normal subjects and those with sarcopenia. Five risk-sarcopenia (age: 66.20 ± 4.44), five healthy (age: 69.00 ± 2.35), and twelve young (age: 21.33 ± 1.15) participants were selected. Each participant performed knee extension exercises at a 50% level of maximal voluntary isometric contraction. Next, electromyogram (EMG) signals were collected, and information on each parameter—e.g., motor unit number, recruitment threshold, the slope of the mean firing rate to recruitment threshold, y-intercept, firing rate per unit force, and mean motor unit firing rate (MFR)—was extracted to analyze muscle fiber discrimination (MFD). Meanwhile, force variance was used to observe the stability between two muscle groups. The results suggested that there was no difference between the three groups for motor unit number, recruitment threshold, y-intercept, mean firing rate, and motor unit discrimination (p > 0.05). However, the slope of MFR and firing rate per unit force in the risk-sarcopenia group were significantly higher than in the young group (p < 0.05). Regarding muscle performance, the force variance in the non-sarcopenia group was significantly higher than the young group (p < 0.05), while the risk-sarcopenia group showed a higher trend than the young group. This study demonstrated some neuromuscular characters between sarcopenia and healthy elderly and young people when performing the same level of leg exercise tasks. This difference may provide some hints for discovering aging-related strength and function loss. Future studies should consider combining the in vivo measurement of muscle fiber type to clarify whether this EMG difference is related to the loss of muscle strength or mass before recruiting symptomatic elderly participants for further investigation.

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