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.

Activation of type-identified motor units during centrally evoked contractions in the cat medial gastrocnemius muscle. III. Muscle-unit force modulation

1996 ◽  
Vol 75 (1) ◽  
pp. 51-59 ◽  
Author(s):  
K. E. Tansey ◽  
A. K. Yee ◽  
B. R. Botterman
Keyword(s):  
Firing Rate ◽  
Motor Unit ◽  
Muscle Force ◽  
Motor Units ◽  
Medial Gastrocnemius ◽  
Rate Variation ◽  
Unit Force ◽  
Rate Modulation ◽  
Force Modulation ◽  
Whole Muscle

1. The aim of this study was to examine the extent of muscle-unit force modulation due to motoneuron firing-rate variation in type-identified motor units of the cat medial gastrocnemius (MG) muscle, and to investigate the contribution of muscle-unit force modulation to whole-muscle force regulation. The motoneuron discharge patterns recorded from 8 pairs of motor units during 12 smoothly graded muscle contractions evoked by stimulation of the mesencephalic locomotor region (MLR) were used to reactivate those units in isolation to estimate what their force profiles would have been like during the evoked whole-muscle contractions. 2. For most motor units, muscle-unit force modulation was similar to motoneuron firing-rate modulation, in that muscle-unit force increased over a limited range (120-600 g) of increasing whole-muscle tension and was then maintained at a near maximal (> 70%) output level as muscle force continued to rise. Most muscle units also decreased their force outputs over a slightly larger range of declining whole-muscle force before relaxing. This second finding was best explained by the counterclockwise hysteresis recorded in the motor units' frequency-tension (f-t) relationships. 3. In those instances when whole-muscle force fluctuated just above the recruitment threshold of a motor unit, a substantial percentage (10-25%) of the change in whole-muscle force could be accounted for by force modulation in that motor unit alone. This finding suggested that few motor units in the pool were simultaneously simultaneously undergoing force modulation. To evaluate this possibility, the extent of parallel muscle-unit force modulation within the 8 pairs of simultaneously active motor units was evaluated. As with parallel motoneuron firing-rate modulation, the extent of parallel muscle-unit force modulation was limited to unit pairs of the same physiological type and recruitment threshold. In several instances, pairs of motor units displayed parallel motoneuron firing-rate modulation but did not show parallel muscle-unit force modulation because of the nature of the motor units' f-t relationships. 4. The limited extent of parallel muscle-unit force modulation seen in these experiments implies that the major strategy for force modulation in the cat MG muscle, involving contractions estimated to reach 30-40% of maximum, may be motor-unit recruitment rather than motor-unit firing-rate variation with resulting force modulation. Given, however, that the majority of motor units are already recruited at these output levels (< 40%), it is proposed that motor-unit firing-rate variation with resulting force modulation may take over as the major muscle force modulating strategy at higher output levels.

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.

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.

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.

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 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 The influence of input excitation on the inter- and intra-day reliability of the motor unit firing rate versus recruitment threshold relationship

2018 ◽  
Vol 120 (6) ◽  
pp. 3131-3139 ◽  
Author(s):  
Ryan J. Colquhoun ◽  
Patrick M. Tomko ◽  
Mitchel A. Magrini ◽  
Tyler W. D. Muddle ◽  
Nathaniel D. M. Jenkins
Keyword(s):  
Firing Rate ◽  
Motor Unit ◽  
Vastus Lateralis ◽  
Intraclass Correlation ◽  
Correlation Coefficients ◽  
Semg Signal ◽  
Recruitment Threshold ◽  
Firing Behavior ◽  
The Mean ◽  
Ramp Contractions

Decomposition of the surface electromyographic (sEMG) signal is commonly used to examine motor unit (MU) firing behavior. However, the intra- and inter-day reliability of these measurements has yet to be quantified or reported. This investigation 1) examined the effect of input excitation on the mean firing rate (MFR) vs. recruitment threshold (RT) relationship and 2) determined the inter- and intra-day reliability of the MFR vs. RT relationship at 30%, 50%, and 70% of maximum voluntary isometric strength (MVIC). Twenty-eight healthy males (23 ± 3 yr) completed two experimental visits, during which they performed MVIC testing and isometric ramp contractions at 30%, 50%, and 70% MVIC. sEMG signals were recorded from the vastus lateralis during the ramp contractions and decomposed to establish the MFR vs. RT relationship for the detected MUs. Intra- and inter-day reliability was then established for the slopes and y-intercepts of the MFR vs. RT relationship at each contraction intensity. All participants displayed significant MFR vs. RT relationships ( r range: −0.662 to −0.999; P ≤ 0.001–0.006). Intra- and inter-day intraclass correlation coefficients (ICCs) ranged from 0.766–0.824 and 0.867–0.919 for the slopes and from 0.780–0.915 and 0.804–0.927 for the y-intercepts, respectively. Furthermore, the slope coefficient was significantly greater at 70% than at 30% MVIC, and the y-intercepts increased with increasing contraction intensities. Changes in input excitation to the MU pool alter the magnitude, but not the reliability, of the slopes and y-intercepts of the MFR vs. RT relationship. NEW & NOTEWORTHY The firing behavior of the motor unit (MU) pool is often characterized using the mean firing rate vs. recruitment threshold relationship of the active MUs. Although this relationship has been widely used, this is the first study to report the effects of input excitation (contraction intensity) on the intra- and inter-day reliability of this relationship. The criteria used for MU analysis and the model utilized in this study allow for generalization to outside investigators and laboratories.

Properties of motor units after self-reinnervation of the cat superior oblique muscle

1995 ◽  
Vol 74 (6) ◽  
pp. 2309-2318 ◽  
Author(s):  
R. F. Waldeck ◽  
E. H. Murphy ◽  
M. J. Pinter
Keyword(s):  
Motor Unit ◽  
Muscle Fiber ◽  
Motor Units ◽  
Oblique Muscle ◽  
Unit Force ◽  
Cross Sectional ◽  
Conduction Velocities ◽  
Polyneuronal Innervation ◽  
Superior Oblique Muscle ◽  
Superior Oblique

1. The mechanical properties of motor units of the cat superior oblique muscle and axonal conduction velocities of trochlear motoneurons have been studied at several postoperative times after intracranial axotomy of the trochlear nerve. 2. Whole muscle twitch forces were generally within the normal range by approximately 4 mo postoperative, indicating that reinnervation is complete at this time. 3. Among animals studied 3.5-4.5 months after trochlear axotomy, average motor-unit tetanic forces were increased by a factor of approximately 2.5 compared with units studied in normal superior oblique muscle. Average motor-unit tetanic forces in animals studied 14.5-23 mo after axotomy were also increased relative to normal, but the difference was not significant. Among all reinnervated motor units, there was a tendency for increased twitch time-to-peak relative to control. Reinnervated motor-unit fatigue properties were similar to normal. 4. Average trochlear motoneuron conduction velocities for animals at all postoperative intervals remained significantly lower than the average conduction velocities from three of four normal animals. 5. Counts of Nissl-stained cell bodies in axotomized and control, contralateral trochlear nuclei showed that some cell loss had occurred, averaging approximately 17% 3.5-4.5 mo postoperative and 24% 14.5-23 mo postoperative. Associated with this loss was an increase (10%) of axotomized motoneuron soma cross-sectional area. 6. Muscle fiber cross-sectional areas (CSA) were measured in reinnervated superior oblique muscles and compared with CSAs from contralateral, control muscles. Average CSA was significantly decreased in all reinnervated muscles, with the relative decreases ranging from approximately 10 to 28%. 7. The results are discussed in terms of factors that determine motor-unit force; muscle fiber CSA, specific force, and innervation ratio. We conclude that the increases of average motor-unit force in short-term reinnervated superior oblique muscles are most likely related to polyneuronal innervation of muscle fibers and that the return of these forces to normal levels in long-term muscles is related to synapse elimination. Our results are compared with those of other self-reinnervation studies, and the potential role played by the time muscle remains denervated in determining the persistence of polyneuronal innervation after reinnervation is considered.

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