scholarly journals Motor-Unit Synchronization Increases EMG Amplitude and Decreases Force Steadiness of Simulated Contractions

2000 ◽  
Vol 83 (1) ◽  
pp. 441-452 ◽  
Author(s):  
Wanxiang Yao ◽  
Rew J. Fuglevand ◽  
Roger M. Enoka
Keyword(s):  
Motor Unit ◽  
Action Potentials ◽  
Isometric Force ◽  
Discharge Rate ◽  
Motor Units ◽  
Average Force ◽  
Unit Discharge ◽  
Motor Unit Synchronization ◽  
Motor Unit Discharge ◽  
High Level

The purpose of the study was to determine the effect of motor-unit synchronization on the surface electromyogram (EMG) and isometric force using a computer model of muscle contraction. The EMG and force were simulated by generating muscle fiber action potentials, defining motor-unit mechanical characteristics and territories, estimating motor-unit action potentials, specifying motor-unit discharge times, and imposing various levels of motor-unit synchronization. The output (EMG and force) was simulated at 11 levels of excitation, ranging from 5 to 100% of maximum. To synchronize motor-unit activity, selected motor-unit discharge times were adjusted; however, the number of motor units recruited and the average discharge rate of each unit was constant across synchronization conditions for a given level of excitation. Two levels of synchronization were imposed on the discharge times: a moderate and a high level, which approximated the experimentally observed range of motor-unit synchronization. The moderate level of synchrony caused the average EMG to increase by ∼65%, whereas the high level caused a 130% increase in the EMG with respect to the no-synchrony condition. Neither synchrony condition influenced the magnitude of the average force. However, motor-unit synchronization did increase the amplitude of the fluctuations in the simulated force, especially at intermediate levels of excitation. In conclusion, motor-unit synchronization increased the amplitude of the average rectified EMG and decreased the steadiness of the force exerted by the muscle in simulated contractions.

Fluctuations in Isometric Force Are Associated With Motor Unit Discharge Rate Variability in Older Adults

2004 ◽  
Vol 36 (Supplement) ◽  
pp. S123 ◽  
Author(s):  
Jennifer L. Stephenson ◽  
Katrina S. Maluf ◽  
Brian L. Tracy ◽  
Sandra K. Hunter ◽  
Roger M. Enoka
Keyword(s):  
Older Adults ◽  
Motor Unit ◽  
Isometric Force ◽  
Discharge Rate ◽  
Unit Discharge ◽  
Motor Unit Discharge

Aging, Resistance Training, and Motor Unit Discharge Behavior

2005 ◽  
Vol 30 (3) ◽  
pp. 341-351 ◽  
Author(s):  
Gary Kamen
Keyword(s):  
Motor Unit ◽  
Muscular Strength ◽  
Discharge Rate ◽  
Recruitment Rate ◽  
Unit Discharge ◽  
Discharge Rates ◽  
Motor Unit Synchronization ◽  
Motor Unit Discharge ◽  
Rate Coding ◽  
Neural Factors

Researchers have alluded to the existence of "neural factors" in the expression and development of muscular strength. Candidate neural factors including motor unit recruitment, rate coding, doublet firing, and motor unit synchronization are discussed in this review. Aging is generally accompanied by lower motor unit discharge rates. However, both young and older adults exhibit rapid changes in muscular strength with repeated strength testing. These strength changes occur with concomitant albeit transient increases in motor unit discharge rate. These and other neural factors may contribute to the initial increases in muscular strength observed during the early phases of resistance exercise training. Key words: firing rate, muscle, exercise

scholarly journals The relative strength of common synaptic input to motor neurons is not a determinant of the maximal rate of force development in humans

2019 ◽  
Vol 127 (1) ◽  
pp. 205-214 ◽  
Author(s):  
Alessandro Del Vecchio ◽  
Deborah Falla ◽  
Francesco Felici ◽  
Dario Farina
Keyword(s):  
Motor Unit ◽  
Motor Neurons ◽  
Synaptic Input ◽  
Discharge Rate ◽  
Rate Of Force Development ◽  
Force Generation ◽  
Unit Discharge ◽  
Force Development ◽  
Motor Unit Synchronization ◽  
Motor Unit Discharge

Correlation between motor unit discharge times, often referred to as motor unit synchronization, is determined by common synaptic input to motor neurons. Although it has been largely speculated that synchronization should influence the rate of force development, the association between the degree of motor unit synchronization and rapid force generation has not been determined. In this study, we examined this association with both simulations and experimental motor unit recordings. The analysis of experimental motor unit discharges from the tibialis anterior muscle of 20 healthy individuals during rapid isometric contractions revealed that the average motor unit discharge rate was associated with the rate of force development. Moreover, the extent of motor unit synchronization was entirely determined by the average motor unit discharge rate ( R > 0.7, P < 0.0001). The simulation model demonstrated that the relative proportion of common synaptic input received by motor neurons, which determines motor unit synchronization, does not influence the rate of force development ( R = 0.03, P > 0.05). Nonetheless, the estimates of correlation between motor unit spike trains were significantly correlated with the rate of force generation ( R > 0.8, P < 0.0001). These results indicate that the average motor unit discharge rate, but not the degree of motor unit synchronization, contributes to most of the variance of human contractile speed among individuals. In addition, estimates of correlation between motor unit discharge times depend strongly on the number of identified motor units and therefore are not indicative of the strength of common input. NEW & NOTEWORTHY It is commonly assumed that motor unit synchronization has an impact on the rate of force development of a muscle. Here we present computer simulations and experimental data of human tibialis anterior motor units during rapid contractions that show that motor unit synchronization is not a determinant of the rate of force production. This conclusion clarifies the neural determinants of rapid force generation.

Motor unit discharge behavior in older adults during maximal-effort contractions

1995 ◽  
Vol 79 (6) ◽  
pp. 1908-1913 ◽  
Author(s):  
G. Kamen ◽  
S. V. Sison ◽  
C. C. Du ◽  
C. Patten
Keyword(s):  
Older Adults ◽  
Motor Unit ◽  
Discharge Rate ◽  
Force Production ◽  
Motor Units ◽  
Older Individuals ◽  
Unit Discharge ◽  
Maximal Force ◽  
Motor Unit Discharge ◽  
Common Observation

A reduction in maximal force production is a common observation in older individuals. In an effort to determine whether aging is accompanied by reductions in central motoneuron drive limiting motor performance, motor unit discharge records were obtained from seven young (21–33 yr) and seven older (> 67 yr) adults. Informed consent was obtained from all subjects. The task required the subject to perform a maximal abduction of the second digit under isometric conditions. Motor unit potentials in the first dorsal interosseous were monitored by using a selective four-wire needle electrode and identified off-line with the aid of a Dantec electromyograph. The maximal discharge rate in the older adults (31.1 impulses/s) was significantly smaller (P < 0.05) than that in the younger subjects (50.9 impulses/s). These findings suggest that reductions in maximal force capability in older adults are partially due to an impaired ability to fully drive the surviving motor units.

Fluctuations in Isometric Force Are Associated With Motor Unit Discharge Rate Variability in Older Adults

2004 ◽  
Vol 36 (Supplement) ◽  
pp. S123
Author(s):  
Jennifer L. Stephenson ◽  
Katrina S. Maluf ◽  
Brian L. Tracy ◽  
Sandra K. Hunter ◽  
Roger M. Enoka
Keyword(s):  
Older Adults ◽  
Motor Unit ◽  
Isometric Force ◽  
Discharge Rate ◽  
Unit Discharge ◽  
Motor Unit Discharge

scholarly journals Prolonged muscle vibration increases stretch reflex amplitude, motor unit discharge rate, and force fluctuations in a hand muscle

2005 ◽  
Vol 99 (5) ◽  
pp. 1835-1842 ◽  
Author(s):  
Minoru Shinohara ◽  
Chet T. Moritz ◽  
Michael A. Pascoe ◽  
Roger M. Enoka
Keyword(s):  
Motor Unit ◽  
Stretch Reflex ◽  
Discharge Rate ◽  
Motor Units ◽  
Muscle Vibration ◽  
Constant Force ◽  
Unit Discharge ◽  
Force Fluctuations ◽  
Motor Unit Discharge ◽  
Hand Muscle

The purpose of this study was to compare the influence of prolonged vibration of a hand muscle on the amplitude of the stretch reflex, motor unit discharge rate, and force fluctuations during steady, submaximal contractions. Thirty-two young adults performed 10 isometric contractions at a constant force (5.0 ± 2.3% of maximal force) with the first dorsal interosseus muscle. Each contraction was held steady for 10 s, and then stretch reflexes were evoked. Subsequently, 20 subjects had vibration applied to the relaxed muscle for 30 min, and 12 subjects received no vibration. The muscle vibration induced a tonic vibration reflex. The intervention (vibration or no vibration) was followed by 2 sets of 10 constant-force contractions with applied stretches (After and Recovery trials). The mean electromyogram amplitude of the short-latency component of the stretch reflex increased by 33% during the After trials ( P < 0.01) and by 38% during the Recovery trials ( P < 0.01). The standard deviation of force during the steady contractions increased by 21% during the After trials ( P < 0.05) and by 28% during the Recovery trials ( P < 0.01). The discharge rate of motor units increased from 10.3 ± 2.7 pulses/s (pps) before vibration to 12.2 ± 3.1 pps ( P < 0.01) during the After trials and to 11.9 ± 2.6 pps during the Recovery trials ( P < 0.01). There was no change in force fluctuations or stretch reflex magnitude for the subjects in the Control group. The results indicate that prolonged vibration increased the short-latency component of the stretch reflex, the discharge rate of motor units, and the fluctuations in force during contractions by a hand muscle. These adjustments were necessary to achieve the target force due to the vibration-induced decrease in the force capacity of the muscle.

Factors Affecting the Common Modulation of Bilateral Motor Unit Discharge in Human Soleus Muscles

2007 ◽  
Vol 97 (6) ◽  
pp. 3917-3925 ◽  
Author(s):  
G. Mochizuki ◽  
T. D. Ivanova ◽  
S. J. Garland
Keyword(s):  
Motor Unit ◽  
Discharge Rate ◽  
Motor Units ◽  
Unit Discharge ◽  
Eyes Closed ◽  
Single Motor Units ◽  
Eyes Open ◽  
Motor Unit Discharge ◽  
Common Drive ◽  
Left And Right

The purpose of this study was to determine the factors that influence the co-modulation of motor unit discharge rate in soleus muscles of both legs during upright standing. Single motor units were recorded from the left and right soleus muscles under three experimental conditions: standing quietly with the eyes open and closed, standing with the eyes closed while vibration was applied to one Achilles tendon, and swaying voluntarily or producing variable low-force isometric contractions at a frequency of 0.05 Hz. Correlations in motor unit discharge rate between left and right soleus motor units were assessed using common drive analysis. The results showed that common drive to motoneurons of the two muscles did not differ between standing with the eyes open or closed, but there was an order effect with the second task having significantly lower common drive than the first. Common drive was also significantly lower when vibration was applied to one leg compared with when no vibration was applied. Common drive was higher as subjects swayed anteriorly as compared with when they swayed posteriorly. There were no significant differences in common drive across phases of the variable isometric force contraction. Common drive was higher during voluntary sway than during variable force production; both of these values were significantly lower than those derived from the quiet standing task. These results suggest that proprioceptive and sub-cortical inputs contribute to the co-modulation of the firing rate of soleus motor unit pairs of the left and right leg during standing posture.

Frequency Modulation of Motor Unit Discharge Has Task-Dependent Effects on Fluctuations in Motor Output

2005 ◽  
Vol 94 (4) ◽  
pp. 2878-2887 ◽  
Author(s):  
Carol J. Mottram ◽  
Evangelos A. Christou ◽  
François G. Meyer ◽  
Roger M. Enoka
Keyword(s):  
Motor Unit ◽  
Frequency Modulation ◽  
Discharge Rate ◽  
Motor Output ◽  
Unit Discharge ◽  
Rate Of Increase ◽  
Output Force ◽  
Position Task ◽  
Motor Unit Discharge ◽  
Target Force

The rate of change in the fluctuations in motor output differs during the performance of fatiguing contractions that involve different types of loads. The purpose of this study was to examine the contribution of frequency modulation of motor unit discharge to the fluctuations in the motor output during sustained contractions with the force and position tasks. In separate tests with the upper arm vertical and the elbow flexed to 1.57 rad, the seated subjects maintained either a constant upward force at the wrist (force task) or a constant elbow angle (position task). The force and position tasks were performed in random order at a target force equal to 3.6 ± 2.1% (mean ± SD) of the maximal voluntary contraction (MVC) force above the recruitment threshold of an isolated motor unit from the biceps brachii. Each subject maintained the two tasks for an identical duration (161 ± 93 s) at a mean target force of 22.4 ± 13.6% MVC. As expected, the rate of increase in the fluctuations in motor output (force task: SD for detrended force; position task: SD for vertical acceleration) was greater for the position task than the force task ( P < 0.001). The amplitude of the coefficient of variation (CV) and the power spectra for motor unit discharge were similar between tasks ( P > 0.1) and did not change with time ( P > 0.1), and could not explain the different rates of increase in motor output fluctuations for the two tasks. Nonetheless, frequency modulation of motor unit discharge differed during the two tasks and predicted ( P < 0.001) both the CV for discharge rate (force task: 1–3, 12–13, and 14–15 Hz; position task: 0–1, and 1–2 Hz) and the fluctuations in motor output (force task: 5–6, 9–10, 12–13, and 14–15 Hz; position task: 6–7, 14–15, 17–19, 20–21, and 23–24 Hz). Frequency modulation of motor unit discharge rate differed for the force and position tasks and influenced the ability to sustain steady contractions.

Blockade of 5-HT2 receptors suppresses rate of torque development and motor unit discharge rate during rapid contractions

2021 ◽  
Author(s):  
Benjamin Ian Goodlich ◽  
Sean A Horan ◽  
Justin J Kavanagh
Keyword(s):  
Motor Unit ◽  
Unit Activity ◽  
Repeated Measures ◽  
Discharge Rate ◽  
Voluntary Contraction ◽  
Unit Discharge ◽  
Motor Unit Activity ◽  
Rate Of Torque Development ◽  
Motor Unit Discharge ◽  
Torque Development

Serotonin (5-HT) is a neuromodulator that is critical for regulating the excitability of spinal motoneurons and the generation of muscle torque. However, the role of 5-HT in modulating human motor unit activity during rapid contractions has yet to be assessed. Nine healthy participants (23.7 ± 2.2 yr) ingested 8 mg of the competitive 5-HT2 antagonist cyproheptadine in a double-blinded, placebo-controlled, repeated-measures experiment. Rapid dorsiflexion contractions were performed at 30%, 50% and 70% of maximal voluntary contraction (MVC), where motor unit activity was assessed by high-density surface electromyographic decomposition. A second protocol was performed where a sustained, fatigue-inducing dorsiflexion contraction was completed prior to undertaking the same 30%, 50% and 70% MVC rapid contractions and motor unit analysis. Motor unit discharge rate (p < 0.001) and rate of torque development (RTD; p = 0.019) for the unfatigued muscle were both significantly lower for the cyproheptadine condition. Following the fatigue inducing contraction, cyproheptadine reduced motor unit discharge rate (p < 0.001) and RTD (p = 0.024), where the effects of cyproheptadine on motor unit discharge rate and RTD increased with increasing contraction intensity. Overall, these results support the viewpoint that serotonergic effects in the central nervous system occur fast enough to regulate motor unit discharge rate during rapid powerful contractions.

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