scholarly journals Muscle fatigue increases beta-band coherence between the firing times of simultaneously active motor units in the first dorsal interosseous muscle

2016 ◽  
Vol 115 (6) ◽  
pp. 2830-2839 ◽  
Author(s):  
Lara McManus ◽  
Xiaogang Hu ◽  
William Z. Rymer ◽  
Nina L. Suresh ◽  
Madeleine M. Lowery
Keyword(s):  
Motor Units ◽  
Surface Emg ◽  
Relative Increase ◽  
Wavelet Coherence ◽  
First Dorsal Interosseous ◽  
Beta Band ◽  
Indirect Measures ◽  
First Dorsal Interosseous Muscle ◽  
Active Motor ◽  
Dorsal Interosseous Muscle

Synchronization between the firing times of simultaneously active motor units (MUs) is generally assumed to increase during fatiguing contractions. To date, however, estimates of MU synchronization have relied on indirect measures, derived from surface electromyographic (EMG) interference signals. This study used intramuscular coherence to investigate the correlation between MU discharges in the first dorsal interosseous muscle during and immediately following a submaximal fatiguing contraction, and after rest. Coherence between composite MU spike trains, derived from decomposed surface EMG, were examined in the delta (1–4 Hz), alpha (8–12 Hz), beta (15–30 Hz), and gamma (30–60 Hz) frequency band ranges. A significant increase in MU coherence was observed in the delta, alpha, and beta frequency bands postfatigue. In addition, wavelet coherence revealed a tendency for delta-, alpha-, and beta-band coherence to increase during the fatiguing contraction, with subjects exhibiting low initial coherence values displaying the greatest relative increase. This was accompanied by an increase in MU short-term synchronization and a decline in mean firing rate of the majority of MUs detected during the sustained contraction. A model of the motoneuron pool and surface EMG was used to investigate factors influencing the coherence estimate. Simulation results indicated that changes in motoneuron inhibition and firing rates alone could not directly account for increased beta-band coherence postfatigue. The observed increase is, therefore, more likely to arise from an increase in the strength of correlated inputs to MUs as the muscle fatigues.

scholarly journals Beta-band motor unit coherence and nonlinear surface EMG features of the first dorsal interosseous muscle vary with force

2019 ◽  
Vol 122 (3) ◽  
pp. 1147-1162 ◽  
Author(s):  
Lara McManus ◽  
Matthew W. Flood ◽  
Madeleine M. Lowery
Keyword(s):  
Motor Unit ◽  
Motor Units ◽  
Surface Emg ◽  
Sample Entropy ◽  
Beta Band ◽  
Motoneuron Pool ◽  
Motor Unit Firing ◽  
First Dorsal Interosseous Muscle ◽  
Nonlinear Surface ◽  
Dorsal Interosseous Muscle

Motor unit firing times are weakly coupled across a range of frequencies during voluntary contractions. Coherent activity within the beta-band (15–35 Hz) has been linked to oscillatory cortical processes, providing evidence of functional connectivity between the motoneuron pool and motor cortex. The aim of this study was to investigate whether beta-band motor unit coherence is altered with increasing abduction force in the first dorsal interosseous muscle. Coherence between motor unit firing times, extracted from decomposed surface electromyography (EMG) signals, was investigated in 17 subjects at 10, 20, 30, and 40% of maximum voluntary contraction. Corresponding changes in nonlinear surface EMG features (specifically sample entropy and determinism, which are sensitive to motor unit synchronization) were also examined. A reduction in beta-band and alpha-band coherence was observed as force increased [ F(3, 151) = 32, P < 0.001 and F(3, 151) = 27, P < 0.001, respectively], accompanied by corresponding changes in nonlinear surface EMG features. A significant relationship between the nonlinear features and motor unit coherence was also detected ( r = −0.43 ± 0.1 and r = 0.45 ± 0.1 for sample entropy and determinism, respectively; both P < 0.001). The reduction in beta-band coherence suggests a change in the relative contribution of correlated and uncorrelated presynaptic inputs to the motoneuron pool, and/or a decrease in the responsiveness of the motoneuron pool to synchronous inputs at higher forces. The study highlights the importance of considering muscle activation when investigating changes in motor unit coherence or nonlinear EMG features and examines other factors that can influence coherence estimation. NEW & NOTEWORTHY Intramuscular alpha- and beta-band coherence decreased as muscle contraction force increased. Beta-band coherence was higher in groups of high-threshold motor units than in simultaneously active lower threshold units. Alterations in motor unit coherence with increases or decreases in force and with the onset of fatigue were accompanied by corresponding changes in surface electromyography sample entropy and determinism. Mixed-model analysis indicated mean firing rate and number of motor units also influenced the coherence estimate.

Motor-unit recruitment in human first dorsal interosseous muscle for static contractions in three different directions

1986 ◽  
Vol 55 (5) ◽  
pp. 1017-1029 ◽  
Author(s):  
C. K. Thomas ◽  
B. H. Ross ◽  
R. B. Stein
Keyword(s):  
Motor Unit ◽  
Index Finger ◽  
Rank Order ◽  
Motor Units ◽  
Motor Unit Recruitment ◽  
First Dorsal Interosseous ◽  
First Dorsal Interosseous Muscle ◽  
Contraction Times ◽  
Static Contractions ◽  
Dorsal Interosseous Muscle

Spike-triggered averaging was used to extract the twitch tensions and contraction times of 144 motor units from the first dorsal interosseous muscle of four subjects for three different directions of static contraction: abduction of the index finger, flexion of the index finger, and adduction of the thumb coupled with flexion of the index finger (hereafter referred to as adduction). Although the twitch tensions were generally largest for the abduction contraction, all units contributed tension to all three directions of contraction. A linear correlation was found for twitch tensions of motor units for the three directions of static contractions. Linear correlations were also found between twitch tension and threshold force of these motor units for each direction, which suggests that an orderly pattern of recruitment, according to increasing twitch size, adequately describes the function of human first dorsal interosseous muscle for all contraction directions. No clear evidence was found for separate groups of motor units in the muscle that were selectively activated for the different tasks. Rank order of recruitment for motor units in the three directions of contraction was correlated, but was not identical. The scatter in our data is discussed in relation to earlier reports of altered motor-unit recruitment during different movements.

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