Adaptations in biceps brachii motor unit activity after repeated bouts of eccentric exercise in elbow flexor muscles

2011 ◽  
Vol 105 (3) ◽  
pp. 1225-1235 ◽  
Author(s):  
Tamara J. Dartnall ◽  
Michael A. Nordstrom ◽  
John G. Semmler
Keyword(s):  
Muscle Damage ◽  
Motor Unit ◽  
Eccentric Exercise ◽  
Unit Activity ◽  
Biceps Brachii ◽  
Elbow Flexor ◽  
Exercise Session ◽  
Isometric Contractions ◽  
Motor Unit Activity ◽  
Motor Unit Synchronization

The purpose of this study was to examine changes in motor unit activity in the biceps brachii muscle after an initial ( Bout 1) and repeated ( Bout 2) session of eccentric exercise separated by 1 wk. Eight subjects (aged 22 ± 2 yr) participated in experimental assessments of neuromuscular function obtained before, immediately after, 24 h after, and 7 days after each exercise bout. Each experimental session involved assessments of elbow-flexor force and biceps and triceps brachii electromyography during maximum voluntary isometric contractions (MVCs) and constant-force isometric contractions at five contraction intensities (5–50% MVC), along with indicators of muscle damage (muscle pain and passive tension). In addition, motor unit recordings were obtained before exercise, 7 days after Bout 1, and 24 h after Bout 2 to assess motor unit synchronization and recruitment thresholds. Following a single eccentric exercise session that elicited significant indicators of muscle damage, we found a 57% increase in motor unit synchronization 7 days later compared with before exercise, despite the recovery of maximal strength, soreness, and relaxed elbow-joint angle at this time. Furthermore, a second bout of the same eccentric exercise resulted in reduced indicators of muscle damage and a decline in the strength of motor unit synchronization (24 h after Bout 2) toward levels observed before both exercise sessions. In contrast, no changes in motor unit recruitment thresholds were observed 7 days after Bout 1 or 24 h after Bout 2 compared with before exercise. The increased motor unit synchronization 7 days after a single eccentric exercise session provides new evidence of changes in motor unit activity during the putative repair and regeneration phase following eccentric muscle damage.

Motor Unit Synchronization Is Increased in Biceps Brachii After Exercise-Induced Damage to Elbow Flexor Muscles

2008 ◽  
Vol 99 (2) ◽  
pp. 1008-1019 ◽  
Author(s):  
Tamara J. Dartnall ◽  
Michael A. Nordstrom ◽  
John G. Semmler
Keyword(s):  
Muscle Damage ◽  
Motor Unit ◽  
Eccentric Exercise ◽  
Biceps Brachii ◽  
Maximum Voluntary Contraction ◽  
Motor Units ◽  
Voluntary Contraction ◽  
Constant Force ◽  
Biceps Brachii Muscle ◽  
Motor Unit Synchronization

The purpose of this study was to determine the effect of eccentric exercise on correlated motor unit discharge (motor unit synchronization and coherence) during low-force contractions of the human biceps brachii muscle. Eight subjects (age, 25 ± 7 yr) performed three tasks involving isometric contraction of elbow flexors while EMG (surface and intramuscular) records were obtained from biceps brachii. Tasks were 1) maximum voluntary contraction (MVC); 2) constant-force contraction at various submaximal targets; and 3) sustained discharge of pairs of concurrently active motor units for 2–5 min. These tasks were performed before, immediately after, and 24 h after fatiguing eccentric exercise. MVC force declined 46% immediately after eccentric exercise and remained depressed (31%) 24 h later, which is indicative of muscle damage. For the constant-force task, biceps brachii EMG (∼100% greater) and force fluctuations (∼75% greater) increased immediately after exercise, and both recovered by ∼50% 24 h later. Motor unit synchronization, quantified by cross-correlation of motor unit pairs during low-force (1–26% MVC) contractions, was 30% greater immediately after ( n = 105 pairs) and 24 h after exercise ( n = 92 pairs) compared with before exercise ( n = 99 pairs). Similarly, motor unit coherence at low (0–10 Hz) frequencies was 20% greater immediately after exercise and 34% greater 24 h later. These results indicate that the series of events leading to muscle damage from eccentric exercise alters the correlated behavior of human motor units in biceps brachii muscle for ≥24 h after the exercise.

scholarly journals Motor unit activity in biceps brachii of left-handed humans during sustained contractions with two load types

2016 ◽  
Vol 116 (3) ◽  
pp. 1358-1365 ◽  
Author(s):  
Jeffrey R. Gould ◽  
Brice T. Cleland ◽  
Diba Mani ◽  
Ioannis G. Amiridis ◽  
Roger M. Enoka
Keyword(s):  
Motor Unit ◽  
Unit Activity ◽  
Position Control ◽  
Biceps Brachii ◽  
Motor Units ◽  
Motor Unit Activity ◽  
Time Interaction ◽  
Left Handed ◽  
Position Task ◽  
Task Time

The purpose of the study was to compare the discharge characteristics of single motor units during sustained isometric contractions that required either force or position control in left-handed individuals. The target force for the two sustained contractions (24.9 ± 10.5% maximal force) was identical for each biceps brachii motor unit ( n = 32) and set at 4.7 ± 2.0% of maximal voluntary contraction (MVC) force above its recruitment threshold (range: 0.5–41.2% MVC force). The contractions were not sustained to task failure, but the duration (range: 60–330 s) was identical for each motor unit and the decline in MVC force immediately after the sustained contractions was similar for the two tasks (force: 11.1% ± 13.7%; position: 11.6% ± 9.9%). Despite a greater increase in the rating of perceived exertion during the position task (task × time interaction, P < 0.006), the amplitude of the surface-recorded electromyogram for the agonist and antagonist muscles increased similarly during the two tasks. Nonetheless, mean discharge rate of the biceps brachii motor units declined more during the position task (task × time interaction, P < 0.01) and the variability in discharge times (coefficient of variation for interspike interval) increased only during the position task (task × time interaction, P < 0.008). When combined with the results of an identical study on right-handers (Mottram CJ, Jakobi JM, Semmler JG, Enoka RM. J Neurophysiol 93: 1381–1392, 2005), the findings indicate that handedness does not influence the adjustments in biceps brachii motor unit activity during sustained submaximal contractions requiring either force or position control.

Eccentric Muscle Damage Has Variable Effects on Motor Unit Recruitment Thresholds and Discharge Patterns in Elbow Flexor Muscles

2009 ◽  
Vol 102 (1) ◽  
pp. 413-423 ◽  
Author(s):  
Tamara J. Dartnall ◽  
Nigel C. Rogasch ◽  
Michael A. Nordstrom ◽  
John G. Semmler
Keyword(s):  
Muscle Damage ◽  
Motor Unit ◽  
Eccentric Exercise ◽  
Biceps Brachii ◽  
Discharge Rate ◽  
Motor Units ◽  
Motor Unit Recruitment ◽  
Recruitment Threshold ◽  
Flexor Muscles ◽  
Elbow Flexor Muscles

The purpose of this study was to determine the effect of eccentric muscle damage on recruitment threshold force and repetitive discharge properties of low-threshold motor units. Ten subjects performed four tasks involving isometric contraction of elbow flexors while electromyographic (EMG) data were recorded from human biceps brachii and brachialis muscles. Tasks were 1) maximum voluntary contraction (MVC); 2) constant-force contraction at various submaximal targets; 3) motor unit recruitment threshold task; and 4) minimum motor unit discharge rate task. These tasks were performed on three separate days before, immediately after, and 24 h after eccentric exercise of elbow flexor muscles. MVC force declined (42%) immediately after exercise and remained depressed (29%) 24 h later, indicative of muscle damage. Mean motor unit recruitment threshold for biceps brachii was 8.4 ± 4.2% MVC, ( n = 34) before eccentric exercise, and was reduced by 41% (5.0 ± 3.0% MVC, n = 34) immediately after and by 39% (5.2 ± 2.5% MVC, n = 34) 24 h after exercise. No significant changes in motor unit recruitment threshold were observed in the brachialis muscle. However, for the minimum tonic discharge rate task, motor units in both muscles discharged 11% faster (10.8 ± 2.0 vs. 9.7 ± 1.7 Hz) immediately after ( n = 29) exercise compared with that before ( n = 32). The minimum discharge rate variability was greater in brachialis muscle immediately after exercise (13.8 ± 3.1%) compared with that before (11.9 ± 3.1%) and 24 h after exercise (11.7 ± 2.4%). No significant changes in minimum discharge rate variability were observed in the biceps brachii motor units after exercise. These results indicate that muscle damage from eccentric exercise alters motor unit recruitment thresholds for ≥24 h, but the effect is not the same in the different elbow flexor muscles.

scholarly journals Electrical nerve stimulation modulates motor unit activity in contralateral biceps brachii during steady isometric contractions

2018 ◽  
Vol 120 (5) ◽  
pp. 2603-2613 ◽  
Author(s):  
Landon D. Hamilton ◽  
Diba Mani ◽  
Awad M. Almuklass ◽  
Leah A. Davis ◽  
Taian Vieira ◽  
...  
Keyword(s):  
Motor Unit ◽  
Pulse Duration ◽  
Nerve Stimulation ◽  
Unit Activity ◽  
Biceps Brachii ◽  
Elbow Flexors ◽  
Stimulus Pulse ◽  
Electrical Nerve Stimulation ◽  
Motor Unit Activity ◽  
The Right

The purpose of our study was to compare the influence of five types of electrical nerve stimulation delivered through electrodes placed over the right biceps brachii on motor unit activity in the left biceps brachii during an ongoing steady isometric contraction. The electrical stimulation protocols comprised different combinations of pulse duration (0.2 and 1.0 ms), stimulus frequency (50 and 90 Hz), and stimulus current (greater or less than motor threshold). The electrical nerve stimulation protocols were applied over the muscle of the right elbow flexors of 13 participants (26 ± 3 yr) while they performed voluntary contractions with the left elbow flexors to match a target force set at 10% of maximum. All five types of electrical nerve stimulation increased the absolute amplitude of the electromyographic (EMG) signal recorded from the left biceps brachii with high-density electrodes. Moreover, one stimulation condition (1 ms, 90 Hz) had a consistent influence on the centroid location of the EMG amplitude distribution and the average force exerted by the left elbow flexors. Another stimulation condition (0.2 ms, 90 Hz) reduced the coefficient of variation for force during the voluntary contraction, and both low-frequency conditions (50 Hz) increased the duration of the mean interspike interval of motor unit action potentials after the stimulation had ended. The findings indicate that the contralateral effects of electrical nerve stimulation on the motor neuron pool innervating the homologous muscle can be influenced by both stimulus pulse duration and stimulus frequency. NEW & NOTEWORTHY Different types of electrical nerve stimulation delivered through electrodes placed over the right biceps brachii modulated the ongoing motor unit activity in the left biceps brachii. Although the effects varied with stimulus pulse duration, frequency, and current, all five types of electrical nerve stimulation increased the amplitude of the electromyographic activity in the left biceps brachii. Moreover, most of the effects in the left arm occurred after the electrical nerve stimulation of the right arm had been terminated.

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